Method for producing polyester film and method for producing polyester pellets

Abstract

To provide a polyester film which has an extremely small amount of a resin derived from a coating layer to be attached to the polyester film while is obtained by subjecting the coating layer to peeling treatment, and can be formed into a pellet having the quality equivalent to a virgin pellet when the polyester film is re-melted and is formed into a pellet, and a polyester pellet using the same which has the quality equivalent to a virgin polyester pellet.SOLUTION: A polyester film has a polyvinyl alcohol removal index calculated from the following analysis method 1 of 0.10 or less (excluding 0). Analysis method 1: When a solution where a polyester film is dissolved is filtered, and then a filter medium is analyzed by a total reflection measurement method of infrared spectroscopy, a ratio (a / b) of a maximum absorbance a detected in the range of 3,200 to 3,650 cm-1 to a maximum absorbance b detected in the range of 2,840 to 3,000 cm-1 is defined as a polyvinyl alcohol removal index.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a polyester film, a method for producing a polyester film, polyester pellets, and a method for producing polyester pellets. [Background technology]

[0002] Polyester films are used in fields such as electronic components, optical components, labels, release films, and transfer films. Polyester films used in these applications often have a coating layer on their surface. Examples of coating layers include functional layers such as antistatic layers, hard coat layers, antireflection layers, antiglare layers, release layers, and printing layers, as well as easy-adhesion layers for ensuring adhesion between the polyester film and the functional layer or between the polyester film and a laminating adhesive. In addition, in fields such as release films and transfer films, a release layer is provided on the surface of a polyester film or on a polyester film, and a functional layer or various thin film layers are then laminated thereon. The laminated functional layer or thin film layer is then peeled off from the polyester film, and the peeled functional layer or thin film layer is used in the above-mentioned fields. In this specification, these functional layers, easy-adhesion layers, and thin film layers are sometimes collectively referred to as coating layers.

[0003] In the process of manufacturing polyester films with such easy-adhesion layers or functional layers, or in the process of manufacturing polyester films laminated with functional or thin film layers for release or transfer purposes, the polyester film may not meet specifications after a coating layer is laminated on the surface. While films that do not meet specifications are usually discarded, it is desirable to peel (remove) the coating layer from the polyester film and recycle them for efficient resource utilization. Even if they do not meet specifications, polyester films for release or transfer purposes, or polyester films used as protective films for processes, are discarded after use, but it is desirable to recycle these as well.

[0004] Technologies for recycling such films are disclosed, for example, in Patent Documents 1 and 2. Patent Document 1 describes a method for removing a foreign layer formed on a synthetic resin substrate in an alkaline aqueous solution at 105°C or higher, followed by treatment with an aqueous solution containing 0.1% to 10% by mass of peroxide to remove the foreign layer. Specifically, a so-called batch method is employed in which the substrate to be treated is first selected, the entire synthetic resin substrate is pre-cut into chips of appropriate size, crushed, and then subjected to a predetermined alkaline treatment. Patent Document 2 also describes a method for recycling the release film by removing green sheet residue from the release film after use. Specifically, the green sheet residue from the release film after use is removed by contacting the film with an adhesive member such as an adhesive roll, employing a so-called roll-to-roll method. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 3270037 [Patent Document 2] Japanese Patent Application Laid-Open No. 2011-104986 Summary of the Invention [Problem to be solved by the invention]

[0006] The method of Patent Document 1 requires the substrate to be crushed into chips in advance, which causes the problem of taking a long time to remove the heterogeneous layer. In addition, crushing into chips increases the surface area and makes it easy for foreign matter to get mixed in. In addition, the method of Patent Document 2 requires the adhesive roll When the adhesive strength of the adhesive member such as the adhesive agent decreases, the removal rate of the green sheet residue decreases.

[0007] Various materials are used for coating layers, and even after the coating layer is peeled from the polyester film, foreign matter originating from the coating layer may still remain on the surface of the polyester film. Meanwhile, when recycling polyester film, the recovered polyester film is melted and molded into pellets, and the resulting pellets are then remelted to form a film. Therefore, if foreign matter originating from the coating layer adheres to the surface of the polyester film to be recycled, it is thought that the recycled polyester film will be contaminated with foreign matter originating from the coating layer. However, no attention has been paid to these foreign matter contaminating the recycled polyester film. Furthermore, the recycled polyester film is not of sufficient quality in terms of foreign matter, etc., to be used in the same high-precision applications as the raw polyester film.

[0008] Patent Document 1 proposes using an alkaline treatment liquid to remove the coating layer, but this does not completely remove foreign matter from the coating layer, and the recycled polyester film may contain a large amount of foreign matter.

[0009] For example, polyvinyl alcohol may be used as a material for the coating layer. Because polyvinyl alcohol has a high affinity for aqueous compounds, it is used as an easy-adhesion layer for applying aqueous adhesives or coating agents to polyester films, as an aqueous ink-receiving layer for inkjet printers and the like, a release layer, or as an alignment layer for thin-film polarizers or liquid crystal compounds. When the coating layer contains polyvinyl alcohol, the polyvinyl alcohol may remain on the surface of the polyester film even after the coating layer is peeled off from the polyester film.

[0010] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a polyester film in which the coating layer has been subjected to a release treatment, but which exhibits very little adhesion of resin originating from the coating layer, and which can be remelted and pelletized to produce pellets having a quality comparable to that of virgin pellets, and to provide polyester pellets using the same having a quality comparable to that of virgin polyester pellets.

[0011] Another object of the present invention is to provide a method for producing a polyester film that can thoroughly remove deposits from a coating-layer-laminated polyester film that has a coating layer, particularly a coating layer containing polyvinyl alcohol, laminated on at least one side of the polyester film and is wound into a roll, and a method for producing polyester pellets with a significantly reduced amount of foreign matter obtained from this polyester film. [Means for solving the problem]

[0012] The present invention is configured as follows. [1] A polyester film having a polyvinyl alcohol removal index of 0.10 or less (excluding 0) calculated by the following analytical method 1. (Analysis method 1) After filtering a solution containing dissolved polyester film, the filter media was analyzed by infrared spectroscopy total reflection measurement. -1 The maximum absorbance a detected in the range of 2840 to 3000 cm -1 The ratio (a / b) of the maximum absorbance detected in the range of a to b is the polyvinyl alcohol removal index. [2] The polyester film has a coating layer containing polyvinyl alcohol laminated on at least one surface of the polyester film and is wound into a roll. The polyester film according to [1], which is produced from [3] A method for producing a polyester film, comprising: an alkali treatment step of contacting a coating layer-laminated polyester film, which has a coating layer containing polyvinyl alcohol laminated on at least one surface of a polyester film and is wound into a roll, with an alkaline treatment liquid; and a water-washing step of contacting the polyester film after the alkali treatment with water. A method for producing a polyester film having a polyvinyl alcohol removal index of 0.10 or less (excluding 0) calculated by the following analytical method 1. (Analysis method 1) After filtering a solution containing dissolved polyester film, the filter media was analyzed by infrared spectroscopy total reflection measurement. -1 The maximum absorbance a detected in the range of 2840 to 3000 cm -1 The ratio (a / b) of the maximum absorbance detected in the range of a to b is the polyvinyl alcohol removal index. [4] The method according to [3], further comprising a winding step of winding the washed polyester film into a roll. [5] The method according to [3] or [4], wherein the alkaline treatment step involves contacting the coating layer-laminated polyester film with a 20 to 60% by mass alkaline treatment liquid at a temperature of 90 to 140°C for 1 to 120 seconds. [6] The method according to any one of [3] to [5], wherein the water-washing step comprises contacting the alkali-treated polyester film with water at 90° C. or less at least once. [7] The method according to any one of [3] to [6], wherein the water-washing step comprises spraying water onto the alkali-treated polyester film. [8] Polyester pellets are melt-molded from polyester film obtained by peeling off the coating layer from a coated polyester film having a coating layer laminated on the surface, and the number of foreign objects calculated by the following analysis method 2 is 100 / mm 2 Is less than or equal to The polyester pellets are characterized by: (Analysis method 2) The polyester pellets were melted and observed under a phase contrast optical microscope at a magnification of 10x, with an observation field area of ​​1mm2 Measure the number of foreign particles with a diameter of 3 μm or more and less than 10 μm per do. [9] The polyester pellet according to [8], wherein the b value is 13 or less.

[10] A method for producing a polyester film, comprising: an alkaline treatment step of contacting a coating layer-laminated polyester film, which has a coating layer laminated on at least one surface of a polyester film and is wound into a roll, with an alkaline treatment liquid; a water-washing step of contacting the polyester film after the alkaline treatment with water; and a pelletizing step of melting and molding the polyester film after the water-washing into pellets. The number of foreign particles calculated using the following analysis method 2 is 100 / mm 2 Polyester pelle Manufacturing method of the cot. (Analysis method 2) The polyester pellets were melted and observed under a phase contrast optical microscope at a magnification of 10x, with an observation field area of ​​1mm 2 Measures the number of foreign particles with a diameter of 3 μm or more and less than 10 μm per do.

[11] The manufacturing method according to

[10] , wherein the coating layer contains polyvinyl alcohol, and the polyester film after water washing has a polyvinyl alcohol removal index of 0.10 or less (excluding 0) calculated by the following analytical method 1. (Analysis method 1) After filtering a solution containing dissolved polyester film, the filter media was analyzed by infrared spectroscopy total reflection measurement. -1 The maximum absorbance a detected in the range of 2840 to 3000 cm -1 The ratio (a / b) of the maximum absorbance detected in the range of a to b is the polyvinyl alcohol removal index.

