Method for depolymerizing polyester film

Abstract

To provide a depolymerization method that allows efficient removal of a functional layer from a polyester film having a functional layer and enhances polyester recovery efficiency.SOLUTION: A depolymerization method of a polyester film comprising the following steps (I) to (IV): (I) depolymerizing a polyester film containing a functional layer to obtain a crude depolymerization solution A; (II) subjecting the obtained crude depolymerization solution A to specific gravity separation into a purified depolymerization solution X and a crude depolymerization solution B containing free matter; (III) separating free matter from the crude depolymerization solution B to obtain a purified depolymerization solution Y; and (IV) sending the purified depolymerization solution Y to a depolymerization tank.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for depolymerizing a polyester film. [Background technology]

[0002] Polyesters have excellent mechanical properties, thermal properties, chemical resistance, electrical properties, and moldability, and are used in a variety of applications. Among polyesters, polyethylene terephthalate (hereinafter referred to as PET) in particular has excellent transparency and processability, making it widely used in applications requiring high quality, such as optical films and release films. However, since process films such as release films are discarded after use, there is a demand for reducing the environmental impact.

[0003] Chemical recycling is gaining attention as a method for regenerating polyester without compromising its quality by chemically breaking down polyester into raw materials and intermediates without using additional petroleum feedstocks and then conducting a polycondensation reaction again. While chemical recycling maintains the quality of polyester, it requires many steps, such as separating and purifying impurities, which increases energy and costs, and reduces the yield of polyester due to the recycling process. In particular, process films such as release films often have a layer (functional layer) on their surface that performs specific functions depending on the application, and the method for separating the functional layer in the chemical recycling process is a challenge. Thus, to recover polyester components from polyester film, a method for efficiently separating the functional layer is required.

[0004] To address these issues, Patent Documents 1 and 2 disclose techniques for depolymerizing polyester films containing functional layers. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2005-264113 A [Patent Document 2] Patent Publication No. 2009-167266 Summary of the Invention [Problem to be solved by the invention]

[0006] Patent Document 1 discloses a technique in which a polyester film having a functional layer formed thereon is pre-immersed in glycol and heat-treated to dissolve and peel off a portion of the functional layer. However, this method requires an additional pretreatment step before depolymerization, which may result in a lower polyester recovery rate and requires larger equipment. Patent Document 2 also discloses a technique for depolymerizing a polyester film laminated with a layer coated with a silicone-containing release agent. This technique discloses a method for efficiently recovering ester monomers without the need to separate and remove the release layer in advance by specially positioning the stirring blades. However, this method directly separates and removes the floating silicone-containing release agent-coated layer from the depolymerization liquid, which makes it impossible to recover the depolymerization liquid associated with the silicone release layer, resulting in poor yield of the depolymerization liquid and frequent filter clogging, resulting in poor recovery efficiency.

[0007] An object of the present invention is to provide a depolymerization method that can efficiently remove a functional layer from a polyester film containing a functional layer and improve the efficiency of polyester recovery. [Means for solving the problem]

[0008] As a result of extensive research aimed at solving the above problems, the present inventors have arrived at the method for depolymerizing a polyester film of the present invention. The object of the present invention is achieved by the following means. (1) A method for depolymerizing a polyester film, comprising the following steps (I) to (IV): (I) A step of depolymerizing a polyester film including a functional layer to obtain a crude depolymerized solution A. (II) A step of gravity separating the obtained crude depolymerized solution A into a pure depolymerized solution X and a crude depolymerized solution B containing liberated substances. (III) A step of separating liberated substances from the crude depolymerized solution B to obtain a pure depolymerized solution Y. (IV) A step of sending the pure depolymerized solution Y to a depolymerization tank. (2) The depolymerization method according to (1), wherein the method for separating the liberated substances from the crude depolymerization solution B is filtration and / or centrifugation. (3) The depolymerization method according to (2), wherein the filtration is carried out in two or more stages. (4) The depolymerization method according to (1), wherein the functional layer is a silicone-containing release layer. (5) The depolymerization method according to (1), wherein the depolymerization is glycololysis. (6) The depolymerization method according to (1), wherein the polyester is polyethylene terephthalate. (7) A polyester resin composition obtained by polycondensing the pure depolymerization solution X described in (1). (8) A method for producing a polyester resin composition, comprising polycondensing the pure depolymerized solution X described in (1). [Effects of the Invention]

