Method for decomposing polyester resin

Abstract

To improve the recovery rate of alcohol, which is a constituent monomer of polyester resin, in a method for decomposing polyester resin using high-temperature water. [Solution] A method for decomposing polyester resin, comprising step (i) of hydrothermally treating polyester resin in the presence of polyamide resin, wherein step (i) includes proceeding with hydrolysis of the polyester resin at a temperature of 150°C or higher.

Description

Technical Field

[0001] The present disclosure relates to a method for decomposing polyester resins.

Background Art

[0002] Polyester resins such as polyethylene terephthalate (PET) are used as recyclable resins in various plastic materials such as packaging films. As methods for recycling polyester resins, a material recycling method and a chemical recycling method are known. Among these, the chemical recycling method has attracted attention as a method for obtaining high-quality recycled materials.

[0003] In the chemical recycling method, the polyester resin is decomposed (depolymerized) into constituent monomers and recovered, and the recovered monomers are reused as raw materials for polyester resins or the like. As a method for decomposing polyester resins, a method of hydrolyzing polyester resins using high-temperature water is known (see Non-Patent Document 1).

Prior Art Documents

Non-Patent Documents

[0004]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] An object of the present disclosure is to improve the recovery rate of alcohol, which is a constituent monomer of the polyester resin, in a method for decomposing a polyester resin using high-temperature water.

Means for Solving the Problems

[0006] Typically, in chemical recycling, the hydrolysis of polyester resin is performed on polyester resin alone to avoid contamination with other components. From the perspective of improving the decomposition efficiency of polyester resin, the inclusion of other resin components such as polyamide resin was considered undesirable. However, as a result of the inventors' research, it was surprisingly revealed that the recovery rate of alcohol is improved by performing the hydrolysis of polyester resin using high-temperature water in the presence of polyamide resin. This disclosure is based on these findings.

[0007] Some aspects of this disclosure provide the following [1] to [9]. [1] A method for decomposing a polyester resin, comprising step (i) of hydrothermally treating the polyester resin in the presence of a polyamide resin, wherein step (i) includes proceeding with hydrolysis of the polyester resin at a temperature of 150°C or higher. [2] The method according to [1], wherein the temperature is 400°C or less. [3] The method according to [1] or [2], wherein the polyester resin comprises at least one selected from the group consisting of polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate. [4] The method according to any one of [1] to [3], wherein the polyamide resin comprises at least one selected from the group consisting of nylon 6 and nylon 66. [5] The method according to any one of [1] to [4], wherein the ratio of the amount of polyamide resin used to the amount of polyester resin used is 0.1 to 10 by mass. [6] The method according to any one of [1] to [5], wherein the ratio of the amount of water used to the total amount of polyester resin and polyamide resin used is 1 to 100 by mass. [7] The method according to any one of [1] to [6], wherein the polyester resin and the polyamide resin are resins derived from waste materials. [8] The method according to any one of [1] to [7], comprising the step (ii) of recovering the alcohol which is a constituent monomer of the polyester resin from the reaction mixture obtained in step (i). [9] The method according to [8], wherein step (ii) is a step of separating the reaction mixture into solid and liquid (ii-a), a step of treating the solid recovered in step (ii-a) with an alkaline treatment solution to obtain the treated solid and the treated alkaline treatment solution, respectively (ii-b), and a step of treating the alkaline treatment solution obtained in step (ii-b) with an acid (ii-c). [Effects of the Invention]

[0008] According to this disclosure, in a method for decomposing polyester resin using high-temperature water, the recovery rate of alcohol, which is a constituent monomer of the polyester resin, can be improved. [Modes for carrying out the invention]

[0009] The following describes exemplary embodiments of this disclosure. However, this disclosure is not limited in any way to the embodiments described below. In this specification, numerical ranges indicated using "~" indicate a range that includes the numbers before and after "~" as the minimum and maximum values, respectively. Also, unless otherwise explicitly stated, the units of the numbers before and after "~" are the same. Furthermore, in numerical ranges described in stages in this specification, the upper or lower limit of a numerical range in one stage may be replaced with the upper or lower limit of a numerical range in another stage. Also, in numerical ranges described in this specification, the upper or lower limit of that numerical range may be replaced with the values ​​shown in the examples. Furthermore, the upper and lower limits described individually can be combined in any way.

