Method for depolymerizing polyalkylene terephthalate resins, and its applications

Abstract

Depolymerization of polyalkylene terephthalate resin using Alcosilis, providing a superior depolymerization method. [Solution] The method for depolymerizing a polyalkylene terephthalate resin according to the present disclosure is a method for depolymerizing a polyalkylene terephthalate resin by alcoholization of the polyalkylene terephthalate resin, wherein the polyalkylene terephthalate resin includes a polybutylene terephthalate resin, and the alcohol used for alcoholization contains tetrahydrofuran in a proportion of more than 0 ppm by mass and 100,000 ppm or less by mass.

Description

[Technical Field]

[0001] This invention relates to a method for depolymerizing polyalkylene terephthalate resins, as well as a method for producing polyalkylene terephthalate resins. Furthermore, it relates to a method for producing terephthalic acid esters and a method for producing diols. [Background technology]

[0002] Polyalkylene terephthalate resins, such as polyethylene terephthalate resin and polybutylene terephthalate resin, are widely used in various industrial fields. As a method for reusing polyalkylene terephthalate resins, chemical recycling has been proposed, which utilizes monomers recovered by depolymerization of polyalkylene terephthalate resin as raw materials (for example, Patent Documents 1-3). [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] International Publication No. 2021 / 229470 [Patent Document 2] Japanese Patent Publication No. 2023-111250 [Patent Document 3] Japanese Patent Publication No. 2023-75033 [Overview of the project] [Problems that the invention aims to solve]

[0004] Alcolysis is a known method for the depolymerization of polyalkylene terephthalate resins. Alcolysis is a method in which polyalkylene terephthalate resins are transesterified in the presence of a large amount of alcohol. However, there is still room for improvement in the depolymerization of polyalkylene terephthalate resins using Alcosilis. The present invention aims to solve the aforementioned problems and to provide a superior depolymerization method for polyalkylene terephthalate resins using Alcosilis. [Means for solving the problem]

[0005] Based on the above problems, the inventors investigated and found that the above problems were solved by the following means. [1] A method for depolymerizing a polyalkylene terephthalate resin by alcoholis of the polyalkylene terephthalate resin, The polyalkylene terephthalate resin comprises polybutylene terephthalate resin, A method for depolymerizing a polyalkylene terephthalate resin, wherein the alcohol used for the alcoholization contains tetrahydrofuran in a proportion of more than 0 ppm by mass and 100,000 ppm by mass or less. [2] The method for depolymerizing a polyalkylene terephthalate resin according to [1], wherein the alcohol used for alcoholissis contains tetrahydrofuran in a proportion of 100 ppm by mass or more and 20,000 ppm by mass or less. [3] A reaction step in which a raw material containing polyalkylene terephthalate resin and alcohol are supplied to a reactor to perform alcoholosis, thereby obtaining a reaction product containing terephthalate ester, diol, tetrahydrofuran, and unreacted alcohol, A solid-liquid separation step to remove solid components from the reaction product to obtain liquid components, A first separation step involves distilling the liquid to separate it into a volatile fraction containing the alcohol and the tetrahydrofuran, and a residual fraction containing the terephthalate ester and the diol. A method for depolymerizing a polyalkylene terephthalate resin according to [1] or [2], comprising a circulation step of circulating at least a portion of the volatile fraction obtained in the first separation step back to the reactor as a source of the alcohol in the reaction step. [4] The method for depolymerizing a polyalkylene terephthalate resin according to any one of [1] to [3], wherein the alcohol used for alcoholissis is an alcohol separated from the depolymer product after depolymerization of the polyalkylene terephthalate resin. [5] The method for depolymerizing a polyalkylene terephthalate resin according to any one of [1] to [4], wherein the depolymerization temperature of the polyalkylene terephthalate resin is 150 to 230°C. [6] The method for depolymerizing a polyalkylene terephthalate resin according to any one of [1] to [5], wherein the polyalkylene terephthalate resin is waste material containing an inorganic filler. [7] A method for depolymerizing a polyalkylene terephthalate resin according to [6], comprising removing solids from the depolymerized product after depolymerizing the polyalkylene terephthalate resin. [8] A method for depolymerizing a polyalkylene terephthalate resin according to any one of [1] to [7], wherein the alcohol used for the alcoholization is methanol. [9] A method for depolymerizing a polyalkylene terephthalate resin according to any one of [1] to [8], wherein the alcohol used for alcoholissis contains water in a proportion of more than 0 ppm by mass and 5,000 ppm by mass or less. A method for producing a polyalkylene terephthalate resin, comprising polymerizing a terephthalic acid ester and a diol obtained by a depolymerization method for polyalkylene terephthalate resin described in any one of

[10] [1] to [9].

[11] The process involves depolymerizing the polyalkylene terephthalate resin by alcoholis of the polyalkylene terephthalate resin, The polyalkylene terephthalate resin comprises polybutylene terephthalate resin, A method for producing terephthalate esters, wherein the alcohol used for the alcoholization contains tetrahydrofuran in a proportion of more than 0 ppm by mass and 1,000,000 ppm by mass or less.

[12] The process involves depolymerizing the polyalkylene terephthalate resin by alcoholis of the polyalkylene terephthalate resin, The polyalkylene terephthalate resin contains polybutylene terephthalate resin, A method for producing a diol, wherein the alcohol used in the alcoholysis contains tetrahydrofuran at a ratio of more than 0 mass ppm and not more than 100,000 mass ppm.

Advantages of the Invention

[0006] It is possible to provide a more excellent depolymerization method, which is the depolymerization of a polyalkylene terephthalate resin by alcoholysis.

Brief Description of the Drawings

[0007] [Figure 1] FIG. 1 is a scheme showing an example of the depolymerization method of the present embodiment. [Figure 2] FIG. 2 shows the process of the example.

Modes for Carrying Out the Invention

[0008] Hereinafter, modes for carrying out the present invention (hereinafter simply referred to as "the present embodiment") will be described in detail. The following present embodiment is an exemplification for explaining the present invention, and the present invention is not limited only to the present embodiment. In this specification, "~" is used in the sense of including the numerical values described before and after it as the lower limit value and the upper limit value. "A~B" means A or more and B or less. Further, any combination of the upper limit value and the lower limit value of the numerical values in this specification can be cited as an example of the present embodiment.

[0009] In this specification, "mass%" indicates the content ratio of a predetermined component contained in the total amount of 100 mass%. Also, "mass%" and "mass ppm" are synonymous.

[0010] In this specification, "arbitrary" or "optionally" means that the subsequently described situation may or may not occur. Therefore, the description includes both the case where the situation occurs and the case where it does not occur. In this specification, unless otherwise specified, all kinds of physical property values and characteristic values are those at 23°C.

[0011] In this specification, the term "step" includes not only an independent step but also a step whose intended function can be achieved even if it cannot be clearly distinguished from other steps. Unless otherwise specified in this specification or clearly inconsistent with the context, all the steps described in this specification can be performed in any suitable order. In this specification, ppm means mass ppm.

