A method for preparing aliphatic-aromatic copolyester by using waste PET as raw material

Abstract

This invention provides a method for preparing aliphatic-aromatic copolyesters from waste PET, comprising: S1, mixing waste PET, glycol materials, carbonates, and a catalyst under a protective atmosphere and stirring, performing an esterification reaction by heating once, followed by a depolymerization reaction by heating again, to obtain a PET depolymerization product; the carbonate is selected from dimethyl carbonate, diethyl carbonate, or methyl ethyl carbonate; S2, mixing dicarboxylic acid or dicarboxylic acid esters, glycol materials, and a catalyst under a protective atmosphere and stirring, performing an esterification reaction to obtain dicarboxylic acid diol ester oligomers; S3, mixing the PET depolymerization product and the dicarboxylic acid diol ester oligomers, performing an esterification reaction at normal pressure, reducing the system pressure, and performing a vacuum polycondensation reaction by heating to obtain an aliphatic-aromatic copolyester. This method can remove ethylene glycol in situ during PET depolymerization without requiring a large amount of alcoholysis agent, improving PET resource utilization efficiency, and obtaining high-performance polyesters.

Description

Technical Field

[0001] This invention belongs to the field of waste PET recycling technology, and particularly relates to a method for preparing aliphatic-aromatic copolyesters from waste PET as raw material. Background Technology

[0002] There are two main types of recycling for waste PET: physical recycling and chemical recycling.

[0003] Physical recycling is the most widely used method, with mechanical recycling being a typical application. This involves washing, crushing, melting, extruding, and granulating waste PET bottles to produce recycled PET particles. Since the chemical structure remains unchanged, this recycling method is simple and low-cost. However, its disadvantage is that the molecular weight of the recycled material can undergo a certain degree of thermal oxidative degradation due to repeated heating, shearing, or the presence of impurities, leading to a decrease in its mechanical properties and transparency. It is usually unsuitable for direct use in food-grade packaging and needs to be downgraded or blended with virgin materials.

[0004] Chemical recycling can not only produce high-quality recycled PET from waste PET through depolymerization and polycondensation, but it can also process waste PET into products with higher added value, thus achieving upgraded recycling of waste PET. Because chemical recycling processes are more complex and the equipment is relatively expensive compared to physical recycling methods, it is difficult to promote industrial-scale application. However, due to its numerous advantages, it has great development potential. Chemical recycling requires additional distillation or separation of ethylene glycol and a large amount of alcoholysis agent, increasing process complexity and energy consumption. Summary of the Invention

[0005] In view of this, the purpose of the present invention is to provide a method for preparing aliphatic-aromatic copolyesters from waste PET as raw material. This method can remove ethylene glycol in situ during the PET depolymerization process without the need for a large amount of alcoholysis agent, thereby improving the utilization efficiency of PET resources and obtaining high-performance polyester.

[0006] This invention provides a method for preparing aliphatic-aromatic copolyesters from waste PET, comprising the following steps: S1. Under a protective atmosphere and with stirring, waste PET, glycol materials, carbonate and catalyst are mixed, heated once to carry out an esterification reaction, and heated again to carry out a depolymerization reaction to obtain PET depolymerization product; the carbonate is selected from dimethyl carbonate, diethyl carbonate or methyl ethyl carbonate. S2. Under a protective atmosphere and with stirring, dicarboxylic acid or dicarboxylic acid ester, diol and catalyst are mixed and esterified to obtain dicarboxylic acid diol ester oligomers. S3. The PET depolymerization product and the dicarboxylic acid diol ester oligomer are mixed and esterified under normal pressure. The system pressure is reduced and the temperature is increased to carry out a vacuum polycondensation reaction to obtain an aliphatic-aromatic copolyester.

[0007] Preferably, in step S1, the molar ratio of the waste PET, glycol materials and carbonate is 1:(2~2.2):(1~1.1).

[0008] Preferably, in step S2, the molar ratio of dicarboxylic acid and diol is 1:1 to 1.1; The molar ratio of dicarboxylic acid esters and diols is 1:1 to 1.1.

[0009] Preferably, in step S1, the temperature of the depolymerization reaction is 220~235℃, and the time of the depolymerization reaction is 3.5~4.5h.