[0010] One aspect of this disclosure is a method for decomposing a polyester resin, comprising the step (i) of hydrothermally treating a polyester resin in the presence of a polyamide resin, wherein step (i) includes proceeding with hydrolysis of the polyester resin at a temperature (T1) of 150°C or higher.

[0011] According to the above aspects, in a method for decomposing polyester resin using high-temperature water, the recovery rate of alcohol, which is a constituent monomer of polyester resin, can be improved.

[0012] The above method is suitably used for the chemical recycling of waste materials containing polyester resin. Furthermore, the above method is also suitably used for the chemical recycling of waste materials (waste composite materials) containing polyester resin and polyamide resin. Such waste materials have traditionally been considered difficult to process using chemical recycling methods. Therefore, the above method is industrially useful because it can be used for the chemical recycling of waste materials containing polyester resin and polyamide resin.

[0013] The above method may also include, in addition to step (i), step (ii) of recovering the alcohol, which is a constituent monomer of the polyester resin, from the reaction mixture obtained in step (i) (hereinafter also referred to as "reaction mixture (i)"). Steps (i) and (ii) will be described in detail below.

[0014] [Step (i)] In step (i), a reaction mixture (i) containing hydrolysis products of the polyester resin, the polyamide resin and / or its hydrolysis products, and water is obtained by hydrothermally treating the polyester resin in the presence of a polyamide resin. Here, "hydrothermal treatment" is a heat treatment method that involves holding the object to be treated in high-temperature water for a certain period of time. In hydrothermal treatment, the object to be treated may also be held in water under high temperature and pressure for a certain period of time.

[0015] The polyester resin may be an aliphatic polyester resin or an aromatic polyester resin. Examples of the aliphatic polyester resin include, for example, polylactic acid (PLA) and polybutylene succinate (PBS). Examples of the aromatic polyester resin include, for example, polyethylene terephthalate (PET), polybutylene terephthalate (PBT), and polyethylene naphthalate (PEN). As the polyester resin, one of these may be used alone, or two or more thereof may be used in combination. Further, a copolymer (for example, PET / PEN) containing one or more of the polyesters exemplified above can also be used.

[0016] From the viewpoint of obtaining the remarkable effects of the present invention, the polyester resin preferably contains at least one selected from the group consisting of polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate, more preferably contains polyethylene terephthalate, and even more preferably contains a homopolymer of polyethylene terephthalate. Here, that the polyester resin contains polyethylene terephthalate means that the polyester resin has a repeating structure represented by the following formula (1), that the polyester resin contains polybutylene terephthalate means that the polyester resin has a repeating structure represented by the following formula (2), and that the polyester resin contains polyethylene naphthalate means that the polyester resin has a repeating structure represented by the following formula (3). In the following formula (1), n1, in the following formula (2), n2, and in the following formula (3), n3 represent the number of repetitions (an integer of 2 or more) of the structure within [].

Chemical formula

Chemical formula

Chemical formula

[0017] The polyamide resin may be an aliphatic polyamide resin or an aromatic polyamide resin. Examples of the aliphatic polyamide resin include nylon 6, nylon 66, nylon 11, and nylon 12. Examples of the aromatic polyamide resin include polyphenylene terephthalamide and polymetaphenylene isophthalamide. As the polyamide resin, one of these may be used alone, or two or more thereof may be used in combination. Further, a copolymer (for example, nylon 6 / 66) containing one or more of the polyamides exemplified above can also be used.

