Recycled polybutylene terephthalate and method for producing the same

Abstract

To provide a recycled polybutylene terephthalate and a method for producing the same.SOLUTION: There is provided a recycled polybutylene terephthalate (PBT) obtained by subjecting a recycled polyethylene terephthalate and 1,4-butanediol to chemical depolymerization and ester exchange using a titanium catalyst chelated by a polyacid or a polyol, followed by condensation polymerization. Thereby, a recycled polybutylene terephthalate having good physical properties can be obtained while reducing the production process.SELECTED DRAWING: None

Description

Technical Field

[0001] The present disclosure relates to a method for producing polybutylene terephthalate (PBT) having good physical properties by recycling polyethylene terephthalate (PET), and to recycled polybutylene terephthalate produced by the method.

Background Art

[0002] Waste plastics have become a global environmental problem to be solved. In order to reduce the amount of waste polyethylene terephthalate (PET), there is a method to recycle and reuse polyethylene terephthalate by using PET bottles and 1,4-butanediol (BDO) as raw materials and obtaining polybutylene terephthalate (PBT) through chemical depolymerization, transesterification, and polymerization. However, in this method, a large amount of 1,4-butanediol needs to be used, that is, the molar ratio of 1,4-butanediol to the required polyethylene terephthalate exceeds 3.2, which causes problems such as an increase in production cost and a large consumption of raw materials, so it has not been commercialized.

[0003] Patent Document 1 describes that after chemically depolymerizing polyethylene terephthalate with ethylene glycol (EG), transesterification and polymerization reactions are carried out by adding 1,4-butanediol, so that the amount of 1,4-butanediol used can be effectively reduced, enabling the commercialization of the technology. However, in the method disclosed in Patent Document 1, diethylene glycol (DEG) is produced in excess during the depolymerization with ethylene glycol, so the melting point of the produced polybutylene terephthalate may decrease and its physical properties may be affected.

Prior Art Documents

Patent Document

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] The present disclosure provides a technique capable of obtaining recycled polybutylene terephthalate having good physical properties while reducing the manufacturing process.

Means for Solving the Problems

[0006] In order to solve the problems of the prior art as described above and achieve the object of the present disclosure, the present disclosure directly uses 1,4-butanediol to perform depolymerization and transesterification by specially selecting a depolymerization catalyst, and then performs polycondensation to obtain polybutylene terephthalate. In addition, the amount of 1,4-butanediol used can be significantly reduced, and the molar ratio of 1,4-butanediol to the required polyethylene terephthalate can be reduced to 2.0. Furthermore, directly using 1,4-butanediol to perform depolymerization and transesterification has the advantage of reducing the process steps. And compared with performing alcoholysis of ethylene glycol, the reaction temperature required for alcoholysis of 1,4-butanediol is further reduced, so the whole process becomes more energy-saving and the economic value is further increased.

[0007] Catalysts commonly used for depolymerization include manganese (Mn), zinc (Zn), antimony (Sb), titanium (Ti), etc. However, catalysts such as manganese (Mn), zinc (Zn), and antimony (Sb) are not commonly used as materials for polybutylene terephthalate. When using catalysts as described above, there are drawbacks such as the resulting polybutylene terephthalate becoming yellowish in color and having poor heat resistance. Therefore, it is preferable to use a titanium (Ti) catalyst as the depolymerization catalyst. However, when using a general titanium (Ti) catalyst, such as tetraisopropyl titanate (TiPT), tetra-n-butyl titanate (TnBT), or a general product, tetrabutyl titanate (TBT), the hue of polyethylene terephthalate may not be good. The present disclosure selects a titanium (Ti) catalyst chelated with a polyacid (e.g., citric acid) or a polyol (e.g., mannitol) in order to solve the problem of poor hue when adding a titanium (Ti) catalyst to the depolymerization of polyethylene terephthalate. When using a titanium (Ti) catalyst chelated with a polyacid or a polyol as the depolymerization catalyst, side reactions of other metal catalysts with respect to polybutylene terephthalate can be avoided. Furthermore, such a catalyst has selectivity and can improve the effects of depolymerization of polyethylene terephthalate and transesterification of 1,4-butanediol. As a result, the amount of 1,4-butanediol used can be reduced, and the recycled polybutylene terephthalate has a good hue and physical properties corresponding to newly manufactured (i.e., not recycled) polybutylene terephthalate.

[0008] One embodiment of the present disclosure is to mix polyethylene terephthalate and 1,4-butanediol in the presence of a titanium catalyst chelated by a polyacid or a polyol at normal pressure, 220 °C (or 210 - 230 °C), and perform a depolymerization and transesterification reaction to form a reaction mixture containing at least tetrahydrofuran (THF), water, ethylene glycol, and 1,4-butanediol in a first step, and then gradually evacuate (reduce the pressure) to remove the tetrahydrofuran and a part of the ethylene glycol from the reaction mixture (by evacuation, at least 90% or more of the tetrahydrofuran and the ethylene glycol can be removed), and perform a polycondensation reaction at less than 0.1 atm and 245 °C (235 - 255 °C) to obtain recycled polybutylene terephthalate in a second step. A method for producing recycled polybutylene terephthalate (recycled PBT) is provided.

[0009] In one specific example, the polyacid for chelating the titanium catalyst includes citric acid or tartaric acid, preferably citric acid.

[0010] In one specific example, the polyol for chelating the titanium catalyst includes xylitol, sorbitol, maltitol, erythritol, or mannitol, preferably mannitol.

[0011] In one specific example, by mixing the polyethylene terephthalate and the 1,4-butanediol, a depolymerization and transesterification reaction is carried out, and the molar ratio of the 1,4-butanediol to the polyethylene terephthalate is 2:1 - 3.5:1, preferably 2:1 - 3:1, and more preferably 2:1 - 2.5:1.

[0012] In one specific embodiment, in the reaction mixture formed in the first step of the method for producing recycled polybutylene terephthalate according to the present disclosure, the amount of the ethylene glycol is 30% by weight or more based on the total amount of the tetrahydrofuran, water, ethylene glycol and 1,4-butanediol.

[0013] In one specific embodiment, in the reaction mixture formed in the first step of the method for producing recycled polybutylene terephthalate according to the present disclosure, the total amount of the tetrahydrofuran and water is 70% by weight or less based on the total amount of the tetrahydrofuran, water, ethylene glycol and 1,4-butanediol.

