Synthesis method of PETG

Abstract

The invention provides a synthetic method of PETG (polyethylene terephthalate glycol), which comprises the following steps: ester exchange: carrying out ester exchange on bis (2-hydroxyethyl) terephthalate and 1, 4-cyclohexanedimethanol under the catalysis of an ester exchange catalyst to obtain an ester exchange product; pre-polycondensation: constructing a reaction system by using the bis (2-hydroxyethyl) terephthalate, the ester exchange product obtained in the ester exchange step, a polymerization catalyst and a stabilizer, heating the reaction system to 270-273 DEG C, reducing the pressure to be less than or equal to 10kPa, and carrying out pre-polycondensation; medium polycondensation: after the pre-polycondensation is completed, heating the reaction system to 273-275 DEG C, continuously reducing the pressure to be less than or equal to 1kPa, and continuously performing polycondensation; and final polycondensation: after the intermediate polycondensation is completed, heating the reaction system to 280-283DEG C, reducing the pressure to be less than or equal to 100Pa, and carrying out final polycondensation to obtain the PETG. BHET and ester exchange products are subjected to condensation polymerization in a multi-stage vacuum and stepped heating mode, small molecule by-products are effectively removed in the condensation polymerization process, and the polymerization degree is promoted to be improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of high polymer materials, and particularly relates to a synthesis method of PETG. BACKGROUND

[0002] Polyethylene terephthalate-1,4-cyclohexanedimethanol (PETG) is a non-crystalline copolyester transparent plastic, which has outstanding thermoforming performance, toughness and excellent chemical resistance and impact resistance. The industrial production process thereof mainly includes a purified terephthalic acid (PTA) direct ester exchange method and a dimethyl terephthalate (DMT) ester exchange method.

[0003] The PTA direct ester exchange method first forms bis-hydroxyethyl terephthalate (BHET) monomers by esterification of purified PTA and ethylene glycol (EG), and then completes chain growth by removing EG through polycondensation of BHET and copolymer monomer 1,4-cyclohexanedimethanol (CHDM) at high temperature. The core defect of the PTA direct ester exchange method is derived from the contradiction between the high-temperature simplified process and the control of side reactions. In order to omit the intermediate step, the PTA direct ester exchange method needs to force PTA and EG to react directly at high temperature, but the high-temperature environment causes EG to be oxidized to form aldehyde substances, and to have side reactions with carboxyl groups to form oligomers or branched structures, which causes the molecular weight distribution to become wide (the polydispersity index increases), and finally reduces the impact resistance and transparency of the material. At the same time, the reaction by-product water is difficult to completely discharge in the high-temperature system, and the residual water causes the hydrolysis reverse reaction of ester bonds, destroys the polymer chain growth balance, limits the increase of the polymerization degree, and forces the subsequent reliance on solid-phase polycondensation to increase the viscosity, which increases the production cost. In addition, trace metal ions (such as iron and titanium) in the PTA raw material catalyze the chain scission reaction at high temperature, which may also cause accumulation of carboxyl end groups and release of CO2, further aggravating the decrease of melt stability and yellowing of the finished product, so that it is difficult to meet the performance requirements of high-purity optical materials.

[0004] The DMT ester exchange method first generates BHET and releases methanol by ester exchange of DMT and EG, and then the chain extension is completed by polycondensation of BHET and CHDM at high temperature. The shortcomings of the DMT ester exchange method are rooted in the dual constraints of efficiency and purity of the multi-step process. The method needs to generate an intermediate BHET by ester exchange of DMT and EG, and the byproduct methanol needs to rely on a complex recovery system, which increases the equipment investment and energy consumption cost, and the flammable nature of methanol requires strict explosion-proof design, further increasing the operation and maintenance burden. In the multi-step reaction, the third monomer (such as CHDM) is prone to epoxidation or dehydration decomposition under long-time high temperature, resulting in deviation of copolymerization ratio and affecting the batch consistency of glass transition temperature (Tg) of the material. In addition, if the intermediate BHET is not purified enough, the residual methanol or catalyst (such as zinc acetate) will cause melt foaming or local gelation in the polycondensation stage, resulting in defects such as surface defects of the finished product, which limits its application in precision injection molding, 3D printing and other fields. SUMMARY

[0005] The purpose of the present application is to provide a synthesis method of PETG, which solves the problems of wide molecular weight distribution, yellow finished product, high equipment and energy consumption cost in the prior art.