[0018] From the viewpoint of obtaining the remarkable effects of the present invention, the polyamide resin preferably contains at least one selected from the group consisting of nylon 6 and nylon 66, more preferably contains nylon 6, and even more preferably contains a homopolymer of nylon 6. Here, that the polyamide resin contains nylon 6 means that the polyamide resin has a repeating structure represented by the following formula (4), and that the polyamide resin contains nylon 66 means that the polyamide resin has a repeating structure represented by the following formula (5). In the following formula (4), n4 and in the following formula (5), n5 indicate the number of repetitions (an integer of 2 or more) of the structure within [].

Chemical formula

Chemical formula

[0019] The polyester resin and polyamide resin may be resins derived from waste materials such as packaging films. That is, in step (i), waste materials containing polyester resin and / or processed products thereof (e.g., crushed material or pellets) and waste materials containing polyamide resin and / or processed products thereof (e.g., crushed material or pellets) may be used as processing raw materials, or waste materials containing polyester resin and polyamide resin and / or processed products thereof (e.g., crushed material or pellets) may be used as processing raw materials. The waste materials containing polyester resin are, for example, polyester films such as PET films. The waste materials containing polyamide resin are, for example, polyamide films such as nylon 6 films. The waste materials containing polyester resin and polyamide resin are, for example, laminated films containing polyester film and polyamide film.

[0020] From the viewpoint of obtaining the effects of the present invention, the ratio of the amount of polyamide resin used to the amount of polyester resin used is preferably 0.1 to 10 by mass, more preferably 0.1 to 2, and even more preferably 0.2 to 1.

[0021] From the viewpoint of obtaining the effects of the present invention, the ratio of the amount of water used to the total amount of polyester resin and polyamide resin used is preferably 1 to 100 by mass, more preferably 5 to 100, and even more preferably 5 to 10.

[0022] The hydrolysis of the polyester resin in step (i) may be carried out in a reaction system containing other optional components in addition to the polyamide resin and water. That is, step (i) may be a step of hydrothermally treating the polyester resin in the presence of the polyamide resin and other optional components. Examples of optional components include olefin resins and styrene resins. Examples of olefin resins include polyethylene (PE) and polypropylene (PP). An example of a styrene resin is polystyrene (PS).

[0023] The above optional components may be resins derived from waste materials. Examples of waste materials containing olefin resins include polyolefin films such as PE film, composite films of polyester film and polyolefin film (e.g., PET / PE composite film), composite films of polyamide film and polyolefin film (e.g., nylon 6 / PE composite film), and composite films of polyester film, polyamide film and polyolefin film (e.g., PET / nylon / PE composite film).

[0024] The temperature (T1) at which the hydrolysis of the polyester resin proceeds in step (i) refers to the temperature within the reaction system described above. The temperature (T1) is 150°C or higher, and may be 200°C or higher, 225°C or higher, 250°C or higher, or 280°C or higher, from the viewpoint of easily obtaining a higher monomer yield. The temperature (T1) may be 400°C or lower, 350°C or lower, or 320°C or lower, from the viewpoint of easily obtaining a higher monomer yield, energy saving, and suppressing the generation of by-products. From the above viewpoint, the temperature (T1) may be 150 to 400°C, 200 to 350°C, 225 to 350°C, 250 to 320°C, or 280 to 320°C. However, even when the temperature (T1) is 150 to 400°C, step (i) may include proceeding with the hydrolysis of the polyester resin at a temperature outside the above range (for example, a temperature above 400°C), as long as it does not hinder the effects of the present invention.

[0025] The holding time at temperature (T1) may be 5 to 30 minutes, or 5 to 20 minutes or 10 to 15 minutes, from the viewpoint of achieving higher process efficiency.

[0026] In step (i), hydrothermal treatment may be performed using a heat source of 150°C or higher in order to bring the temperature (T1) within the above range. The temperature of the heat source may be adjusted to obtain the desired temperature (T1). Specifically, for example, it may be 150-400°C, 200-350°C, 250-320°C, or 280-320°C.