[0014] Another embodiment of the present disclosure provides a recycled polybutylene terephthalate produced by the above method, which is a copolymer and a polymer formed by polymerizing two or more monomers. The copolymer includes terephthalic acid units, isophthalic acid units, and butanediol units. Further, the copolymer further includes ethylene glycol units, and the amount of the ethylene glycol units is less than 0.1% by weight based on the total amount of the recycled polybutylene terephthalate. Preferably, the copolymer does not contain ethylene glycol units.

[0015] In one specific embodiment, the recycled polybutylene terephthalate has an intrinsic viscosity (IV) in the range of 0.5 to 1.0 dl / g, preferably in the range of 0.85 to 0.9 dl / g, a melting point of 215 °C or higher, preferably 216 °C or higher, and excellent physical properties close to the melting point of 223 °C of a novel material that is not recycled polybutylene terephthalate.

[0016] In one specific embodiment, the CIE L * a * b * The lightness (L) of the coordinates of the color model is 74.0 or higher, and b* The value is 4.0 or less, that is, it has a good hue.

[0017] In one specific embodiment, the recycled polybutylene terephthalate contains diethylene glycol units, and the amount of the diethylene glycol units is less than 0.3% by weight, preferably less than 0.27% by weight, more preferably less than 0.25% by weight, based on the total amount of the recycled polybutylene terephthalate.

[0018] In one specific embodiment, the amount of butanediol units contained in the recycled polybutylene terephthalate is more than 40.0% by weight based on the total amount of the recycled polybutylene terephthalate.

[0019] Yet another embodiment of the present disclosure provides a polyester fiber containing the recycled polybutylene terephthalate according to the present disclosure, which is formed by process steps such as melting and spinning using the recycled polybutylene terephthalate as a raw material for ester pellets.

[0020] Yet another embodiment of the present disclosure provides an ester thin film containing the recycled polybutylene terephthalate.

Advantages of the Invention

[0021] According to the present disclosure, it is possible to obtain a recycled polybutylene terephthalate having good physical properties while reducing the manufacturing process.

Modes for Carrying Out the Invention

[0022] Hereinafter, each example described in the present disclosure will be described in detail. It should be understood that the invention according to the present disclosure can be embodied in various aspects and is not limited to the examples described in the present disclosure. The examples provided in the present disclosure are for making the content of the present disclosure sufficient and complete, and enabling those skilled in the art to understand and implement the present disclosure.

[0023] Any numerical values such as the concentrations or concentration ranges described in this disclosure should be understood to be modified by the term "about" in all cases. "About" means within an acceptable error range for a particular value determined by those skilled in the art, which will depend in part on how that value is measured or determined, i.e., on the limitations of the measurement system. In the context of a particular test, result, or example, unless otherwise explicitly stated elsewhere in the example or the specification, "about" means either within one standard deviation or within a maximum of 5%, whichever is greater, in accordance with the convention in the technical field of this disclosure.

[0024] Embodiments described by terms such as "one embodiment", "some embodiments", etc. may include certain features, structures, aspects, or characteristics, but not all embodiments necessarily include such particular features, structures, aspects, or characteristics. Furthermore, such expressions do not necessarily refer to the same embodiment. Additionally, when a particular feature, structure, aspect, or characteristic is described in relation to an embodiment, in this disclosure, whether or not it is explicitly described, it is within the knowledge of those skilled in the art, and thus, such a feature, structure, aspect, or characteristic is considered to be achievable in relation to other embodiments.

[0025] All technical and scientific terms mentioned in the specification and claims are definitions known to those skilled in the art unless otherwise defined. Singular terms such as "one", "the", or their similar terms refer to two or more objects unless otherwise explained. The terms "or", "and", "with" used in this disclosure refer to "or / and" unless otherwise explained. Also, the terms "comprising", "containing" are non - restrictive open - ended connectives. Also, the above definitions are only for explaining the definitions of the terms and should not be construed as limitations on the subject matter. Unless otherwise explained, all materials used in this disclosure are commercially available and easily obtainable.

[0026] The recycled polybutylene terephthalate in the present disclosure is a copolymer formed by subjecting the recovered polyethylene terephthalate to depolymerization, transesterification, and polycondensation, and is close to pure polybutylene terephthalate (pure PBT). It has good hue and physical properties and can be used in the manufacture of polyester fibers, polyester panels or thin film materials.

[0027] <Method for manufacturing recycled polybutylene terephthalate> One embodiment of the present disclosure provides a method for manufacturing recycled polybutylene terephthalate by manufacturing recycled polybutylene terephthalate in two steps with respect to the recovered polyethylene terephthalate.

[0028] (Step 1) In the presence of a titanium catalyst chelated with a polyacid or a polyol as a depolymerization catalyst at 1 atm, 210°C to 230°C (for example, 220°C), 1,4-butanediol and polyethylene terephthalate are introduced and mixed to carry out a depolymerization and transesterification reaction, thereby obtaining a low polymer of polyethylene terephthalate (low polymer PET), a low polymer of ethylene isophthalate (low polymer PEI), a low polymer of butylene terephthalate (low polymer PBT), a low polymer of butylene isophthalate (low polymer PBI), ethylene glycol (EG), 1,4-butanediol (BDO), tetrahydrofuran (THF), and water. The reaction temperature may be 210°C, 211°C, 212°C, 213°C, 214°C, 215°C, 216°C, 217°C, 218°C, 219°C, 220°C, 221°C, 222°C, 223°C, 224°C, 225°C, 226°C, 227°C, 228°C, 229°C, 230°C, or any value or range between any two of the above values.

[0029] TIFF2025100449000001.tif26153

[0030] (Step 2) The mixed solution formed in the first step is gradually evacuated to remove a part of ethylene glycol and tetrahydrofuran, and a polycondensation reaction is carried out under the conditions that the pressure is lower than 0.1 atm or close to vacuum and the temperature is 235°C to 255°C (preferably 245°C), thereby forming recycled polybutylene terephthalate (r-PBT). In one embodiment, after evacuation, at least 90% or more of ethylene glycol and tetrahydrofuran can be removed. The reaction temperature can be 235°C, 236°C, 237°C, 238°C, 239°C, 240°C, 241°C, 242°C, 243°C, 244°C, 245°C, 246°C, 247°C, 248°C, 249°C, 250°C, 251°C, 252°C, 253°C, 254°C, 255°C, or any value or range between any two of the above values.

[0031] TIFF2025100449000002.tif23153

[0032] In the present disclosure, "depolymerization" mainly refers to the ability to decompose into low polymers or monomers of dihydric alcohol and dibasic acid by cleaving the bonds of the ester groups of polyethylene terephthalate and recycle them as raw materials for subsequent polymerization.

[0033] In the present disclosure, "transesterification" mainly refers to the substitution of the ethylene glycol group of the initial ester with a butanediol group.