[0012] When the measurement methods and the like described by the standards shown in this specification vary from year to year, unless otherwise specified, they are based on the standards as of January 1, 2024. When the measurement methods and the like described by the standards shown in this specification have been abolished as of January 1, 2024, they are based on the standards at the time of abolition.

[0013] The method for depolymerizing a polyalkylene terephthalate resin according to this embodiment is a method for depolymerizing a polyalkylene terephthalate resin by alcoholysis of the polyalkylene terephthalate resin, wherein the polyalkylene terephthalate resin contains a polybutylene terephthalate resin, and the alcohol used for the alcoholysis contains tetrahydrofuran at a ratio of more than 0 mass ppm and 100,000 mass ppm or less (or 20,000 mass ppm or less).

[0014] More specifically, the above method is a method for depolymerizing polyalkylene terephthalate resin, comprising: a reaction step of supplying a raw material containing polyalkylene terephthalate resin and alcohol to a reactor to perform alcoholosis and obtain a reaction product containing terephthalate ester, diol, tetrahydrofuran, and unreacted alcohol; a solid-liquid separation step of removing solid components from the reaction product to obtain a liquid component; a first separation step of distilling the liquid component to separate it into a volatile fraction containing the alcohol and tetrahydrofuran and a residual fraction containing the terephthalate ester and diol; and a circulation step of circulating at least a portion of the volatile fraction obtained in the first separation step back to the reactor as a source of alcohol in the reaction step.

[0015] When depolymerizing polyalkylene terephthalate resins by alcoholis, the transesterification reaction proceeds, for example, in the following sequence. [ka] In the above formula, R 1 R is an alkylene group having 1 to 8 carbon atoms. 2 is an aliphatic hydrocarbon group having 1 to 10 carbon atoms, where n is an integer and m is 1 or 2.

[0016] R 1 When R is an ethylene group (-CH2CH2-), the polyalkylene terephthalate resin to be depolymerized becomes polyethylene terephthalate resin (PET). 1 When the group is a butylene group (-CH2CH2CH2CH2-), it becomes polybutylene terephthalate resin (PBT).

[0017] In this specification, the material containing the polyalkylene terephthalate resin that is to be depolymerized and introduced into the depolymerization reaction system is referred to as raw material A. Alcohol is added to raw material A, and alcoholosis proceeds. Therefore, raw material A may consist only of the polyalkylene terephthalate resin that is to be depolymerized, but it is usually a resin material or resin molded product containing polyalkylene terephthalate resin.

[0018] When polyalkylene terephthalate resins are depolymerized, the depolymerized product contains not only the intended target products, terephthalate esters and diols, but also by-reactants and unwanted substances. Therefore, it is desirable to be able to remove these by-reactants. More specifically, as shown in Figure 1, when a methanolysis reaction is carried out using methanol (MeOH) with a raw material (raw material A), a depolymer containing dimethyl terephthalate (DMT) and butanediol (BG) is obtained. Such a depolymer usually contains by-products such as tetrahydrofuran, water, inorganic fillers and catalysts derived from raw material A. Of these components, the inorganic fillers and catalysts derived from raw material A, which are solid components, can be separated by solid-liquid separation of the depolymer.

[0019] Furthermore, since the transesterification reaction is carried out in the presence of a large amount of methanol (MeOH) during the alcoholiseration process, it would be beneficial if the methanol after the transesterification reaction could be recovered and reused in the alcoholiseration. The methanol (MeOH) used in the alcoholiseration can be separated as crude methanol (crude MeOH) by distilling the liquid remaining after the solid-liquid separation of the depolymerized product. However, the methanol after the transesterification reaction contains by-reactants such as tetrahydrofuran and water. Tetrahydrofuran and water can be reduced to some extent during the distillation. However, the inventors' investigations revealed that the crude methanol (crude MeOH) contains an excess amount of tetrahydrofuran, considering that the methanol will be reused in the depolymerization of polyalkylene terephthalate. Repeated use of methanol containing such tetrahydrofuran in the depolymerization process increases the amount of THF in the depolymerization reaction system, which is thought to reduce the productivity of terephthalic acid esters and diols by depolymerization. Furthermore, it is thought that if alcoholissis is performed using methanol in this manner, the polymers obtained by polymerizing the terephthalate ester and diol obtained by depolymerization (especially the polymers obtained by polymerizing dimethyl terephthalate and butanediol) will take on a yellowish tint. On the other hand, completely removing tetrahydrofuran from crude methanol (crude MeOH) is not practical due to cost and other factors.

[0020] Under these circumstances, this embodiment presents a method for depolymerizing polybutylene terephthalate resin that allows for the successful recovery of terephthalate esters and diols even when using alcohols containing tetrahydrofuran. Specifically, it is based on the premise of using alcohols containing a small amount of tetrahydrofuran, and the range in which polyalkylene terephthalate resin can be appropriately depolymerized by alcoholis has been identified. As a result, the alcohol used for alcoholosis can be reused, reducing the amount of new alcohol used, thus reducing waste and carbon dioxide emissions. Furthermore, the components obtained by distillation and removal of alcohol contain dimethyl terephthalate (DMT) and butanediol (BG), which are the target products of the depolymerization. These are preferably further purified.

[0021] On the other hand, the generation of high-boiling point byproducts can be effectively suppressed by adjusting the amount of tetrahydrofuran in the alcohol. Although the above explanation has been given in reference to Figure 1, it goes without saying that the present invention is not limited to the embodiment shown in Figure 1. The reference numerals in Figure 1 correspond to the reference numerals in Figure 2.

[0022] The embodiments of the present invention will be described in detail below, but the description of the constituent elements described below is merely one example of an embodiment of the present invention and is not limited to these.

[0023] The depolymerization method of this embodiment depolymerizes a polyalkylene terephthalate resin and includes at least a polybutylene terephthalate resin. That is, the polyalkylene terephthalate resin to be depolymerized may be a polybutylene terephthalate resin alone, or it may include other polyalkylene terephthalate resins.

[0024] In this embodiment, the polybutylene terephthalate resin is preferably composed mainly of a polycondensation of terephthalic acid or its ester-forming derivative with butanediol, i.e., more than 50% by mass of the total resin, preferably 70% by mass or more, more preferably 80% by mass or more, even more preferably 90% by mass or more, even more preferably 95% by mass or more, and even more preferably 99% by mass or more. In addition to terephthalic acid or its ester-forming derivatives, other aromatic dicarboxylic acids (e.g., isophthalic acid) can be used in combination. Furthermore, in small amounts, one or more aliphatic dicarboxylic acids such as adipic acid, azelaic acid, dodecanedionic acid, and sebacic acid, or alicyclic dicarboxylic acids such as 1,2-cyclohexanedicarboxylic acid, 1,3-cyclohexanedicarboxylic acid, and 1,4-cyclohexanedicarboxylic acid can be used in combination.

[0025] In addition to butanediol, other aliphatic diols and aromatic diols can also be used as dihydroxy compounds. In addition to the difunctional monomers mentioned above, small amounts of trifunctional monomers such as trimellitic acid, trimesic acid, pyromellitic acid, pentaerythritol, and trimethylolpropane can also be used to introduce branched structures, as well as monofunctional compounds such as fatty acids to adjust molecular weight.