[0006] To achieve one of the above-mentioned purposes, an embodiment of the present application provides a synthesis method of PETG, comprising the following steps: Ester exchange: ester exchange of bis-hydroxyethyl terephthalate and 1,4-cyclohexane dimethanol under the catalysis of an ester exchange catalyst to obtain an ester exchange product; Pre-polycondensation: constructing a reaction system with bis-hydroxyethyl terephthalate, the ester exchange product obtained in the ester exchange step, a polymerization catalyst and a stabilizer, heating the reaction system to 270-273℃, and pre-polycondensing under pressure reduction to ≤10kPa; Medium polycondensation: after the pre-polycondensation is completed, the reaction system is heated to 273-275℃, and the polycondensation is continued under pressure reduction to ≤1kPa; Final polycondensation: after the medium polycondensation is completed, the reaction system is heated to 280-283℃, and the final polycondensation is carried out under pressure reduction to ≤100Pa to obtain PETG.

[0007] As a further improvement of the embodiment of the present application, an alcoholysis step is further included before the ester exchange step: The recycled polyethylene terephthalate and ethylene glycol are heated and subjected to alcoholysis under the catalysis of an alcoholysis catalyst to obtain bis-hydroxyethyl terephthalate; The bis-hydroxyethyl terephthalate in the ester exchange step and the pre-polycondensation step is obtained by alcoholysis of the recycled polyethylene terephthalate.

[0008] As a further improvement of the embodiment of the present application, the alcoholysis catalyst is one or a combination of two or more of potassium carbonate, zinc oxide, and zinc acetate.

[0009] As a further improvement of the embodiment of the present application, the mass ratio of the recovered polyethylene terephthalate to the alcoholysis catalyst is 1:(0.001-0.003).

[0010] As a further improvement of the embodiment of the present application, in the alcoholysis step, the recovered polyethylene terephthalate is alcoholized at a temperature of 180-220°C for 2-3h.

[0011] As a further improvement of the embodiment of the present application, the ester exchange catalyst is a titanium catalyst; and the polymerization catalyst is one or both of a titanium catalyst and an antimony catalyst. The titanium catalyst includes one or a combination of two or more of tetrabutyl titanate, isopropyl titanate, modified titanium dioxide, titanium-metal dinuclear catalyst, and modified titanium alkoxide. The antimony catalyst is one or a combination of two or more of antimony trioxide, ethylene glycol antimony, and antimony acetate.

[0012] As a further improvement of the embodiment of the present application, in the ester exchange step, the reaction temperature of the bis-hydroxyethyl terephthalate and the 1,4-cyclohexane dimethanol is 220-240°C, the reaction pressure is ≤100Pa, and the reaction time is 2-3h.

[0013] As a further improvement of the embodiment of the present application, before the pre-polymerization step, a pre-mixing step is further included. The bis-hydroxyethyl terephthalate obtained by alcoholysis of the recovered polyethylene terephthalate and the ester exchange product obtained by ester exchange are mixed at 150-180°C for 30-60min.

[0014] As a further improvement of the embodiment of the present application, after the alcoholysis step, the product obtained by alcoholysis is sequentially filtered through filtering devices with a precision of 20-40μm and 0.3-0.5μm.

[0015] As a further improvement of the embodiment of the present application, the stabilizer in the pre-polymerization step is one or a combination of two or more of triethyl phosphate, triphenyl phosphate, triphenyl phosphite, and tris(nonyl)phosphite.

