A method for the preparation of a recycled low-melting copolyester

Abstract

The application belongs to the technical field of degradable polyester copolymer modification, and relates to a preparation method of regenerated low-melting-point copolyester. First, dimethyl isophthalate, dimethyl carbonate and 1,4-butanediol are esterified and pre-polycondensed according to a certain molar ratio to prepare depolymerization ester, then waste polyethylene terephthalate and the depolymerization ester are mixed according to a certain proportion and then depolymerization reaction is carried out, intermediate esterification product is prepared after the reaction is completed, and finally, the intermediate esterification product is subjected to polycondensation reaction under negative pressure to prepare regenerated low-melting-point copolyester. In view of the characteristics that dimethyl carbonate has a low boiling point and is easy to form a ring with ethylene glycol, the application prepares depolymerization ester through an esterification-pre-polycondensation process, the carbonate ester structure of dimethyl carbonate is stably connected to the depolymerization ester chain segment, the depolymerization ester containing a carbonate bond is introduced to effectively reduce the rigidity of the molecular chain, the prepared copolyester has low melting point and excellent bonding performance, and the high-value recycling of waste PET is realized.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of degradable polyester copolymer modification, and relates to a preparation method of regenerated low-melting-point copolyester. BACKGROUND

[0002] Polyethylene terephthalate (PET) is widely used in the fields of bottle flakes, fibers and films due to its excellent mechanical properties, transparency and chemical stability. However, the large-scale use of PET has brought about increasingly serious environmental problems. Therefore, efficient recycling and high-value reuse of waste PET have become a current research hotspot. One of the effective ways to increase the added value of waste PET is to prepare low-melting-point copolyester through chemical modification.

[0003] Low-melting-point polyester (LMPET) has a melting point generally controlled between 80 and 160 DEG C, and has a wide range of applications in textile composites, packaging and sealing, hot melt adhesives, powder coatings, plastic compatibilizers and green recycling due to its low-temperature melting and convenient processing. The preparation technology of LMPET mainly includes copolymerization modification, chain segment disordering and precise control of molecular weight to destroy the crystalline integrity and realize low-temperature melting. The conventional copolymerization modification method usually directly introduces a third or even a fourth monomer during the PET polycondensation process. The currently available modification monomers for low-melting-point copolyester are mainly divided into three categories: aliphatic dibasic acids (such as succinic acid, glutaric acid and adipic acid); aromatic dibasic acids (such as isophthalic acid and phthalic acid); and long-chain diols (such as propylene glycol, butanediol and neopentyl glycol). By introducing these monomers, the regularity of the molecular chain is destroyed, thereby reducing the crystallinity and melting point of the polyester.

[0004] The existing technology for preparing regenerated low-melting-point polyester is mainly carried out in two steps. First, the polyester is subjected to alcoholysis depolymerization to obtain small molecules, and then the low-melting-point modification component is added for copolymerization after purification. For example, patent CN117624570A obtains alcoholysis products by alcoholysis of regenerated polyester in the presence of an alcoholysis catalyst, and the alcoholysis products are subjected to a polycondensation reaction to obtain low-melting-point regenerated polyester. ZL201911005873.0 and ZL201310091292.X also disclose this method, i.e., first alcoholysis and then purification and separation of the products as raw materials for copolymerization with low-melting-point modification components. However, this method has problems such as complex process, poor controllability, easy decomposition or volatilization of the modification monomers, and difficulty in accurately controlling the properties of the products for the regenerated system using waste PET as raw material.

[0005] Dimethyl carbonate (DMC), as a green chemical raw material, possesses a unique monocarbonyl structure in its carbonate bonds. The additional oxygen atoms in these bonds allow for more flexible bond angles, effectively disrupting the rigidity of the polyester molecular chain and resulting in a significant reduction in melting point. This makes it an ideal monomer for lowering the melting point. However, due to its low boiling point and its tendency to undergo cyclic side reactions with ethylene glycol during the alcoholysis of PET, forming cyclic compounds such as ethylene carbonate, the utilization rate of DMC is extremely low. This makes it difficult to stably and efficiently introduce its carbonate structure into the PET macromolecular backbone, thus failing to achieve the desired significant reduction in melting point.

[0006] Therefore, it is of great significance to study a method for preparing recycled low-melting-point copolyester in order to solve the problems existing in the prior art. Summary of the Invention

[0007] A method for preparing recycled low-melting-point copolyester involves mixing waste PET with depolymerized polyester and then performing a depolymerization reaction to obtain an intermediate esterified product. The intermediate esterified product is then subjected to a polycondensation reaction to obtain the recycled low-melting-point copolyester.