[0010] Preferably, in steps S1 and S2, the diol material is independently selected from butanediol or hexanediol.

[0011] Preferably, in step S1, the process of carrying out the esterification reaction by heating once includes: The temperature was increased from 130℃ to 190℃ within 8 hours, with a heating rate of 10~20℃ / h; The esterification reaction continues until the distillate reaches at least 90 mol of the theoretical amount.

[0012] Preferably, in step S2, the esterification reaction includes: The temperature was increased from 130℃ to 190℃ in 5~8 hours, with a heating rate of 10~20℃ / h. The esterification reaction continues until the distillate reaches at least 90 mol of the theoretical amount.

[0013] Preferably, the catalyst in steps S1 and S2 is selected from one or more of titanium-based catalysts, zinc-based catalysts, and tin-based catalysts; more preferably, it is selected from one or more of tetrabutyl titanate, tetraisopropyl titanate, zinc acetate, antimony trifluoride, and antimony acetate. The catalyst mentioned in step S1 accounts for 0.1% to 0.3% of the mass of waste PET; The catalyst mentioned in step S2 accounts for 0.1~0.2% of the mass of waste PET.

[0014] Preferably, in step S3, the temperature of the vacuum polycondensation reaction is 230~250℃, and the time of the vacuum polycondensation reaction is 3~6h.

[0015] Preferably, in step S1, the PET depolymerization products include ethylene glycol, butanediol, ethylene carbonate, homopolymer, and copolymer; The monomers of the homopolymer are selected from butylene terephthalate, ethylene terephthalate, butylene carbonate, or ethylene carbonate. The copolymer's polymeric monomers are selected from at least two of butylene terephthalate, ethylene terephthalate, butylene carbonate, and ethylene carbonate.

[0016] This invention provides a method for preparing aliphatic-aromatic copolyesters from waste PET, comprising the following steps: S1, under a protective atmosphere and stirring, waste PET, glycol materials, carbonates, and a catalyst are mixed, and an esterification reaction is carried out by heating once, followed by a depolymerization reaction by heating again to obtain a PET depolymerization product; the carbonate is selected from dimethyl carbonate, diethyl carbonate, or methyl ethyl carbonate; S2, under a protective atmosphere and stirring, dicarboxylic acid or dicarboxylic acid esters, glycol materials, and a catalyst are mixed and an esterification reaction is carried out to obtain dicarboxylic acid diol ester oligomers; S3, the PET depolymerization product and the dicarboxylic acid diol ester oligomers are mixed and an esterification reaction is carried out at normal pressure, the system pressure is reduced, and a vacuum polycondensation reaction is carried out by heating to obtain an aliphatic-aromatic copolyester. This method can remove ethylene glycol in situ during PET depolymerization without requiring a large amount of alcoholysis agent, improving the utilization efficiency of PET resources and obtaining high-performance polyesters. Attached Figure Description

[0017] Figure 1 The product PBAT prepared in Example 1 of this invention 1 H-NMR spectrum; Figure 2 EC is a byproduct obtained in Example 1 of this invention. 1 H-NMR spectrum; Figure 3 The GPC spectrum test results of PBAT, the product prepared in Example 1 of this invention; Figure 4 Mechanical property test diagrams of PBAT prepared in Example 1 and commercial PBAT; Figure 5 TGA curves of PBAT prepared in Example 1 and commercial PBAT; Figure 6 DSC curves of PBAT prepared in Example 1 and commercial PBAT; Figure 7 The polyesters prepared in Example 1 and the comparative example 1 H-NMR spectrum comparison test. Detailed Implementation

[0018] This invention provides a method for preparing aliphatic-aromatic copolyesters from waste PET, comprising the following steps: S1. Under a protective atmosphere and with stirring, waste PET, glycol materials, carbonate and catalyst are mixed, heated once to carry out an esterification reaction, and heated again to carry out a depolymerization reaction to obtain PET depolymerization product; the carbonate is selected from dimethyl carbonate, diethyl carbonate or methyl ethyl carbonate. S2. Under a protective atmosphere and with stirring, dicarboxylic acid or dicarboxylic acid ester, diol and catalyst are mixed and esterified to obtain dicarboxylic acid diol ester oligomers. S3. Mix the PET depolymerization product and the dicarboxylic acid diol ester oligomer, perform esterification under normal pressure, reduce the system pressure, and perform vacuum polycondensation reaction to obtain an aliphatic-aromatic copolyester.