[0026] As described above, the polyalkylene terephthalate resin used in this embodiment may contain polyalkylene terephthalate resins other than polybutylene terephthalate resin. Polyethylene terephthalate resin is preferred as the polyalkylene terephthalate resin other than polybutylene terephthalate resin. The polyethylene terephthalate resin is preferably one that consists mainly of a polycondensation of terephthalic acid or its ester-forming derivative with ethylene glycol, i.e., more than 50% by mass of the total resin, preferably 70% by mass or more, more preferably 80% by mass or more, even more preferably 90% by mass or more, even more preferably 95% by mass or more, and even more preferably 99% by mass or more. For dicarboxylic acid compounds other than terephthalic acid or its ester-forming derivatives, and for components other than dihydroxy compounds other than ethylene glycol and bifunctional monomers, the matters described in the section on polybutylene terephthalate resin can be considered.

[0027] The polyalkylene terephthalate resin used in this embodiment preferably contains polybutylene terephthalate resin as its main component, and it is preferable that the amount of polybutylene terephthalate resin exceeds 50% by mass. The proportion of polybutylene terephthalate resin in the polyalkylene terephthalate resin is preferably 60% by mass or more, more preferably 70% by mass or more, and may be 80% by mass or more, 90% by mass or more, 95% by mass or more, or 99% by mass or more. Furthermore, it is preferable that 90% or more (more preferably 95% or more, and especially 99% or more) of the alkylene terephthalate resin other than polybutylene terephthalate resin is polyethylene terephthalate resin.

[0028] Raw material A may contain residues of the polymerization catalyst used when synthesizing the polyalkylene terephthalate resin. The polymerization catalyst is not particularly limited as long as it is a catalyst used in the polymerization reaction of polyalkylene terephthalate resin. In a typical example, the polymerization catalyst includes at least one selected from the group consisting of titanium compounds, magnesium compounds, zinc compounds, tin compounds, germanium compounds, and antimony compounds, and may particularly include titanium compounds.

[0029] Examples of polymerization catalysts include titanium compounds such as tetrabutoxytitanium, tetraisobutoxytitanium, tetraisopropoxytitanium, tetrapropoxytitanium, tetraethoxytitanium, tetramethoxytitanium, titanium acetylacetonate, and titanium diisopropoxide bis(acetylacetonate). Titanium compounds may be used individually or in combination of two or more.

[0030] Examples of polymerization catalysts include magnesium compounds such as magnesium oxide, magnesium hydroxide, magnesium methoxide, magnesium ethoxide, magnesium carbonate, magnesium acetate, and magnesium acetylacetonate. Magnesium compounds may be used individually or in combination of two or more.

[0031] Examples of polymerization catalysts include zinc oxide, zinc hydroxide, zinc acetate, zinc trifluoroacetate, and zinc acetylacetonate. Zinc compounds may be used individually or in combination of two or more.

[0032] Examples of polymerization catalysts include tin oxide, tin acetate, tin butoxide, tin acetylacetonate, and dibutyltin oxide. The tin compound may be used alone or in combination of two or more types.

[0033] Examples of polymerization catalysts include germanium oxide, germanium methoxide, germanium ethoxide, germanium propoxide, germanium isopropoxide, and germanium butoxide. Germanium compounds may be used individually or in combination of two or more.

[0034] Examples of polymerization catalysts include antimony compounds such as antimony oxide, antimony acetate, antimony methoxide, antimony ethoxide, antimony isopropoxide, and antimony butoxide. Antimony compounds may be used individually or in combination of two or more.

[0035] The content of the polymerization catalyst (preferably a titanium compound) is preferably such that the amount of titanium atoms is typically 1 ppm or more, more preferably 3 ppm or more, even more preferably 5 ppm or more, and usually 200 ppm or less, more preferably 100 ppm or less, and even more preferably 50 ppm or less, per 100 parts by mass of the polyalkylene terephthalate resin to be depolymerized. Setting the content above the lower limit tends to further improve the reactivity when depolymerizing the polyalkylene terephthalate resin. Setting the content below the upper limit tends to effectively suppress the repolymerization of terephthalic acid esters and diols, which are the depolymerized products, during distillation purification.

[0036] In this embodiment, raw material A, which contains a polyalkylene terephthalate resin, is introduced into the depolymerization reaction system. One example of the aforementioned raw materials is waste material. Waste material includes scraps from the polymerization process of polyalkylene terephthalate resin, defective products and scraps generated during the molding of polyalkylene terephthalate resin, unused polyalkylene terephthalate resin material over time, and molded products of polyalkylene terephthalate resin derived from products that have been used in the market. Examples of such waste material include heat-molded products such as injection-molded products and extruded products. Examples of waste materials derived from such used products include waste materials from connectors, home appliances, power meter housings, battery transport trays, relays, sensors, toothbrushes, general merchandise, automotive parts, textiles, films, tubes, and other extruded parts. Examples of unused polyalkylene terephthalate resin materials over time include pellets, resin sheets, and prepregs.

[0037] When raw material A is waste material, it often contains components other than polyalkylene terephthalate resin. Examples of waste material raw material A include, in addition to polyalkylene terephthalate resin, at least one of the following: thermoplastic resins other than polyalkylene terephthalate resin, inorganic fillers, and resin additives. Each of these components may be present individually or in combination of two or more.

[0038] Examples of thermoplastic resins other than polyalkylene terephthalate resin include elastomers, polystyrene resins, polycarbonate resins, and polyamide resins. Details of elastomers can be found in paragraphs 0025 to 0029 of Japanese Patent Application Publication No. 2020-19950 and paragraphs 0043 to 0067 of Japanese Patent Application Publication No. 2016-132772, and this content is incorporated herein by reference. The content of thermoplastic resins other than polyalkylene terephthalate resin is preferably 0 to 40 parts by mass, more preferably 0 to 30 parts by mass, even more preferably 0 to 20 parts by mass, even more preferably 0 to 10 parts by mass, even more preferably 0 to 5 parts by mass, and even more preferably 0 to 3 parts by mass, based on 100 parts by mass of polyalkylene terephthalate resin contained in raw material A.

[0039] Examples of inorganic fillers include fibrous fillers and non-fibrous fillers. As fibrous fillers, glass fibers, carbon fibers, basalt fibers, wollastonite, potassium titanate fibers, etc., can be used, with glass fibers and / or carbon fibers being preferred, and glass fibers being more preferred. Non-fibrous fillers can include granular or amorphous fillers such as calcium carbonate, titanium oxide, feldspar minerals, clay, organic clay, and glass beads; plate-like fillers such as talc; and flake-like fillers such as glass flakes, mica, and graphite. The inorganic filler content is preferably 150 parts by mass or less, more preferably 120 parts by mass or less, even more preferably 100 parts by mass or less, even more preferably 80 parts by mass or less, even more preferably 70 parts by mass or less, and may be 60 parts by mass or less, based on 100 parts by mass of polyalkylene terephthalate resin contained in raw material A. The lower limit of the inorganic filler content may be 0 parts by mass, based on 100 parts by mass of polyalkylene terephthalate resin contained in raw material A, but if inorganic fillers are included, it is preferably 10 parts by mass or more, more preferably 15 parts by mass or more, preferably 20 parts by mass or more, and even more preferably 25 parts by mass or more.