[0027] The heating time in hydrothermal treatment varies depending on the water temperature at the start of heating, but may be 10 to 40 minutes, 10 to 30 minutes, or 15 to 20 minutes.

[0028] Hydrothermal treatment may be carried out under an atmosphere of an inert gas such as argon or nitrogen, from the viewpoint of improving monomer yield by suppressing oxidation reactions.

[0029] The method of hydrothermal treatment is not particularly limited. For example, a treatment solution (treatment liquid) containing polyester resin, polyamide resin, water, and the above-mentioned optional components may be prepared, and the polyester resin may be hydrothermally treated by heating the treatment solution using a known method. In this case, the temperature of the treatment solution should be set to the above-mentioned temperature (T1). The method of heating the treatment solution should be one that can heat the treatment solution to a temperature of 150°C or higher. One such method is to seal the treatment solution in a reaction vessel and heat the reaction vessel. With this method, a portion of the treatment solution heated in the sealed reaction vessel vaporizes, increasing the pressure inside the reaction vessel, which allows the temperature of the treatment solution to reach 150°C or higher. The above-mentioned inert gas may be sealed inside the reaction vessel. The pressure inside the reaction vessel may be adjusted to, for example, 1.4 to 40 MPa or 1.5 to 20 MPa. The pressure inside the reaction vessel can be adjusted by the size (internal volume) of the reaction vessel, the amount of treatment solution, the amount of gas sealed inside, etc. The means for heating the treatment liquid are not particularly limited, and various heating furnaces such as molten salt baths and electric furnaces, and heaters can be used. After heating is complete, the reactor may be cooled and the treatment liquid (reaction mixture (i)) may be recovered from the reactor. The treatment liquid may be recovered from the reaction vessel using water (e.g., distilled water).

[0030] Process (i) may be carried out in batch, semi-batch, or continuous flow manner, or a combination of two or three of these may be used.

[0031] Step (i) may include allowing the hydrolysis of the polyamide resin to proceed. That is, the reaction mixture (i) may contain amines and carboxylic acids, which are hydrolysis products of the polyamide resin. Since the hydrolysis of polyamide resin usually proceeds at its melting point (225°C in the case of nylon 6), if the temperature (T1) is set to 225°C or higher, the hydrolysis of the polyester resin may proceed in the presence of the hydrolysis products of the polyamide resin (amines and carboxylic acids). When the hydrolysis of the polyester resin is carried out at 225°C or higher, the recovery rate of alcohol may be particularly significantly improved.

[0032] [Step (ii)] In step (ii), the alcohol, which is a constituent monomer of the polyester resin, is recovered from the reaction mixture (i). Specifically, if the polyester resin is polyethylene terephthalate, ethylene glycol is recovered as the alcohol. Ethylene glycol may also be recovered as its condensate (e.g., diethylene glycol). Ethylene glycol can be recovered by distillation or the like.

[0033] Step (ii) may include recovering a carboxylic acid, which is a constituent monomer of the polyester resin, from the reaction mixture (i). Specifically, if the polyester resin is polyethylene terephthalate, terephthalic acid is recovered as the carboxylic acid.

[0034] Step (ii) may include recovering the constituent monomers of the polyamide resin from the reaction mixture (i). Specifically, if the polyamide resin is nylon 6, one or both of 6-aminocaproic acid and its cyclic product ε-caprolactam are recovered as constituent monomers.

[0035] In step (ii), if the reaction mixture (i) contains a solid, a step (ii-a) of solid-liquid separation of the reaction mixture (i) may be performed. In this step, the solid (hereinafter referred to as "solid a") and liquid (hereinafter referred to as "liquid a") remaining in the reaction mixture (i) are separated and recovered. Solid a includes, for example, unreacted resin components and non-water-soluble components (e.g., terephthalic acid) from the decomposition products of the resin components. The unreacted resin components are, for example, polyester resin and polyamide resin, and the decomposition products of the resin components are, for example, constituent monomers of polyester resin and constituent monomers of polyamide resin. Liquid a is an aqueous solution and includes, for example, water-soluble components (e.g., ethylene glycol, ε-caprolactam) from the decomposition products of the resin components.