[0008] The polyester is identified as butylene isophthalate-co-carbonate, and its chemical structural formula is as follows:

[0009] ;

[0010] The values ​​of x and y are both 1 to 4, and the sum of x and y is 3 to 5.

[0011] This invention designs an oligomer containing a dimethyl carbonate structure (i.e., butylene isophthalate-co-carbonate) as a depolymerized polyester, thereby achieving both depolymerization and functional copolymerization modification of PET. Specifically, as... Figure 1 As shown, the two end groups of the depolymerized polyester are hydroxyl groups, which can depolymerize waste PET to obtain intermediate esters. Since the depolymerized polyester contains dimethyl carbonate structures, the obtained intermediate esters also contain dimethyl carbonate structures. During the polycondensation reaction of the intermediate esters, the dimethyl carbonate structures are stably integrated into the PET backbone, thus obtaining a recycled low-melting-point copolyester. Because dimethyl carbonate is used as one of the raw materials to produce the depolymerized polyester, the problem of dimethyl carbonate easily undergoing cyclic side reactions with the product ethylene glycol when using dimethyl carbonate alone to depolymerize PET is avoided. At the same time, since dimethyl carbonate is chemically bonded to the depolymerized polyester macromolecule, its physical properties are changed, and it no longer volatilizes at the reaction temperature, solving the problem of low utilization rate caused by its low boiling point. Therefore, compared with directly using dimethyl carbonate for depolymerization, this invention has a higher utilization rate of dimethyl carbonate.

[0012] As a preferred technical solution:

[0013] The preparation method of the regenerated low-melting copolyester as described above, the added amount of depolymerization ester is 10-30wt% of the mass of waste PET.

[0014] If the added amount of depolymerization ester is too low (less than 10wt%), the problems of insufficient melting point reduction effect, low adhesion performance and low depolymerization efficiency may occur, which are analyzed as follows:

[0015] (a) One of the core roles of depolymerization ester is to introduce flexible carbonate bonds and isophthalic acid structures to destroy the regularity of PET molecular chains, thereby significantly reducing the melting point. If the added amount is too small, the introduced flexible units are insufficient, which cannot effectively disrupt the crystalline structure, resulting in that the melting point of the final copolyester does not decrease significantly;

[0016] (b) The depolymerization ester is not only a depolymerization agent, but also a modified monomer. If the added amount is too small, its depolymerization and chain reconstruction effects on PET macromolecules are limited;

[0017] (c) The hydroxyl group of the end group of the depolymerization ester attacks the ester bond of PET. If the added amount is insufficient, it means that the reactive sites are insufficient, which may lead to incomplete depolymerization reaction, slow speed, and uneven polymerization degree of intermediate ester.

[0018] If the added amount of depolymerization ester is too high (more than 30wt%), the problems of reduced mechanical properties of the final product and subsequent processing difficulties may occur, which are analyzed as follows:

[0019] (I) Excessive addition (more than 30wt%) may weaken the molecular chain strength of the regenerated low-melting copolyester due to the excessive flexibility of the carbonate bond, resulting in a significant deterioration of the mechanical properties (such as tensile strength and temperature resistance) of the material, which cannot meet the requirements of subsequent processing and application scenarios for basic properties. At the same time, excessive flexible segments may reduce the adhesion stability;

[0020] (II) The depolymerization ester is an oligomer with low molecular weight. Excessive addition (more than 30wt%) may increase the proportion of low molecular weight components in the system, not only increasing the waste of raw materials, but also possibly leading to fluctuations in the composition of the system, making it difficult to accurately control the composition and final performance (such as melting point and molecular weight distribution) of the copolyester.

[0021] The preparation method of the regenerated low-melting copolyester as described above, the preparation process of the depolymerized polyester is: dimethyl isophthalate, dimethyl carbonate and 1,4-butanediol are used as raw materials to perform esterification reaction and pre-polycondensation reaction in sequence, and the depolymerized polyester is obtained; wherein the esterification reaction temperature is 140-180℃, the esterification reaction time is 2-5h, and the esterification reaction pressure is 0.1MPa; the pre-polycondensation reaction temperature is 200-240℃, the pre-polycondensation reaction time is 30-90min, and the pre-polycondensation reaction pressure is 1-100Pa; the molar ratio of dimethyl isophthalate to dimethyl carbonate is 1:5-5:1; and the ratio of the molar amount of 1,4-butanediol to the total molar amount of dimethyl isophthalate and dimethyl carbonate is 1.2-1.5.