[0034] In the present disclosure, "polycondensation" is a polycondensation reaction that includes the loss of molecules and the formation of polyesters in the process of two molecules bonding to form a new molecule due to changes in functional groups in a chemical reaction, which means that water molecules are eliminated from the carboxyl group (-COOH) or hydroxyl group (-OH) to form an ester group and polymerize.

[0035] In one embodiment, 1,4-butanediol and polyethylene terephthalate are introduced and mixed, and a depolymerization and transesterification reaction is carried out. The molar ratio of the 1,4-butanediol to the polyethylene terephthalate is 2:1 to 3.5:1, for example, 2:1, 2.1:1, 2.2:1, 2.3:1, 2.4:1, 2.5:1, 2.6:1, 2.7:1, 2.8:1, 2.9:1, 3.0:1, 3.1:1, 3.2:1, 3.3:1, 3.4:1, 3.5:1. In this embodiment, the molar ratio of the 1,4-butanediol to the polyethylene terephthalate is 2:1 to 3:1, preferably 2:1 to 2.5:1.

[0036] In one embodiment, in the reaction mixture of the first step, the amount of ethylene glycol is 30% by weight or more based on the total amount of tetrahydrofuran, water, ethylene glycol and 1,4-butanediol, for example, 30.0% by weight, 30.5% by weight, 31.0% by weight, 31.5% by weight, 32.0% by weight, 32.5% by weight, 33.0% by weight, 33.5% by weight, 34.0% by weight, 34.5% by weight, 35.0% by weight, 35.5% by weight, 36.0% by weight, 36.5% by weight, 37.0% by weight, 37.5% by weight, 38.0% by weight, 38.5% by weight, 39.0% by weight, 39.5% by weight, 40.0% by weight, or any value or range between any two of the above values. In the reaction mixture of the first step, the total amount of tetrahydrofuran and water is 70% by weight or less based on the total amount of tetrahydrofuran, water, ethylene glycol and 1,4-butanediol, for example, 70.0% by weight, 69.5% by weight, 69.0% by weight, 68.5% by weight, 68.0% by weight, 67.5% by weight, 67.0% by weight, 66.5% by weight, 66.0% by weight, 65.5% by weight, 65.0% by weight, 64.5% by weight, 64.0% by weight, 63.5% by weight, 63.0% by weight, 62.5% by weight, 62.0% by weight, 61.5% by weight, 61.0% by weight, 60.5% by weight, 60.0% by weight, or any value or range between any two of the above values.

[0037] <Recycled Polybutylene Terephthalate> Other embodiments of the present disclosure are recycled polybutylene terephthalate produced by the above method, which is a copolymer containing terephthalic acid units, isophthalic acid units and butanediol units. The physical properties of the recycled polybutylene terephthalate in this example can be defined by measuring various parameters of the recycled polybutylene terephthalate, the yarns and fabrics produced from the recycled polybutylene terephthalate as follows.

[0038] (1) Intrinsic viscosity Intrinsic Viscosity (IV) measures the inherent resistance to the flow of a polymer and can be measured by a general method in the technical field of the present disclosure. For example, it can be measured according to ASTM D2857 and is usually expressed in dl / g. For example, the intrinsic viscosity of a polyester such as polyethylene terephthalate can be obtained by measuring the viscosity of a polymer with o-chlorophenol as a solvent at 35 °C and a concentration of 1 g / dL in a glass capillary viscometer. Alternatively, a thermoplastic polyester elastomer is dissolved in phenol / tetrachloroethane (50 / 50 wt) as a solvent to prepare a 0.5 wt% solution, and the viscosity is measured with an Ubbelohde viscometer at 35 °C.

[0039] According to the present disclosure, the intrinsic viscosity is measured in accordance with ASTM D2857. The recycled polybutylene terephthalate in this example has an intrinsic viscosity in the range of 0.5 to 1.0 dl / g, for example, 0.50 dl / g, 0.55 dl / g, 0.60 dl / g, 0.65 dl / g, 0.70 dl / g, 0.75 dl / g, 0.80 dl / g, 0.81 dl / g, 0.82 dl / g, 0.83 dl / g, 0.84 dl / g, 0.85 dl / g, 0.86 dl / g, 0.87 dl / g, 0.88 dl / g, 0.89 dl / g, 0.90 dl / g, 0.91 dl / g, 0.92 dl / g, 0.93 dl / g, 0.94 dl / g, 0.95 dl / g, 0.96 dl / g, 0.97 dl / g, 0.98 dl / g, 0.99 dl / g, 1.00 dl / g, or any value or range between any two of the above values may be acceptable.

[0040] (2) Coordinates of the CIE color model The coordinates of the CIE color model (also referred to as "L * a * b * ") are shown based on the coordinate diagram of the color space defined by the International Commission on Illumination (CIE). Among the three coordinates, L * represents the lightness of the color (L * = 0 represents black, and L * = 100 represents white), a * represents the position between red and green (a * a negative value indicates green, and a positive value indicates red), and b * represents the position between yellow and blue (b * a negative value indicates blue, and a positive value indicates yellow). When the value of L * is small, it indicates low lightness, and when the value of b * is large, it indicates a bias towards yellow.

[0041] The recycled polybutylene terephthalate in this example has CIE L * a * b *The lightness (L) of the coordinates of the color model is 74.0 or more, for example, 74.0, 74.5, 75.0, 75.5, 76.0, 76.5, 77.0, 77.5, 78.0, 78.5, 79.0, 79.5, 80.0, or any value or range between any two of the above values, and b * The value is 4.0 or less, for example, 4.0, 3.5, 3.0, 2.5, 2.0, 1.5, 1.0, or any value or range within any two of the above values is also acceptable.

[0042] (3) Melting point Based on the measurement method of ISO11357-1, the melting point of the measurement object is measured by differential scanning calorimetry (DSC). Its basic principle is that when a phase transition, glass transition, or chemical reaction occurs in the sample, heat absorption and release proceed. Therefore, the corrector can measure how to increase or decrease the heat flow to maintain the temperature of the sample and the reference object constant. The recycled polybutylene terephthalate of this embodiment has a melting point of 215°C or more, for example, 215°C, 215.5°C, 216°C, 216.5°C, 217°C, 217.5°C, 218°C, 218.5°C, 219°C, 219.5°C, 220°C, 220.5°C, 221°C, 221.5°C, 222°C, 222.5°C, 223°C, or any value or range between any two of the above values is also acceptable.