[0036] The method for separating solid a and liquid a may be any conventionally known method, such as filtration using a membrane filter, centrifugal separation, or precipitation. Each component in liquid a can be separated and recovered by methods such as distillation.

[0037] Step (ii) may further include step (ii-b) in which the solid a recovered in step (ii-a) is treated with an alkaline treatment solution to obtain the treated solid (hereinafter referred to as "solid b") and the treated alkaline treatment solution, and may further include step (ii-c) in which the alkaline treatment solution obtained in step (ii-b) is treated with an acid. The non-water-soluble component contained in solid a is usually an acid and is alkali-soluble, so by these series of treatments, the unreacted resin component, the non-water-soluble component which is a decomposition product of the resin component, and the water-soluble component which is a decomposition product of the resin component can be recovered, respectively. Steps (ii-b) and (ii-c) will be described below.

[0038] In step (ii-b), the solid a is first treated with an alkaline treatment solution. Specifically, for example, solid a is added to the alkaline treatment solution and stirred to dissolve the alkali-soluble components (water-insoluble components which are decomposition products of the resin component) in solid a into the alkaline treatment solution. As the alkaline treatment solution, for example, an aqueous solution of sodium hydroxide, an aqueous solution of potassium hydroxide, an aqueous solution of ammonia, etc., can be used. The treatment time may be adjusted as appropriate so that the alkali-soluble components (e.g., acidic components) in solid a can be dissolved in the alkaline treatment solution, for example, it may be 1 to 5 hours or 2 to 4 hours.

[0039] Next, solid b and the alkaline treatment solution are separated and recovered. The recovered solid b is, for example, unreacted resin components, and may include polyester resin and polyamide resin. The alkaline treatment solution may contain water-soluble components, which are decomposition products of the resin components, as well as water-soluble components, which are also decomposition products of the resin components. These water-soluble components may be, for example, those incorporated into solid a, or those generated during hydrothermal treatment by hydrolysis of esters such as 2-hydroxyethyl terephthalic acid and bis(2-hydroxyethyl) terephthalate by the alkaline treatment solution. The method for separating solid b and the alkaline treatment solution may be a conventionally known method similar to that described above.

[0040] In step (ii-b), the alkali-soluble components in solid a may be dissolved in the alkaline treatment solution by washing solid a with the alkaline treatment solution. In this case, the operation of separating solid b from the alkaline treatment solution is unnecessary, and the alkaline treatment solution is recovered as the washing solution.

[0041] In step (ii-c), the alkaline treatment solution obtained in step (ii-b) is subjected to acid treatment. Specifically, for example, an acid is added to the alkaline treatment solution to make the pH of the treatment solution acidic, and then the treatment solution is left to stand at a temperature below room temperature. This allows components that precipitate due to the acid in the alkaline treatment solution (e.g., terephthalic acid) to be precipitated. As the acid, for example, hydrochloric acid, sulfuric acid, nitric acid, etc. can be used. The treatment time may be adjusted as appropriate to allow sufficient precipitation of the above components, for example, it may be 6 to 24 hours, 8 to 16 hours, or 8 to 12 hours.

[0042] If precipitates are obtained, they may be separated and recovered from the treatment solution (acidic solution). The acidic solution obtained after recovering the precipitates may contain constituent monomers of polyester resin and polyamide resin. Therefore, if there are components dissolved in the acidic solution, the acidic solution may be neutralized and then the dissolved components may be separated and recovered by distillation or other methods.

[0043] The reaction mixture (i) may contain oligomers such as 2-hydroxyethyl terephthalic acid or bis(2-hydroxyethyl) terephthalate. If the reaction mixture (i) contains oligomers, the oligomers can be recovered by the operations of steps (ii-a) to (ii-c) described above. Alternatively, the oligomers may be subjected to the same hydrothermal treatment as in step (i) again.