[0022] The core role of dimethyl carbonate is to provide carbonate bonds, and the additional oxygen atoms in the molecule can increase the freedom of chain segment movement and destroy the regularity of PET molecular chain (core mechanism of the invention), which is the key to realizing the low melting point of the copolyester; if the molar ratio of dimethyl isophthalate to dimethyl carbonate is less than 1:5, that is, the proportion of dimethyl carbonate is too high, although the flexibility of the chain segment can be maximized, the proportion of the rigid aromatic structure (from dimethyl isophthalate) in the molecular chain of the depolymerized polyester will be too low, which will reduce the structural stability of the depolymerized polyester itself; the isophthalic acid chain segment can give the depolymerized polyester a certain rigidity and mechanical stability, and if the molar ratio of dimethyl isophthalate to dimethyl carbonate is greater than 5:1, that is, the proportion of dimethyl carbonate is too low, the content of carbonate bonds is insufficient, and the flexibility modification ability of the depolymerized polyester is weakened, which cannot effectively destroy the crystalline integrity of PET when mixed with PET later, resulting in insufficient reduction of the melting point of the final copolyester.

[0023] 1,4-butanediol is used as a diol reactant to perform ester exchange and esterification reaction with the two esters (dimethyl isophthalate and dimethyl carbonate). The esterification / ester exchange reaction is a reversible reaction, according to Le Chatelier's principle, increasing the amount of 1,4-butanediol can promote the forward movement of the equilibrium, and improve the conversion rate of the two esters, so that they can be completely reacted as much as possible, therefore, the molar ratio should be greater than 1. If the amount of 1,4-butanediol is insufficient (molar ratio <1.2), the two esters will not be completely reacted, and the residual monomers will be easily volatilized or undergo side reactions in the subsequent pre-polycondensation reaction stage, reducing the yield of the depolymerized polyester; if the amount of 1,4-butanediol is excessive (molar ratio >1.5), the residual amount of unreacted 1,4-butanediol in the system will be too high, which will be volatilized in the pre-polycondensation reaction stage, causing waste of raw materials.

[0024] The preparation method of the regenerated low-melting copolyester as described above, in the preparation process of the depolymerized polyester, a first catalyst is further added before the esterification reaction; the first catalyst is zinc acetate, manganese acetate, sodium acetate, sodium methoxide, potassium chloride or lithium chloride; and the addition amount of the first catalyst is 200-400ppm of the total mass of dimethyl isophthalate and dimethyl carbonate.

[0025] The preparation method of the regenerated low-melting copolyester as described above, the depolymerization degree of the depolymerized polyester is 3-5, the molecular weight distribution index is 2.5-3.5, the terminal carboxyl group content is 2-7 mg / g, and the boiling point is greater than 300 DEG C.

[0026] The preparation method of the regenerated low-melting copolyester as described above, the temperature of the depolymerization reaction is 200-240 DEG C, the time of the depolymerization reaction is 2-4 h, and the pressure of the depolymerization reaction is 0.1 MPa (i.e. one standard atmosphere).

[0027] The preparation method of the regenerated low-melting copolyester as described above, the polymerization degree of the intermediate ester is 10-15, and the molecular weight distribution index is 3-4.

[0028] The preparation method of the regenerated low-melting copolyester as described above, the temperature of the polycondensation reaction is 200-260 DEG C, the time of the polycondensation reaction is 2-6 h, and the pressure of the polycondensation reaction is 1-100 Pa.

[0029] The preparation method of the regenerated low-melting copolyester as described above, a second catalyst is further added to the intermediate ester before the polycondensation reaction; the second catalyst is antimony glycol, antimony trioxide, tetrabutyl titanate or titanium glycol; and the addition amount of the second catalyst is 200-400 ppm of the mass of the waste PET.

[0030] The preparation method of the regenerated low-melting copolyester as described above, the melting point of the regenerated low-melting copolyester is 80-150 DEG C, and the peeling strength after hot adhesion with polyester fabric is greater than or equal to 5 N / cm.

[0031] One of the core application fields of the low-melting regenerated copolyester prepared by the method is to be used as a hot melt adhesive or a bonding fiber; in these applications, the material is heated and melted to be coated or placed between two substrates, and after cooling and solidification, the substrates are bonded together through the adhesive force and cohesive force of the material itself. The peeling strength measures the maximum force (unit: N / cm) per unit width when the bonding joint is peeled off at a certain angle and speed. The peeling strength greater than or equal to 5 N / cm means that the bonding interface between the copolyester and the polyester fabric has sufficient bonding force, which can resist external forces (such as stretching and friction) in daily use or processing, and avoid problems such as "delamination" and "delamination".