[12] A coating in which a coating layer is laminated on at least one side of a polyester film. a water-washing step of bringing the alkali-treated polyester film into contact with water; a pelletizing step of melting and molding the water-washed polyester film into pellets; and a film-forming step of melting and molding the pellets into a film, the method comprising the steps of: 2 A method for producing a polyester film, comprising: [Effects of the Invention]

[0013] According to the above-mentioned configuration, the present invention can provide a polyester film having quality comparable to that of a virgin product, and a method for producing the same. Also, the present invention can provide polyester pellets having quality comparable to that of a virgin product, and a method for producing the same. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a schematic diagram of a roll-to-roll apparatus used in one embodiment of the present invention. [Figure 2] FIG. 2 shows the infrared absorption spectrum of the release-treated polyethylene terephthalate film after washing with water. [Figure 3] FIG. 3 shows the infrared absorption spectrum of the coating layer-laminated polyethylene terephthalate film before alkali treatment. DETAILED DESCRIPTION OF THE INVENTION

[0015]

[0003] The inventors' investigations have revealed that even in polyester films in which a coating layer has been laminated on at least one side thereof and from which the coating layer has been peeled, components derived from the coating layer still remain. When polyester pellets are formed using polyester films with such components attached, the pellets contain a large amount of foreign matter, resulting in a decrease in quality. Hereinafter, a film with a coating layer laminated thereon will be referred to as a "coated layer-laminated film," and a film from which the coating layer has been peeled will be referred to as a "release-treated film." Furthermore, when the film is a polyester film, the polyester film with a coating layer laminated thereon will be referred to as a "coated layer-laminated polyester film," and the polyester film from which the coating layer has been peeled will be referred to as a "release-treated polyester film."

[0016] Furthermore, the inventors' investigations have revealed that when the resin component contained in the coating layer is not easily decomposed in an alkaline treatment solution, for example, when the resin component contained in the coating layer is a resin with a carbon-carbon bond as the main chain, a silicone resin, or a cellulose-based resin such as cellulose acetate or nitrocellulose, or when the resin contained in the coating layer contains a crosslinking agent that is prone to react and form aggregates, such as melamine or an isocyanate compound, these resins peel off from the coating layer-laminated polyester film and remain suspended in the alkaline treatment solution in large amounts, thereby reattaching to the release-treated film, making it difficult to reduce the amount of foreign matter even under strict alkaline treatment conditions. In particular, when the resin component is polyvinyl alcohol, it swells in the alkaline treatment solution or in the water used for washing, becoming pasty and easily adhering to the film. Furthermore, when a release-treated film with these resin components adhering to its surface is melted and molded, it is likely to be included as foreign matter in pellets.

[0017] The present invention will be described in detail below. The following description will be focused on a typical example in which a coating layer containing polyvinyl alcohol is laminated on at least one surface of a polyester film. Hereinafter, the coating layer containing polyvinyl alcohol may be referred to as a PVA-containing coating layer.

[0018] The release-treated polyester film of the present invention has a polyvinyl alcohol removal index of 0.10 or less (not including 0) calculated by the following analytical method 1. (Analysis method 1) After filtering the solution in which the release-treated polyester film was dissolved, the filter media was analyzed by the total reflection measurement method of infrared spectroscopy. -1 The maximum absorbance a detected in the range of 2840 to 3000 cm -1 The ratio (a / b) of the maximum absorbance detected in the range of a to b is the polyvinyl alcohol removal index.

[0019] The polyvinyl alcohol removal index (hereinafter sometimes referred to as the PVA removal index) calculated by the above analytical method 1 indicates the amount of polyvinyl alcohol contained in the release-treated polyester film. A PVA removal index of 0.10 or less can achieve quality equivalent to that of virgin products. The PVA removal index is preferably 0.05 or less, more preferably 0.03 or less. The smaller the PVA removal index, the better. However, when a release-treated polyester film is produced from a polyester film having a PVA-containing coating layer laminated on at least one side of the polyester film (hereinafter sometimes referred to as a PVA-containing coating-laminated polyester film), a PVA removal index of 0 is not necessarily desirable from the standpoint of economical industrial production; a PVA removal index of 0.001 or more is usually preferred. The PVA removal index may be 0.003 or more.

[0020] The PVA removal index was measured by filtering a solution containing a release-treated polyester film, analyzing the filter media using infrared spectroscopy (IR) attenuated total reflection (ATR) and measuring the wavelength of light from 3200 to 3650 cm -1 The maximum absorbance a detected in the range of 2840 to 3000 cm -1 It can be calculated as the ratio (a / b) of the maximum absorbance b detected in the range of 3200 to 3650 cm -1The peak detected in the range of 2840–3000 cm is due to the OH stretching vibration of polyvinyl alcohol. -1 The peak detected in the range is due to the CH stretching vibration of the filter material.

[0021] As a solvent for dissolving the release-treated polyester film, for example, 1,1,1,3,3,3-hexafluoro-2-propanol can be used.

[0022] As the filter medium, for example, a membrane filter made of polytetrafluoroethylene (PTFE) can be used.

[0023] Examples of polyester films include polyethylene terephthalate (PET) films, polybutylene terephthalate (PBT) films, polyethylene naphthalate (PEN) films, polybutylene naphthalate (PBN) films, and polypropylene terephthalate (PPT) films, with PET films being preferred.

[0024] The thickness of the polyester film is not particularly limited, but is preferably 5 μm or more and 500 μm or less in consideration of handling, etc. The thickness is more preferably 7 μm or more, more preferably 400 μm or less, even more preferably 350 μm or less, and particularly preferably 300 μm or less.

[0025] The release-treated polyester film of the present invention is preferably produced from a PVA-containing coating layer-laminated polyester film wound into a roll. The PVA-containing coating layer may be provided on a part of at least one surface of the polyester film, or on the entire surface. The PVA-containing coating layer may be provided on at least one surface of the polyester film, or may be provided on both surfaces.

[0026] The release-treated polyester film of the present invention is a recycled product, but has an extremely low content of foreign matter, and therefore can be used in a variety of fields, and is particularly suitable for optical applications.

[0027] Next, a method for producing such a release-treated polyester film will be described.

[0028] The release-treated polyester film of the present invention can be produced by a method including: an alkaline treatment step in which a coating-layer-laminated polyester film (PVA-containing coating-laminated polyester film) having a PVA-containing coating layer laminated on at least one surface of the polyester film and wound into a roll is brought into contact with an alkaline treatment solution; and a water-washing step in which the alkali-treated polyester film is brought into contact with water. The method may further include a winding step in which the water-washed release-treated polyester film obtained above is wound into a roll. The production method of the present invention differs from the method of treating a substrate film pulverized into chips with an alkaline treatment solution described in Patent Document 1 above in that the coating-layer-laminated polyester film wound into a roll is brought into contact with an alkaline treatment solution in a long state, and then brought into contact with water.

[0029] Each step will be described below.

[0030] (Alkali treatment process) In the alkaline treatment step, a polyester film having a PVA-containing coating layer laminated on at least one side thereof and wound into a roll (a PVA-containing coating layer laminated polyester film) is brought into contact with an alkaline treatment liquid. "Contacting with an alkaline treatment liquid" refers to contacting the polyester film with an alkaline treatment liquid of a predetermined concentration, for example, by immersion, application, spraying, or other methods. "Contacting with an alkaline treatment liquid" also includes heating the polyester film by contacting it with a heated alkaline treatment liquid.

[0031] The alkaline treatment liquid contains an alkali source such as sodium hydroxide or potassium hydroxide. The alkali sources may be used alone or in combination. These alkali sources have the effect of dissolving and swelling the PVA-containing coating layer adhered to the polyester film, thereby peeling (removing) the PVA-containing coating layer from the polyester film. As the alkaline treatment liquid, an aqueous solution such as an aqueous sodium hydroxide solution or an aqueous potassium hydroxide solution is preferred, with an aqueous potassium hydroxide solution being more preferred.

[0032] The alkaline treatment liquid may further contain an alkaline treatment aid to improve the peeling efficiency of the PVA-containing coating layer. As the alkaline treatment aid, those capable of increasing the heating temperature of the alkaline treatment liquid, such as boiling point elevating agents, are preferably used. Examples of such alkaline treatment aids include surfactants, water-soluble inorganic compounds, water-soluble organic compounds, and water-soluble solvents. Specific examples include nonionic surfactants, water-soluble inorganic salts, water-soluble organic salts, water-soluble polymers, polysaccharides, alcohols, glycols (ethylene glycol, diethylene glycol, propylene glycol, etc.), water-soluble organic solvents (dimethylformamide (DMF), dimethyl sulfoxide (DMSO), ethylene glycol ethers (various cellosolves)), and the like. Among these, glycols are preferred. These may be used alone or in combination.

[0033] The alkaline treatment conditions can be appropriately selected depending on the type, thickness, etc. of the polyester film. In order to obtain a high-quality release-treated polyester film with little foreign matter using small, simple equipment, which is a preferred embodiment, without extending the overall treatment time, it is preferable to contact (e.g., immerse) the polyester film in a 20 to 60 mass % alkaline treatment solution heated to a temperature of 90 to 140°C for 1 to 120 seconds.

[0034] When the alkaline processing liquid contains both an alkali source and an alkaline processing aid, the concentration of the alkaline processing liquid containing these is adjusted to 20 to 60 mass %, and the processing liquid is heated to a temperature of 90 to 140°C, and the PVA-containing coating layer-laminated polyester film is mixed with the processing liquid for 1 to 12 minutes. It is preferable to contact for 0 seconds.