[0019] This invention introduces a specific type of carbonate as an in-situ conversion reagent for ethylene glycol. The carbonate reacts with the ethylene glycol released during the reaction via transesterification, producing low-boiling-point ethanol and easily separable ethylene carbonate. This cleverly disrupts the reaction equilibrium, achieving efficient conversion of waste PET into the target polyester. The tensile strength, elongation at break, and thermodynamic properties of the target polyester obtained by this method are not significantly different from those of commercially available products.

[0020] In this method, carbonate reacts in situ with ethylene glycol depolymerized from PET to produce ethylene carbonate, eliminating the need for a large amount of depolymerizing agent. This method does not require complete depolymerization of waste PET into monomers; the depolymerization products are mostly oligomers and ethylene carbonate, which can be directly fed into the next reaction step.

[0021] This method uses carbonates to react in situ to remove the ethylene glycol produced by depolymerization. At the end of the reaction, only the terephthalic acid segments of the original PET remain in the reaction system. Adding any glycol can yield a new polymer, and adding any other polyester prepolymer can yield a copolyester.

[0022] This invention involves mixing waste PET, glycol materials, carbonates, and a catalyst under a protective atmosphere and with stirring. The mixture undergoes an esterification reaction by heating once, followed by a depolymerization reaction by heating again, to obtain a PET depolymerization product. The carbonate is selected from dimethyl carbonate, diethyl carbonate, or methyl ethyl carbonate.

[0023] The waste PET mentioned in this invention refers to one or more of waste PET granules, waste PET bottle flakes, and waste PET fibers. The glycol material mentioned in this invention is selected from butanediol or hexanediol; the hexanediol is 1,6-hexanediol (HDO). The molar ratio of waste PET, glycol material, and carbonate in this invention is 1:(2~2.2):(1~1.1), where the molar amount of PET is based on its repeating unit mass of 192.164 g / mol. By controlling the amount of raw materials and using glycol materials, this invention allows the waste PET depolymerization product to be used in the next step of polycondensation without purification to obtain monomers; it also eliminates the need for excessive addition of depolymerizing agents.

[0024] The catalyst described in this application is one or more of titanium-based catalysts, zinc-based catalysts, and tin-based catalysts; preferably one or more of tetrabutyl titanate, tetraisopropyl titanate, zinc acetate, antimony trifluoride, and antimony acetate; the catalyst in step S1 accounts for 0.1~0.3% of the mass of waste PET, specifically 0.1%, 0.15%, 0.2%, 0.25%, or 0.3%.

[0025] The esterification reaction process in this invention, which involves a single temperature increase, includes: The temperature was increased from 130℃ to 190℃ within 8 hours, with a heating rate of 10~20℃ / h.

[0026] The esterification reaction is carried out by heating once, and the distillate reaches at least 90 mol% of the theoretical amount, thus ending the esterification reaction; the distillate is ethanol.

[0027] In this invention, after the esterification reaction is completed, the temperature is raised again to carry out the depolymerization reaction; the temperature of the depolymerization reaction is 220~235℃, and the time of the depolymerization reaction is 3.5~4.5h.

[0028] The stirring rate used in step S1 of this invention is 400~500 rpm, specifically 400 rpm, 450 rpm or 500 rpm.

[0029] In step S1 of this invention, the PET depolymerization products include a large amount of ethylene carbonate and oligomers, and a small amount of monomers; the oligomers include homopolymers and copolymers. Preferably, the PET depolymerization products include ethylene glycol, butanediol, ethylene carbonate, homopolymers, and copolymers; The monomers of the homopolymer are selected from butylene terephthalate, ethylene terephthalate, butylene carbonate, or ethylene carbonate. The copolymer's polymeric monomers are selected from at least two of butylene terephthalate, ethylene terephthalate, butylene carbonate, and ethylene carbonate.

[0030] This invention involves mixing dicarboxylic acids or dicarboxylic acid esters, diols, and a catalyst under a protective atmosphere and with stirring, followed by an esterification reaction to obtain dicarboxylic acid diol ester oligomers.