[0032] The core mechanism of the reduced melting point of the regenerated low-melting copolyester is that the carbonate bond provided by the dimethyl carbonate destroys the molecular chain rigidity; if the utilization rate of dimethyl carbonate is low, the proportion of carbonate structure in the system is insufficient, and the melting point cannot be significantly and stably reduced, so the reduction of the melting point of the low-melting polyester can indirectly prove that the carbonate structure of dimethyl carbonate has been effectively integrated into the molecular chain, and the utilization rate has reached the process design requirement.

[0033] Beneficial effects:

[0034] (1) The present invention provides a method for preparing a recycled low-melting-point copolyester. By reacting dimethyl carbonate monomer, which has a low boiling point and is prone to cyclic side reactions with ethylene glycol, with dimethyl isophthalate and 1,4-butanediol, a structurally stable depolymerized polyester butylene isophthalate-co-carbonate is prepared, which is then used for PET depolymerization. This process fundamentally avoids the decomposition and side reaction problems when dimethyl carbonate directly participates in the reaction, ensuring that its carbonate structure can be efficiently and quantitatively integrated into the PET molecular backbone, and achieving precise control over the composition of the copolyester.

[0035] (2) The method for preparing a recycled low-melting-point copolyester of the present invention, compared with the traditional multi-step method of first alcoholysis and then purification and then copolymerization modification, combines the depolymerizing agent and the modifying monomer into one, so that the depolymerization process of waste PET and its low-melting-point copolymerization modification process are completed continuously in the same system, achieving the purpose of adding multiple effects at one time, greatly shortening the production process, and reducing energy consumption and production costs.

[0036] (3) A method for preparing a recycled low-melting-point copolyester according to the present invention, wherein the prepared recycled low-melting-point copolyester has a significantly reduced melting point due to the successful introduction of the carbonate structure, which effectively disrupts the regularity of the molecular chain, while maintaining good bonding performance.

[0037] (4) The present invention provides a method for preparing a recycled low-melting-point copolyester by "fixing" the volatile dimethyl carbonate monomer in a stable oligomer chain. The polyester itself has an extremely high boiling point and will not volatilize at the depolymerization reaction temperature of PET (200~240℃), thus fundamentally avoiding the problem of volatilization loss due to its low boiling point during the high-temperature reaction. Attached Figure Description

[0038] Figure 1 The reaction mechanism diagram for preparing the recycled low-melting-point copolyester of the present invention.

[0039] Figure 2 This is the 1H NMR spectrum of the recycled low-melting-point copolyester in Example 1 of the present invention. Detailed Implementation

[0040] The present invention will be further described below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0041] The testing methods involved in the performance indicators of this invention are as follows:

[0042] Carboxyl end group content: tested according to the test method of 5.4 carboxyl end group in GB / T 14190-2017.

[0043] Peeling strength: 50 mg of the recycled low-melting copolyester chip was taken and hot bonded with a national standard polyester fabric, and then tested according to GB / T 2791-1995.

[0044] NMR hydrogen spectrum test: 5-10 mg of the dried sample was weighed and placed in a 5 mm NMR tube, 0.5 mL of deuterated trifluoroacetic acid (CF3COOD) reagent was added for dissolution, and then tested by using a German Bruker Avance-400 type NMR spectrometer, with tetramethylsilane (TMS) as the internal standard, and the test frequency was set to 600 MHz.

[0045] Example 1

[0046] A preparation method of a recycled low-melting copolyester, the specific steps are as follows:

[0047] (1) Preparation of depolymerized polyester:

[0048] Under the action of zinc acetate, dimethyl isophthalate, dimethyl carbonate and 1,4-butanediol were used as raw materials for esterification reaction, and then pre-polycondensation reaction was carried out to obtain the depolymerized polyester, the molecular weight distribution index of the depolymerized polyester was 2.5, the carboxyl end group content was 7 mg / g, and the boiling point was 302℃;