[0016] The one or more technical solutions provided by the present application have at least the following technical effects or advantages: The synthesis method of PETG provided in the application is obtained by transesterification of 1,4-cyclohexanedimethanol and bis-hydroxyethyl terephthalate, and then the transesterification product and bis-hydroxyethyl terephthalate are subjected to multi-stage vacuum and stepwise heating, so that the feeding ratio of the two can be adjusted, small molecule by-products are effectively removed during the polycondensation process, and the polymerization degree is promoted. The process route is highly consistent with the existing PET / PETG production process, the reaction conditions are scientific and reasonable, no special equipment or harsh conditions are needed, and it has good industrialization feasibility and stability. In addition, the carboxyl end group content and the color of the product are also significantly reduced, which avoids yellowing of the product and degradation of the performance. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 is the liquid chromatogram of the purified BHET in Example 1 of the application.

[0018] Figure 2 is the Fourier infrared spectrum of the purified BHET in Example 1 of the application.

[0019] Figure 3 is the Fourier infrared spectrum of the regenerated PETG in Example 1 of the application.

[0020] Figure 4 is the DSC graph of the regenerated PETG chip in Example 3 of the application.

[0021] Figure 5 is the DSC graph of the PET standard sample. DETAILED DESCRIPTION

[0022] The technical solutions in the embodiments of the application will be described below in a clear and complete manner. Obviously, the described embodiments are only a part of the embodiments of the application, rather than all the embodiments. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the application.

[0023] The application provides a synthesis method of PETG, comprising the following steps: transesterification: transesterification of bis-hydroxyethyl terephthalate and 1,4-cyclohexanedimethanol under the catalysis of a transesterification catalyst to obtain a transesterification product; pre-polycondensation: constructing a reaction system of bis-hydroxyethyl terephthalate, the transesterification product obtained in the transesterification step, a polymerization catalyst and a stabilizer, heating the reaction system to 270-273℃, and reducing the pressure to ≤10kPa for pre-polycondensation; middle polycondensation: after the pre-polycondensation is completed, the reaction system is heated to 273-275℃, and the pressure is continuously reduced to ≤1kPa for further polycondensation; Final polycondensation: after the intermediate polycondensation is completed, the reaction system is heated to 280-283℃, and the pressure is reduced to ≤100 Pa for final polycondensation to obtain PETG.

[0024] The present application uses bis-hydroxyethyl terephthalate (also referred to as bis(hydroxyethyl) terephthalate, hereinafter referred to as BHET) and 1,4-cyclohexanedimethanol (hereinafter referred to as CHDM) to perform ester exchange, so that CHDM replaces the ethylene glycol monomer in BHET to obtain an ester exchange product, which is mainly composed of bis(1,4-cyclohexanedimethanol) terephthalate (hereinafter referred to as BHCT) structure (wherein the byproduct ethylene glycol is not included in the polycondensation reaction and calculation), and provides another reaction monomer BHCT in addition to BHET for subsequent polycondensation reaction.

[0025] BHET and the ester exchange product are subjected to polycondensation reaction in a multi-stage vacuum and stepwise heating mode, which effectively removes small molecule byproducts and promotes the increase of polymerization degree during the polycondensation process.

[0026] The present application performs pre-polycondensation and intermediate polycondensation at a relatively low temperature (270-273℃, 273-275℃), which avoids the side reaction of the byproduct ethylene glycol with the carboxyl group to form oligomers or branched structures after producing aldehyde substances at high temperature, and also avoids the problem of CHDM epoxidation or dehydration decomposition at high temperature in the traditional synthesis method. In addition, it can reduce the time of multimers, BHET and ester exchange product at high temperature in the final polycondensation, and reduce the occurrence of chain scission side reactions.

[0027] The step-by-step acquisition of BHET and BHCT for re-polycondensation can precisely control the proportion and ensure uniform molecular weight distribution of the product. Moreover, using BHET and ester exchange product as polycondensation reactants, there is no methanol byproduct in the production process, and no additional methanol separation system is needed, which reduces equipment investment and energy consumption cost.