[0031] The dicarboxylic acid described in this invention is succinic acid or adipic acid; the dicarboxylic acid ester is succinic acid diester or adipic acid diester; the adipic acid ester in this invention is dimethyl adipic acid (DMA) or diethyl adipic acid (DEA); if adipic acid or succinic acid is used, the distillate of the esterification reaction is water; if dimethyl adipic acid is used, the distillate of the esterification reaction is methanol; if diethyl adipic acid is used, the distillate of the esterification reaction is ethanol.

[0032] The diols used in the esterification reaction with dicarboxylic acids or dicarboxylic acid esters are butanediol or hexanediol. The diols used in step S2 are the same as those used in step S1.

[0033] In this invention, the molar ratio of dicarboxylic acid and diol is 1:1 to 1.1; the molar ratio of dicarboxylic acid ester and diol is 1:1 to 1.1. The catalyst in step S2 is selected from one or more of titanium-based, zinc-based, and tin-based catalysts; preferably one or more of tetrabutyl titanate, tetraisopropyl titanate, zinc acetate, antimony trifluoride, and antimony acetate. The catalyst accounts for 0.1% to 0.2% of the mass of waste PET, specifically 0.1%, 0.11%, 0.12%, 0.13%, 0.14%, 0.15%, 0.16%, 0.17%, 0.18%, 0.19%, or 0.2%.

[0034] In this invention, steps S1 and S2 are not in any particular order. Step S1 can be performed first, or step S2 can be performed first, or steps S1 and S2 can be performed simultaneously.

[0035] After obtaining the PET depolymerization product and the dicarboxylic acid diol ester oligomer, the present invention mixes the PET depolymerization product and the dicarboxylic acid diol ester oligomer, performs an esterification reaction under normal pressure, reduces the system pressure, and performs a vacuum polycondensation reaction by raising the temperature to obtain an aliphatic-aromatic copolyester.

[0036] The present invention preferably carries out the esterification reaction at 145~155℃ under normal pressure until the distillate accounts for more than 90 mol% of the theoretical amount.

[0037] The present invention preferably reduces the system pressure to an absolute pressure of less than 100 Pa.

[0038] In this invention, the temperature of the vacuum polycondensation reaction is 230~250℃, and the reaction time is 3~6 hours. During the vacuum polycondensation reaction, the invention continuously removes the slightly excess diol and cyclic ethylene carbonate (EC) added during the initial feeding.

[0039] The present invention involves a vacuum polycondensation reaction until the stirring torque reaches a set value to obtain the target aliphatic-aromatic copolyester; the stirring torque is preferably 1.0~1.5 N·m.

[0040] The aliphatic-aromatic copolyester prepared by this invention can be polybutylene terephthalate (PBAT), polyhexylene terephthalate (PHAT), or polybutylene terephthalate (PBST).

[0041] The aliphatic-aromatic copolyester is specifically PBAT, with a number-average molecular weight (Mn) of 30-70 kDa and a molecular weight distribution of 1.5-2.0. The polyester segments contain a molar percentage of polybutylene adipate units greater than 48%, a molar percentage of terephthalate units greater than 48%, a carbonate residue of less than 2%, a ethylene glycol residue of less than 1%, a polyester tensile strength greater than 15 MPa, an elongation at break greater than 2000%, and thermodynamic properties that are not significantly different from commercially available PBAT.

[0042] To further illustrate the present invention, the following detailed description, in conjunction with embodiments, of a method for preparing aliphatic-aromatic copolyesters from waste PET provided by the present invention, should not be construed as limiting the scope of protection of the present invention.

[0043] All raw materials used in the following examples need to be dehydrated. Waste PET is dried in an 80°C vacuum oven for 5-8 hours. BDO, HDO, carbonate, TBT, AA, DMA, DEA, etc. need to be dehydrated by molecular sieve or calcium hydride to a water content of less than 50 ppm before use.