[0043] Note that the recycled polybutylene terephthalate in this example further contains an ethylene glycol unit, and the amount of the ethylene glycol unit is less than 0.1% by weight based on the total amount of the recycled polybutylene terephthalate, for example, 0.099% by weight, 0.095% by weight, 0.09% by weight, 0.08% by weight, 0.07% by weight, 0.06% by weight, 0.05% by weight, 0.04% by weight, 0.03% by weight, 0.02% by weight, 0.01% by weight, 0.005% by weight, 0.001% by weight, or any value or range between any two of the above values is also acceptable.

[0044] In one embodiment, the recycled polybutylene terephthalate substantially does not contain ethylene glycol units.

[0045] In one embodiment, the amount of butanediol units in the recycled polybutylene terephthalate in this example is more than 40.0 wt% based on the total amount of the recycled polybutylene terephthalate, for example, 40.05 wt%, 40.1 wt%, 40.2 wt%, 40.3 wt%, 40.4 wt%, 40.5 wt%, 40.6 wt%, 40.7 wt%, 40.8 wt%, 40.9 wt%, 41.0 wt%, 41.1 wt%, 41.2 wt%, 41.3 wt%, 41.4 wt%, 41.5 wt%, 42.0 wt%, or any value or range between any two of the above values may be acceptable.

[0046] <Polyester fiber> In another embodiment of the present disclosure, there is provided a polyester fiber containing the recycled polybutylene terephthalate obtained by the above example, or a polyester fiber manufactured from the recycled polybutylene terephthalate manufactured by the above example. For the recycled polybutylene terephthalate obtained by the above example, after heating and melting by a screw extruder, it is quantitatively extruded from a spinning nozzle at a spinning temperature of about 160 - 300 °C, cooled and solidified by cooling air, and after adding oil, spinning and drawing are carried out to obtain polyester fibers.

[0047] <Polyester film> In another embodiment of the present disclosure, there is provided a polyester film containing the recycled polybutylene terephthalate obtained by the above example, or a polyester film manufactured from the recycled polybutylene terephthalate manufactured by the above example. For the recycled polybutylene terephthalate obtained by the above example, it is melted and extruded at a temperature of 230 - 300 °C to manufacture a polyester film, and further stretching processing is carried out to obtain a polyester film.

[0048] Except for the content described in the above embodiments, other detailed content of each embodiment of the present disclosure can be implemented with reference to the general technology in the field. Hereinafter, several specific embodiments will be given for further explanation, but the present disclosure is not limited to these embodiments. The embodiments exemplified in the present disclosure are for the purpose of convenience of explanation only, and the technology applied to the invention according to the present disclosure can also be applied to similar products with the same concept. When implementing or measuring each embodiment and / or each specific example of the present disclosure, any method and material similar or equivalent to those described in the present disclosure can be used, but the preferred methods and materials are mentioned in the present disclosure. All documents mentioned in the present disclosure are hereby incorporated by reference in their entirety into this specification.

[0049] (Example 1) 1,4-butanediol and polyethylene terephthalate are charged into a reactor at a molar ratio of 3.2:1, and mixed under the conditions of 1 atm, 220 °C, and an appropriate amount of titanium citrate chelate added as a catalyst to carry out depolymerization and transesterification reactions. After the completion of the depolymerization and transesterification reactions, the pressure is gradually reduced to remove a part of ethylene glycol and tetrahydrofuran, and a polycondensation reaction is carried out under the conditions of less than 0.1 atm and 245 °C. After the polycondensation reaction reaches the target viscosity IV = 0.88 ± 0.02 dl / g, the cooled product is taken out of the reactor and granulated to produce recycled polybutylene terephthalate granules, that is, recycled PBT ester pellets. The melting point (Tm) of this recycled polybutylene terephthalate pellet is 216 ± 1 °C.

[0050] (Example 2) 1,4-Butanediol and polyethylene terephthalate are charged into a reactor at a molar ratio of 2.5:1, and mixed under the conditions of 1 atm, 220 °C, and addition of an appropriate amount of titanium citrate chelate as a catalyst to carry out depolymerization and transesterification reactions. After the completion of the depolymerization and transesterification reactions, gradually evacuate to remove a part of ethylene glycol and tetrahydrofuran, and carry out a polycondensation reaction under the conditions of lower than 0.1 atm and 245 °C. After the polycondensation reaction reaches the target viscosity IV = 0.88 ± 0.02 dl / g, the cooled product is taken out from the reactor and granulated to produce recycled polybutylene terephthalate pellets. The melting point of this recycled polybutylene terephthalate pellet is 216 ± 1 °C.

[0051] (Comparative Example 1) 1,4-Butanediol and polyethylene terephthalate are charged into a reactor at a molar ratio of 3.2:1, and mixed under the conditions of 1 atm, 220 °C, and without catalyst addition to carry out depolymerization and transesterification reactions. After the completion of the depolymerization and transesterification reactions, gradually evacuate to remove a part of ethylene glycol and tetrahydrofuran, and carry out a polycondensation reaction under the conditions of lower than 0.1 atm and 245 °C. After the polycondensation reaction reaches the target viscosity IV = 0.88 ± 0.02 dl / g, the cooled product is taken out from the reactor and granulated to produce recycled polybutylene terephthalate pellets. The melting point of this recycled polybutylene terephthalate pellet is 214 ± 1 °C.

[0052] (Comparative Example 2) 1,4-butanediol and polyethylene terephthalate are charged into a reactor at a molar ratio of 3.2:1, and mixed under the conditions of 1 atm, 220 °C, and addition of an appropriate amount of zinc (Zn) as a catalyst to carry out a depolymerization and transesterification reaction. After the completion of the depolymerization and transesterification reactions, the pressure is gradually reduced to remove a part of ethylene glycol and tetrahydrofuran, and a polycondensation reaction is carried out under the conditions of less than 0.1 atm and 245 °C. After the polycondensation reaction reaches the target viscosity IV = 0.88 ± 0.02 dl / g, the cooled product is taken out from the reactor and granulated to produce recycled polybutylene terephthalate pellets. The melting point of this recycled polybutylene terephthalate pellets is 214 ± 1 °C.

[0053] (Comparative Example 3) 1,4-butanediol and polyethylene terephthalate are charged into a reactor at a molar ratio of 3.2:1, and mixed under the conditions of 1 atm, 220 °C, and addition of an appropriate amount of manganese (Mn) as a catalyst to carry out a depolymerization and transesterification reaction. After the completion of the depolymerization and transesterification reactions, the pressure is gradually reduced to remove a part of ethylene glycol and tetrahydrofuran, and a polycondensation reaction is carried out under the conditions of less than 0.1 atm and 245 °C. After the polycondensation reaction reaches the target viscosity IV = 0.88 ± 0.02 dl / g, the cooled product is taken out from the reactor and granulated to produce recycled polybutylene terephthalate pellets. The melting point of this recycled polybutylene terephthalate pellets is 214 ± 1 °C.