[0009] The present invention is capable of highly efficiently depolymerizing a polyester film including a functional layer. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a flow diagram of the depolymerization method of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0011] The present invention will be described in detail below. The method for depolymerizing a polyester film of the present invention must include the following steps (I) to (IV) in order to obtain the depolymerization with high efficiency. (I) A step of depolymerizing a polyester film including a functional layer to obtain a crude depolymerized solution A. (II) A step of gravity separating the obtained crude depolymerized solution A into a pure depolymerized solution X and a crude depolymerized solution B containing liberated substances. (III) A step of separating liberated substances from the crude depolymerized solution B to obtain a pure depolymerized solution Y. (IV) A step of sending the pure depolymerized solution Y to a depolymerization tank.

[0012] The polyester film in the present invention is a film made using a polyester resin composition obtained by polycondensation of a dicarboxylic acid component and a diol component.

[0013] Examples of the dicarboxylic acid component include various dicarboxylic acid components such as aromatic dicarboxylic acids, linear aliphatic dicarboxylic acids, and alicyclic dicarboxylic acids. Among these, from the viewpoint of reactivity during depolymerization of the polyester film, terephthalic acid, isophthalic acid, naphthalenedicarboxylic acid, and ester-forming derivatives thereof are preferred, and terephthalic acid is most preferred.

[0014] Examples of diol components include aliphatic diols such as ethylene glycol, 1,2-propanediol, 1,3-propanediol, butanediol, and neopentyl glycol; alicyclic diols such as cyclohexanedimethanol and cyclohexanediethanol; and aromatic diols such as bisphenol A, bisphenol S, styrene glycol, 9,9-bis(4-(2-hydroxyethoxy)phenyl)fluorene, and 9,9'-bis(4-hydroxyphenyl)fluorene. Among these, ethylene glycol is particularly preferred from the viewpoint of reactivity during depolymerization.

[0015] Furthermore, within the scope of not impairing the effects of the present invention, a polyester resin composition obtained by copolymerizing a plurality of the above dicarboxylic acid components, diol components, and hydroxycarboxylic acids may be used for the film.

[0016] In step (I) of the present invention, a polyester film including a functional layer is depolymerized to obtain a crude depolymerization solution A. The polyester film used in the present invention may be either a single-layer film or a laminated film having two or more layers laminated therein. The stretching form is not particularly limited, and may be an unstretched film, a uniaxially stretched film, or a biaxially stretched film. However, it is necessary for the polyester film to include a functional layer. A functional layer is a layer provided on at least one surface of a polyester film substrate and exhibits a specific function. Examples of functional layers include a photosensitive layer, a printing layer, a release layer, an adhesive layer, a pressure-sensitive layer, a peeling layer, and a magnetic layer. Among these, release films are used in many industrial fields and are discarded after use. Therefore, a release layer is preferable from the viewpoint of recycling these films. Furthermore, the release layer is preferably a silicone-containing release layer. Silicone is extremely stable against heat and organic solvents, and does not inhibit the depolymerization reaction of the polyester film. During the depolymerization reaction, the release layer components peel off from the polyester substrate in a solid state and float. The floating release layer components are entrained in the depolymerization liquid and are formed near the liquid surface as free matter of various sizes. The free matter containing the floating release layer components can be easily separated by gravity separation or filtration. Specific examples of such release films include release films for the production of multilayer ceramic capacitors (MLCCs), polarizing plate release films, and optical release films.

[0017] There is no limitation on the shape of the polyester film used in the present invention, but from the viewpoint of handling, flakes obtained by cutting the film or pellets obtained by compressing and solidifying the flakes are preferred.

[0018] The polyester film used in the present invention may be a used product, scraps generated in a molding process, or waste material.