[0044] Example 1

[0045] 1. PET alcoholysis prepolymerization stage

[0046] 38.4 g (0.2 mol) of waste PET, 37.9 g (0.42 mol) of 1,4-butanediol (BDO), 24.8 g (0.21 mol) of diethyl carbonate (DEC) and 0.08 g of tetrabutyl titanate (TBT) catalyst were added to a two-necked flask. A protective gas was introduced, and the temperature was raised from 130℃ to 190℃ within 8 h under stirring, with the heating rate controlled at 10℃ / h and the stirring rate at 400 rpm. The distillate was ethanol, and the ethanol content of the distillate was required to reach 90 mol% or more of the theoretical amount. Then, the temperature was raised to 230℃ for depolymerization reaction for 4 h to obtain the PET depolymerization product.

[0047] 2. Adipic acid polycondensation stage

[0048] 29.2 g (0.2 mol) adipic acid (AA), 18.0 g (0.2 mol) BDO and 0.06 g TBT were added to the reactor, and a protective gas was introduced. The temperature was gradually increased from 130℃ to 190℃ under stirring. Timing was started from 130℃, the heating rate was 10℃ / h, the stirring rate was 400 rpm, and the reaction time was 6 h to carry out the esterification reaction to obtain butylene adipate oligomer. Water was removed from the system, and the water content of the distillate was required to reach 90 mol% or more of the theoretical amount.

[0049] 3. Vacuum polycondensation stage

[0050] Butylene adipate oligomer (PBA) was added to the PET depolymerization product. The mechanical stirrer was replaced, and an esterification reaction was carried out at 150°C under normal pressure until the materials were uniformly mixed. The system pressure was gradually reduced to an absolute pressure less than 100 Pa, and the temperature was raised to 250°C for a vacuum polycondensation reaction for 4 hours. During the reaction, excess BDO and cyclic ethylene carbonate (EC) were continuously removed until the stirring torque reached the set value, yielding high molecular weight polyester PBAT.

[0051] The polyester chain segment contains a polybutylene adipate unit with a molar percentage greater than 48%, a terephthalate unit with a molar percentage greater than 48%, a carbonate residue of less than 2%, and an ethylene glycol residue of less than 1%. The structural formula is shown below: .

[0052] The reaction route of Example 1 is shown below: ; Figure 1 The product PBAT prepared in Example 1 of this invention 1 H-NMR spectrum; Figure 2 EC is a byproduct obtained in Example 1 of this invention. 1 H-NMR spectrum; Figure 3 The GPC spectrum test results of PBAT, the product prepared in Example 1 of this invention, show that the Mn of PBAT prepared in Example 1 is 54386, Mw is 90529, Mp is 71360, Mz is 135305, Mz+1 is 183552, and the molecular weight distribution is 1.66.

[0053] Figure 4 Mechanical property test diagrams of PBAT prepared in Example 1 and commercial PBAT; Figure 5 TGA curves of PBAT prepared in Example 1 and commercial PBAT; Figure 6 DSC curves of PBAT prepared in Example 1 and commercial PBAT. Figures 4-6 It can be seen that the tensile strength of the polyester PBAT prepared in Example 1 is higher than 15 MPa, and the elongation at break is higher than 2000%. d，5% The temperature was 343℃, T g -30℃, T m At 126℃, its thermodynamic properties are not significantly different from those of commercial PBAT. Comparative Example 1 1. PET depolymerization stage 38.4 g (0.2 mol) of waste PET, 18.0 g (0.2 mol) of 1,4-butanediol (BDO) and 0.08 g of tetrabutyl titanate (TBT) catalyst were added to a two-necked flask, a protective gas was introduced, and the depolymerization reaction was carried out at 230 °C for 3 h under stirring conditions at a stirring rate of 400 rpm. The distillate was ethylene glycol, and the desired amount was 90 mol% or more of the theoretical amount, to obtain the PET depolymerization product. 2. Adipic acid prepolymerization stage 29.2 g (0.2 mol) adipic acid (AA), 18.0 g (0.2 mol) BDO and 0.06 g TBT were added to a reactor, a protective gas was introduced, and the temperature was gradually increased from 130℃ to 190℃ under stirring at a rate of 10℃ / h and a stirring rate of 400 rpm for 6 h to carry out the esterification reaction. Water was removed from the system, and the desired amount was 90% or more of the theoretical amount, to obtain butylene adipate oligomer (PBA). 3. Vacuum polycondensation stage Add butylene adipate oligomer to the PET depolymerization product, replace the mechanical stirrer, and carry out an esterification reaction at 150°C under normal pressure until the materials are mixed evenly. Gradually reduce the system pressure to an absolute pressure of less than 100 Pa, and raise the temperature to 250°C for a vacuum polycondensation reaction. The reaction time is 4 h to obtain polyester.