[0035] Under the alkaline treatment conditions described above, by setting the heating temperature of the alkaline treatment liquid to 90°C or higher, the PVA-containing coating layer can be sufficiently removed even with small, simple equipment, which is a preferred embodiment. Furthermore, since a high-concentration alkaline treatment liquid is preferably used in the present invention as described above, by increasing the temperature of the alkaline treatment liquid, it is possible to prevent the alkaline component from precipitating and the treatment liquid itself from solidifying. The heating temperature of the alkaline treatment liquid is more preferably 100°C or higher. On the other hand, by setting the heating temperature of the alkaline treatment liquid to 140°C or lower, it is possible to prevent the release-treated polyester film from deteriorating due to contact with the alkaline treatment liquid. The heating temperature of the alkaline treatment liquid is more preferably 135°C or lower.

[0036] Under the alkaline treatment conditions described above, by setting the concentration of the alkaline treatment liquid (when the alkaline treatment liquid contains an alkali source and an alkaline treatment aid, the total concentration of these) to 20% by mass or more, the PVA-containing coating layer can be removed in a short time to obtain a high-quality release-treated polyester film with little foreign matter. This allows the PVA-containing coating layer to be sufficiently removed even using small, simple equipment, which is a preferred embodiment. The concentration of the alkaline treatment liquid is more preferably 25% by mass or more, even more preferably 30% by mass or more, and particularly preferably 35% by mass or more. On the other hand, by setting the concentration of the alkaline treatment liquid to 60% by mass or less, deterioration of the release-treated polyester film due to contact with the alkaline treatment liquid can be prevented. The concentration of the alkaline treatment liquid is more preferably 55% by mass or less, even more preferably 50% by mass or less, particularly preferably 47% by mass or less, and most preferably 45% by mass or less.

[0037] Under the above alkaline treatment conditions, the PVA-containing coating layer can be sufficiently removed by setting the contact time with the alkaline treatment solution to 1 second or longer. The contact time is more preferably 3 seconds or longer, and even more preferably 5 seconds or longer. On the other hand, setting the contact time with the alkaline treatment solution to 120 seconds or shorter enables treatment using a small, convenient roll-to-roll system (details will be described later), which is preferably used. Specifically, this simplifies the process of laying the polyester film in the alkaline treatment tank, allowing for a smaller alkaline treatment tank, and the number of alkaline treatment tanks can be reduced, thereby enabling the overall equipment to be more compact. The contact time is more preferably 90 seconds or shorter, even more preferably 60 seconds or shorter, and particularly preferably 40 seconds or shorter.

[0038] The alkaline treatment conditions (particularly the concentration and time) differ from those described in Patent Document 1. Patent Document 1 employs a batch process in which the substrate is crushed before alkaline treatment. For example, the concentration of the aqueous sodium hydroxide solution is controlled to a low level of 0.1 to 20% by mass to prevent hydrolysis and dissolution of the substrate film due to heating. Furthermore, in Patent Document 1, in order to effectively achieve the peeling effect of the alkaline treatment, the alkaline treatment is performed for a long period of time, for example, 30 to 60 minutes, using a 5% by mass aqueous sodium hydroxide solution. In contrast, in the present invention, aiming for a short peeling process, the polyester film is unwound in a roll and directly contacted with the alkaline treatment solution while still in a long form. Therefore, the concentration of the alkaline treatment solution is 20 to 60% by mass, and the contact time with the alkaline treatment solution is 1 to 120 seconds, which is higher than that of Patent Document 1, enabling alkaline treatment at a shorter time. Thus, both of these methods differ from Patent Document 1 in terms of preferred alkaline treatment conditions.

[0039] The number of alkali treatments is preferably 1 to 5 times, more preferably 1 to 3 times. Here, one alkali treatment refers to the number of times the polyester film is brought into contact with the alkaline treatment solution. When the polyester film is immersed in the alkaline treatment solution, one treatment is from immersion to removal. When the alkali treatment is carried out multiple times using an alkaline treatment tank, the same alkaline treatment tank may be used, or separate alkaline treatment tanks may be used. Using separate alkaline treatment tanks improves the flexibility of the equipment and reduces the amount of alkali treatment in the alkaline treatment tank. This is also preferable in terms of management of the alkaline processing solution.

[0040] The alkali treatment may be carried out in combination with physical means such as brushing or ultrasonic treatment, if necessary.

[0041] In the alkaline treatment step, residues of the PVA-containing coating layer peeled from the surface of the polyester film are present in the alkaline treatment liquid, and therefore it is preferable to carry out the alkaline treatment step while appropriately removing the residues of the PVA-containing coating layer from the alkaline treatment liquid.

[0042] (Water washing process) In the water-washing step, the polyester film after the alkali treatment is brought into contact with water to wash it. By bringing the polyester film into contact with water, the alkaline treatment solution adhering to the polyester film and the residue of the PVA-containing coating layer adhering in the alkali treatment step can be removed.

[0043] The method for contacting the alkali-treated polyester film with water is not particularly limited, and examples thereof include immersing the alkali-treated polyester film in a water washing tank and spraying water onto the alkali-treated polyester film, and it is preferable to use these methods in combination. In the water washing step, it is more preferable to spray water onto the polyester film. By spraying water, the alkaline treatment solution and polyvinyl alcohol adhering to the polyester film can be efficiently removed. The method for spraying water is not particularly limited, and for example, water may be sprayed in a shower-like manner from a nozzle provided toward the polyester film.

[0044] The water washing conditions can be appropriately selected depending on the type, thickness, etc. of the polyester film. However, in order to obtain a high-quality release-treated polyester film with little foreign matter using small, simple equipment, which is a preferred embodiment, without extending the overall treatment time, it is preferable to contact the polyester film with water at 90°C or lower at least once.

[0045] By using water at 90°C or less, the PVA-containing coating layer can be dissolved without damaging the polyester film. Furthermore, since boiling of water can be prevented, safe and stable washing can be achieved. The washing temperature is more preferably 80°C or less, and even more preferably 70°C or less. There is no particular lower limit for the water temperature, but it should be above 0°C because water solidifies below 0°C. The water temperature is more preferably 10°C or higher, even more preferably room temperature (27°C) or higher, and particularly preferably 40°C or higher.

[0046] The number of water washes is preferably two or more, preferably five or fewer, and more preferably three or fewer. Here, one water wash refers to the number of times the polyester film is brought into contact with water. When the polyester film is immersed in water, one wash is counted from the time the film is immersed until it is removed. When spraying water onto the polyester film, for example, it refers to the number of times water is sprayed onto the polyester film from a nozzle. When multiple nozzles are arranged at intervals in the film transport direction, one wash is counted until the water is drained midway. For example, when two nozzles are arranged at intervals in the film transport direction, the number of water washes is counted as one. When, for example, a mangle or the like is arranged between the nozzles to remove water adhering to the film, the number of water washes is counted as two. Furthermore, when the water sprayed onto the film is poured off into a water washing tank, the combination of the water spraying and the treatment in the water washing tank is counted as one wash.

[0047] When washing with water is performed two or more times, the water temperature may be the same for each wash, or different temperatures may be used. When changing the washing temperature, the temperature of the (n+1)th wash may be higher or lower than the temperature of the nth wash, with a lower temperature being preferred. The higher the water temperature, the more easily PVA dissolves. Since PVA is easily dissolved, the removal rate of PVA can be increased by contacting the alkali-treated polyester film with water at a relatively high temperature and then with water at a relatively low temperature.

[0048] The water used in the washing step may contain a surfactant, an antifoaming agent, etc., but it is preferable that these agents not be added to the water used in the final washing step.

[0049] Although the contact time with water is not particularly limited, by setting it to 1 second or more, the alkaline treatment solution and residues of the PVA-containing coating layer can be sufficiently removed. The contact time is more preferably 3 seconds or more, and even more preferably 5 seconds or more. On the other hand, by setting the contact time to 120 seconds or less, washing with water can be performed even with a preferably used small and simple roll-to-roll system (details will be described later). Specifically, since the process of laying the polyester film in the water washing tank can be simplified, the water washing tank can be made smaller, and the number of water washing tanks can be reduced, thereby making the entire equipment more compact. The contact time is more preferably 90 seconds or less, even more preferably 60 seconds or less, and particularly preferably 40 seconds or less.

[0050] When the water-washing step is carried out in multiple steps, it is preferable to set the total treatment time of the water-washing step to, for example, 1 to 120 seconds in order to obtain a high-quality polyester film with less foreign matter without extending the total treatment time using small and simple equipment, which is a preferred embodiment.

[0051] When the alkali-treated polyester film is brought into contact with water by immersing it in a water washing tank, it is preferable to carry out the process while appropriately removing from the water washing tank any residue of the PVA-containing coating layer that has adhered to the film and been carried into the water washing tank.

[0052] The inventors' investigations have revealed that coating layers containing resins whose main chains are resistant to alkali decomposition, such as polyvinyl alcohol, leave a large amount of residue in the alkaline processing solution as fine powder or thin-film-like solids. This residue adheres to the film and is carried from the alkaline treatment tank to the washing tank, resulting in a large amount of coating residue even in the washing tank. If the water in the washing tank is not reused, the water used for washing does not contain any coating residue. However, for the purpose of reducing waste liquid and for economic reasons, the washing water is stored in the washing tank and reused. For these reasons, it is preferable to perform processing while removing the coating residue from the alkaline processing solution and washing water.

[0053] The method for removing the residue can be a suitable combination of methods such as using a filter, centrifugal separation, scraping off suspended matter with a squeegee or the like, settling in a settling tank, or adding a certain amount of fresh alkaline processing solution or washing water and removing the corresponding amount of used alkaline processing solution or washing water by overflow or the like. As will be described later, it is also preferable to use a multi-stage washing step, and in the final stage, filter the solution or rinse the release-treated polyester film with a shower using fresh water free of suspended matter. It is also preferable to add the water used for the shower to the washing tank and then flush it out by overflow or the like.