[0044] The above describes one aspect of the present disclosure regarding the decomposition method of polyester resin, but the present disclosure is not limited to the above.

[0045] Another aspect of this disclosure is a method for recycling waste materials, including a method for decomposing the polyester resin described above. The recycling method may include a step of reusing the constituent monomers and oligomers of the polyester resin recovered as decomposition products of the polyester resin (for example, a step of regenerating a polyester resin-containing material containing the constituent monomers using the constituent monomers).

[0046] The waste materials mentioned above are waste materials containing polyester resin. Examples of waste materials include packaging containers (packaging bags made of film, packaging containers made of film and resin injection molded products, packaging containers made of resin injection molded products, etc.), clothing, etc. A specific example of a packaging container is a liquid bottle.

[0047] The waste material may be a waste material (waste composite material) containing polyester resin and polyamide resin. In this case, the recycling method may include a step of recovering the constituent monomers of the polyamide resin as decomposition products of the polyamide resin, and a step of reusing the recovered constituent monomers of the polyamide resin (for example, a step of regenerating a polyamide resin-containing material containing the constituent monomers using the constituent monomers). [Examples]

[0048] The present invention will be described in more detail below using examples and comparative examples, but the present invention is not limited to the following examples.

[0049] <Preparing the materials> The following materials were used as processing raw materials. • PET film: A film made of polyethylene terephthalate (manufactured by Toyobo Co., Ltd., product name: Toyobo Ester Film E5102) • PA film: A film made of nylon 6 (manufactured by Toyobo Co., Ltd., product name: Harden Film N1100) • PET / PE composite film: A composite film containing polyethylene terephthalate film and polyethylene film in a mass ratio of 1:4 (manufactured by RM Tohcello Co., Ltd., product name: TUX FC-S) • PA / PE composite film: A composite film containing a nylon 6 film and a polyethylene film in a mass ratio of 1:4 (manufactured by RM Tohcello Co., Ltd., product name: TUX FC-S)

[0050] <Example 1> (Hydrothermal treatment) A stainless steel reaction vessel (φ: 1 / 2 inch) with an internal volume of 10 ml was filled with 5.0 g of water, 0.5 g of PET film, and 0.5 g of PA film, and then sealed. At this time, the atmosphere inside the reaction vessel was replaced with nitrogen gas. Next, the reaction vessel was placed in a molten salt bath at 300°C and heated for 15 minutes to hydrothermally treat the polyester resin and promote hydrolysis of the polyester resin. After the hydrothermal treatment, the reaction vessel was cooled with water, and the reaction products were analyzed according to the following procedure.

[0051] (Analysis of reaction products) The reaction products were analyzed according to the following procedures (1) to (6). The results are shown in Table 1. In the following operations, a membrane filter manufactured by Millipore Japan Ltd. (pore size 0.2 μm) was used. (1) The reaction mixture in the reaction tube was collected using distilled water and filtered by suction using a membrane filter to obtain a solid (solid A) and a filtrate (filtrate A). (2) A portion of solid A (approximately 0.2 g) was added to 1.5 mL of 1.5 N aqueous ammonia solution and stirred to obtain a mixture of solid A and aqueous ammonium solution. This mixture was then left at room temperature for 2 hours. The mixture after standing was then filtered by suction using a membrane filter to obtain a solid (solid B) and a filtrate (filtrate B). (3) 3N hydrochloric acid was added dropwise to filtrate B until the pH of the solution became 3, and a mixture of filtrate B and hydrochloric acid was obtained. This mixture was then left to stand overnight (12 hours) at 10°C. Next, the mixture after standing was filtered by suction using a membrane filter to obtain a solid (solid C) and a filtrate (filtrate C). (4) Liquid chromatography (LC) analysis was performed on filtrate A under the following conditions, and the yield of each component was determined from the amount of ethylene glycol (EG) and diethylene glycol (DEG) contained in filtrate A and the number of units of each component contained in the raw material (polyethylene terephthalate). The same analysis was performed on filtrate C, but ethylene glycol (EG) and diethylene glycol (DEG) were not detected. [LC analysis conditions] A high-performance liquid chromatograph (Prominence series, Shimadzu Corporation) with a differential refractive index (RI) detector was used. For ε-CL, EG, and DEG, a Rezex™ RPM-Monosaccharide Pb+2 column (phenomenex, 7.8 mm inner diameter, 300 mm length) was used, and the column temperature was set to 70°C. Water was used as the mobile phase, and the flow rate was 0.6 mL / min. (5) After drying solid B, it was weighed, and assuming that all of solid B was the raw material for processing, the recovery rate of the raw material for processing was determined. (6) The solid C was dried and weighed, and the yield of terephthalic acid was determined assuming that all of the solid C was terephthalic acid (TPA).