[0019] Examples of methods for depolymerizing the polyester film in the present invention include, but are not limited to, a glycolysis method in which depolymerization is performed in the presence of a glycol compound to obtain polyester oligomers, an alcoholysis method in which depolymerization is performed in the presence of a glycol compound followed by alcoholysis to obtain polyester oligomers, and a hydrolysis method in which polyester oligomers are obtained by hydrolysis. Glycolysis using the glycol component of the polyester film used for depolymerization is preferred from the viewpoint of facilitating the subsequent purification step.

[0020] The depolymerization reaction does not require the addition of a catalyst or polyester oligomer, but a conventionally known depolymerization catalyst or polyester oligomer can be added within a range that does not impair the effects of the present invention. When a depolymerization catalyst is used, metal hydroxides such as sodium hydroxide, potassium hydroxide, and magnesium hydroxide, and metal acetates such as magnesium acetate, manganese acetate, cobalt acetate, calcium acetate, and lithium acetate can be used, but the catalyst is not particularly limited to these. The use of a depolymerization catalyst makes it possible to improve the depolymerization reaction rate.

[0021] The polyester oligomers include, but are not limited to, those obtained by esterification or transesterification of a dicarboxylic acid compound such as terephthalic acid, isophthalic acid, dimethyl terephthalate, or dimethyl isophthalate with a glycol compound such as ethylene glycol, or those obtained by depolymerization of a polyester composition. However, because oligomers obtained by esterification or transesterification are produced using new raw materials, it is preferable to use oligomers obtained by depolymerization of a polyester composition from the perspective of environmental impact. By using oligomers, the depolymerization reaction temperature can be increased because they easily dissolve polyesters and do not easily boil, thereby improving the depolymerization rate.

[0022] In step (II) of the present invention, the crude depolymerized liquid A obtained in step (I) is separated by gravity separation into a pure depolymerized liquid X and a crude depolymerized liquid B containing free matter. The crude depolymerized liquid A obtained in step (I) is separated by gravity separation because the functional layer peels off from the polyester substrate and becomes a solid state as free matter floating on the surface of the depolymerized liquid. There are no limitations on the method of gravity separation, but a simple method is to extract the pure depolymerized liquid X by gravity from the bottom of the depolymerization tank after depolymerization. Centrifugation may be performed to accelerate separation due to the difference in specific gravity. When centrifuging, the gravitational acceleration is preferably 1000 G or more, and more preferably 2000 G or more. By setting the gravitational acceleration to 2000 G or more, it is possible to separate the depolymerized liquid adhering to the free matter with high accuracy.

[0023] During gravity separation, a filter may be provided on the flow path for extracting the pure depolymerized liquid X in order to prevent free substances from being mixed in with the pure depolymerized liquid X. There is no limitation on the amount of the pure depolymerized liquid X extracted by gravity separation, but it is preferable to separate the amount so that it is about 90% or less by weight of the crude depolymerized liquid A in order to prevent free substances from being mixed in.

[0024] When performing gravity separation, it is preferable to maintain the temperature in the system at a temperature sufficiently higher than the melting point of the depolymerized liquid, from the viewpoint of preventing precipitation of polyester oligomers from the crude depolymerized liquid X. When the polyester is polyethylene terephthalate, it is preferable to maintain the temperature of the crude depolymerized liquid X at 200°C or higher. In step (III) of the present invention, the liberated matter is separated from the crude depolymerized liquid B obtained in step (II) to obtain a pure depolymerized liquid Y. Methods for obtaining the liberated matter and the pure depolymerized liquid Y from the crude depolymerized liquid B include filtration, centrifugation, distillation, extraction, and the like, and a combination of these methods is also possible. From the viewpoint of separation efficiency, filtration, centrifugation, and centrifugal filtration, which is a combination of these, are preferably performed.

[0025] When filtration is performed, it is preferable to perform the filtration in two or more stages in order to remove the free substances with high efficiency, and by gradually reducing the filter openings, it is possible to efficiently remove the free substances without causing problems such as clogging. Filtration methods include, but are not limited to, gravity filtration, pressure filtration, and suction filtration.