[0054] The method of the present invention makes it possible to remove the residue of the coating layer in a simple manner, and to obtain a release-treated polyester film from which the coating layer has been removed to a low level, and polyester pellets obtained therefrom containing little foreign matter.

[0055] When the alkali-treated polyester film is brought into contact with water multiple times by immersing it in a water washing tank, the same water washing tank may be used or separate water washing tanks may be used. Using separate water washing tanks is preferred in terms of improving the flexibility of the equipment and also in terms of managing the water in the water washing tanks.

[0056] After the water washing step, it is preferable to dry the film. The drying temperature is not particularly limited, but is preferably, for example, 90 to 140°C. By setting the drying temperature to 90°C or higher, drying can be completed in a short time, making it possible to produce a polyester film even with small, simple equipment, which is a preferred embodiment. The drying temperature is more preferably 100°C or higher. On the other hand, by setting the drying temperature to 140°C or lower, deterioration of the polyester film can be prevented. The drying temperature is more preferably 135°C or lower.

[0057] The drying time is not particularly limited, but is preferably 50 to 150 seconds, for example. By setting the drying time to 50 seconds or more, uneven drying can be reduced. The drying time is more preferably 60 seconds or more. On the other hand, by setting the drying time to 150 seconds or less, deformation of the polyester film can be prevented. The drying time is more preferably 140 seconds or less.

[0058] The dried polyester film may be cut into sheets or shredded in a shredder or the like, but is preferably subjected to a winding process.

[0059] (winding process) In the winding step, the release-treated polyester film after water washing is wound into a roll. That is, the coated layer-laminated polyester film wound into a roll is fed out, contacted in a long state with an alkaline treatment solution to peel off the coating layer, contacted with water to wash, and dried, thereby winding into a roll. The length of "long" varies depending on, for example, the type of equipment used, but generally means 100 to 20,000 m. The term "long" includes both an embodiment in which a long coated layer-laminated polyester film wound into a roll is fed out and contacted with an alkaline treatment solution as is, and an embodiment in which the film is contacted with an alkaline treatment solution after being adjusted (treated) to a state that allows contact with the alkaline treatment solution. The former "long state as is" refers to an embodiment in which the film is contacted while maintaining the same width and length as the long coated layer-laminated polyester film wound into a roll. The latter "adjusted state" includes, for example, a mode in which a long, wound-up coated layer-laminated polyester film is cut into a width or length sufficient for contact with the alkaline treatment liquid and then rewound into a roll; or a mode in which, when the cut long coated layer-laminated films are introduced separately into the alkaline treatment liquid, the film is cut into a length sufficient for contact before being brought into contact with the alkaline treatment liquid if the film is too long. Furthermore, "contacting the coated layer-laminated polyester film in a long state with the alkaline treatment liquid" means that the coated layer-laminated polyester film in the above-described state is brought into direct contact with the alkaline treatment liquid without being crushed. By removing the coating layer in a long state in this way, problems associated with conventional methods, such as the coating layer digging into the cut portions during crushing into chips and leaving the coating layer behind, or the peeled coating layer getting caught between the chips or at the cut portions, are less likely to occur, and the coating layer can be effectively removed from the film.

[0060] According to the manufacturing method of the present invention, unlike the batch method in which the base film is crushed in advance as in Patent Document 1, a coating layer-laminated polyester film wound into a roll is directly contacted with an alkaline treatment solution in a long state, thereby enabling efficient peeling of a coating layer containing polyvinyl alcohol and the like in a short time. The roll-to-roll method, in which a roll-shaped coating layer-laminated polyester film is contacted with an alkaline treatment solution and, after the contact, the long polyester film from which the coating layer containing polyvinyl alcohol and the like has been peeled is directly introduced into the next process (water washing and drying) and wound up in a roll, is a roll-to-roll method in which a roll-shaped coating layer-laminated polyester film is continuously transported via multiple guide rolls, and the polyester film that has undergone the required treatment is then wound up into a roll again. The roll-to-roll method allows a series of peeling steps to be performed continuously in a short time, making it highly efficient. Furthermore, transportation is also efficient when the polyester film from which the coating layer has been peeled is further transferred to a next process, such as cutting or melting and pelletizing.

[0061] Examples of the coating layer laminated polyester film wound into a roll include one in which an easy-adhesion layer is provided on at least one surface of the polyester film, one in which a functional layer is provided on the polyester film directly or via an easy-adhesion layer, and one in which a functional layer or thin film layer for peeling is provided on the polyester film directly or via an easy-adhesion layer.

[0062] In the case of a polyester film having a functional layer laminated thereon as a coating layer or a polyester film having a release functional layer or thin film layer laminated thereon, the method may further include a step of removing the functional layer or thin film layer that is a part of the coating layer prior to the alkali treatment step. Alternatively, the polyester film may be subjected to the alkali treatment step while the functional layer or thin film layer is still laminated thereon.

[0063] Examples of functional layers include antistatic layers, adhesive layers, hard coat layers, antireflection layers, antiglare layers, release layers, printing layers, thin film polarizers, alignment layers for liquid crystal compounds, and ink-receiving layers. Examples of thin film layers include thin films such as polyimide precursors, polyarylates, and polysulfones, skin materials, ceramic green sheets, fuel cell electrode material sheets, and metal thin film layers. These coating layers, such as easy-adhesion layers, functional layers, and thin film layers, may be present on at least one side of the polyester film, or may be present on both sides. The coating layers may be present alone, or two or more types may be laminated. The coating layers may have not only a single function but also multiple functions.

[0064] Hereinafter, some examples of the easy-adhesion layer and the functional layer will be specifically described.

[0065] The easy-adhesion layer is provided on a polyester film, particularly on a stretched PET film, and is a layer that improves or adjusts the adhesion (adhesion strength) with a functional layer or adhesive.

[0066] Examples of resins used in the easy-adhesion layer include copolymer polyester, polyester polyurethane, polyether polyurethane, polycarbonate polyurethane, acrylic resin, urethane-modified acrylic resin, polyvinyl alcohol, and cellulose-based resin. The easy-adhesion layer is preferably crosslinked, and examples of crosslinking agents include isocyanate compounds, oxazoline compounds, epoxy compounds, and amino resins such as melamine. In addition, organic or inorganic lubricant particles may be added.

[0067] The adhesive layer is preferably formed inline during the film production process. The thickness of the adhesive layer is preferably 10 to 200 nm, more preferably 20 to 150 nm.

[0068] The resin constituting the antistatic layer is not particularly limited as long as it is a resin commonly used in antistatic layers. The antistatic layer may further contain an antistatic agent. Examples of the antistatic agent include nonionic, cationic, anionic, and amphoteric surfactants; conductive polymers such as polypyrrole, polyaniline, and poly(3,4-ethylenedioxythiophene) / poly(4-styrenesulfonate); SnO2 (Sb-doped), In2O3 (Sn-doped), and ZnO (A). Examples of the antistatic agent include metal oxide fillers such as 1-doped cellulose; and carbon-based materials such as carbon black, graphene, and carbon nanotubes (CNT). These antistatic agents may be used alone or in combination of two or more.

[0069] The adhesive layer contains an adhesive commonly used for adhesive layers. The type of adhesive is not particularly limited, and examples thereof include acrylic adhesives, rubber adhesives, polyurethane adhesives, and silicone adhesives. The adhesive layer may further contain a tackifier (tackifier resin).

[0070] The resin constituting the hard coat layer is not particularly limited as long as it is a resin generally used in hard coat layers, and examples thereof include acrylic resins, urethane resins, epoxy resins, etc. These resins may be used alone or in combination of two or more.

[0071] The resin constituting the release layer is not particularly limited as long as it is a resin commonly used in release layers, and examples thereof include silicone resins such as polydimethylsiloxane, amino resins such as melamine resins and urea resins, acrylic resins, epoxy resins, and alkyd resins. These resins may be used alone or in combination of two or more. The release layer may further contain a release agent such as silicone oil or wax.

[0072] The thickness of the functional layer (if there are multiple functional layers, the total thickness of these layers) is preferably 0.001 μm or more. To effectively exert the predetermined function, the thickness of the functional layer is more preferably 0.005 μm or more, and even more preferably 0.01 μm or more. On the other hand, by making the thickness of the functional layer 50 μm or less, the functional layer can be removed in a short time. The thickness of the functional layer is more preferably 20 μm or less, even more preferably 10 μm or less, and particularly preferably 5 μm or less.

[0073] A surface treatment layer may be laminated on the surface of the functional layer in order to enhance affinity with the alkaline treatment solution. The surface modification treatment used to form the surface treatment layer can be a known method, such as corona treatment, plasma treatment, or flame treatment.

[0074] As mentioned above, coating layers such as functional layers may contain resins whose main chains are difficult to decompose by alkali or may be highly crosslinked, making them difficult to remove. In such cases, it is preferable to subject the surface of the coating layer to a surface treatment. Surface treatment refers to a treatment that hydrophilizes the surface of the coating layer to increase its affinity with an alkaline treatment solution, or deteriorates it in some way to make it more susceptible to alkali decomposition or penetration. Surface treatments are also called oxidation treatments, decomposition treatments, deterioration treatments, hydrophilization treatments, etc. Furthermore, the purpose of surface treatments is not to peel the coating layer itself, but to make it easier to peel the coating layer by alkali treatment.