[0054] (Comparative Example 4) 1,4-butanediol and polyethylene terephthalate are charged into a reactor at a molar ratio of 3.2:1, and mixed under the conditions of 1 atm, 220 °C, and addition of an appropriate amount of antimony (Sb) as a catalyst to perform a depolymerization and transesterification reaction. After the completion of the depolymerization and transesterification reactions, the pressure is gradually reduced to remove a part of ethylene glycol and tetrahydrofuran, and a polycondensation reaction is carried out under the conditions of lower than 0.1 atm and 245 °C. After the polycondensation reaction reaches the target viscosity IV = 0.88 ± 0.02 dl / g, the cooled product is taken out from the reactor and granulated to produce recycled polybutylene terephthalate pellets. The melting point of this recycled polybutylene terephthalate pellet is 214 ± 1 °C.

[0055] (Comparative Example 5) 1,4-butanediol and polyethylene terephthalate are charged into a reactor at a molar ratio of 3.2:1, and mixed under the conditions of 1 atm, 220 °C, and addition of an appropriate amount of non-chelated titanium (Ti) as a catalyst to perform a depolymerization and transesterification reaction. After the completion of the depolymerization and transesterification reactions, the pressure is gradually reduced to remove a part of ethylene glycol and tetrahydrofuran, and a polycondensation reaction is carried out under the conditions of lower than 0.1 atm and 245 °C. After the polycondensation reaction reaches the target viscosity IV = 0.88 ± 0.02 dl / g, the cooled product is taken out from the reactor and granulated to produce recycled polybutylene terephthalate pellets. The melting point of this recycled polybutylene terephthalate pellet is 216 ± 1 °C.

[0056] (Comparative Example 6) 1,4-butanediol and polyethylene terephthalate are charged into a reactor at a molar ratio of 2.5:1, mixed under the conditions of 1 atm, 220 °C, and without adding a catalyst, and a depolymerization and transesterification reaction are carried out. After the completion of the depolymerization and transesterification reactions, the pressure is gradually reduced to remove a part of ethylene glycol and tetrahydrofuran, and a polycondensation reaction is carried out under the conditions of less than 0.1 atm and 245 °C. After the polycondensation reaction reaches the target viscosity IV = 0.88 ± 0.02 dl / g, the cooled product is taken out from the reactor and granulated to produce recycled polybutylene terephthalate pellets. The melting point of this recycled polybutylene terephthalate pellet is 210 ± 1 °C.

[0057] (Comparative Example 7) 1,4-butanediol and polyethylene terephthalate are charged into a reactor at a molar ratio of 2.5:1, mixed under the conditions of 1 atm, 220 °C, and adding an appropriate amount of zinc (Zn) as a catalyst, and a depolymerization and transesterification reaction are carried out. After the completion of the depolymerization and transesterification reactions, the pressure is gradually reduced to remove a part of ethylene glycol and tetrahydrofuran, and a polycondensation reaction is carried out under the conditions of less than 0.1 atm, 245 °C, and retaining the zinc as a catalyst. After the polycondensation reaction reaches the target viscosity IV = 0.88 ± 0.02 dl / g, the cooled product is taken out from the reactor and granulated to produce recycled polybutylene terephthalate pellets. The melting point of this recycled polybutylene terephthalate pellet is 212 ± 1 °C.

[0058] (Comparative Example 8) 1,4-butanediol and polyethylene terephthalate are charged into a reactor at a molar ratio of 2.5:1, and are mixed under the conditions of 1 atm, 220 °C, and an appropriate amount of manganese (Mn) added as a catalyst, and a depolymerization and transesterification reaction are carried out. After the completion of the depolymerization and transesterification reactions, the pressure is gradually reduced to remove a part of ethylene glycol and tetrahydrofuran, and a polycondensation reaction is carried out under the conditions of lower than 0.1 atm and 245 °C. After the polycondensation reaction reaches the target viscosity IV = 0.88 ± 0.02 dl / g, the cooled product is taken out from the reactor and granulated to produce recycled polybutylene terephthalate pellets. The melting point of this recycled polybutylene terephthalate pellet is 212 ± 1 °C.

[0059] (Comparative Example 9) 1,4-butanediol and polyethylene terephthalate are charged into a reactor at a molar ratio of 2.5:1, and are mixed under the conditions of 1 atm, 220 °C, and an appropriate amount of antimony (Sb) added as a catalyst, and a depolymerization and transesterification reaction are carried out. After the completion of the depolymerization and transesterification reactions, the pressure is gradually reduced to remove a part of ethylene glycol and tetrahydrofuran, and a polycondensation reaction is carried out under the conditions of lower than 0.1 atm and 245 °C. After the polycondensation reaction reaches the target viscosity IV = 0.88 ± 0.02 dl / g, the cooled product is taken out from the reactor and granulated to produce recycled polybutylene terephthalate pellets. The melting point of this recycled polybutylene terephthalate pellet is 210 ± 1 °C.

[0060] (Comparative Example 10) 1,4-Butanediol and polyethylene terephthalate are charged into a reactor at a molar ratio of 2.5:1, and mixed under the conditions of 1 atm, 220 °C, and addition of an appropriate amount of non-chelating titanium (Ti) as a catalyst to carry out depolymerization and transesterification reactions. After completion of the depolymerization and transesterification reactions, the pressure is gradually reduced to remove a part of ethylene glycol and tetrahydrofuran, and a polycondensation reaction is carried out under the conditions of lower than 0.1 atm and 245 °C. After the polycondensation reaction reaches the target viscosity IV = 0.88 ± 0.02 dl / g, the cooled product is taken out from the reactor and granulated to produce recycled polybutylene terephthalate pellets. The melting point of this recycled polybutylene terephthalate pellet is 214 ± 1 °C.

[0061] The characteristics of the recycled polybutylene terephthalate pellets obtained with different catalysts or different ratios of 1,4-butanediol are summarized in Table 1.