[0054] This invention involves the processing of polyesters prepared in Example 1 and Comparative Example 1. 1 H-NMR spectrum analysis, see Figure 7 A large number of ethylene glycol segments can be observed to remain, indicating a complex polymer structure and low degree of polymerization, which affects its subsequent applications.

[0055] Example 2

[0056] 1. PET alcoholysis prepolymerization stage

[0057] 38.4 g (0.2 mol, the molar amount of PET here is based on the mass of its repeating unit of 192.164 g / mol) of waste PET, 37.9 g (0.42 mol) of 1,4-butanediol (BDO), 24.8 g (0.21 mol) of diethyl carbonate (DEC) and 0.08 g of tetrabutyl titanate (TBT) catalyst were added to a two-necked flask. A protective gas was introduced, and the temperature was raised from 130℃ to 190℃ over 8 h under stirring, with the heating rate controlled at 10℃ / h and the stirring rate at 400 rpm. The distillate was ethanol, and the ethanol distillation was required to reach 90% or more of the theoretical amount. Then the temperature was raised to 230℃ for depolymerization reaction for 4 h.

[0058] 2. Hexadecanediol prepolymerization stage (PBA)

[0059] 34.8 g (0.2 mol) dimethyl adipate (DMA), 18.0 g (0.2 mol) BDO and 0.06 g TBT were added to the reactor, a protective gas was introduced, and the temperature was gradually increased from 130℃ to 190℃ under stirring at a rate of 10℃ / h and a stirring rate of 400 rpm for 6 h to carry out the esterification reaction. Methanol was discharged from the system, and the methanol distillate was required to reach 90% or more of the theoretical amount.

[0060] 3. Vacuum polycondensation stage

[0061] Butylene adipate oligomer (PBA) was added to the PET depolymerization system. Mechanical stirring was then employed, and an esterification reaction was carried out at 150°C under normal pressure until the materials were uniformly mixed. The system pressure was gradually reduced to an absolute pressure less than 100 Pa, and the temperature was raised to 250°C for a vacuum polycondensation reaction for 4 hours. During the reaction, excess BDO and EC were continuously removed until the stirring torque reached the set value, yielding high molecular weight polyester PBAT.

[0062] Example 3

[0063] 1. PET alcoholysis prepolymerization stage

[0064] 38.4 g (0.2 mol, the molar amount of PET here is based on the mass of its repeating unit of 192.164 g / mol) of waste PET, 37.9 g (0.42 mol) of 1,4-butanediol (BDO), 18.9 g (0.21 mol) of dimethyl carbonate (DMC) and 0.08 g of tetrabutyl titanate (TBT) catalyst were added to a two-necked flask. A protective gas was introduced, and the reaction was carried out at 90 °C for 3 h under stirring. Then, the temperature was raised to 190 °C over 5 h, with the heating rate controlled at 20 °C / h and the stirring rate at 400 rpm. The distillate was methanol, and the methanol distillation was required to reach 90% or more of the theoretical amount. Then, the temperature was raised to 230 °C for depolymerization reaction for 4 h.

[0065] 2. Adipic acid prepolymerization stage (PBA)

[0066] 29.2 g (0.2 mol) adipic acid (AA), 18.0 g (0.2 mol) BDO and 0.06 g TBT were added to the reactor, and a protective gas was introduced. The temperature was gradually increased from 130℃ to 190℃ under stirring. Timing was started from 130℃, the heating rate was 10℃ / h, the stirring rate was 400 rpm, and the reaction time was 6 h to carry out the esterification reaction. Water was removed from the system, and the theoretical amount was required to be 90% or more.

[0067] 3. Vacuum polycondensation stage

[0068] Butylene adipate oligomer (PBA) was added to the PET depolymerization system. Mechanical stirring was then employed, and an esterification reaction was carried out at 150°C under normal pressure until the materials were uniformly mixed. The system pressure was gradually reduced to an absolute pressure less than 100 Pa, and the temperature was raised to 250°C for a vacuum polycondensation reaction for 5 hours. During the reaction, excess BDO and EC were continuously removed until the stirring torque reached the set value, yielding high molecular weight polyester PBAT.