[0028] In an embodiment of the present application, an alcoholysis step is further included before the ester exchange step: The recycled polyethylene terephthalate glycol (hereinafter referred to as PET) is heated and subjected to alcoholysis under the catalysis of an alcoholysis catalyst to obtain BHET. The BHET in the ester exchange step and the pre-polycondensation step is obtained by alcoholysis of recycled PET.

[0029] The waste recycled PET is used for alcoholysis to recover BHET as raw material, which converts waste polyester into high-value PETG product, realizes the closed-loop recycling of "waste plastic → monomer raw material → new material", significantly reduces the demand for virgin raw materials and white pollution, and reduces the consumption of petroleum-based raw materials.

[0030] In an embodiment of the present application, the alcoholysis catalyst is one or a combination of two or more of potassium carbonate, zinc oxide, and zinc acetate.

[0031] Further, the mass ratio of the recovered PET to the alcoholysis catalyst is 1:(0.001-0.003).

[0032] In an embodiment of the present application, in the alcoholysis step, the recovered PET is alcoholysed at a temperature of 180-220°C for 2-3 hours.

[0033] The recovered PET, ethylene glycol, and alcoholysis catalyst are added to the alcoholysis kettle in a mass ratio of 1:(2-4):(0.001-0.003), the alcoholysis kettle is heated to a temperature of 180-220°C, the pressure is 0.1-0.2 MPa, and the alcoholysis catalyst catalyses alcoholysis at a rotation speed of 80-120 rpm for 2-3 hours to obtain BHET monomers, BHET polymers, and partially incompletely depolymerized long chains.

[0034] The alcoholysis product is separated and purified, the alcoholysis product is filtered through a multi-stage filtering device to remove impurities and relatively large BHET polymers or incompletely depolymerized long chains, for example, the filtering precision is 20-40 μm and 0.3-0.5 μm in turn. Then, the alcoholysis product is purified through multi-stage distillation to obtain pure BHET monomers, which are used for subsequent transesterification with CHDM and polycondensation with the transesterification product.

[0035] In an embodiment of the present application, the transesterification catalyst is a titanium catalyst; and the polymerization catalyst is one or both of a titanium catalyst and an antimony catalyst. The titanium catalyst includes one or a combination of two or more of tetrabutyl titanate, isopropyl titanate, modified titanium dioxide, titanium-metal dinuclear catalyst, and modified titanium alkoxide (such as titanium-citric acid complex).

[0036] The antimony catalyst is one or a combination of two or more of antimony trioxide, ethylene glycol antimony, and antimony acetate.

[0037] The amounts of the titanium catalyst and the antimony catalyst used in the transesterification reaction and the polycondensation reaction are different according to the catalytic activity of the catalyst. Specifically, the amount of titanium in the titanium catalyst used as the transesterification catalyst is 20-100 ppm based on the weight of the BHET, and the amount of titanium in the modified titanium alkoxide is 5-20 ppm based on the weight of the BHET, except for the modified titanium alkoxide. The amount of titanium in the titanium catalyst used as the polymerization catalyst is 20-100 ppm based on the total weight of the BHET and the transesterification product, and the amount of titanium in the modified titanium alkoxide is 5-20 ppm based on the total weight of the BHET and the transesterification product, except for the modified titanium alkoxide.

[0038] ​

[0039] The weight of antimony in the antimony catalyst accounts for 180-250 ppm of the total amount of BHET and the ester exchange product.

[0040] In the ester exchange step, the reaction temperature of BHET and CHDM is 220-240°C, the reaction pressure is ≤100 Pa, and the reaction time is 2-3 h.