[0062] [Table 1] Comparison of the characteristics of recycled polybutylene terephthalate pellets under different production conditions TIFF2025100449000003.tif88155

[0063] As is clear from the results in Table 1, when the molar ratio of 1,4-butanediol to polyethylene terephthalate is high (for example, molar ratio = 3.2), as in Example 1 and Comparative Examples 1 to 5, regardless of the presence or absence of catalyst addition or the type of catalyst added, recycled polybutylene terephthalate of a quality close to the melting point (Tm) of pure polybutylene terephthalate was produced. Also, when a catalyst that is not a catalyst generally used for polybutylene terephthalate, such as zinc (Zn), manganese (Mn), antimony (Sb), etc., is added, it affects the hue after polymerization, the lightness (L) decreases and becomes lower than 75 and darker, or the b value increases and exceeds +5.0 and even becomes as high as +10.0 and yellower.

[0064] When the molar ratio of 1,4 - butanediol to polyethylene terephthalate was decreased (for example, molar ratio = 2.5), as in Example 2 and Comparative Examples 6 - 10, except for Example 2 using a chelated titanium catalyst, the melting points of the recycled polybutylene terephthalate produced by other formulations all decreased significantly. From these results, it was found that only when using a chelated titanium catalyst as the catalyst in the depolymerization reaction of polyethylene terephthalate and 1,4 - butanediol, the characteristics of the melting point and hue of the obtained recycled polybutylene terephthalate can be maintained.

[0065] The depolymerization time and polymerization time for the processes of the above - mentioned respective Examples and Comparative Examples are summarized in Table 2.

[0066] [Table 2] Comparison of Depolymerization Time and Polymerization Time under Different Production Conditions TIFF2025100449000004.tif81151

[0067] From Table 2, it was found that for different catalysts, the speeds of depolymerization and transesterification are different, and as a result, the reaction times are also different. When using a chelated titanium (Ti) catalyst as the depolymerization catalyst in Examples 1 and 2, the depolymerization times were both the shortest at 2 hours and 30 minutes, and the subsequent polymerization time was 2 hours. In contrast, when using zinc (Zn), manganese (Mn), or antimony (Sb) catalysts, the depolymerization times were about 3 hours and 30 minutes to 4 hours, and the polymerization time was 2 hours and 30 minutes. All were longer than the depolymerization reaction time and the subsequent polymerization reaction time when using a chelated titanium catalyst as the depolymerization catalyst. Therefore, it is considered that using a chelated titanium catalyst as the depolymerization catalyst can significantly shorten the overall process time. Furthermore, the depolymerization reaction time (2 hours and 30 minutes) when using a chelated titanium catalyst as the depolymerization catalyst in Examples 1 and 2 was 30 minutes shorter than the depolymerization reaction time (3 hours) when using a non - chelated titanium catalyst in Comparative Examples 5 and 10, and it is considered that the overall process time can be significantly shortened.

[0068] The recycled polybutylene terephthalate pellets obtained in the above Examples and Comparative Examples contain terephthalic acid units, isophthalic acid (IPA) units, ethylene glycol (EG) units, diethylene glycol (DEG) units, and 1,4-butanediol (BDO) units, and the respective weight percentages are summarized in Table 3.

[0069] [Table 3] Analysis of Components of Recycled Polybutylene Terephthalate Pellets TIFF2025100449000005.tif84153

[0070] From the results in Table 3, it was found that when the molar ratio of 1,4-butanediol to polyethylene terephthalate is 3.2, the component ratios of the recycled polybutylene terephthalate pellets obtained using different depolymerization catalysts are similar. For example, the component ratios of the ester pellets obtained using zinc (Zn), manganese (Mn), or antimony (Sb) catalysts are similar, and in each case, the ratio of IPA is in the range of 1.03 - 1.11 wt%, the ratio of EG is in the range of 0.60 - 0.67 wt%, the ratio of DEG is in the range of 0.39 - 0.43 wt%, and the ratio of BDO is in the range of 38.96 - 39.11 wt%. Also, these ester pellets have similar melting points, all of which are 214 ± 1 °C. In addition, the component ratios of the recycled polybutylene terephthalate pellets obtained using non-chelated titanium or chelated titanium (Ti) catalysts are also similar, the ratio of IPA is in the range of 1.01 - 1.05 wt%, the ratio of EG is in the range of 0 - 0.10 wt%, the ratio of DEG is in the range of 0.25 - 0.27 wt%, the ratio of BDO is in the range of 40.30 - 40.53 wt%, and they have similar melting points.

[0071] However, when the amount of 1,4 - butanediol used was reduced (for example, molar ratio = 2.5), in the recycled polybutylene terephthalate pellets obtained using a zinc, manganese, antimony or non - chelated titanium catalyst, the remaining amounts of both ethylene glycol and diethylene glycol increased significantly. For example, the remaining amount of ethylene glycol increased to 0.55 - 2.16 wt%, and the remaining amount of diethylene glycol increased to 0.46 - 1.30 wt%, resulting in a significant decrease in the melting point of these ester pellets. When the remaining amount of ethylene glycol was 0.55 wt% and the remaining amount of diethylene glycol was 0.46, the melting point decreased to 214 ± 1°C. When the remaining amount of ethylene glycol increased to 1.11 - 1.14 wt% and the remaining amount of diethylene glycol increased to 0.68 - 0.73 wt%, the melting point decreased to 212 ± 1°C. When the remaining amount of ethylene glycol increased to 2.16 wt% and the remaining amount of diethylene glycol increased to 1.30 wt%, the melting point further decreased to 210 ± 1°C. As is also clear from the results in Table 3, in the recycled polybutylene terephthalate pellets obtained using a chelated titanium catalyst as the depolymerization catalyst, almost no ethylene glycol was contained (Example 2), and furthermore, no ethylene glycol was detected at all (Example 1), and the remaining amount of the by - product diethylene glycol was also the lowest at 0.27 wt% or less. In other words, when titanium chelated by a polyacid or polyol was used as the depolymerization catalyst, even without using so much 1,4 - butanediol, in the recycled polybutylene terephthalate produced by the method for producing recycled polybutylene terephthalate provided in the present disclosure, a low remaining amount of ethylene glycol and diethylene glycol could be maintained, and its melting point was equivalent to that of the recycled polybutylene terephthalate produced with a high dose of 1,4 - butanediol.

[0072] In the above Examples and Comparative Examples, in the distillate of the reaction mixture produced after the depolymerization and transesterification reaction of polyethylene terephthalate and 1,4-butanediol, tetrahydrofuran, water, ethylene glycol, and 1,4-butanediol were included, and the weight percentages of each are summarized in Table 4.