[0069] Example 4

[0070] 1. PET alcoholysis prepolymerization stage

[0071] 38.4 g (0.2 mol, the molar amount of PET here is based on the mass of its repeating unit of 192.164 g / mol) of waste PET, 49.63 g (0.42 mol) of 1,6-hexanediol (HDO), 24.8 g (0.21 mol) of diethyl carbonate (DEC) and 0.08 g of tetrabutyl titanate (TBT) catalyst were added to a two-necked flask. A protective gas was introduced, and the temperature was raised from 130℃ to 190℃ over 8 h under stirring, with the heating rate controlled at 10℃ / h and the stirring rate at 400 rpm. The distillate was ethanol, and the ethanol distillation was required to reach 90% or more of the theoretical amount. Then the temperature was raised to 230℃ for depolymerization reaction for 4 h.

[0072] 2. Adipic acid prepolymerization stage (PHA)

[0073] 29.2 g (0.2 mol) adipic acid (AA), 26.0 g (0.22 mol) 1,6-hexanediol (HDO) and 0.06 g TBT were added to the reactor. A protective gas was introduced, and the temperature was gradually increased from 130℃ to 190℃ under stirring at a rate of 10℃ / h and a stirring rate of 400 rpm for 6 h to carry out the esterification reaction. Water was removed from the system, and the water distillation was required to reach 90% or more of the theoretical amount.

[0074] 3. Vacuum polycondensation stage

[0075] Butylene adipate oligomer (PHA) was added to the PET depolymerization system. Mechanical stirring was then employed, and an esterification reaction was carried out at 150°C under normal pressure until the materials were uniformly mixed. The system pressure was gradually reduced to an absolute pressure less than 100 Pa, and the temperature was raised to 250°C for a vacuum polycondensation reaction for 5 hours. During the reaction, excess BDO and EC were continuously removed until the stirring torque reached the set value, yielding a high molecular weight polyester PHAT.

[0076] Example 5

[0077] 1. PET alcoholysis prepolymerization stage

[0078] 38.4 g (0.2 mol) of waste PET, 37.9 g (0.42 mol) of 1,4-butanediol (BDO), 24.8 g (0.21 mol) of diethyl carbonate (DEC) and 0.08 g of tetrabutyl titanate (TBT) catalyst were added to a two-necked flask. A protective gas was introduced, and the temperature was raised from 130℃ to 190℃ over 8 h with stirring. The heating rate was controlled at 20℃ / h, and the stirring rate was 400 rpm. The distillate was ethanol, and the ethanol distillation was required to reach 90% or more of the theoretical amount. Then, the temperature was raised to 230℃ for depolymerization reaction for 4 h.

[0079] 2. Succinic acid prepolymerization stage (PBS)

[0080] 23.6 g (0.2 mol) succinic acid (SA), 18.0 g (0.2 mol) BDO and 0.06 g TBT were added to the reactor, a protective gas was introduced, and the temperature was gradually increased from 130℃ to 190℃ under stirring at a rate of 10℃ / h and a stirring rate of 400 rpm for 7 h to carry out the esterification reaction. Water was discharged from the system, and the water distillation was required to reach 90% or more of the theoretical amount.

[0081] 3. Vacuum polycondensation stage

[0082] Butyl succinate oligomer (PBS) was added to the PET depolymerization system. Mechanical stirring was then employed, and an esterification reaction was carried out at 150°C under normal pressure until the materials were uniformly mixed. The system pressure was gradually reduced to an absolute pressure less than 100 Pa, and the temperature was raised to 250°C for a vacuum polycondensation reaction for 5 hours. During the reaction, excess BDO and EC were continuously removed until the stirring torque reached the set value, yielding high molecular weight polyester PBST.