[0040] Examples of resin additives include flame retardants, flame retardant enhancers, colorants, stabilizers, mold release agents, transesterification inhibitors, UV absorbers, antistatic agents, antifogging agents, antiblocking agents, flow improvers, plasticizers, and dispersants. The content of the resin additive is preferably 0 to 30 parts by mass, more preferably 0 to 10 parts by mass, even more preferably 0 to 5 parts by mass, and even more preferably 0 to 3 parts by mass, based on 100 parts by mass of the polyalkylene terephthalate resin contained in raw material A.

[0041] The raw material A preferably contains a total of 95% by mass or more, more preferably 99% by mass or more, and may also contain 100% by mass, a total of polyalkylene terephthalate resin, thermoplastic resins other than polyalkylene terephthalate resin, inorganic fillers, and resin additives as needed. In particular, the proportion of polyalkylene terephthalate resin (preferably polybutylene terephthalate resin) in raw material A is preferably 10% by mass or more, more preferably 20% by mass or more, even more preferably 30% by mass or more, even more preferably 40% by mass or more, and also preferably 90% by mass or less, more preferably 80% by mass or less, even more preferably 70% by mass or less, and even more preferably 60% by mass or less.

[0042] In this embodiment, the alcohol used in Alcosilis is not particularly specified as long as it can carry out the transesterification reaction of the polyalkylene terephthalate resin, however R 2 -(OH) m A compound represented by R 2 A compound in which is an aliphatic hydrocarbon group having 1 to 10 carbon atoms and m is 1 or 2 is preferred. 2 The alkyl group is preferably a C1-C10 alkyl group, more preferably a C1-C6 alkyl group, and even more preferably a C1-C3 alkyl group. The alkyl group may be linear, branched, or cyclic, and is preferably a linear or branched alkyl group, and more preferably a linear alkyl group. m is preferably 1. The alcohol is preferably methanol, ethanol, propanol, ethylene glycol, or butanediol, more preferably methanol or ethanol, and even more preferably methanol.

[0043] In this embodiment, the alcohol used in the alkosilis (for example, Stream 2 in Figure 2) contains tetrahydrofuran. The proportion of tetrahydrofuran in the alcohol is greater than 0 ppm by mass and may be 10 ppm or more, 50 ppm or more, 100 ppm or more, 200 ppm or more, 500 ppm or more, 1,000 ppm or more, 5,000 ppm or more, 8,000 ppm or more, 10,000 ppm or more, or 15,000 ppm or more. Furthermore, the proportion of tetrahydrofuran in the alcohol may be 100,000 ppm or less, 80,000 ppm or less, or 60,000 ppm or less, and more preferably 20,000 ppm or less, and more preferably 15,000 ppm or less, 10,000 ppm or less, 8,000 ppm or less, 6,000 ppm or less, or 5,000 ppm or less.

[0044] Alcohols such as methanol act as poor solvents for polyalkylene terephthalate resins, such as polybutylene terephthalate resins, and their depolymerization intermediates, oligomers. Therefore, the solubility of polyalkylene terephthalate oligomers in alcohol is low, and the depolymerization reaction becomes rate-limiting, leading to a tendency for high-boiling point compounds such as hydroxybutylmethyl terephthalate (HBMT) to be formed. However, it has been found that adding a small amount of tetrahydrofuran to alcohols such as methanol improves the solubility of polyalkylene terephthalate oligomers. As a result, the depolymerization reactivity of polyalkylene terephthalate resins increases, and depolymerization proceeds rapidly. This effectively suppresses the formation of the by-product HBMT. Suppressing the formation of high-boiling point byproducts such as HBMT makes it possible to avoid increasing the amount of purging required in the purification process. As a result, the loss of product DMT and BG, and the increase in unnecessary waste disposal costs can be reduced, significantly improving the economics, sustainability, and efficiency of the depolymerization process.

[0045] To obtain the above effects, the THF concentration in the alcohol, such as methanol, is preferably in the range of greater than 0 ppm by mass and less than or equal to 100,000 ppm by mass. At 0 ppm by mass, the effect of suppressing the formation of high-boiling point byproducts by improving the solubility of polyalkylene terephthalate oligomers may not be sufficiently exhibited. At concentrations greater than 100,000 ppm by mass, the amount of MeOH, the main solvent, relatively decreases, which may lower the equilibrium yield of the transesterification reaction and reduce the yield of the target products, DMT and BG. By adding THF within this specific range, it is possible to achieve both the suppression of HBMT formation and the maintenance of DMT and BG yields. In this embodiment, it is particularly preferable that the alcohol used for alcoholissis contains tetrahydrofuran in a proportion of 100 ppm by mass or more and less than or equal to 20,000 ppm by mass.

[0046] The alcohol used for alcoholization is preferably free of water, but it may contain water. The proportion of water in the alcohol used in Alcosilis is preferably 5,000 ppm by mass or less, more preferably 3,000 ppm by mass or less, even more preferably 2,000 ppm by mass or less, and even more preferably 1,700 ppm by mass or less. It may also be 0 ppm by mass, and even more preferably greater than 0 ppm by mass, 50 ppm or more by mass, 100 ppm or more by mass, 500 ppm or more by mass, 1,000 ppm or more by mass, or 1,100 ppm or more by mass. By setting the value above the aforementioned lower limit, the accuracy of fractional distillation during the purification of alcohol for reuse can be reduced, which tends to improve production efficiency. Furthermore, by setting the value below the aforementioned upper limit, the deactivation of the transesterification catalyst can be more effectively suppressed, allowing the depolymerization reaction to proceed more effectively.

[0047] When the amount of alcohol used in alcoholysis is based on 1 equivalent of the total number of moles of ester bonds in the polyalkylene terephthalate resin (preferably polybutylene terephthalate resin), 1 to 500 equivalents is preferable, 2 to 200 equivalents is more preferable, and 5 to 200 equivalents is even more preferable. When the amount of alcohol used is within the above range, R 2 The yields of terephthalic acid (such as dimethyl terephthalate) and diol substituted with are likely to improve, and the formation of tetrahydrofuran also tends to be suppressed.

[0048] The amount of alcohol used in alcoholysis is also preferably 15 to 10,000 parts by mass, more preferably 50 to 5,000 parts by mass, and even more preferably 80 to 2,000 parts by mass with respect to 100 parts by mass of raw material A when adding alcohol. When the amount of alcohol used is within the above range, R 2 The yields of terephthalic acid (such as dimethyl terephthalate) and diol substituted with are likely to improve, and the formation of tetrahydrofuran also tends to be suppressed.