[0049] The esterification reaction temperature was 140℃, the esterification reaction time was 5h, and the esterification reaction pressure was 0.1MPa; the pre-polycondensation reaction temperature was 200℃, the pre-polycondensation reaction time was 90min, and the pre-polycondensation reaction pressure was 100Pa; the molar ratio of dimethyl isophthalate to dimethyl carbonate was 0.2:1; the ratio of the molar amount of 1,4-butanediol to the total molar amount of dimethyl isophthalate and dimethyl carbonate was 1.2; the addition amount of zinc acetate was 200ppm of the total mass of dimethyl isophthalate and dimethyl carbonate;

[0050] The chemical structural formula of the depolymerized polyester is:

[0051] ;

[0052] (2) The waste PET was mixed with the depolymerized polyester to prepare an intermediate esterification product by depolymerization reaction, and the polymerization degree of the intermediate esterification product was 10 and the molecular weight distribution index was 4;

[0053] The addition amount of the depolymerized polyester was 10wt% of the mass of the waste PET; the depolymerization reaction temperature was 200℃, the depolymerization reaction time was 4h, and the depolymerization reaction pressure was 0.1MPa;

[0054] (3) Add antimony glycolate to the intermediate ester to carry out polycondensation reaction to obtain recycled low melting point copolyester;

[0055] The polycondensation reaction was carried out at a temperature of 200℃, for a time of 6 hours, and at a pressure of 100Pa. The amount of antimony glycol added was 200ppm of the mass of waste PET.

[0056] The final recycled low-melting-point copolyester has a melting point of 150℃ and a peel strength of 5 N / cm after thermal bonding with polyester fabric; the chemical structural formula of the recycled low-melting-point copolyester is: a, b, and c represent the degree of polymerization of the repeating units in the polymer chain segment. =72%, =13%, =15%.

[0057] like Figure 2 As shown, the peak at 8.1 ppm corresponds to hydrogen on the aromatic ring in the copolyester molecular chain (from waste PET), the peaks at 7.5 ppm, 8 ppm and 8.6 ppm correspond to substituted hydrogen at different positions on the benzene ring in the depolymerized polyester, the hydrogen at 4.9 ppm corresponds to hydrogen atoms in the methylene group connected by the carbonate bond, the peaks around 4~4.8 ppm correspond to hydrogen in the -CH2-CH2- of the 1,4-butanediol segment, and the peak at 2 ppm corresponds to hydrogen in the -CH2- of the 1,4-butanediol segment or a small amount of hydrogen on the terminal groups.

[0058] Example 2

[0059] A method for preparing recycled low-melting-point copolyester, comprising the following specific steps:

[0060] (1) Preparation of depolyester:

[0061] Under the action of manganese acetate, dimethyl isophthalate, dimethyl carbonate and 1,4-butanediol are used as raw materials for esterification reaction, followed by pre-condensation reaction to obtain depolyester. The molecular weight distribution index of depolyester is 2.8, the end carboxyl group content is 5 mg / g and the boiling point is 305℃.

[0062] The esterification reaction was carried out at a temperature of 150℃ for 4 hours and a pressure of 0.1 MPa. The pre-polymerization reaction was carried out at a temperature of 210℃ for 75 minutes and a pressure of 80 Pa. The molar ratio of dimethyl isophthalate to dimethyl carbonate was 1:1. The molar ratio of 1,4-butanediol to the total molar ratio of dimethyl isophthalate and dimethyl carbonate was 1.25. The amount of manganese acetate added was 250 ppm of the total mass of dimethyl isophthalate and dimethyl carbonate.

[0063] The chemical structural formula of polyester is:

[0064] ;

[0065] (2) the intermediate esterification product is prepared by depolymerization reaction after mixing the waste PET with the depolymerization ester, the polymerization degree of the intermediate esterification product is 11, and the molecular weight distribution index is 3.8;

[0066] The adding amount of the depolymerization ester is 15wt% of the mass of the waste PET; the temperature of the depolymerization reaction is 210℃, the time of the depolymerization reaction is 3.5h, and the pressure of the depolymerization reaction is 0.1MPa;

[0067] (3) the intermediate esterification product is added with antimony trioxide to perform polycondensation reaction, and the regenerated low-melting copolyester is obtained;

[0068] The temperature of the polycondensation reaction is 210℃, the time of the polycondensation reaction is 5h, and the pressure of the polycondensation reaction is 80Pa; the adding amount of the antimony trioxide is 250ppm of the mass of the waste PET;

[0069] The melting point of the regenerated low-melting copolyester finally prepared is 130℃, the peeling strength after hot adhesion with the polyester fabric is 6N / cm; and the chemical structural formula of the regenerated low-melting copolyester is: , =68%, =19%, =13%.