[0026] Centrifugation can be carried out by a known method. It is preferable to carry out this method from the viewpoint that the depolymerized liquid adhering to the liberated matter can be efficiently removed and the yield of the depolymerized liquid can be improved. A more preferable method is to carry out centrifugation and then remove the liberated matter by filtration. Centrifugal filtration, which combines centrifugation and filtration, is also useful. By carrying out centrifugal filtration, the depolymerized liquid adhering to the liberated matter can be separated by centrifugal force and efficiently recovered by filtration. Note that since the purpose of this step is to remove the suspended matter (liberated matter) obtained by centrifugation, it is particularly preferable to carry out the centrifugation while extracting pure depolymerized matter Y, because this improves separation efficiency and allows the depolymerized liquid Y to be recovered in a high yield.

[0027] When centrifugal separation is performed, the gravitational acceleration is preferably 1000 G or more, more preferably 2000 G or more. By setting the gravitational acceleration to 2000 G or more, it becomes possible to separate the depolymerized liquid adhering to the released matter with high accuracy. Furthermore, the time for centrifugal separation is preferably 10 minutes or more, more preferably 20 minutes or more.

[0028] When filtration and centrifugation are performed, it is preferable to maintain the temperature in the system at a temperature sufficiently higher than the melting point of the depolymerized liquid, from the viewpoint of improving the yield of the pure depolymerized liquid Y. When the polyester is polyethylene terephthalate, it is preferable to maintain the temperature of the crude depolymerized liquid B at 200°C or higher.

[0029] In step (IV) of the present invention, the pure depolymerized liquid Y obtained in step (III) is sent to a depolymerization tank and subjected to the depolymerization step (I) again. There are no particular limitations on the method of transport, but it is preferable to add the pure depolymerized liquid Y either before or during the depolymerization reaction in step (I). By performing step (IV), it becomes possible to recover the depolymerized liquid without loss.

[0030] Steps (III) and (IV) may be carried out after steps (I) and (II) have been carried out multiple times and a certain amount of liberated material has accumulated in the depolymerization tank, which can be carried out less frequently and at lower cost than a method in which liberated material is removed each time by filtration or the like.

[0031] The pure depolymerized solution X obtained by the present invention can be purified by a known method and subjected to a polycondensation reaction to produce a polyester resin composition. The polycondensation reaction in the present invention refers to a process in which polyester oligomers are stirred under heating and reduced pressure, glycol components generated from the polyester oligomers are eliminated, and a polyester resin composition is ultimately obtained. The resulting polyester resin composition can be molded and used suitably for fibers, films, sheets, containers, bottles, and the like. Among these, polyester resin compositions obtained using the depolymerized solution obtained by the depolymerization method of the present invention are suitable for polyester films due to their excellent color tone and transparency, and are particularly suitable for use in biaxially oriented polyester films for process release. Since the films used for this purpose are no longer needed after reuse, they can be repeatedly reused as raw materials for recycling, which is preferable from the perspective of a circular economy.

[0032] Specific examples of the depolymerization method of the present invention are given below, but the present invention is not limited thereto. (I) 100 parts by weight of recycled PET raw material, such as PET film scraps coated with a silicone release agent as a release layer, and 15 to 145 parts by weight of ethylene glycol are added to a depolymerization tank and subjected to a depolymerization reaction at a temperature ranging from 190 to 240°C to obtain crude depolymerized liquid A. (II) The depolymerization tank containing the crude depolymerized liquid A is maintained at 210°C, and pure depolymerized liquid X is extracted from the bottom of the depolymerization tank until the crude depolymerized liquid A's weight is 10%. (III) The crude depolymerized liquid B remaining in the depolymerization tank is transferred to a basket-type centrifugal filter and centrifuged at a gravitational acceleration of 2000 g at 210°C for 1 hour. The filter mesh size is 100 to 400 μm. (IV) The pure depolymerized liquid Y obtained is transferred to the depolymerization tank and subjected to step (I) again.