[0073] [Table 4] Analysis of Components of Distillate of Recycled Polybutylene Terephthalate TIFF2025100449000006.tif59153

[0074] In the reaction mixture produced after the depolymerization and transesterification reaction, in addition to components with relatively high boiling points such as low polymers of polyethylene terephthalate, low polymers of ethylene isophthalate, low polymers of butylene terephthalate, and low polymers of butylene isophthalate, there are also components with relatively low boiling points such as tetrahydrofuran, water, ethylene glycol, and 1,4-butanediol. As demonstrated by the analysis results of the components of the esterification distillate in Table 4, the components with low boiling points in these reaction mixtures are present in the distillate of the reaction mixture. Considering that there is a substitution or conversion relationship among the four main compounds such as tetrahydrofuran, water, ethylene glycol, and 1,4-butanediol that make up the distillate (for example, an increase in the distillation amount of ethylene glycol usually means that 1,4-butanediol has successfully substituted the ethylene glycol unit in the low polymer after depolymerization, and accordingly, the content of 1,4-butanediol in the distillate decreases. Also, 1,4-butanediol may also produce tetrahydrofuran by dehydration, resulting in a decrease in 1,4-butanediol and an increase in tetrahydrofuran and water.), and since this substitution or conversion relationship directly affects the components of the final product, from the relative differences among these proportional relationships, the influence of different depolymerization catalysts on the depolymerization and transesterification reactions can be easily and clearly determined.

[0075] From the analysis results of the components of the distillate in Table 4, when no catalyst is added or an antimony (Sb) catalyst is added, the depolymerization and transesterification of 1,4-butanediol are not effective, and the ethylene glycol unit cannot be effectively separated from polyethylene terephthalate and ethylene glycol cannot be distilled. Therefore, it was found that in the mixture produced by the depolymerization and transesterification reactions using such a catalyst, the weight percentage of ethylene glycol in the components of the distillate decreased significantly (less than 20% by weight). For example, when adding the chelated titanium catalyst of Example 2, the ethylene glycol in the components of the distillate by depolymerization and transesterification was 31.50% by weight, while when no catalyst was added in Comparative Example 6 and when an antimony catalyst was added in Comparative Example 10, the ethylene glycol was 16.56% by weight and 17.90% by weight, and the proportion of ethylene glycol decreased significantly by about 13 to 15% by weight. Also, when a zinc (Zn) or manganese (Mn) catalyst is added, the depolymerization and transesterification of 1,4-butanediol contribute to the effect that ethylene glycol slightly exceeding 20% by weight is substituted from polyethylene terephthalate and distilled. However, the zinc (Zn) or manganese (Mn) catalyst has a side effect of promoting the dehydration of 1,4-butanediol and the production of tetrahydrofuran, and the free 1,4-butanediol that can participate in the polymerization reaction is insufficient to completely substitute ethylene glycol. For example, when adding the chelated titanium catalyst of Example 2, the total amount of tetrahydrofuran and water in the measured distillate was 63.74% by weight. However, when the depolymerization catalyst was replaced with a zinc or manganese catalyst as in Comparative Example 7 and Comparative Example 8, the total amount of tetrahydrofuran and water in the measured distillate was 75.10% by weight and 74.61% by weight, which was increased by 11.36% by weight and 10.87% by weight compared to the total amount of tetrahydrofuran and water in Example 2.

[0076] From the analysis results of the components of the distillate in Table 4, regarding the remaining amount of 1,4 - butanediol (BDO), Comparative Example 10 using a non - chelated titanium catalyst (the remaining amount of 1,4 - butanediol is 2.03% by weight) shows a greater effect of depolymerization and transesterification of 1,4 - butanediol than Comparative Example 6 without using any catalyst (the remaining amount of 1,4 - butanediol is 15.33% by weight) and Comparative Example 9 using an antimony catalyst (the remaining amount of 1,4 - butanediol is 16.10% by weight). Also, regarding the remaining amounts of tetrahydrofuran and water, Comparative Example 10 using a non - chelated titanium catalyst has a smaller influence on the production of tetrahydrofuran as a by - product than Comparative Example 7 using a zinc (Zn) catalyst (the total remaining amount of tetrahydrofuran and water is 75.10% by weight) and Comparative Example 8 using a manganese (Mn) catalyst (the total remaining amount of tetrahydrofuran and water is 74.61% by weight) (the total remaining amount of tetrahydrofuran and water was 68.02% by weight). However, compared with the case of using a chelated titanium catalyst, the reaction between polyethylene terephthalate and 1,4 - butanediol has the highest effect of depolymerization and transesterification, and the lowest influence on the production of tetrahydrofuran as a by - product. Taking Example 2 as an example, in Example 2 using a chelated titanium catalyst, the distillate amount of ethylene glycol exceeds 30% by weight with respect to the total amount of polyester distillate or the main components of the distillate, namely tetrahydrofuran, water, ethylene glycol, and 1,4 - butanediol. On the other hand, in Comparative Examples 6 - 10 using a non - chelated titanium catalyst and other catalysts, the distillate amount of ethylene glycol is all lower than 30% by weight, and in particular, in Comparative Example 9 using an antimony catalyst, it is about 16% by weight. From this result, it was found that there is a significant effect on depolymerization and transesterification, and the proportion of ethylene glycol units in the recycled polybutylene terephthalate after the subsequent polymerization reaction can be effectively reduced.In Comparative Examples 7 and 8 using a zinc or manganese catalyst, the total amount of tetrahydrofuran and water relative to the total amount of the polyester distillate or the main components of the distillate is about 75% by weight. In contrast, in Example 2, the total amount of tetrahydrofuran and water relative to the total amount of the polyester distillate or the main components of the distillate is about 65% by weight (less than 70% by weight), significantly lower than in Comparative Examples 7 and 8, and also lower than in Comparative Example 10 using a non-chelated titanium catalyst (about 68 - 69% by weight), indicating that the formation of the by-product tetrahydrofuran was effectively reduced.

[0077] Furthermore, in the depolymerization reaction of polyethylene terephthalate and 1,4-butanediol, different catalysts were introduced, and the respective ratios of the produced 1,4-butanediol, ethylene glycol, tetrahydrofuran, and water are summarized in Table 5.

[0078] [Table 5] Analysis of Components Involved in the Polymerization of Recycled Polybutylene Terephthalate TIFF2025100449000007.tif59152

[0079] From the analysis results in Table 5, in the case of depolymerization using a chelated titanium catalyst, the most free 1,4-butanediol can participate in the polymerization reaction, and it can increase the ratio of butanediol units contained in the recycled polybutylene terephthalate obtained by the subsequent polymerization reaction, resulting in a recycled polybutylene terephthalate product having an optimal melting point.