[0070] Embodiment 3

[0071] A preparation method of a regenerated low-melting copolyester, and the specific steps are as follows:

[0072] (1) preparation of the depolymerization ester:

[0073] Under the action of sodium acetate, dimethyl isophthalate, dimethyl carbonate and 1,4-butanediol are used as raw materials to perform esterification reaction, and then pre-polycondensation reaction is performed to obtain the depolymerization ester, the molecular weight distribution index of the depolymerization ester is 3, the carboxyl end group content is 4mg / g, and the boiling point is 310℃;

[0074] The temperature of the esterification reaction is 160℃, the time of the esterification reaction is 3h, and the pressure of the esterification reaction is 0.1MPa; the temperature of the pre-polycondensation reaction is 220℃, the time of the pre-polycondensation reaction is 60min, and the pressure of the pre-polycondensation reaction is 60Pa; the molar ratio of dimethyl isophthalate to dimethyl carbonate is 2:1; the ratio of the molar amount of 1,4-butanediol to the total molar amount of dimethyl isophthalate and dimethyl carbonate is 1.3; and the adding amount of sodium acetate is 300ppm of the total mass of dimethyl isophthalate and dimethyl carbonate;

[0075] The chemical structural formula of the depolymerization ester is:

[0076] ;

[0077] (2) The waste PET is mixed with the depolymerization ester to perform a depolymerization reaction to obtain an intermediate ester, the polymerization degree of the intermediate ester is 12, and the molecular weight distribution index is 3.6;

[0078] The adding amount of the depolymerization ester is 20wt% of the mass of the waste PET; the temperature of the depolymerization reaction is 220℃, the time of the depolymerization reaction is 3h, and the pressure of the depolymerization reaction is 0.1MPa;

[0079] (3) The intermediate ester is added with tetrabutyl titanate to perform a polycondensation reaction, and a regenerated low-melting-point copolyester is obtained;

[0080] The temperature of the polycondensation reaction is 220℃, the time of the polycondensation reaction is 4h, and the pressure of the polycondensation reaction is 60Pa; the adding amount of the tetrabutyl titanate is 300ppm of the mass of the waste PET;

[0081] The melting point of the regenerated low-melting-point copolyester finally obtained is 120℃, the peeling strength after the regenerated low-melting-point copolyester is hotly bonded with the polyester fabric is 7N / cm; and the chemical structural formula of the regenerated low-melting-point copolyester is: , =66%, =23%, =11%.

[0082] Example 4

[0083] A preparation method of a regenerated low-melting-point copolyester, and the specific steps are as follows:

[0084] (1) Preparation of a depolymerization ester:

[0085] Under the action of sodium methoxide, dimethyl isophthalate, dimethyl carbonate and 1,4-butanediol are used as raw materials to perform an esterification reaction, and then a pre-polycondensation reaction is performed to obtain the depolymerization ester, the molecular weight distribution index of the depolymerization ester is 3.3, the carboxyl end group content is 3mg / g, and the boiling point is 315℃;

[0086] The temperature of the esterification reaction is 170℃, the time of the esterification reaction is 2.5h, and the pressure of the esterification reaction is 0.1MPa; the temperature of the pre-polycondensation reaction is 230℃, the time of the pre-polycondensation reaction is 45min, and the pressure of the pre-polycondensation reaction is 40Pa; the molar ratio of dimethyl isophthalate to dimethyl carbonate is 3:1; the ratio of the molar amount of 1,4-butanediol to the total molar amount of dimethyl isophthalate and dimethyl carbonate is 1.35; and the adding amount of sodium methoxide is 350ppm of the total mass of dimethyl isophthalate and dimethyl carbonate;

[0087] The chemical structural formula of the depolymerization ester is:

[0088] ;

[0089] (2) The waste PET is mixed with depolymerization ester to carry out depolymerization reaction to obtain intermediate esterification product, the polymerization degree of the intermediate esterification product is 13, and the molecular weight distribution index is 3.4;

[0090] The adding amount of the depolymerization ester is 25wt% of the mass of the waste PET; the temperature of the depolymerization reaction is 230℃, the time of the depolymerization reaction is 2.5h, and the pressure of the depolymerization reaction is 0.1MPa;

[0091] (3) The intermediate esterification product is added with titanium glycol to carry out polycondensation reaction to obtain the regenerated low-melting-point copolyester;

[0092] The temperature of the polycondensation reaction is 230℃, the time of the polycondensation reaction is 3.5h, and the pressure of the polycondensation reaction is 40Pa; the adding amount of the titanium glycol is 350ppm of the mass of the waste PET;

[0093] The melting point of the regenerated low-melting-point copolyester finally obtained is 100℃, the peeling strength after the regenerated low-melting-point copolyester is hotly bonded with polyester fabric is 8N / cm; and the chemical structural formula of the regenerated low-melting-point copolyester is: , =64%, =27%, =9%.