[0033] Mesh size is the size of the gaps between the meshes in a filter, and is calculated by multiplying the number of meshes (unit: mesh) per inch (25.4 mm) by the diameter of the filter wire that makes up the filter: 25.4 mm ÷ mesh - filter wire diameter (mm) = mesh size (mm). [Example]

[0034] The present invention will be described in more detail below with reference to examples. The physical properties in the examples were measured by the following methods. (1) Ratio (mass%) of depolymerization liquid attached to the functional layer (determination of depolymerization liquid ratio) The yield was calculated as follows: (weight (g) of the free matter including the accompanying depolymerization liquid obtained by separation and removal - weight (g) of the functional layer in the polyester film subjected to the depolymerization reaction) / (weight (g) of the functional layer in the polyester film subjected to the depolymerization reaction) x 100. The greater the weight of the free matter, the greater the proportion of the weight of the depolymerization liquid contained in the free matter, and the worse the yield of pure depolymerization liquid Y. If the yield is low, it is necessary to add new polyester oligomers derived from petroleum-based materials, which places an environmental burden. Therefore, the yield was evaluated according to the following criteria, with ◎ and ○ being considered as passing. ◎: 0 to less than 20 wt% ○: 20% or more to less than 50% by weight △: 50wt% or more.

[0035] (2) Evaluation of foreign matter in pure depolymerized solutions X and Y Regarding the foreign matter contained in pure depolymerized solutions X and Y, 10 g of cooled and solidified pure depolymer was dissolved in 30 ml of o-chlorophenol, filtered through a 55 mm diameter 5C filter paper, and the number of foreign matter remaining on the filter paper was counted. If any free matter remained, it may cause adverse effects such as clogging in the subsequent purification process, so the results were evaluated according to the following criteria, with ◎ and ○ being considered as passing. ◎: Less than 5 ○: 5 or more but less than 10 △: 10 or more.

[0036] [Reference example] In the depolymerization reaction described as an example of the present invention, a PET film (total thickness: 40 μm, functional layer thickness: 0.5 μm) with a silicone release layer applied to one surface as a functional layer was used. The weight of the base polyester film before applying the functional layer (53.2 g / m) 2 ) The weight of the functional layer is calculated as 2g / m 2 It was.

[0037] Example 1 Approximately 1cm 2 100 g of PET film with a silicone release layer (functional layer weight: 3.6 g) cut into squares and 160 g of ethylene glycol were added to the depolymerization reactor. The temperature was gradually increased from the initial setting of 200 °C to 210 °C while stirring. The depolymerization reaction proceeded with stirring until the PET film and ethylene glycol were uniformly dissolved. The end point was 30 minutes after the system temperature reached 210 °C, yielding crude depolymerized solution A (260 g). Free matter, including the release layer that had peeled off from the PET film, was floating on the liquid surface. The depolymerization reactor was maintained at 210 °C, and 234 g of crude depolymerized solution A was extracted from the bottom of the reactor by gravity separation to obtain pure depolymerized solution X. Subsequently, the remaining crude depolymerized solution B (26 g) was transferred to a basket-type centrifugal filter (Kansai Centrifuge Manufacturing Co., Ltd., KMN-10) and centrifuged at a gravitational acceleration of 4000 g at 210 °C for 1 hour. The filtration was carried out in two stages using filters with openings of 400 μm and 100 μm. The pure depolymerized solution Y obtained by filtration was transferred to a depolymerization tank and subjected to the depolymerization step again.

[0038] The weight of the pure depolymerized liquid Y obtained in Example 1 was 22.3 g, the weight of the free residue was 3.7 g, and the ratio of the depolymerized liquid adhering to the functional layer was 2 wt %. This indicates that the depolymerized liquid could be recovered with high efficiency, and the quality of the pure depolymerized liquid was also satisfactory.

[0039] Examples 2 to 4 Pure depolymerized polymers X and Y were obtained in the same manner as in Example 1, except that the amount of ethylene glycol added to the depolymerization tank, the final temperature reached during depolymerization, and the temperature of the depolymerization tank during gravity separation were changed as shown in Table 1. The pure depolymerized liquid Y obtained by filtration was transferred to the depolymerization tank and subjected to the depolymerization step again.

[0040] The weight of the pure depolymerized liquid Y obtained in Example 2 was 9.3 g, the weight of the free residue was 3.9 g, and the ratio of the depolymerized liquid adhering to the functional layer was 8 wt %. This indicates that the depolymerized liquid could be recovered with high efficiency, and the quality of the pure depolymerized liquid was also satisfactory.