[0075] Examples of surface treatments include wet treatments, dry treatments, mechanical treatments, and swelling treatments. Wet treatments include treatments using aqueous solutions of ozone, hydrogen peroxide, nitric acid, perchloric acid, permanganic acid, and the like. Dry treatments include ozone treatment, hydrogen peroxide gas treatment, corona treatment, plasma treatment, and flame treatment. Mechanical treatments include treatments using a sander roller or a metal brush to increase the contact area with the alkaline treatment liquid. Swelling treatments include treatments involving coating with a solvent or the like. Among these, dry treatments are preferred in terms of safety, ease of handling, and stability of effect, with corona treatment, plasma treatment, and flame treatment being more preferred, and corona treatment being even more preferred.

[0076] Surface treatment is an effective method not only for the roll-type treatment method according to the present invention, but also for batch-type treatment when the coating layer contains a resin whose main chain is difficult to decompose by alkali, as described above, or when the coating layer is highly crosslinked. However, in terms of the high removal ability achieved by using a high concentration of alkali and the ability to remove residues of the removed coating layer, it is preferable to perform surface treatment in combination with the roll-type treatment method according to the present invention. In the case of a batch-type method, the concentration of the alkaline treatment liquid is particularly preferably 20% by mass or less, more preferably 15% by mass or less, and even more preferably 10% by mass or less. In the case of a batch-type method, the contact time with the alkaline treatment liquid is long, and if the concentration of the alkaline treatment liquid becomes too high, the polyester of the substrate may be decomposed. In the case of a batch-type method, The contact time with the alkaline treatment liquid is preferably 5 minutes or more, more preferably 10 minutes or more, even more preferably 15 minutes or more, and is preferably 150 minutes or less, more preferably 120 minutes or less.

[0077] Next, the method for producing a release-treated polyester film according to the present invention will be described in more detail with reference to Fig. 1. Fig. 1 shows an example of an apparatus 100 preferably used in a so-called roll-to-roll system, but the method is not limited to this as long as the effects of the present invention are effectively exhibited. For example, the number of rolls shown in Fig. 1 can be changed as appropriate.

[0078] First, a coating-layer-laminated polyester film 1a having a PVA-containing coating layer laminated on at least one surface and wound into a roll is prepared, and introduced into an alkaline treatment tank 3 containing an alkaline treatment liquid via a plurality of guide rolls 2. The details of the treatment method using the alkaline treatment liquid are as described above.

[0079] The manner of introduction into the alkaline treatment tank 3 is not particularly limited. To adjust the contact time with the alkaline treatment solution, for example, the coated layer-laminated polyester film may be passed over multiple guide rolls in a W-shape, and then brought into contact with the alkaline treatment solution. While FIG. 1 shows an example of a configuration in which one alkaline treatment tank 3 is provided, multiple alkaline treatment tanks 3 may be provided, and the contact time with the alkaline treatment solution may be adjusted by passing the film through multiple alkaline treatment tanks. That is, by using multiple guide rolls to form a W-shape after introduction into the alkaline treatment tank 3, or by using multiple alkaline treatment tanks 3, the contact time with the alkaline treatment solution can be extended without changing the conveying speed, thereby enabling treatment without extending the overall treatment time.

[0080] Next, the polyester film is pulled out of the alkali treatment tank 3 and introduced into a water washing tank 4, where the residue of the PVA-containing coating layer that was once peeled off from the surface of the polyester film by the alkali treatment and then reattached is removed by washing with water.

[0081] In FIG. 1, two washing tanks 4 are arranged, washing tank 4a stores hot water at 40 to 90° C., and washing tank 4b stores water at a lower temperature than the hot water stored in washing tank 4a.

[0082] In the water washing tank 4b, nozzles 5a and 5b are arranged on the path of the polyester film introduced from the water washing tank 4a into the water washing tank 4b, and water is sprayed from the nozzles onto the polyester film. The number of nozzles is not limited to two, and may be one, or three or more. The nozzles may be arranged so as to spray water onto both sides of the polyester film as shown in FIG. 1, or so as to spray water onto only one side. Furthermore, nozzles may be arranged in the water washing tank 4a, or in both the water washing tank 4a and the water washing tank 4b.

[0083] FIG. 1 shows an example in which the polyester film is immersed in water stored in a water washing tank 4, but the present invention is not limited to this as long as the above-mentioned object is achieved, and for example, showering may also be performed.

[0084] As shown in Fig. 1, a mangle 6 may be disposed between the alkaline treatment tank 3 and the washing tank 4a. The polyester film obtained after treatment in the alkaline treatment tank 3 is squeezed with the mangle 6 to remove the alkaline treatment solution adhering to the polyester film, thereby improving the efficiency of washing in the washing tank 4.

[0085] FIG. 1 shows an embodiment in which the coating material is introduced into the water washing tank 4 immediately after the alkaline treatment tank 3, but this is not limiting. For example, between the alkaline treatment tank 3 and the water washing tank 4, a brush, ultrasonic waves, a water jet, or the like may be used to remove the residue of the PVA-containing coating layer that was not removed in the alkaline treatment tank 3. The auxiliary vessel may be provided with any physical means.

[0086] The release-treated polyester film washed in the water washing tank 4b is introduced into a drying furnace 7, dried, and then wound into a roll, to obtain a release-treated polyester film 1b wound into a roll.

[0087] As shown in Fig. 1, a mangle 8 may be placed inside the drying furnace 7. The release-treated polyester film obtained by washing in the water washing tank 4b is squeezed with the mangle 8 to remove water adhering to the release-treated polyester film, thereby shortening the drying time.

[0088] The release-treated polyester film thus obtained may be used as a film for various applications as it is. It is also preferable to melt and mold it. When melting and molding it, it is preferable to pulverize the release-treated polyester film. When pulverizing, the dried release-treated polyester film may be pulverized without being wound up, but it is preferable to first wind it up into a roll and then pulverize the release-treated polyester film unwound from the roll using a shredder or the like. The pulverized release-treated polyester film may be fed into an extruder to form a molded product such as a film, but it is preferable to first process it into pellets and then use these pellets to form a film or the like. By processing it into pellets, supply stability can be ensured when it is fed into the extruder. Furthermore, when it is mixed with virgin pellets and then fed into the extruder, segregation is less likely to occur, and molded products of stable quality can be obtained.

[0089] The method for pelletizing the release-treated polyester film is not particularly limited, but for example, it is preferable to feed the release-treated polyester film pulverized with a shredder or the like into an extruder, melt it, and pelletize it. The extruder may be a single-screw or twin-screw extruder. The melting temperature is equal to or higher than the melting point of the polyester raw material of the film, preferably "melting point + 10°C" or higher, and equal to or lower than the decomposition temperature, preferably "decomposition temperature - 10°C" or lower. Specifically, when the polyester is PET, the melting temperature is preferably equal to or higher than 260°C, more preferably equal to or higher than 270°C, even more preferably equal to or higher than 275°C, and is preferably equal to or lower than 300°C, even more preferably equal to or lower than 290°C.

[0090] The molten polyester is extruded through a die in the form of a strand, cooled with water, and cut into pellets, or extruded into water and cut into pellets by an underwater cutter installed at the die outlet, where it is cooled. The pellets are then dried.

[0091] The shape of the pellets is not particularly limited, and examples thereof include dice-shaped, bale-shaped, flat bale-shaped, spherical, and rugby ball-shaped. The size of the pellets is preferably, for example, 1 to 5 mm in each side or diameter (longer diameter, shorter diameter), more preferably 2 to 4 mm. The shape and size of the pellets are preferably the same as those of virgin pellets so that segregation is less likely to occur when mixed with virgin pellets.

[0092] To prevent the pelletization process from producing foreign matter, it is preferable to prevent resin retention in the extruder, piping, and die. It is also preferable to use water for cooling the chips that contains as little foreign matter as possible by filtering the water.

[0093] The polyester pellets obtained by melt molding the release-treated polyester film of the present invention (hereinafter sometimes referred to as recycled pellets) have a foreign matter count of 100 pieces / mm3 as calculated by the following analysis method 2. 2 It is preferable that: (Analysis method 2) The polyester pellets were melted and observed under a phase contrast optical microscope at a magnification of 10x, with an observation field area of ​​1mm 2 Measures the number of foreign particles with a diameter of 3 μm or more and less than 10 μm per The diameter refers to the maximum diameter.

[0094] The number of foreign objects in the recycled pellets is 100 / mm 2 By doing the following, optical poly When used as a polyester film that serves as the base material for release films for precision thin films such as ester films, ceramic green sheets, and liquid crystal compound thin films, the number of defects can be reduced to the same level as films made only from virgin pellets. The number of foreign objects in the recycled pellets is 80 / mm2 Preferably less than 60 pieces / mm 2 Less than 50 / mm 2 Less than 40 pieces / mm 2 The following is the result.

[0095] It is preferable that the number of foreign matters in the recycled pellets is small. In addition to the above-mentioned coating layer peeling method, it is also possible to reduce the number of foreign matters by improving the precision of the filter for the molten resin used in the extruder. However, because there are also foreign matters such as polyester degradation products, catalyst and lubricant aggregates, etc., and it is sometimes difficult to completely eliminate coating residues, and the filter pore size cannot be made smaller than necessary from the viewpoint of industrial productivity, the number of foreign matters in the recycled pellets is limited to 3 / mm. 2 More preferably, 5 pieces / mm 2 More preferably, 8 pieces / mm 2 More than 10 pieces / mm 2 That's all.

[0096] For the reasons mentioned above, the foreign matter in the recycled pellets may contain material derived from the coating layer, or may contain material not derived from the coating layer, such as degradation products of the polyester, the raw material for the film, or aggregates of polyester polymerization catalysts or lubricants. Whether the foreign matter is derived from the coating layer can be determined by dissolving the recycled pellets in a solvent such as HFIP, filtering them through a membrane filter such as PTFE, and then measuring the foreign matter on the filter using microscopic IR or fluorescent X-rays to determine whether or not there are characteristic peaks or elements derived from the coating layer.