[0049] In particular, in this embodiment, it is preferable that the alcohol used in alcoholysis contains alcohol separated from the depolymerized product (preferably the depolymerized product after removing the solid content) after depolymerizing the polyalkylene terephthalate resin. By using such alcohol, the reuse of alcohol becomes possible. In this embodiment, it is preferable that 50% by mass or more, 70% by mass or more, 90% by mass or more, 95% by mass or more of the alcohol used in alcoholysis is alcohol separated from the solution after depolymerizing the polyalkylene terephthalate resin. Also, it is preferable to use virgin alcohol, which is an unused product, in combination with the alcohol used in alcoholysis and adjust it to a desired tetrahydrofuran concentration. In this embodiment, the alcohol used in alcoholysis can also be put into the reaction system for depolymerizing the polyalkylene terephthalate resin to continuously advance the depolymerization reaction. Details of the method for separating the alcohol to be reused will be described later.

[0050] It is preferable to use a transesterification catalyst during depolymerization. By using a transesterification catalyst, the depolymerization reaction can be carried out effectively. As the transesterification catalyst, at least one selected from the group consisting of magnesium compounds, zinc compounds, calcium compounds, barium compounds, strontium compounds, aluminum compounds, sodium compounds, potassium compounds, iron compounds, copper compounds, manganese compounds, zirconium compounds, germanium compounds, and titanium compounds is preferred. These transesterification catalysts tend to improve the yield of terephthalic acid esters and diols, and also tend to suppress the formation of tetrahydrofuran.

[0051] In further preferred examples, the transesterification catalyst may include at least one selected from the group consisting of oxides, hydroxides, compounds having alkoxy groups, carbonates, acetates, and acetylacetonate compounds. These transesterification catalysts tend to improve the yield of terephthalic acid esters and diols, and also tend to suppress the formation of tetrahydrofuran.

[0052] In further preferred examples, the transesterification catalyst may include at least one selected from the group consisting of alkali metal trifluoroacetates and trifluoromethanesulfonates. These transesterification catalysts tend to improve the yield of terephthalic acid esters and diols, and also tend to suppress the formation of tetrahydrofuran. In particular, they can effectively suppress the formation of tetrahydrofuran.

[0053] Examples of transesterification catalysts include magnesium compounds such as magnesium oxide, magnesium hydroxide, magnesium methoxide, magnesium ethoxide, magnesium carbonate, magnesium acetate, and magnesium acetylacetonate. Magnesium compounds may be used individually or in combination of two or more.

[0054] Examples of transesterification catalysts include sodium methoxide, sodium ethoxide, sodium carbonate, sodium bicarbonate, sodium acetate, sodium trifluoroacetate, and sodium trifluoromethanesulfonate. Sodium compounds may be used individually or in combination of two or more.

[0055] Examples of transesterification catalysts include titanium compounds such as tetrabutoxytitanium, tetraisobutoxytitanium, tetraisopropoxytitanium, tetrapropoxytitanium, tetraethoxytitanium, tetramethoxytitanium, titanium acetylacetonate, and titanium diisopropoxide bis(acetylacetonate). Titanium compounds may be used individually or in combination of two or more.

[0056] As the transesterification catalyst, at least one selected from tetrabutoxytitanium, magnesium hydroxide, and sodium carbonate is preferred.

[0057] The amount of transesterification catalyst used to decompose raw material A is preferably 0.0001 mol% or more, more preferably 0.0005 mol% or more, even more preferably 0.001 mol% or more, and may also be 0.1 mol% or more, 0.5 mol% or more, or 0.8 mol% or more, and may also be 10 mol% or less, more preferably 5 mol% or less, and may also be 2.5 mol% or less. When the amount of transesterification catalyst used is within the aforementioned numerical range, the yields of terephthalic acid esters and diols tend to improve, and the formation of tetrahydrofuran tends to be suppressed. One or more transesterification catalysts may be used; if two or more are used, it is preferable that the total amount is within the above range.

[0058] In this embodiment, the components introduced into the alcoholis reaction system preferably consist of raw material A and alcohol totaling 90% by mass or more, more preferably 95% by mass or more, and may also consist of 99% by mass or more.

[0059] In this embodiment, the depolymerization temperature is preferably 100°C or higher, more preferably 140°C or higher, even more preferably 150°C or higher, even more preferably 160°C or higher, and may also be 170°C or higher. Furthermore, it is preferably 230°C or lower, more preferably 210°C or lower, even more preferably 200°C or lower, and even more preferably 190°C or lower. These upper and lower temperature limits can be arbitrarily combined, preferably 100 to 230°C, more preferably 140 to 210°C, even more preferably 150 to 200°C, and even more preferably 160 to 190°C. Higher temperatures tend to improve the yield of terephthalic acid esters and diols. Lower temperatures tend to more effectively suppress the formation of tetrahydrofuran and reduce the energy load required for depolymerization.

[0060] The pressure during depolymerization is preferably 1 MPa or higher, more preferably 1.2 MPa or higher, even more preferably 1.5 MPa or higher, and also preferably 10 MPa or lower, more preferably 8 MPa or lower, and even more preferably 7 MPa or lower. These upper and lower pressure limits can be combined arbitrarily, preferably 1 to 10 MPa, more preferably 1.2 to 8 MPa, and even more preferably 1.5 to 7 MPa. Higher pressure conditions tend to improve the yield of terephthalic acid esters and diols. Lower pressure conditions tend to more effectively suppress the formation of tetrahydrofuran and reduce the energy load required for depolymerization.

[0061] Next, the depolymer obtained from the alcoholis reaction is preferably subjected to solid-liquid separation to remove the solid content from the depolymer, if necessary. The solid content in the depolymerized product includes inorganic fillers contained in raw material A, catalysts, and metallic or nonmetallic foreign matter. The removal of solid components from the depolymer is preferably carried out by solid-liquid separation. The solid-liquid separation method is not particularly limited, and known methods can be used. As shown in Figure 1, it may be pressure filtration or vacuum filtration, which are filtration methods using filters, or sedimentation separation, which is separation based on the difference in specific gravity between the solid components and the solution, or centrifugation, or a combination of these methods can be used. The filtration method is not particularly limited and includes natural filtration, vacuum filtration, pressure filtration, centrifugal filtration, and cross-flow filtration. The filters used for filtration are not particularly limited and can include filter paper, ceramic filters, glass filters, membrane filters, polytetrafluoroethylene filter cloth, polyphenylene sulfide filter cloth, polypropylene filter cloth, polyester filter cloth, nylon filter cloth, etc. Porous filter aids such as activated clay and activated carbon can be used during filtration. Furthermore, the centrifugation method is not particularly limited and can include disc centrifuges, nozzle-type disc centrifuges, decanter centrifuges, cylindrical centrifuges, etc. By performing solid-liquid separation, solid components such as inorganic fillers and catalysts contained in raw material A can be easily separated.

[0062] It is preferable to distill the depolymerized product, or, if necessary, the depolymerized product from which the solid content has been removed. Distillation allows for the separation of the alcohol used in the alcoholisesis reaction from the depolymerized products, namely the terephthalic acid ester and the diol.

[0063] The top temperature of the column during distillation of the depolymer (for example, the top temperature when fractionating from reference numeral 6 to reference numeral 7 and reference numeral 8 in Figure 1 or Figure 2) is preferably 30°C or higher, more preferably 40°C or higher, even more preferably 50°C or higher, even more preferably 60°C or higher, and preferably 80°C or lower, more preferably 75°C or lower, and even more preferably 70°C or lower. Setting the temperature above the lower limit tends to allow the distillate to be recovered as a liquid without excessive cooling. Setting the temperature below the upper limit tends to allow the depolymer to be purified with energy savings.