[0083] The tensile strength and elongation at break of the aliphatic-aromatic copolyesters prepared in Examples 1-5 were tested in this invention, and the results are as follows: Table 1

[0084] As can be seen from the above embodiments, the present invention provides a method for preparing aliphatic-aromatic copolyesters from waste PET, comprising the following steps: S1, under a protective atmosphere and stirring, waste PET, glycol materials, carbonates and catalysts are mixed, and an esterification reaction is carried out by heating once, followed by a depolymerization reaction by heating again to obtain PET depolymerization products; the carbonates are selected from dimethyl carbonate, diethyl carbonate or methyl ethyl carbonate; S2, under a protective atmosphere and stirring, dicarboxylic acid or dicarboxylic acid esters, glycol materials and catalysts are mixed, and an esterification reaction is carried out to obtain dicarboxylic acid diol ester oligomers; S3, the PET depolymerization products and dicarboxylic acid diol ester oligomers are mixed, and an esterification reaction is carried out under normal pressure. The system pressure is reduced, and a vacuum polycondensation reaction is carried out by heating to obtain aliphatic-aromatic copolyesters. This method can remove ethylene glycol in situ during PET depolymerization without the need for a large amount of alcoholysis agent, improving the utilization efficiency of PET resources and obtaining high-performance polyesters.

[0085] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing aliphatic-aromatic copolyesters from waste PET, comprising the following steps: S1. Under a protective atmosphere and with stirring, waste PET, glycol materials, carbonate and catalyst are mixed, heated once to carry out an esterification reaction, and heated again to carry out a depolymerization reaction to obtain PET depolymerization product; the carbonate is selected from dimethyl carbonate, diethyl carbonate or methyl ethyl carbonate. S2. Under a protective atmosphere and with stirring, dicarboxylic acid or dicarboxylic acid ester, diol and catalyst are mixed and esterified to obtain dicarboxylic acid diol ester oligomers. S3. The PET depolymerization product and the dicarboxylic acid diol ester oligomer are mixed and esterified under normal pressure. The system pressure is reduced and the temperature is increased to carry out a vacuum polycondensation reaction to obtain an aliphatic-aromatic copolyester.

2. The method according to claim 1, characterized in that, In step S1, the molar ratio of the waste PET, glycol materials and carbonate is 1:(2~2.2):(1~1.1).

3. The method according to claim 1, characterized in that, In step S2, the molar ratio of dicarboxylic acid and diol is 1:1 to 1.1; The molar ratio of dicarboxylic acid esters and diols is 1:1 to 1.

1.

4. The method according to claim 1, characterized in that, In step S1, the temperature of the depolymerization reaction is 220~235℃, and the time of the depolymerization reaction is 3.5~4.5h.

5. The method according to claim 1, characterized in that, In steps S1 and S2, the diols are independently selected from butanediol or hexanediol.

6. The method according to claim 1, characterized in that, In step S1, the esterification reaction process, which involves a single temperature increase, includes: The temperature was increased from 130℃ to 190℃ within 8 hours, with a heating rate of 10~20℃ / h; The esterification reaction continues until the distillate reaches at least 90 mol of the theoretical amount.

7. The method according to claim 1, characterized in that, In step S2, the esterification reaction includes: The temperature was increased from 130℃ to 190℃ in 5~8 hours, with a heating rate of 10~20℃ / h. The esterification reaction continues until the distillate reaches at least 90 mol of the theoretical amount.

8. The method according to claim 1, characterized in that, The catalysts described in steps S1 and S2 are selected from one or more of titanium-based catalysts, zinc-based catalysts, and tin-based catalysts; preferably one or more of tetrabutyl titanate, tetraisopropyl titanate, zinc acetate, antimony trifluoride, and antimony acetate. The catalyst mentioned in step S1 accounts for 0.1% to 0.3% of the mass of waste PET; The catalyst mentioned in step S2 accounts for 0.1~0.2% of the mass of waste PET.

9. The method according to claim 1, characterized in that, In step S3, the temperature of the vacuum polycondensation reaction is 230~250℃, and the time of the vacuum polycondensation reaction is 3~6h.

10. The method according to claim 1, characterized in that, In step S1, the PET depolymerization products include ethylene glycol, butanediol, ethylene carbonate, homopolymer, and copolymer; The monomers of the homopolymer are selected from butylene terephthalate, ethylene terephthalate, butylene carbonate, or ethylene carbonate. The copolymer's polymeric monomers are selected from at least two of butylene terephthalate, ethylene terephthalate, butylene carbonate, and ethylene carbonate.

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