[0097] In addition, the recycled pellets contain 2 foreign particles per mm with a diameter of 10 μm or more. 2 The following is preferred: 1 piece / mm 2 The following is more preferred, and most preferred is that none is observed:

[0098] The method for keeping the amount of foreign matter in the recycled pellets within the above range can be carried out in the same manner as the above-mentioned method for peeling the coating layer, even if the coating layer does not contain polyvinyl alcohol.

[0099] Furthermore, although the amount of polyvinyl alcohol contained in the release-treated polyester film was evaluated using the PVA removal index, even if the coating layer does not contain polyvinyl alcohol, the extent to which the resin originating from the coating layer has been removed can be confirmed at the release-treated polyester film stage by dissolving the release-treated polyester film in an appropriate solvent, filtering it through a membrane filter, and measuring the residue on the filter by IR. Furthermore, the extent to which the resin originating from the coating layer has been removed can also be confirmed by measuring the surface of the release-treated polyester film by ATR-IR, X-ray fluorescence, or ESCA.

[0100] When the coating layer does not contain polyvinyl alcohol, there are no particular restrictions on the resin used in the coating layer. However, as described above, resins that are resistant to decomposition in an alkaline treatment solution and are prone to becoming foreign matter include resins with a carbon-carbon bond as the main chain. For example, resins commonly used as coating agents include acrylic resins, various modified acrylic resins such as silicone-modified acrylic resins, polyvinyl acetal, ethylene-vinyl acetate copolymers, and ethylene-vinyl alcohol copolymers. Furthermore, examples of resins other than those with a carbon-carbon bond as the main chain include silicone resins and cellulose-based resins such as cellulose acetate and nitrocellulose.

[0101] The recycled pellets of the present invention preferably have a b value in the Hunter Lab color space of 13 or less. The b value indicates the yellow / blue color coordinate, with +b indicating yellow and -b indicating blue. The b value is thought to be affected by the content of foreign matter originating from the coating layer, including foreign matter in the polyester film that cannot be observed with a microscope, etc., and a b value of 13 or less is an indicator of a low amount of foreign matter. By keeping the b value at the above level or less, not only can it be used for various purposes like virgin pellets, but when blended with virgin pellets to produce molded products, molded products with a consistent color tone can be obtained even if the ratio of virgin pellets to recycled pellets varies. The b value is more preferably 12 or less. The lower limit of the b value is preferably -2 or more, and even more preferably 0 or more. The method for measuring the b value will be explained in the Examples section.

[0102] The polyester pellets obtained from the release-treated polyester film of the present invention (i.e., recycled pellets) contain little foreign matter and can be used as materials for molded products for a variety of applications. In particular, they are preferably used as polyester films that serve as the base material for coated-layer laminated polyester films, particularly as the base material for optical polyester films and release films. By using recycled pellets in polyester films for the same applications as the coated-layer laminated polyester film to be subjected to release treatment, the production of the film can be achieved with a stable material balance with little surplus material, and a stable, high-quality polyester film can be obtained.

[0103] When recycled pellets are used in a molded product, they may be used alone or blended with virgin pellets. When blended with virgin pellets, the proportion of recycled pellets relative to the total is preferably 5 to 95 mass%, more preferably 10 to 90 mass%, and even more preferably 15 to 80 mass%.

[0104] When recycled pellets are used to form molded articles, the manufacturing conditions are the same as when virgin pellets are used. For example, in the case of a stretched polyethylene terephthalate film, PET pellets containing recycled pellets are melted in an extruder at 270 to 300°C and extruded onto a cooling roll to form a sheet. The resulting sheet is heated to 70 to 130°C and stretched, and then heat-set at 150 to 230°C to form a stretched film. The stretching may be uniaxial or biaxial, and roll stretching is preferably used in the machine direction (MD) of the film formation, while tenter stretching is preferably used in the transverse direction (TD) perpendicular to the MD. Simultaneous biaxial stretching using a tenter may also be used.

[0105] The thickness of a film produced using recycled pellets is preferably 5 μm or more and 500 μm or less, more preferably 7 μm or more, more preferably 400 μm or less, even more preferably 350 μm or less, and particularly preferably 300 μm or less.

[0106] The film produced using recycled pellets is preferably provided with a coating layer. The coating layer of the film may be formed by an in-line method during film formation or an offline method separately performed after film formation. In one preferred embodiment, the coating layer is the same as the coating layer of the coated-layer laminated polyester film to be subjected to the release treatment.

[0107] Foreign matter in films made from recycled pellets was melted and observed under a phase contrast optical microscope at a magnification of 10x, with an observation field area of ​​1mm 2 The diameter of each particle is 3 μm The number of foreign particles of 10 μm or more and less than 100 / mm 2 It is preferable that the number of foreign objects is less than or equal to the number of foreign objects. Number: 80 / mm 2 More preferably, 60 or less per mm 2 Less than 50 pieces / mm 2 Less than or equal to 40 / mm 2 The number of foreign particles in the film is 3 / mm 2More preferably, 5 pieces / mm 2 More preferably, 8 pieces / mm 2 More than 10 pieces / mm 2 In addition, the number of foreign particles in the film with a diameter of 10 μm or more is 2 / mm 2 Preferably less than 1 piece / mm 2 The following is more preferable, and it is most preferable that no foreign matter is observed. The procedure is the same as in Analysis Method 2, except that approximately 30 mg of the film cut into approximately 5 mm cubes is used. [Example]

[0108] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples and can be practiced with modifications within the scope of the above-mentioned or below-mentioned aims, all of which are included in the technical scope of the present invention. In the following, unless otherwise specified, "%" means % by mass.

[0109] (Coated layer laminated polyethylene terephthalate film A) A polyester film having a PVA-containing coating layer laminated on at least one side was prepared using the following procedure. Specifically, polyethylene terephthalate pellets (raw material pellets) were fed into an extruder, melted at 285°C, and extruded onto a cooling roll to produce a sheet. The sheet was stretched 3 times in the MD direction at 90°C between rolls operating at different peripheral speeds. The resulting uniaxially stretched film was then coated on both sides with a coating solution containing a copolymerized polyester, polyvinyl alcohol, and blocked isocyanate crosslinking agent in a solids ratio of 9:3:1. The solvent was then removed in a dryer. The film was then introduced into a tenter and stretched 3.3 times at 100°C. It was then heat-set at 200°C while crosslinking and curing the coating layer, producing a coated-layer-laminated polyethylene terephthalate film A. The coating layers formed on both sides of the coated-layer-laminated polyethylene terephthalate film A each had a thickness of approximately 60 nm. The obtained coating layer laminated polyethylene terephthalate film A had a thickness of 75 μm, a width of 60 cm and a length of 3000 m, and was wound in a roll shape around a plastic core.

[0110] (No.1) The coating-laminated polyethylene terephthalate film A thus prepared was subjected to an alkali treatment, water washing, drying, and winding using the apparatus shown in Figure 1 to produce a release-treated polyethylene terephthalate film. Specifically, the alkaline treatment solution stored in the alkaline treatment tank 3 contained potassium hydroxide as an alkali source, had a concentration of 42%, and was heated to 115°C. The alkaline treatment step was performed once. The contact time between the coating-laminated polyethylene terephthalate film A and the alkaline treatment solution was 30 seconds. Polyvinyl alcohol floating on the water surface in the alkaline treatment tank 3 was removed appropriately by scooping it up with a stainless steel mesh. The water washing tank 4a contained warm water at a temperature of 60°C, and the water washing tank 4b contained water at room temperature (27°C). The contact time between the polyethylene terephthalate film and the warm water in the water washing tank 4a was 30 seconds, and the contact time between the polyethylene terephthalate film and the water in the water washing tank 4b was 30 seconds. The total contact time in the water washing step was 60 seconds. Polyvinyl alcohol floating on the water surface of washing tank 4a was scooped up with a stainless steel mesh and appropriately removed. Shower nozzles 5a to 5d were placed in washing tank 4b, and fresh water was sprayed onto both sides of the polyethylene terephthalate film at a flow rate of 50 L / min. The sprayed water flowed into washing tank 4b, and excess water from washing tank 4b was allowed to overflow and be discharged (not shown).

[0111] (No.2) A release-treated polyethylene terephthalate film was produced under the same conditions as in No. 1, except that shower nozzles 5a and 5b were provided and shower nozzles 5c and 5d were not provided.

[0112] (No.3) A release-treated polyethylene terephthalate film was produced under the same conditions as in No. 1 above, except that the temperature of the water stored in the water tank 4a was set to room temperature (27°C).

[0113] (No.4) In the above No. 1, a release-treated polyethylene terephthalate film was produced under the same conditions as No. 1, except that the polyvinyl alcohol floating on the surface of the water in the alkaline treatment tank 3 was not properly removed, the polyvinyl alcohol floating on the surface of the water in the water washing tank 4a was not properly removed, and shower nozzles 5a to 5d were not placed in the water washing tank 4b (water was not sprayed).