[0094] Example 5

[0095] A preparation method of a regenerated low-melting-point copolyester, and the specific steps are as follows:

[0096] (1) Preparation of depolymerization ester:

[0097] Under the action of potassium chloride, dimethyl isophthalate, dimethyl carbonate and 1,4-butanediol are used as raw materials to carry out esterification reaction, and then pre-polycondensation reaction is carried out to obtain the depolymerization ester, the molecular weight distribution index of the depolymerization ester is 3.5, the carboxyl end group content is 2mg / g, and the boiling point is 325℃;

[0098] The temperature of the esterification reaction is 180℃, the time of the esterification reaction is 2h, and the pressure of the esterification reaction is 0.1MPa; the temperature of the pre-polycondensation reaction is 240℃, the time of the pre-polycondensation reaction is 30min, and the pressure of the pre-polycondensation reaction is 1Pa; the molar ratio of dimethyl isophthalate to dimethyl carbonate is 5:1; the ratio of the molar amount of 1,4-butanediol to the total molar amount of dimethyl isophthalate and dimethyl carbonate is 1.5; and the adding amount of potassium chloride is 400ppm of the total mass of dimethyl isophthalate and dimethyl carbonate;

[0099] The chemical structural formula of the depolymerization ester is:

[0100] ;

[0101] (2) the waste PET is mixed with depolymerization ester to carry out depolymerization reaction to obtain intermediate esterification product, the polymerization degree of the intermediate esterification product is 15, and the molecular weight distribution index is 3;

[0102] The adding amount of the depolymerization ester is 30wt% of the mass of the waste PET; the temperature of the depolymerization reaction is 240℃, the time of the depolymerization reaction is 2h, and the pressure of the depolymerization reaction is 0.1MPa;

[0103] (3) the intermediate esterification product is added with titanium glycol to carry out polycondensation reaction to obtain the regenerated low-melting-point copolyester;

[0104] The temperature of the polycondensation reaction is 240℃, the time of the polycondensation reaction is 3h, and the pressure of the polycondensation reaction is 1Pa; the adding amount of the titanium glycol is 400ppm of the mass of the waste PET;

[0105] The melting point of the regenerated low-melting-point copolyester finally obtained is 80℃, the peeling strength after the regenerated low-melting-point copolyester is hotly bonded with polyester fabric is 10N / cm; and the chemical structural formula of the regenerated low-melting-point copolyester is: , =62%, =30%, =8%.

[0106] Example 6

[0107] A preparation method of a regenerated low-melting-point copolyester, and the specific steps are as follows:

[0108] (1) preparation of depolymerization ester:

[0109] Under the action of lithium chloride, dimethyl isophthalate, dimethyl carbonate and 1,4-butanediol are used as raw materials to carry out esterification reaction, and then pre-polycondensation reaction is carried out to obtain the depolymerization ester, the molecular weight distribution index of the depolymerization ester is 2.9, the carboxyl end group content is 6mg / g, and the boiling point is 320℃;

[0110] The temperature of the esterification reaction is 155℃, the time of the esterification reaction is 3.5h, and the pressure of the esterification reaction is 0.1MPa; the temperature of the pre-polycondensation reaction is 220℃, the time of the pre-polycondensation reaction is 60min, and the pressure of the pre-polycondensation reaction is 20Pa; the molar ratio of dimethyl isophthalate to dimethyl carbonate is 4:1; the ratio of the molar amount of 1,4-butanediol to the total molar amount of dimethyl isophthalate and dimethyl carbonate is 1.4; and the adding amount of lithium chloride is 320ppm of the total mass of dimethyl isophthalate and dimethyl carbonate;

[0111] The chemical structural formula of the depolymerization ester is:

[0112] ;

[0113] (2) the intermediate esterification product is prepared by depolymerization reaction after mixing waste PET and depolymerization ester, the polymerization degree of the intermediate esterification product is 14, and the molecular weight distribution index is 3.2;

[0114] The adding amount of the depolymerization ester is 18wt% of the mass of the waste PET; the temperature of the depolymerization reaction is 200℃, the time of the depolymerization reaction is 4h, and the pressure of the depolymerization reaction is 0.1MPa;

[0115] (3) the intermediate esterification product is added with antimony glycol to perform polycondensation reaction, and the regenerated low-melting copolyester is obtained;

[0116] The temperature of the polycondensation reaction is 260℃, the time of the polycondensation reaction is 2h, and the pressure of the polycondensation reaction is 20Pa; the adding amount of the antimony glycol is 320ppm of the mass of the waste PET;

[0117] The melting point of the regenerated low-melting copolyester finally prepared is 90℃, the peeling strength after hot adhesion with polyester fabric is 9N / cm; and the chemical structural formula of the regenerated low-melting copolyester is: , =65%, =25%, =10%.