[0064] The column top pressure during distillation is preferably 20 kPa or higher, more preferably 35 kPa or higher, even more preferably 50 kPa or higher, even more preferably 80 kPa or higher, and also preferably 200 kPa or lower, more preferably 150 kPa or lower, and even more preferably 120 kPa or lower. Setting the pressure above the lower limit tends to allow the distillate to be recovered as a liquid without excessive cooling. Setting the pressure below the upper limit tends to allow the depolymer to be purified with less energy. The top pressure refers to the pressure applied at the top temperature of the column as described above (the same applies to top pressure hereafter).

[0065] As described above, the alcohol separated by distillation is preferably used in the alcoholosis of the depolymerization method for the polyalkylene terephthalate resin of this embodiment. Here, the alcohol separated by distillation usually contains tetrahydrofuran and water. Therefore, the alcohol in this state is not suitable for use in the alcoholosis of the depolymerization method of this embodiment. In this embodiment, it is preferable to distill the alcohol separated by distillation again. By performing such distillation, the amount of tetrahydrofuran and water in the alcohol can be reduced. Furthermore, as mentioned above, the alcohol used in the depolymerization reaction can be used in combination with fresh alcohol to reduce the tetrahydrofuran and water content in the alcohol.

[0066] The top temperature of the column during the distillation of the fractionated alcohol (for example, during distillation to separate it into reference numerals 14 and 15, or reference numerals 16 and 17 in Figure 2) is preferably 30°C or higher, more preferably 40°C or higher, even more preferably 50°C or higher, even more preferably 60°C or higher, and preferably 80°C or lower, more preferably 75°C or lower, and even more preferably 70°C or lower. Setting the temperature above the lower limit tends to further improve the effect of recovering the distillate as a liquid without excessive cooling. Also, setting the temperature below the upper limit tends to further improve the effect of purifying the depolymer with energy savings.

[0067] The top pressure of the column during distillation of the fractionated alcohol is preferably 20 kPa or higher, more preferably 35 kPa or higher, even more preferably 50 kPa or higher, even more preferably 80 kPa or higher, and also preferably 200 kPa or lower, more preferably 150 kPa or lower, and even more preferably 120 kPa or lower. Setting the pressure above the lower limit tends to allow the distillate to be recovered as a liquid without excessive cooling. Setting the pressure below the upper limit allows for more efficient purification of the depolymer with reduced energy consumption.

[0068] The depolymer product (containing terephthalic acid ester and diol, etc.) obtained after distillation and separation of alcohol is preferably further purified. Purification makes it possible to separate the terephthalic acid ester and diol, respectively. Purification here can be carried out by distillation as shown in Figure 1, as well as by crystallization, recrystallization, or reprecipitation using a solvent. Distillation can also be performed by pressure swing distillation or by adding a third component as an entrainer. Diols can be purified not only by precision distillation (vaporization) but also by adsorbents such as activated carbon or ion exchange resins. In this embodiment, it is preferable to further purify the depolymer product after the alcohol has been separated by distillation by distillation. Distillation purification makes it possible to separate the terephthalic acid ester and the diol, respectively. The distillation may be a single-stage distillation or a two-stage or more-stage distillation. That is, for example, the reaction system may pass through one distillation column or two or more distillation columns.

[0069] Alternatively, the depolymerized product (containing terephthalate ester and diol, etc.) obtained after distillation and separation of alcohol may be further purified and used as a raw material for the synthesis of polyalkylene terephthalate resin without separating the terephthalate ester and diol. Furthermore, the depolymerized product may contain hydroxybutyl methyl terephthalate (HBMT). If the depolymerized product contains HBMT, HBMT may also be used as a raw material for the synthesis of polyalkylene terephthalate resin.

[0070] Furthermore, it is also preferable to produce a polyalkylene terephthalate resin by polymerizing the terephthalic acid ester and diol obtained by the depolymerization method of the polyalkylene terephthalate resin of this embodiment. In this case, it is also preferable to produce the polyalkylene terephthalate resin by including other terephthalic acid or its ester-forming derivative or other diols in addition to the terephthalic acid ester and diol obtained by the depolymerization method.

[0071] Furthermore, it goes without saying that the terephthalic acid ester and diol obtained by the depolymerization method of this embodiment may be used as separate industrial materials. In other words, the following method is also disclosed in this embodiment. A method for producing terephthalate esters, comprising depolymerizing a polyalkylene terephthalate resin by alcoholis of the polyalkylene terephthalate resin, wherein the polyalkylene terephthalate resin includes a polybutylene terephthalate resin, and the alcohol used for alcoholis contains tetrahydrofuran in a proportion of more than 0 ppm by mass and 100,000 ppm by mass or less (or 20,000 ppm by mass or less). A method for producing a diol, comprising depolymerizing a polyalkylene terephthalate resin by alcoholis of the polyalkylene terephthalate resin, wherein the polyalkylene terephthalate resin includes a polybutylene terephthalate resin, and the alcohol used for alcoholis contains tetrahydrofuran in a proportion of more than 0 ppm by mass and 100,000 ppm by mass or less (or 20,000 ppm by mass or less). The details of these methods are similar to those of preferred methods for the depolymerization of polyalkylene terephthalate resins, resulting in the acquisition of excellent terephthalate ester diols. [Examples]

[0072] The present invention will be described in more detail below with reference to examples. The materials, amounts used, proportions, processing content, and processing procedures shown in the following examples can be modified as appropriate, as long as they do not depart from the spirit of the present invention. Therefore, the scope of the present invention is not limited to the specific examples shown below. If the measuring instruments used in the examples are difficult to obtain due to discontinuation or other reasons, measurements can be taken using other instruments with equivalent performance.

[0073] In this example, polybutylene terephthalate resin may be referred to as PBT, methanol as MeOH, hydroxybutylmethyl terephthalate as HBMT, dimethyl terephthalate as DMT, butanediol as BG, tetrahydrofuran as THF, and glass fiber as GF.

[0074] <Ingredients of ingredient A> A composition mainly composed of PBT, containing 30% by mass of glass fiber, other polymers, and carbon black.

[0075] <Compound made from raw material A> The raw material components of raw material A, excluding glass fiber, were placed in a stainless steel tumbler and stirred and mixed for 1 hour. The resulting mixture was fed into the main hopper of a 30 mm vented twin-screw extruder (manufactured by Japan Steel Works, Ltd., "TEX30α"). Glass fiber (GF) was supplied from the hopper through the 7th side feeder. The mixture was kneaded and extruded into strands under the following conditions: extruder barrel temperature C1~C15 at 260°C, die at 250°C, screw rotation speed at 200 rpm, and discharge rate at 40 kg / hour. The mixture was then rapidly cooled in a water bath and pelletized using a pelletizer to obtain pellets of the resin composition.