[0080] Refer to Table 3 showing the analysis results of the components of the ester pellets of recycled polybutylene terephthalate. Convert the weight percentages of each component (isophthalic acid unit, ethylene glycol unit, diethylene glycol unit, 1,4-butanediol unit, and terephthalic acid unit) in the ester pellets to mole percentages (mole percent). Further, when the molar ratio of 1,4-butanediol to polyethylene terephthalate is 2.5, the total amount of ethylene glycol unit and diethylene glycol unit contained in the ester pellets and the mole percent occupied by the 1,4-butanediol unit were analyzed and the results are summarized in Table 6.

[0081] [Table 6] Mole% Analysis of EG, DEG, and BDO in the Ester Pellets of Recycled Polybutylene Terephthalate TIFF2025100449000008.tif51153

[0082] From the results in Table 6, when the molar ratio of 1,4-butanediol to polyethylene terephthalate is 2.5, in Comparative Example 6 without using a catalyst and Comparative Examples 7 to 10 using a zinc, manganese, antimony, or non-chelated titanium catalyst, the total of EG and DEG in the ester pellets of the obtained recycled polybutylene terephthalate is in the range of 1.63 to 5.80 mol%, and BDO is 49.37 to 53.83 mol%. In contrast, in Example 2 using a chelated titanium catalyst, the total of EG and DEG in the ester pellets of the obtained recycled polybutylene terephthalate is 0.47 mol%, and BDO is 55.44 mol%. Therefore, it was found that the total of EG and DEG in the ester pellets of recycled polybutylene terephthalate can be effectively reduced by 1.16 to 5.33%, and BDO can be increased by 1.61 to 6.07%. In other words, when the molar ratio of the used 1,4-butanediol is 2.5, using a chelated titanium catalyst can effectively improve the effect of substituting the ethylene glycol unit of polyethylene terephthalate (PET) with 1,4-butanediol and avoid the generation of diethylene glycol units. Therefore, the obtained recycled polybutylene terephthalate will have good hue and physical properties.

[0083] For purposes of interpretation, in the above description, specific terms are used so that the described examples can be fully understood. However, those skilled in the art should understand that the above examples can be implemented without many specific details. Therefore, the above description of the specific examples of the present disclosure is presented for purposes of illustration and explanation. This is not intended to be exhaustive or to limit the described examples to a particular form. Those skilled in the art should understand that many modifications or changes are possible from the above description.

[0084] The summary and content of the invention are not all of the present disclosure contemplated herein, but may illustrate one or more exemplary examples. Accordingly, it is not intended to limit the present disclosure and the appended claims in any way.

[0085] The above description of specific examples fully reveals the general essence of the present disclosure, and others can easily modify and / or substitute such examples and specific examples for various uses by applying the knowledge in the technical field without the need for undue experimentation and without departing from the spirit of the present disclosure. Accordingly, such modifications and substitutions based on the description or teachings of the present disclosure are intended to be within the meaning and scope of equivalents of the described examples and specific examples. Therefore, the terms or expressions in the present disclosure are for illustrative purposes only and not for limitation, and should be understood to be interpreted by those skilled in the art from the description or teachings of the present disclosure. The content and scope of the present disclosure should not be limited by the above exemplary examples or specific examples, but should be defined based on either the appended claims and their equivalent scope.

Claims

1. In the presence of a titanium catalyst chelated with a polyacid or a polyol, polyethylene terephthalate and 1,4-butanediol are mixed, and a depolymerization and transesterification reaction is carried out to form a reaction mixture containing at least tetrahydrofuran, water, ethylene glycol and 1,4-butanediol. The first step is After reducing the pressure to remove a part of the tetrahydrofuran and the ethylene glycol from the reaction mixture, a polycondensation reaction is carried out to obtain recycled polybutylene terephthalate. The second step is A method for producing recycled polybutylene terephthalate.

2. The method for producing recycled polybutylene terephthalate according to claim 1, wherein the polyacid contains citric acid or tartaric acid.

3. The method for producing recycled polybutylene terephthalate according to claim 1, wherein the polyol contains xylitol, sorbitol, maltitol, erythritol or mannitol.

4. The method for producing recycled polybutylene terephthalate according to claim 1, wherein the molar ratio of the 1,4-butanediol to the polyethylene terephthalate used in the first step is 2:1 to 2.5:

1.

5. The method for producing recycled polybutylene terephthalate according to claim 1, wherein in the reaction mixture, the amount of ethylene glycol is 30% by weight or more based on the total amount of tetrahydrofuran, water, ethylene glycol and 1,4-butanediol.

6. The method for producing recycled polybutylene terephthalate according to claim 1, wherein in the reaction mixture, the total amount of tetrahydrofuran and water is 70% by weight or less based on the total amount of tetrahydrofuran, water, ethylene glycol and 1,4-butanediol.

7. The method for producing recycled polybutylene terephthalate according to claim 1, wherein the reaction pressure of the depolymerization and transesterification reaction is about 1 atm, and the reaction temperature is 210 to 230 °C.

8. The method for producing recycled polybutylene terephthalate according to claim 1, wherein the reaction pressure of the polycondensation reaction is lower than 0.1 atm, and the reaction temperature is 235 to 255 °C.

9. A recycled polybutylene terephthalate produced by the method for producing recycled polybutylene terephthalate according to any one of claims 1 to 8, which is a copolymer containing terephthalic acid units, isophthalic acid units and butanediol units. Recycled polybutylene terephthalate.

10. The recycled polybutylene terephthalate according to claim 9, wherein the intrinsic viscosity is in the range of 0.5 to 1.0 dl / g and the melting point is 215 °C or higher.

11. CIE L ＊ a ＊ b ＊ The lightness (L) of the coordinates of the color model is 74.0 or more, and the b ＊ value is 4.0 or less. The recycled polybutylene terephthalate according to claim 9.

12. The recycled polybutylene terephthalate according to claim 9, further containing diethylene glycol units, wherein the amount of the diethylene glycol units is less than 0.3% by weight based on the total amount of the recycled polybutylene terephthalate.

13. The recycled polybutylene terephthalate according to claim 9, further containing ethylene glycol units, wherein the amount of the ethylene glycol units is less than 0.1% by weight based on the total amount of the recycled polybutylene terephthalate.

14. The recycled polybutylene terephthalate according to claim 9, which does not contain ethylene glycol units.

15. The recycled polybutylene terephthalate according to claim 9, wherein the amount of the butanediol units is more than 40.0% by weight based on the total amount of the recycled polybutylene terephthalate.

16. Containing the recycled polybutylene terephthalate according to any one of claims 9 to 15, Polyester fiber.

17. Containing the recycled polybutylene terephthalate according to any one of claims 9 to 15, Polyester film.

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