[0041] BHET and CHDM are added to the ester exchange kettle in a molar ratio of (1.1-1.3):1 and an ester exchange catalyst, the ester exchange kettle is heated to 220-240°C, and vacuum is extracted to a pressure below 100 Pa in the ester exchange kettle. The ester exchange is catalyzed by the ester exchange catalyst for 2-3 h. Excessive BHET is used to ensure complete ester exchange of CHDM, and the excessive BHET participates in the reaction in the polycondensation reaction to improve the raw material conversion rate and utilization rate.

[0042] In an embodiment of the present application, a premixing step is further included before the pre-polycondensation step: The BHET obtained by recycling PET and the ester exchange product obtained by ester exchange are mixed at 150-180°C for 30-60 min.

[0043] Both ester exchange reaction and polycondensation reaction require BHET, so the alcoholysis step must be performed first. The BHET obtained by the alcoholysis step needs to be temporarily stored and mixed with the ester exchange product after the ester exchange reaction for polycondensation reaction. To avoid decomposition or oxidation of BHET in the high-temperature state of alcoholysis for a long time, after the completion of the alcoholysis step and the ester exchange step, the BHET and the ester exchange product are transferred into a temporarily stored container (such as a temporary kettle) through a melt delivery pump and a heat exchanger for cooling. The BHET and the ester exchange product are mixed uniformly at a lower temperature, which can quickly produce a pre-polycondensation reaction and avoid color deterioration caused by high-temperature oxidation, thereby ensuring stable color of the copolyester.

[0044] In an embodiment of the present application, the stabilizer in the pre-polycondensation step is one or two or more of triethyl phosphate, triphenyl phosphate, triphenyl phosphite, and trisnonyl phosphite.

[0045] The amount of triphenyl phosphate is 100-200 ppm of the total amount of BHET and the ester exchange product, and the amount of triethyl phosphate is 50-100 ppm of the total amount of BHET and the ester exchange product.

[0046] The technical solutions of the present application will be further described below in combination with some specific embodiments.

[0047] Example 1 Alcoholysis: 3000g of waste PET, 6000g of EG, 3g of zinc acetate were put into the alcoholysis kettle, and alcoholysis was carried out at 200℃, 120rpm, 0.15MPa for 120min. After multistage filtration and purification, BHET was obtained, and the purity of BHET was 98.652% by liquid chromatography.

[0048] Trans-esterification: 144g of CHDM, 305g of BHET and 0.11g of tetrabutyl titanate were put into the trans-esterification kettle, and trans-esterification was carried out at 230℃, 100rpm, vacuum degree≤100Pa (later stabilized at 80Pa) for 180min to obtain the trans-esterification product, which was then transported to the temporary storage kettle through the heat exchanger by the melt dosing pump.

[0049] Premixing: 2300g of BHET was put into the temporary storage kettle, and after being mixed at 160℃, 120rpm for 60min, it was transported to the polycondensation kettle for pre-polycondensation.

[0050] Pre-polycondensation: the mixed melt was put into the pre-polycondensation kettle, 1g of tetrabutyl titanate and 0.3g of triphenyl phosphate were added, the temperature in the kettle was raised to 273℃, and the vacuum degree in the kettle was slowly reduced to 10kPa, and pre-polycondensation was carried out for 40min.

[0051] Medium polycondensation: the temperature was raised to 275℃, and the vacuum degree in the kettle was slowly reduced to 1kPa, and the reaction was carried out for 60min.

[0052] Final polycondensation: the temperature was raised to 283℃, and the vacuum degree in the kettle was slowly reduced to 80Pa, and after 180min, the polymerization was completed and the particles were cut to obtain the regenerated PETG.

[0053] The intrinsic viscosity of the regenerated PETG was 0.72dL / g, the carboxyl end group content was 25.32mol / t, the color (L:83.28, a:0.25, b:6.43), and the diethylene glycol content was 2.1%.

[0054] As shown in Figure 1 , it is the liquid chromatogram of the purified BHET.