[0052] <Example 2> The hydrothermal treatment of the polyester resin was carried out in the same manner as in Example 1, except that the amount of PA film used was changed to 0.1 g, and the resulting reaction product was analyzed. The results are shown in Table 1.

[0053] <Comparative Example 1> The hydrothermal treatment of the polyester resin was carried out in the same manner as in Example 1, except that PA film was not used, and the resulting reaction products were analyzed. The results are shown in Table 1.

[0054] [Table 1]

[0055] <Examples 3-4, Comparative Example 2> Hydrothermal treatment of polyester resin was carried out in the same manner as in Examples 1 and 2 and Comparative Example 1, except that the heating time was changed to 30 minutes, and the resulting reaction products were analyzed. However, for Comparative Example 2, steps (2), (3), (5), and (6) in the analysis of the reaction products were not performed. The results are shown in Table 2.

[0056] [Table 2]

[0057] <Examples 5-6, Comparative Example 3> Hydrothermal treatment of polyester resin was carried out in the same manner as in Examples 1 and 2 and Comparative Example 1, except that the heating time was changed to 45 minutes, and the resulting reaction products were analyzed. However, for Comparative Example 3, steps (2), (3), (5), and (6) in the analysis of the reaction products were not performed. The results are shown in Table 3.

[0058] [Table 3]

[0059] <Examples 7-8, Comparative Example 4> Hydrothermal treatment of polyester resin was carried out in the same manner as in Examples 1-2 and Comparative Example 1, except that the heating time was changed to 60 minutes, and the resulting reaction products were analyzed. However, for Comparative Example 4, steps (2), (3), (5), and (6) in the analysis of the reaction products were not performed. The results are shown in Table 4.

[0060] [Table 4]

[0061] <Examples 9-10, Comparative Example 5> Hydrothermal treatment of polyester resin was carried out in the same manner as in Examples 1-2 and Comparative Example 1, except that the temperature of the molten salt bath was changed to 325°C, and the resulting reaction products were analyzed. However, for Comparative Example 5, steps (2), (3), (5), and (6) in the analysis of the reaction products were not performed. The results are shown in Table 5.

[0062] [Table 5]

[0063] <Examples 11-12, Comparative Example 6> Hydrothermal treatment of polyester resin was carried out in the same manner as in Examples 9-10 and Comparative Example 5, except that the heating time was changed to 30 minutes, and the resulting reaction products were analyzed. However, for Comparative Example 6, steps (2), (3), (5), and (6) in the analysis of the reaction products were not performed. The results are shown in Table 6.

[0064] [Table 6]

[0065] <Examples 13-14, Comparative Example 7> Hydrothermal treatment of polyester resin was carried out in the same manner as in Examples 1 and 2 and Comparative Example 1, except that the temperature of the molten salt bath was changed to 350°C, and the resulting reaction products were analyzed. However, for Comparative Example 7, steps (2), (3), (5), and (6) in the analysis of the reaction products were not performed. The results are shown in Table 7.