[0041] The weight of the pure depolymerized liquid Y obtained in Example 3 was 38.2 g, the weight of the free residue was 3.8 g, and the ratio of the depolymerized liquid adhering to the functional layer was 5 wt %. This indicates that the depolymerized liquid could be recovered with high efficiency, and the quality of the pure depolymerized liquid was also satisfactory.

[0042] The weight of the pure depolymerized liquid Y obtained in Example 4 was 64.3 g, the weight of the free residue was 3.9 g, and the ratio of the depolymerized liquid attached to the functional layer was 8 wt %. This indicates that the depolymerized liquid could be recovered with high efficiency, and the quality of the pure depolymerized liquid was also satisfactory.

[0043] Example 5 Pure depolymerized polymers X and Y were obtained in the same manner as in Example 1, except that filtration was not performed in the separation step of the crude depolymerized liquid B. The pure depolymerized liquid Y obtained by filtration was transferred to a depolymerization tank and subjected to the depolymerization step again.

[0044] The weight of the pure depolymerized liquid Y obtained in Example 5 was 22.1 g, the weight of the residual free matter was 3.9 g, and the ratio of the depolymerized liquid adhering to the functional layer was 8 wt %. Thus, the depolymerized liquid could be recovered with high efficiency, but a small amount of foreign matter that was thought to be part of the free matter was found in the pure depolymerized liquid Y.

[0045] [Table 1]

[0046] Examples 6 to 9 Pure depolymerized polymers X and Y were obtained in the same manner as in Example 1, except that in the separation step of the crude depolymerized liquid B, the gravitational acceleration during centrifugation was changed as shown in Table 1, or centrifugation was not performed. The pure depolymerized liquid Y obtained by filtration was transferred to a depolymerization tank and subjected to the depolymerization step again.

[0047] The weight of the pure depolymerized liquid Y obtained in Example 6 was 21.5 g, the weight of the free residue was 4.5 g, and the proportion of the depolymerized liquid adhering to the functional layer was 24 wt %. Since the acceleration during centrifugation was 500 G, the loss of the depolymerized liquid was somewhat high, but there was no problem with the quality of the pure depolymerized liquid.

[0048] The weight of the pure depolymerized liquid Y obtained in Example 7 was 21.6 g, the weight of the free residue was 4.4 g, and the ratio of the depolymerized liquid adhering to the functional layer was 21 wt %. Since the acceleration during centrifugation was 1500 G, the loss of the depolymerized liquid was somewhat high, but there was no problem with the quality of the pure depolymerized liquid.

[0049] The weight of the pure depolymerized liquid Y obtained in Example 8 was 22.3 g, the weight of the free residue was 3.7 g, and the ratio of the depolymerized liquid adhering to the functional layer was 2 wt %. This indicates that the depolymerized liquid could be recovered with high efficiency, and the quality of the pure depolymerized liquid was also satisfactory.

[0050] The weight of the pure depolymerized liquid Y obtained in Example 9 was 21.3 g, the weight of the residual free matter was 4.7 g, and the ratio of the depolymerized liquid adhering to the functional layer was 30 wt %. Although the loss of the depolymerized liquid was high because centrifugation was not performed, there was no problem with the quality of the pure depolymerized liquid.

[0051] Example 10 Pure depolymerized polymers X and Y were obtained in the same manner as in Example 1, except that filtration was performed only once using a filter with a mesh size of 400 μm in the separation step of the crude depolymerized liquid B. The pure depolymerized liquid Y obtained by filtration was transferred to a depolymerization tank and subjected to the depolymerization step again.

[0052] The weight of the pure depolymerized liquid Y obtained in Example 10 was 22.1 g, the weight of the free residue was 3.9 g, and the ratio of the depolymerized liquid adhering to the functional layer was 8 wt %. This indicates that the depolymerized liquid could be recovered with high efficiency, and the quality of the pure depolymerized liquid was also satisfactory.

[0053] (Comparative Example 1) The crude depolymerized liquid A (260 g) obtained in the same manner as in Example 1 was withdrawn entirely from the bottom of the depolymerization tank while the depolymerization tank was kept at 210° C., to obtain a pure depolymerized liquid X.