[0055] As shown in Figure 2 , it is the Fourier infrared spectrum of the purified BHET, in which: 3441.17 -1 corresponds to the hydroxyl stretching vibration of the two hydroxyethyl groups in the BHET molecule, 1713.44cm -1 corresponds to the anti-symmetric stretching vibration of the carbonyl group (—O—CO—O—) in the terephthalate unit, 1249.75cm -1 corresponds to the asymmetric stretching vibration of the ether bond (—O—) in the ester group, and 1110.20cm -1This corresponds to the symmetrical vibration of the C—O bond in the ester group, 1505.11 cm -1 、1456.40cm -1 Corresponding to the stretching vibration of the benzene ring in the terephthalic acid unit, 2879.81 cm -1 ~2963.65cm -1 The multiple peaks within the range correspond to the symmetric and antisymmetric stretching vibrations of -CH2- in the ethylene glycol chain segment.

[0056] like Figure 3 The following is a Fourier infrared image of PETG: 1712.59cm -1 Corresponding to the (—O—CO—O—) stretching vibration of terephthalate group, 1241.87 cm -1 Corresponding to the asymmetric stretching vibration of the ether bond (—O—) in the ester group, 1505.24 cm -1 Corresponding to the stretching vibration of the benzene ring in the terephthalic acid unit, 2967.22m -1 The multiple peaks near it correspond to the stretching vibration of saturated C—H (—CH2— and cyclohexane ring) in CHDM, 1089.18m -1 The corresponding peaks are the superposition of the C—C skeleton vibration and C—O—C vibration of the cyclohexane ring.

[0057] Example 2 Alcoholysis: 3000g of waste PET, 7500g of EG, and 4.5g of zinc acetate were placed in a 10L alcoholysis reactor and subjected to alcoholysis for 120 min at 205°C, 120 rpm, and 0.1 MPa. The alcoholysis product was filtered and purified to obtain BHET, which had a purity of 97.322% as determined by liquid chromatography.

[0058] Transesterification: 288 g CHDM, 560 g BHET and 0.05 g titanium-citric acid complex titanium catalyst (titanium content 11.1%) were placed in a transesterification kettle. The transesterification was carried out at 220 ° C, 100 rpm, and a vacuum degree of ≤ 100 Pa (later stabilized at 80 Pa) for 180 min to obtain the transesterification product, which was then transported to the temporary storage kettle through a melt metering pump via a heat exchanger.

[0059] Premixing: 2050 g of BHET was put into a temporary storage kettle, mixed thoroughly at 160°C and 120 rpm for 45 min, and then transferred to a polycondensation kettle for pre-polycondensation.

[0060] Precondensation: The mixed melt enters the precondensation kettle and 0.95g of ethylene glycol antimony and 0.15g of triethyl phosphate are added at the same time. The temperature in the precondensation kettle is raised to 273°C, and the vacuum degree in the kettle is slowly reduced to 10kPa. Precondensation is carried out for 40 minutes.

[0061] Precondensation: temperature was raised to 275℃, and the vacuum degree in the kettle was slowly reduced to 1 kPa, and precondensation was 60 min.

[0062] Final condensation: temperature was raised to 283℃, and the vacuum degree in the kettle was slowly reduced to 80 Pa, and the polymerization was completed after 210 min, and the regenerated PETG was obtained.

[0063] The intrinsic viscosity of the regenerated PETG was measured to be 0.78 dL / g, the carboxyl end group content was 18.15 mol / t, the color (L: 85.13, a: 0.22, b: 5.55), and the diethylene glycol content was 1.5%.

[0064] Example 3 Alcoholysis: 3000 g of waste PET, 6000 g of EG, and 3 g of zinc acetate were put into the alcoholysis kettle, and alcoholysis was carried out at 205℃, 120 rpm, and 0.1 MPa for 120 min. The alcoholysis product was filtered and purified to obtain BHET, and the purity of BHET was measured by liquid chromatography to be 98.455%. Ester exchange: 432 g of CHDM, 1385 g of BHET, and 0.5 g of tetrabutyl titanate were put into the ester exchange kettle, and ester exchange was carried out at 220℃, 100 rpm, and a vacuum degree of ≤100 Pa (stabilized at 80 Pa later) for 180 min to obtain an ester exchange product, which was then directly transported to the polycondensation kettle through a heat exchanger by a melt dosing pump.