Claims

1. A method for the preparation of a recycled low-melting copolyester, characterized in that, The waste PET is mixed with the depolymerization ester, and then subjected to a depolymerization reaction to obtain an intermediate esterification product; and then the intermediate esterification product is subjected to a polycondensation reaction to obtain the regenerated low-melting-point copolyester. The chemical structural formula of the depolymerization ester is as follows: ； wherein the value range of x is 1-4, the value range of y is 1-4, and the value range of the sum of x and y is 3-5. The addition amount of the depolymerization ester is 10-30wt% of the mass of the waste PET.

2. The method of claim 1, wherein the low-melting copolyester is regenerated by heating the low-melting copolyester to a temperature of 100°C to 150°C. The preparation process of the depolymerization ester is as follows: dimethyl isophthalate, dimethyl carbonate and 1,4-butanediol are used as raw materials to sequentially perform esterification reaction and pre-polycondensation reaction, and the depolymerization ester is obtained; wherein the temperature of the esterification reaction is 140-180℃, the time of the esterification reaction is 2-5h, and the pressure of the esterification reaction is 0.1MPa; the temperature of the pre-polycondensation reaction is 200-240℃, the time of the pre-polycondensation reaction is 30-90min, and the pressure of the pre-polycondensation reaction is 1-100Pa; the molar ratio of dimethyl isophthalate to dimethyl carbonate is 1:5-5:1; and the ratio of the molar amount of 1,4-butanediol to the total molar amount of dimethyl isophthalate and dimethyl carbonate is 1.2-1.

5.

3. The method of claim 2, wherein the low-melting copolyester is regenerated by heating the low-melting copolyester to a temperature of 100°C to 150°C. In the preparation process of the depolymerization ester, a first catalyst is further added before the esterification reaction; the first catalyst is zinc acetate, manganese acetate, sodium acetate, sodium methoxide, potassium chloride or lithium chloride; and the addition amount of the first catalyst is 200-400ppm of the total mass of dimethyl isophthalate and dimethyl carbonate.

4. The method of claim 3, wherein the low-melting copolyester is regenerated by heating the low-melting copolyester to a temperature of 100°C to 150°C. The molecular weight distribution index of the depolymerization ester is 2.5-3.5, the carboxyl end group content is 2-7mg / g, and the boiling point is greater than 300℃.

5. The method of claim 1, wherein the low-melting copolyester is regenerated by heating the low-melting copolyester to a temperature of about 100°C to about 150°C. The temperature of the depolymerization reaction is 200-240℃, the time of the depolymerization reaction is 2-4h, and the pressure of the depolymerization reaction is 0.1MPa.

6. The method of claim 5, wherein the low-melting copolyester is regenerated by heating the low-melting copolyester to a temperature of about 100°C to about 150°C. The polymerization degree of the intermediate esterification product is 10-15, and the molecular weight distribution index is 3-4.

7. The method of claim 1, wherein the low-melting copolyester is regenerated by heating the low-melting copolyester to a temperature of about 100°C to about 150°C. The temperature of the polycondensation reaction is 200-260℃, the time of the polycondensation reaction is 2-6h, and the pressure of the polycondensation reaction is 1-100Pa.

8. The method of claim 7, wherein the low-melting copolyester is regenerated by heating the low-melting copolyester to a temperature of about 100°C to about 150°C. A second catalyst is further added to the intermediate esterification product before the polycondensation reaction; the second catalyst is antimony glycol, antimony trioxide, tetrabutyl titanate or titanium glycol; and the addition amount of the second catalyst is 200-400ppm of the mass of the waste PET.

9. The method of claim 8, wherein the low-melting copolyester is regenerated by heating the low-melting copolyester to a temperature of about 100°C to about 150°C. The melting point of the regenerated low-melting-point copolyester is 80-150℃, and the peeling strength after hot adhesion with the polyester fabric is ≥5N / cm.

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