[0066] [Table 7]

[0067] <Examples 15-16, Comparative Example 8> Hydrothermal treatment of polyester resin was carried out in the same manner as in Examples 1-2 and Comparative Example 1, except that the temperature of the molten salt bath was changed to 365°C and the heating time was changed to 10 minutes, and the resulting reaction products were analyzed. However, for Comparative Example 8, steps (2), (3), (5), and (6) in the analysis of the reaction products were not performed. The results are shown in Table 8.

[0068] [Table 8]

[0069] <Examples 17-18> Hydrothermal treatment of polyester resin was carried out in the same manner as in Examples 1 and 2, except that the temperature of the molten salt bath was changed to 285°C, and the resulting reaction products were analyzed. The results are shown in Table 9.

[0070] <Example 19> Hydrothermal treatment of polyester resin was carried out in the same manner as in Example 1, except that the temperature of the molten salt bath was changed to 275°C, and the resulting reaction product was analyzed. The results are shown in Table 9.

[0071] [Table 9]

[0072] <Example 20> The polyester resin was subjected to hydrothermal treatment in the same manner as in Example 1, except that PET / PE composite film and PA / PE composite film were used instead of PET film and PA film. The resulting reaction products were then analyzed. The results are shown in Table 10.

[0073] <Example 21> The hydrothermal treatment of the polyester resin was carried out in the same manner as in Example 20, except that the amount of PA / PE composite film used was changed to 0.1 g, and the resulting reaction product was analyzed. The results are shown in Table 10.

[0074] <Example 22> The hydrothermal treatment of the polyester resin was carried out in the same manner as in Example 20, except that the amount of PET / PE composite film used was changed to 1.18 g, the amount of PA / PE composite film used was changed to 0.23 g, and the amount of water used was changed to 3.0 g. The resulting reaction products were then analyzed. The results are shown in Table 10.

[0075] <Comparative Example 9> The hydrothermal treatment of the polyester resin was carried out in the same manner as in Example 20, except that a PA / PE composite film was not used, and the resulting reaction products were analyzed. The results are shown in Table 10.

[0076] <Example 23> Hydrothermal treatment of polyester resin was carried out in the same manner as in Example 20, except that the temperature of the molten salt bath was changed to 325°C, and the resulting reaction products were analyzed. The results are shown in Table 10.

[0077] Table 10

Claims

1. The process includes (i) a step of hydrothermally treating a polyester resin in the presence of a polyamide resin, A method for decomposing a polyester resin, wherein step (i) includes carrying out hydrolysis of the polyester resin at a temperature of 150°C or higher.

2. The method according to claim 1, wherein the temperature is 400°C or less.

3. The method according to claim 1 or 2, wherein the polyester resin comprises at least one selected from the group consisting of polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate.

4. The method according to claim 1 or 2, wherein the polyamide resin comprises at least one selected from the group consisting of nylon 6 and nylon 66.

5. The method according to claim 1 or 2, wherein the ratio of the amount of polyamide resin used to the amount of polyester resin used is 0.1 to 10 by mass.

6. The method according to claim 1 or 2, wherein the ratio of the amount of water used to the total amount of polyester resin and polyamide resin used is 1 to 100 by mass.

7. The method according to claim 1 or 2, wherein the polyester resin and the polyamide resin are resins derived from waste materials.

8. The method according to claim 1 or 2, further comprising the step (ii) of recovering the alcohol which is a constituent monomer of the polyester resin from the reaction mixture obtained in step (i).

9. The above step (ii) is, The process of separating the reaction mixture into solid and liquid (ii-a), The process involves treating the solid recovered in step (ii-a) with an alkaline treatment solution, and then obtaining the treated solid and the treated alkaline treatment solution, respectively, in step (ii-b). The method according to claim 8, further comprising the step (ii-c) of treating the alkaline treatment solution obtained in the above step (ii-b) with acid.

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