[0076] <Alcolisis reaction at the laboratory level> The mass ratio of raw material A (compound) to alcohol was set to one of the three patterns described below, and a methanolysis reaction (depolymerization) was carried out for 5 hours. Magnesium acetate was used as the transesterification catalyst during depolymerization. The mass ratio of raw material A (compound) to alcohol in the alcoholisesis reaction system was set to one of the following three patterns. Details are shown in Examples 1 to 3 below.

[0077] <Example 1 (equivalent to a laboratory experiment)> 8 g of raw material A (compound), 8 g of methanol, and 54 mg (0.25 mmol) of magnesium acetate as a transesterification catalyst were placed in a 70 ml magnetic stirrer autoclave. After replacing the gas phase in the autoclave with nitrogen gas, nitrogen gas was injected to a pressure of 3 MPa. The autoclave was placed in an electric furnace heated to 170 °C, and the reaction was carried out for 5 hours while stirring with a stirrer at a rotation speed of 600 rpm. The reaction pressure was 6 MPa. The autoclave was cooled to room temperature, and the reaction product was dissolved in 50 mL of 1,4-dioxane and collected in a vial. 0.1 mL of triglime was added to 1.5 mL of the supernatant of the reaction solution as an internal standard, and the amounts of THF, BG, HBMT, and DMT contained in the solution after the depolymerization reaction were analyzed by gas chromatography.

[0078] <Example 2 (equivalent laboratory experiment)> The transesterification reaction was carried out using the same method as in Example 1, except that the amount of raw material A (compound) added was changed to 4g.

[0079] <Example 3 (equivalent laboratory experiment)> The transesterification reaction was carried out using the same method as in Example 1, except that the amount of raw material A (compound) added was changed to 2g.

[0080] <Measurement method> (Yields for THF, BG, HBMT, and DMT) The measurements were performed using gas chromatography (Shimadzu Corporation product "GC-2025"). The measurement conditions were as follows: • Column: HP-5 30m x 0.320mm x 0.25μm, ID (manufactured by Agilent Technologies) Carrier gas: Helium, 2.34 mL / min • Column temperature: After being held at 50°C for 5 minutes, the temperature was increased at 10°C / min until it reached 300°C, at which point it was held for 20 minutes. ·Inlet temperature: 320℃ ·Injection volume: 0.2μL • Split ratio: 1:50 Detector temperature: 320℃ • Detector: FID

[0081] [Table 1]

[0082] Details of each item in Table 1 above are as follows: Ratio of raw material to raw material + MeOH: The ratio of raw material A when the total of raw material A and methanol is 100% by mass (unit: mass%) Ratio of PBT to raw materials + MeOH: The ratio of PBT when the total of raw materials A and methanol is set to 100% by mass (unit: mass%). Catalyst metal concentration (Mg vs. ester bond): The number of moles of Mg contained in the catalyst relative to the total number of moles of ester bonds in PBT (unit: mol%) Catalyst metal concentration (ratio of Mg to raw material): The percentage of Mg contained in the catalyst when raw material A is considered to be 100% by mass (unit: ppm by mass). Catalyst metal concentration (ratio of Mg to raw materials + MeOH): The percentage of Mg contained in the catalyst when the total amount of raw material A and methanol is set to 100% by mass (unit: ppm by mass). Amount of PBT used (PBT content in raw material A) (unit: g) (unit: mmol) BG·DMT·HBMT (Yield) The yields of BG·DMT·HBMT after the depolymerization reaction was carried out for 5 hours under conditions of 170°C and 6MPa pressure (units: g) (units: mmol). BG·DMT·HBMT (Yield) Yield of BG·DMT·HBMT relative to the amount of PBT charged after the depolymerization reaction was carried out for 5 hours under conditions of 170°C and 6MPa pressure (unit: mol%)

[0083] Based on the results of Examples 1-3, the composition of the depolymerized solution after the methanolysis reaction (reactor outlet composition after depolymerization) was calculated (Table 2). The amount (g) of each component after the depolymerization reaction was calculated, and the composition (mass%) was calculated assuming all components were 100% by mass. <Calculation conditions for the amount (g) of each component> PBT: Completely depolymerized by the depolymerization reaction. BG: Refer to the results in Table 1. DMT: Refer to the results in Table 1. HBMT: Refer to the results in Table 1. THF: Refer to the results in Table 1. H2O: The amount of H2O produced by depolymerization (mol) was calculated as equal to the amount of THF produced (mol). GF: This refers to the amount of glass fiber in raw material A used in the depolymerization reaction. Others: This refers to the amount of PBT and glass fiber removed from raw material A used in the depolymerization reaction. Oligomer: The amount of oligomer produced (mol) was calculated as follows: Amount of PBT in raw material A used in the depolymerization reaction (mol) - Amount of DMT produced (mol) - Amount of HBMT produced (mol). Catalyst: This refers to the amount of catalyst used in the depolymerization reaction. MeOH: This amount was calculated by subtracting the sum of the amounts of each component (PBT + BG + DMT + HBMT + THF + H2O + GF + Others + Oligomer + Catalyst) from the total amount of each component used in the depolymerization reaction. <Molecular weight Mw> PBT(Unit):220.22 Oligomer (Unit): 220.22 DMT:194.19 BG: 90.12 HBMT:252.3

[0084] In this embodiment, process simulation was performed using the process simulator "AspenPlus" manufactured by Aspen Technology, Inc. AspenPlus is process modeling software that includes a large database of physical property estimation models and pure substance parameters for common chemicals, electrolytes, solids, and polymers. It is software that can reproduce data equivalent to that of an actual plant by inputting the design conditions and operating conditions of distillation and solvent extraction in an actual plant and calculating it based on chemical engineering methods.

[0085] The composition of the depolymerized solution after the methanolysis reaction (Table 2), calculated above, was applied as the composition of the Reactor outlet after depolymerization, and Aspen was calculated. The numbers shown in Figure 2 and Table 3 represent the following substances: 1. Raw material A (compound product of PBT and glass fiber, etc.), and transesterification catalyst. 2. Feed methanol (including recycled methanol and fresh methanol) 3. Depolymerized product after metallization reaction 4. Substances removed by filtration (glass fibers, solid matter, catalyst) 5. Filtrate 6. The liquids of item 5 above 7. Light boiling distillation fraction (methanol, tetrahydrofuran, water) 8. Distillation residue (still liquid) (dimethyl terephthalate, butanediol, polymers, etc.) 9. Distilled fraction (methanol) 10. Distillation fraction (butanediol) 11. Distillation residue (dimethyl terephthalate (DMT), heavy substances (HBMT, etc.)) 12. Distillation fraction (purified dimethyl terephthalate (purified DMT)) 13. Distillation residue heavy material (HBMT, etc.) 14. Distillation fraction (methanol, tetrahydrofuran) 15. Distillation residue (water, methanol) 16. Recycled methanol (purified methanol) after two distillations. 17. Distillation residue (water, methanol) 18 methanol 19 Purification BG 20 Fresh methanol GF glass fiber cat. catalyst

[0086] [Table 2]

[0087] In Table 2, "Others" refers to elastomers. "Oligomer" refers to low molecular weight compounds resulting from the partial depolymerization of PBT other than HBMT.