[0054] The obtained pure depolymerized solution X contained free substances and a large amount of foreign matter, resulting in poor quality.

[0055] (Comparative Example 2) The depolymerization tank was kept at 210°C for the crude depolymerized liquid A (260 g) obtained in the same manner as in Example 1, and pure depolymerized liquid X was extracted from the bottom of the depolymerization tank by gravity separation until the weight of the crude depolymerized liquid A reached 10 wt % (234 g). The quality of the obtained pure depolymerized liquid X was satisfactory, but the residue remaining in the depolymerization tank contained liberated substances and had to be discarded, resulting in a large loss of the depolymerized liquid.

[0056] Example 11 The depolymerization tank was kept at 210°C, and the crude depolymerized liquid A (260 g) obtained in the same manner as in Example 1 was passed through a filter having a mesh size of 70 μm from the bottom of the depolymerization tank, and a pure depolymerized liquid X (234 g) was extracted by gravity separation until the weight of the crude depolymerized liquid A became 10%. Thereafter, the same operation as in Example 1 was performed, and a pure depolymerized liquid Y was obtained. The pure depolymerized liquid Y obtained by filtration was transferred to the depolymerization tank and subjected to the depolymerization step again.

[0057] The weight of the pure depolymerized liquid Y obtained in Example 11 was 22.0 g, the weight of the free residue was 4.0 g, and the ratio of the depolymerized liquid adhering to the functional layer was 10 wt %. This indicates that the depolymerized liquid could be recovered with high efficiency, and the quality of the pure depolymerized liquid was also satisfactory.

[0058] (Comparative Example 3) The depolymerization tank was kept at 210°C, and the crude depolymerized liquid A (260 g) obtained in the same manner as in Example 1 was passed through a filter having a mesh size of 70 μm from the bottom of the depolymerization tank, and the entire amount was extracted as pure depolymerized liquid X by gravity separation. During the process, clogging of the filter occurred due to free matter, and although the quality of the obtained pure depolymerized liquid X was not affected, a large amount of free matter remained as residue at the bottom of the depolymerization tank, resulting in a large loss of the depolymerized liquid.

[0059] [Table 2] [Industrial Applicability]

[0060] The depolymerized polymer thus obtained can be subjected to an appropriate purification step and then subjected to a polycondensation reaction again to obtain a polyester composition. The resulting polyester resin composition is useful for optical applications, agricultural materials, horticultural materials, fishing materials, civil engineering and construction materials, stationery, medical supplies, automotive parts, electrical and electronic parts, and other applications, and is particularly suitable for process release films that require high quality. [Explanation of symbols]

[0061] 1. Polyester film with functional layer 2 Depolymerization (crude depolymerization liquid A) 3 Specific gravity separation 4 Pure depolymerization liquid X 5. Crude depolymerization solution B (including free substances) 6 separation 7 Free Matter 8 Pure depolymerization liquid Y 9 Reload

Claims

1. A method for depolymerizing a polyester film, comprising the following steps (I) to (IV): (I) A step of depolymerizing a polyester film including a functional layer to obtain a crude depolymerized solution A. (II) A step of gravity separating the obtained crude depolymerized liquid A into a pure depolymerized liquid X and a crude depolymerized liquid B containing liberated substances. (III) A step of separating liberated substances from the crude depolymerized solution (B) to obtain a pure depolymerized solution (Y). (IV) A step of sending the pure depolymerized solution Y to a depolymerization tank.

2. 2. The depolymerization method according to claim 1, wherein the method for separating the liberated substances from the crude depolymerization solution B is filtration and / or centrifugation.

3. 3. The method for depolymerization according to claim 2, wherein the filtration is carried out in two or more stages.

4. 2. The method for depolymerization according to claim 1, wherein the functional layer is a silicone-containing release layer.

5. 2. The method for depolymerization according to claim 1, wherein the depolymerization is glycolysis.

6. 2. The method for depolymerization according to claim 1, wherein the polyester is polyethylene terephthalate.

7. A polyester resin composition obtained by polycondensing the pure depolymerization solution X according to claim 1.

8. A method for producing a polyester resin composition, comprising polycondensing the pure depolymerized solution X according to claim 1.

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