[0114] (No.5) Approximately 10 kg of the coated polyethylene terephthalate film A was prepared and cut into chips measuring 0.5 to 3 cm square. These chips were then introduced into a tank equipped with an agitator containing alkaline treatment liquid 1. Alkaline treatment liquid 1 contained 0.7% by mass of potassium hydroxide as an alkali source and 0.01% by mass of a polyethylene glycol ether of a higher alcohol, and its temperature was 115°C. The contact time between the coated polyethylene terephthalate film A and the alkaline treatment liquid 1 was 30 seconds. During this time, the film was stirred at 105°C and approximately 100 rpm for 15 minutes. The alkaline treatment liquid 1 was then removed from the pipe at the bottom of the tank, and water was added to the tank. The film was then stirred at approximately 100 rpm for 5 minutes to rinse the film. Subsequently, alkaline treatment liquid 2 was added to the tank and stirred at 120°C and approximately 200 rpm for 40 minutes. The alkaline treatment liquid 2 contained 1.0% by mass of potassium hydroxide and 0.05% by mass of a polyethylene glycol ether of a higher alcohol. After this, the alkaline treatment liquid 2 was extracted from a pipe installed at the bottom of the tank, and water was supplied from a pipe installed at the bottom of the tank. While the water was allowed to overflow, the chips were washed with water by stirring at a rotation speed of about 100 rpm for 10 minutes. The chips were then removed from the tank and dried at 100°C for 60 minutes to produce peeled chips.

[0115] Next, the PVA removal index was calculated for the release-treated polyethylene terephthalate films obtained in Nos. 1 to 4 using the following analytical method 1. The results are shown in Table 1. Specifically, approximately 200 m of the surface layer of the wound release-treated polyethylene terephthalate film was unwound, and five points were randomly cut from this section. The average of the a / b ratios measured at these five points was used as the PVA removal index. In the release treatments in Nos. 1 to 3, the foreign matter state in the water washing tank stabilized after the middle of the treatment. Therefore, samples of the release-treated polyethylene terephthalate films in Nos. 1 to 3 were taken from the section after the treatment in which the foreign matter state had stabilized. Specifically, the middle of the treatment refers to the middle of the three equal parts (first half, middle, and second half) of the entire treatment time. The term "stable foreign matter state" refers to a state in which the amount of foreign matter removed and the amount of foreign matter removed by scooping or overflowing are approximately equal, and the amount of foreign matter in the alkaline treatment solution and water in the water washing tank are approximately constant.

[0116] (Analysis method 1) A sample (1 cm x 1 cm) cut from a release-treated polyethylene terephthalate film was dissolved in 5 ml of 1,1,1,3,3,3-hexafluoro-2-propanol, and the resulting solution was filtered using a membrane filter (Membrane Solutions Limited, "PTFE025045 (trade name)", pore size 0.45 μm). The filter material was then measured by total reflection infrared spectroscopy. The infrared spectroscopy measurement device used was the "FT / IR-4200" manufactured by JASCO Corporation. The measured infrared absorption spectrum was measured from 3200 to 3650 cm. -1 The maximum absorbance a detected in the range of 2840 to 3000 cm -1 The ratio (a / b) of the maximum absorbance b detected in the range of

[0117] In addition, five pieces were randomly selected from the peel-treated chips obtained in No. 5 and cut into 1 cm x 1 cm pieces to use as samples. The PVA removal index was calculated using the above analytical method 1, and the results are shown in Table 1 below.

[0118] Next, the infrared absorption spectrum was measured using the release-treated polyethylene terephthalate film obtained in No. 1 after alkali treatment and water washing. The measurement results are shown in Figure 2. In addition, in No. 1, the infrared absorption spectrum was measured using the coating layer-laminated polyethylene terephthalate film A before alkali treatment. The measurement results are shown in Figure 3. Comparing Figures 2 and 3, the infrared absorption spectrum was measured from 3200 to 3650 cm -1 It can be seen that the peaks detected in the range of 100 to 1500 nm were almost completely removed by the alkali treatment and water washing. This is thought to be because the polyvinyl alcohol was peeled off and removed from the surface of the polyethylene terephthalate film by the alkali treatment and water washing.

[0119] Next, the number of foreign objects was calculated for recycled pellets obtained by melt-molding the release-treated polyethylene terephthalate films obtained in Nos. 1 to 4 using the following analytical method 2. The recycled pellets were prepared by rewinding the release-treated polyethylene terephthalate films obtained by alkali treatment and water washing in Nos. 1 to 4, and then cutting the rewound release-treated polyethylene terephthalate film into chips measuring 0.5 to 3 cm square while unwinding it. These chips were then sequentially fed into a single-screw extruder. The polyethylene terephthalate was melted in the extruder at a resin temperature of 285°C, and the strand-like polyethylene terephthalate extruded from the die was cooled in water and then cut into pellets. The pellets used in the following analytical method 2 were those obtained just before the end of pelletization, which corresponds to the latter half of the release treatment.

[0120] (Analysis method 2) A single recycled pellet (approximately 35 mg) obtained from peel-treated polyethylene terephthalate film was sandwiched between two cover glasses (Matsunami microcover glasses, 25 mm x 25 mm, 0.2 mm thick), heated to approximately 300 °C on a hot plate, pressed to a thickness of 0.8–0.9 mm, and immediately quenched to prepare a specimen for observation. The center of the sample was observed using a Nikon phase-contrast optical microscope and an objective lens (10x magnification, 0.5 numerical aperture). Images were captured via a CCD camera and analyzed by an image analyzer (Nireco Luzex-FS) to count particles 3 μm to less than 10 μm and particles 10 μm or larger. Particle size was defined as the diameter of a circle equivalent to the area. Similar measurements were performed 20 times, changing the field of view, to determine the total particle count for each measurement. The number of particles 3 μm to less than 10 μm and particles 10 μm or larger per square mm of field of view was calculated, and this was used to count the number of foreign matter in the recycled pellet.

[0121] The number of foreign objects in the recycled pellets obtained from the peeled chips obtained in No. 5 was also calculated using the above analytical method 2. The peeled chips obtained in No. 5 were directly fed into a single-screw extruder and pelletized.

[0122] Next, the b-value was measured for the recycled pellets obtained by melt molding the release-treated polyethylene terephthalate film obtained in No. 1 to 4, and for the recycled pellets obtained from the release-treated chips obtained in No. 5. The b-value was measured using a colorimeter (ZE6000) manufactured by Nippon Denshoku Industries Co., Ltd. A φ30 mm round cell placed in a cell case was filled with resin up to the level, a cap was attached to the cell case, and the cell was placed on a reflective sample stage, and the Hunter Lab was measured. This procedure was repeated three times with different pellets, and the average value was calculated.

[0123] [Table 1]

[0124] The following can be inferred from Table 1. Nos. 1 to 3 are examples that satisfy the requirements of the present invention. The obtained recycled pellets were of the same quality as the raw material pellets used in coating-laminated polyethylene terephthalate film A and are considered to be usable as substitutes for virgin products. Furthermore, the b-values ​​of the recycled pellets obtained in Nos. 1 to 3 were 13 or less, indicating high whiteness. On the other hand, No. 4 had an insufficient alkali treatment and water washing, resulting in a PVA removal index of over 0.10. As a result, the number of foreign particles with a particle size of 3 μm or more and less than 10 μm contained in the obtained recycled pellets exceeded 100. Furthermore, the b-value of the recycled pellets obtained in No. 4 exceeded 13. No. 5 was an example in which the alkali treatment and water washing were performed batchwise, and the PVA removal index exceeded 0.10. As a result, the number of foreign particles with a particle size of 3 μm or more and less than 10 μm contained in the obtained peel-treated pellets exceeded 100. Furthermore, the b-value of the pellets obtained in No. 5 exceeded 13.

[0125] Next, approximately 200 mg of recycled pellets obtained by melt molding the release-treated polyethylene terephthalate film obtained in No. 1 were dissolved in 2 ml of 1,1,1,3,3,3-hexafluoro-2-propanol, and the resulting solution was filtered using a PTFE membrane filter (manufactured by Membrane Solutions Limited, pore size 0.5 μm). Ten particles were then collected from the foreign matter remaining on the filter, and IR measurements of each particle were performed using the microscopic transmission method. As a result, two of these foreign matter particles had an IR spectrum of 3200 to 3650 cm. -1 A broad peak was observed in the range of , and it was found that some of these foreign matters were derived from the PVA contained in the coating layer.

[0126] (No.6) Coated polyethylene terephthalate film B was produced in the same manner as coated polyethylene terephthalate film A, except that the coating solution used in the in-line coating was changed to a coating solution with a solids ratio of polycarbonate polyurethane:copolymer polyester:isocyanate compound of 5:3:2, and the film thickness was changed. The coating layers formed on one side and the other side of coated polyethylene terephthalate film B each had a thickness of approximately 60 nm. The resulting coated polyethylene terephthalate film B was 40 μm thick, 60 cm wide, and 3,000 m long, and was wound into a roll on a plastic core.

[0127] The prepared coating layer-laminated polyethylene terephthalate film B was subjected to alkali treatment, water washing, drying, and winding using the apparatus shown in Figure 1 under the same conditions as No. 1 above to produce a release-treated polyethylene terephthalate film.

[0128] (No.7) The same coated layer laminated polyethylene terephthalate as in No. 6 except that no coating liquid was applied. A biaxially stretched polyethylene terephthalate film prepared in the same manner as in Film B was coated with a release layer, which was a 150 nm thick cured layer of ultraviolet-curable silicone resin, to produce Coating Layer-laminated Polyethylene Terephthalate Film C.

[0129] The release layer surface of the coated layer laminated polyethylene terephthalate film C was corona treated (discharge amount 50 w m 2 / min) to modify the surface of the release layer, and then, as in No. 1 above, The polyethylene terephthalate film was subjected to alkali treatment, water washing, drying, and winding using the apparatus shown in 1. The temperature of the alkaline treatment solution was set to 100°C.

[0130] (No.8) A release-treated polyethylene terephthalate film was produced in the same manner as in No. 7, except that the release layer was a cured layer of a thermal addition type silicone resin.