[0088] Tables 3 to 5 show the ASPEN calculation results. "Stream" refers to the substance corresponding to each number in Figure 2. Temperature, Pressure, Mass flow, and Density represent the temperature, pressure, flow rate, and density in the process for obtaining each substance. For example, Stream 3 shows the temperature, pressure, flow rate, and density of the depolymerized solution after the metallization reaction. Furthermore, "Mass composition" indicates the composition of the substance in each number. [Table 3] [Table 4] [Table 5]

[0089] As is clear from the above results, the depolymerization method of the present invention was found to be able to carry out the depolymerization reaction successfully. In other words, it was confirmed that the depolymerization of PBT had proceeded sufficiently in the depolymerized product Stream3 after the metallization reaction. Furthermore, regarding the methanol in Stream 16 after distillation, it was confirmed that methanol could be separated with high purity, although it contained water and THF. In addition, it was found that Stream 2, which contains feed methanol, recycled methanol, and fresh methanol reused for alcoholisization, had an even higher methanol purity, with THF content of 4400 ppm by mass or less and water content of 1200 ppm by mass or less.

[0090] <Example 4> 2.5 g of PBT and 8 g of methanol containing 200 ppm by mass of THF were charged into a 70 ml magnetic stirrer autoclave. After replacing the gas phase in the autoclave with nitrogen gas, nitrogen gas was injected to a pressure of 3 MPa. The autoclave was placed in an electric furnace heated to 170 °C, and the reaction was carried out for 3 hours while stirring with a stirrer at a rotation speed of 600 rpm. The reaction pressure was approximately 6 MPa. The autoclave was cooled to room temperature, and the reaction product was dissolved in 50 mL of 1,4-dioxane and collected in a vial. 0.1 mL of triglime was added to 1.5 mL of the supernatant of the reaction mixture as an internal standard, and the product was analyzed by gas chromatography according to the method described above. The results are shown in Table 6.

[0091] <Example 5> The transesterification reaction was carried out using the same method as in Example 4, except that 8 g of methanol containing 10,000 ppm by mass of THF was used. The results are shown in Table 6.

[0092] <Example 6> The transesterification reaction was carried out using the same method as in Example 4, except that 8 g of methanol containing 20,000 ppm by mass of THF was used. The results are shown in Table 6.

[0093] <Example 7> The transesterification reaction was carried out using the same method as in Example 4, except that 8 g of methanol containing 50,000 ppm by mass of THF was used. The results are shown in Table 6.

[0094] <Comparative Example 1> The transesterification reaction was carried out using the same method as in Example 4, except that 8g of methanol was used. The results are shown in Table 6.

[0095] [Table 6]

[0096] Methanol acts as a poor solvent for polybutylene terephthalate resin and its depolymerization intermediate, polybutylene terephthalate oligomer. Therefore, the solubility of polybutylene terephthalate oligomer in methanol is low, and its depolymerization reaction becomes the rate-limiting step, leading to a tendency for high-boiling-point compounds such as HBMT to be formed. However, by adding a small amount of THF to methanol, the solubility of polybutylene terephthalate oligomer improved. As a result, the depolymerization reactivity of polybutylene terephthalate oligomer increased, and depolymerization proceeded rapidly. This effectively suppressed the formation of the by-product HBMT. Specifically, as the THF concentration in methanol increased, the amount of HBMT produced decreased, and in Examples 6 and 7, it was confirmed that the HBMT yield was saturated. Suppressing the formation of high-boiling point byproducts such as HBMT makes it possible to avoid increasing the amount of purging required in the purification process. As a result, the loss of product DMT and BG, and the increase in unnecessary waste disposal costs are reduced, significantly improving the economics, sustainability, and efficiency of the depolymerization process.

[0097] Although the present invention has been described in detail using specific embodiments, it will be apparent to those skilled in the art that various modifications are possible without departing from the intent and scope of the invention.

Claims

1. A method for depolymerizing a polyalkylene terephthalate resin by alcoholis of the polyalkylene terephthalate resin, The polyalkylene terephthalate resin comprises polybutylene terephthalate resin, A method for depolymerizing a polyalkylene terephthalate resin, wherein the alcohol used for the alcoholization contains tetrahydrofuran in a proportion of more than 0 ppm by mass and 100,000 ppm by mass or less.

2. The method for depolymerizing a polyalkylene terephthalate resin according to claim 1, wherein the alcohol used for the alcoholization contains tetrahydrofuran in a proportion of 100 ppm by mass or more and 20,000 ppm by mass or less.

3. A reaction step involves supplying a raw material containing polyalkylene terephthalate resin and alcohol to a reactor to perform alcoholosis and obtain a reaction product containing terephthalate ester, diol, tetrahydrofuran, and unreacted alcohol. A solid-liquid separation step to remove solid components from the reaction product to obtain liquid components, A first separation step involves distilling the liquid to separate it into a volatile fraction containing the alcohol and the tetrahydrofuran, and a residual fraction containing the terephthalate ester and the diol. A method for depolymerizing a polyalkylene terephthalate resin according to claim 1, comprising a circulation step of circulating at least a portion of the volatile fraction obtained in the first separation step back to the reactor as a source of alcohol in the reaction step.

4. The method for depolymerizing a polyalkylene terephthalate resin according to claim 1 or 3, wherein the alcohol used for the alcoholization includes an alcohol separated from the depolymerized product after depolymerizing the polyalkylene terephthalate resin.

5. The method for depolymerizing a polyalkylene terephthalate resin according to claim 1 or 3, wherein the depolymerization temperature of the polyalkylene terephthalate resin is 150 to 230°C.

6. The method for depolymerizing a polyalkylene terephthalate resin according to claim 1 or 3, wherein the polyalkylene terephthalate resin is waste material containing an inorganic filler.

7. A method for depolymerizing a polyalkylene terephthalate resin according to claim 6, comprising removing solid content from the depolymerized product after depolymerizing the polyalkylene terephthalate resin.

8. The method for depolymerizing a polyalkylene terephthalate resin according to claim 1 or 3, wherein the alcohol used for the alcoholization is methanol.

9. The method for depolymerizing a polyalkylene terephthalate resin according to claim 1 or 3, wherein the alcohol used for the alcoholization contains water in a proportion of more than 0 ppm by mass and 5,000 ppm by mass or less.

10. A method for producing a polyalkylene terephthalate resin, comprising polymerizing a terephthalic acid ester and a diol obtained by the depolymerization method of a polyalkylene terephthalate resin described in claim 1 or 3.

11. This includes depolymerizing a polyalkylene terephthalate resin by alcoholis, The polyalkylene terephthalate resin comprises polybutylene terephthalate resin, A method for producing terephthalate esters, wherein the alcohol used for the alcoholization contains tetrahydrofuran in a proportion of more than 0 ppm by mass and 1,000,000 ppm by mass or less.

12. This includes depolymerizing a polyalkylene terephthalate resin by alcoholis, The polyalkylene terephthalate resin comprises polybutylene terephthalate resin, A method for producing a diol, wherein the alcohol used for the alcoholization contains tetrahydrofuran in a proportion of more than 0 ppm by mass and 100,000 ppm by mass or less.

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