[0131] Next, the removal rate of the release layer was calculated for the release-treated polyethylene terephthalate films obtained in Nos. 7 and 8. The removal rate of the release layer was calculated based on the following formula, measuring the amount of Si element on the surface of the release-treated polyethylene terephthalate film using a wavelength dispersive X-ray fluorescence analyzer (Supermini200 manufactured by Rigaku Corporation). Release layer removal rate (%) = {(AC) - (BC) / (AC)} × 100 In the formula, A represents the Si intensity detected by irradiating the release layer surface of a laminate of a release layer and a polyethylene terephthalate substrate with X-rays before the release layer is provided, B represents the Si intensity detected by irradiating the release-treated polyethylene terephthalate film surface with X-rays after alkali treatment, and C represents the Si intensity detected by irradiating the substrate film surface with X-rays before the release layer is provided. The above formula was designed taking into account the amount of Si originally contained in the substrate film (defined as "C" in the formula), and in the formula, (AC) represents the amount of Si contained in the release layer before alkali treatment, and (BC) represents the amount of Si derived from the release layer remaining on the substrate film after alkali treatment.

[0132] Next, the release-treated polyethylene terephthalate films obtained in Nos. 6 to 8 were pelletized, and approximately 200 mg of each of the obtained recycled pellets was dissolved in 2 ml of 1,1,1,3,3,3-hexafluoro-2-propanol. The resulting solution was filtered using a PTFE membrane filter (manufactured by Membrane Solutions Limited, pore size 0.5 μm). Then, the foreign matter remaining on the filter was collected in the same manner as above, and IR measurement was performed using the microscopic transmission method. As a result, from No. 6, -1 and 1440cm -1 A peak was observed around 1260cm from Nos. 7 and 8, indicating the presence of foreign matter derived from the reaction product of an isocyanate compound. -1 Near 800cm -1 Around 1090cm -1 Near 1020cm -1Foreign matter derived from silicone was observed with a peak around this area, and it was found that some of these foreign matter originated from the coating layer.

[0133] The release-treated polyethylene terephthalate films obtained in Nos. 6 to 8 were pelletized, and the number of foreign objects in each of the resulting recycled pellets was calculated using the above-mentioned analytical method 2. The b value was also measured. The results are shown in Table 2 below.

[0134] [Table 2]

[0135] Next, the recycled pellets obtained from Nos. 1, 2, 6, 7, and 8 were blended with virgin pellets (the raw material pellets) to produce biaxially stretched polyethylene terephthalate films. Film production was carried out under the same conditions as for coating-laminated polyethylene terephthalate film A for the recycled pellets obtained from No. 1 and 2, under the same conditions as for coating-laminated polyethylene terephthalate film B for the recycled pellets obtained from No. 6, and under the same conditions as for coating-laminated polyethylene terephthalate film C for the recycled pellets obtained from Nos. 7 and 8. The recycled pellets were obtained from the latter half of the peeling treatment, which was divided into three equal parts (first half, middle, and second half). For the examples using the recycled pellets from No. 2, the mixing ratio of recycled pellets to virgin pellets was varied (Nos. 2-1 and 2-2) as shown in Table 3 below.

[0136] Foreign matter in films made from recycled pellets was analyzed in the same manner as in Analysis Method 2 above, except that the in-line coating layer was removed with a razor, the film surface was rinsed with pure water, and then approximately 30 mg of the sample was cut into approximately 5 mm squares. The results are shown in Table 3 below.

[0137] The color tone of the end face of the obtained film roll was also visually evaluated. A rating of ◯ was given if the color tone was the same as that of the end faces of coating layer-laminated polyethylene terephthalate films A and B, a rating of △ if the color tone was slightly yellowish, and an rating of × if the color tone was clearly yellowish. The results are shown in Table 3 below.

[0138] [Table 3]

[0139] The polyethylene terephthalate film obtained using recycled pellets was free of foreign matter and abnormal color tone on the edges, and had the same quality in terms of strength and elongation as polyethylene terephthalate film using virgin pellets, and it was found that it could be used without problems as a film for the same purpose. Furthermore, a release layer was provided on the film using the recycled pellets obtained from No. 7 in the same manner as No. 7, and it was made into a release film. As a result, it was found that the ceramic capacitor It was found that it can be used as a release film without any problems. [Explanation of symbols]

[0140] 1a: A coated laminated polyester film wound in a roll. 2 guide rolls 3. Alkaline treatment tank 4, 4a, 4b washing tank 5a~5d Nozzles 6, 8 Mangle 7 Drying oven

Claims

1. an alkaline treatment step of bringing a coating layer-laminated polyester film, which has a coating layer containing polyvinyl alcohol laminated on at least one surface of a polyester film and is wound into a roll, into contact with 20 to 60% by mass of an alkaline treatment solution; a water-washing step of bringing the polyester film after the alkali treatment step into contact with water; Including, a step of removing a residue of the coating layer from the water brought into contact with the polyester film in the water-washing step; A method for producing a polyester film having a polyvinyl alcohol removal index of 0.10 or less (excluding 0) calculated by the following analytical method 1. (Analysis method 1) After filtering the solution containing the polyester film, the filter media was analyzed by infrared spectroscopy attenuated total reflection. 3200-3650cm －１ The maximum absorbance a detected in the range 2840~3000cm －１ The ratio (a / b) of the maximum absorbance b detected in the range of a to b is defined as the polyvinyl alcohol removal index.

2. The method according to claim 1 , further comprising a winding step of winding the polyester film into a roll after the water-washing step.

3. 3. The method according to claim 1, wherein the alkaline treatment step comprises contacting the coating layer-laminated polyester film with the alkaline treatment solution at 90 to 140° C. for 1 to 120 seconds.

4. 4. The method according to claim 1, wherein the water-washing step comprises contacting the polyester film after the alkali treatment step with water at 90° C. or less at least once.

5. 5. The method according to claim 1, wherein the water-washing step comprises spraying water onto the polyester film after the alkali treatment step.

6. 6. The method according to claim 1, further comprising removing residue of the coating layer from the alkaline treatment solution brought into contact with the coating layer-laminated polyester film in the alkaline treatment step.

7. an alkaline treatment step of bringing a coating layer-laminated polyester film, which has a coating layer containing polyvinyl alcohol laminated on at least one surface of a polyester film and is wound into a roll, into contact with 20 to 60% by mass of an alkaline treatment solution; a water-washing step of contacting the polyester film after the alkali treatment step with water; and a pelletizing step of melting and molding the polyester film after the water washing step into pellets; Including, removing residue of the coating layer from the water that has been brought into contact with the polyester film in the water-washing step; The polyester film after the water-washing step is characterized in that the polyvinyl alcohol removal index calculated by the following analysis method 1 is 0.10 or less (not including 0). The number of foreign particles calculated by the following analysis method 2 is 100 / mm ２ A method for producing polyester pellets, which is as follows: (Analysis method 1) After filtering the solution containing the polyester film, the filter media was analyzed by infrared spectroscopy attenuated total reflection. a maximum absorbance a detected in the range of 3200 to 3650 cm −1 ; The ratio (a / b) of the maximum absorbance detected in the range of 2840 to 3000 cm −1 to b is defined as the polyvinyl alcohol removal index. (Analysis method 2) The polyester pellets were melted and observed under a phase contrast optical microscope at a magnification of 10 times, with an observation field area of ​​1 mm ２ The number of foreign particles with a diameter of 3 μm or more and less than 10 μm contained per unit area is measured.

8. 8. The method according to claim 7, wherein the alkaline treatment step comprises contacting the coating layer-laminated polyester film with the alkaline treatment solution at 90 to 140° C. for 1 to 120 seconds.

9. The method according to claim 7 or 8, wherein the water-washing step comprises contacting the polyester film after the alkali treatment step with water at 90°C or less at least once.

10. The method according to any one of claims 7 to 9, wherein the water-washing step comprises spraying water onto the polyester film after the alkali treatment step.

11. The method according to any one of claims 7 to 10, further comprising removing residue of the coating layer from the alkaline treatment solution brought into contact with the coating layer-laminated polyester film in the alkaline treatment step.

12. an alkaline treatment step of bringing a coating layer-laminated polyester film, which has a coating layer containing polyvinyl alcohol laminated on at least one surface of a polyester film and is wound into a roll, into contact with 20 to 60% by mass of an alkaline treatment solution; a water-washing step of bringing the polyester film after the alkali treatment step into contact with water; a pelletizing step of melting and molding the polyester film after the water washing step into pellets; a film-forming step of melting and molding the pellets into a film; Including, removing residue of the coating layer from the water that has been brought into contact with the polyester film in the water-washing step; The polyester film after the water-washing step is characterized in that the polyvinyl alcohol removal index calculated by the following analysis method 1 is 0.10 or less (not including 0). The number of foreign objects is 100 / mm ２ A method for producing a polyester film, comprising: (Analysis method 1) After filtering the solution containing the polyester film, the filter media was analyzed by infrared spectroscopy attenuated total reflection. a maximum absorbance a detected in the range of 3200 to 3650 cm −1 ; The ratio (a / b) of the maximum absorbance detected in the range of 2840 to 3000 cm −1 to b is defined as the polyvinyl alcohol removal index.

13. The method according to claim 12, wherein the alkaline treatment step comprises contacting the coating layer-laminated polyester film with the alkaline treatment solution at 90 to 140° C. for 1 to 120 seconds.

14. The method according to claim 12 or 13, wherein the water-washing step comprises contacting the polyester film after the alkali treatment step with water at 90° C. or less at least once.

15. The method according to any one of claims 12 to 14, wherein the water-washing step comprises spraying water onto the polyester film after the alkali treatment step.

16. The method according to any one of claims 12 to 15, further comprising removing residue of the coating layer from the alkaline treatment solution brought into contact with the coating layer-laminated polyester film in the alkaline treatment step.

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