[0065] Pre-mixing: 1800 g of BHET was put into the temporary storage kettle, and was mixed at 160℃ and 120 rpm for 60 min, and then was transported to the polycondensation kettle for precondensation.

[0066] Precondensation: the mixed melt was put into the precondensation kettle, 1.1 g of tetrabutyl titanate and 0.2 g of triethyl phosphate were added, the temperature in the kettle was raised to about 273℃, and the vacuum degree in the kettle was slowly reduced to 10 kPa, and precondensation was 40 min.

[0067] Medium condensation: the temperature was raised to 275℃, and the vacuum degree in the kettle was slowly reduced to 1 kPa, and the reaction was 60 min.

[0068] Final condensation: temperature was raised to 283℃, and the vacuum degree in the kettle was slowly reduced to 80 Pa, and the polymerization was completed after 240 min, and the regenerated PETG was obtained.

[0069] The intrinsic viscosity of the regenerated PETG was measured to be 0.75 dL / g, the carboxyl end group content was 15.71 mol / t, the color (L: 84.09, a: 0.19, b: 4.20), and the diethylene glycol content was 0.9%. Reason for change in intrinsic viscosity: With the increase of CHDM copolymerization content, the introduction of larger cyclohexane structure significantly reduces the crystallization ability of PETG, which maintains a longer melting state during polycondensation, thereby promoting the full growth of molecular chains and improving the intrinsic viscosity. However, the steric hindrance effect of cyclohexane groups may also reduce the reactivity of chain ends at high content, prolonging the reaction time required to reach the target molecular weight. In addition, the actual copolymerization content of CHDM is often slightly lower than the feeding ratio, because the removal of small molecule byproducts during the reaction may change the equilibrium content of the comonomer. In the examples (such as Example 1), the copolymerization feeding ratio is 18.25%, and the actual content may be slightly lower. If chain scission or esterification side reactions occur at this time, it will limit the further growth of molecular weight, making the intrinsic viscosity slightly lower than expected.

[0070] Reasons for changes in carboxyl end group content: The carboxyl end group content reflects the concentration of carboxylic acid groups that are not fully condensed at the end of the polyester chain. A lower content indicates a higher degree of polymerization and a more stable end structure. The introduction of CHDM copolymerization significantly reduces the proportion of ethylene glycol in the system, thereby reducing the opportunity for side reactions such as esterification and hydrolysis during polycondensation, and further reducing the probability of formation of additional carboxylic end groups. In addition, the use of CHDM copolymerization with BHET is more conducive to the stability of the chain end structure and the reduction of side reactions, so the carboxyl end group content of recycled PETG is lower than that of traditional PET under the same process conditions. At the same time, by adding a suitable amount of phosphate stabilizer in the pre-polycondensation stage, the metal catalyst active sites are effectively passivated and the free carboxyl groups are neutralized, further reducing the carboxyl end group content of the polymer and improving the thermal stability and degradation resistance of the material.

[0071] Reasons for changes in color: The color change of recycled PETG is mainly due to the production of color-causing byproducts (such as conjugated double bonds, aldehyde and ketone impurities) by thermal oxidative degradation and further oxidation reactions caused by catalyst residues. In this patent process, phosphate stabilizers are added to effectively chelate metal catalyst residues, capture active carboxyl groups and free radicals, significantly inhibit high-temperature thermal oxidative degradation, and improve the color of the material. In addition, the introduction of CHDM chain segments reduces the melting processing temperature of PETG, directly reducing the possibility of generating color-causing impurities at high temperatures. At the same time, since CHDM is a saturated cyclic diol structure, it has a lower tendency to form aldehyde impurities by dehydration at high temperatures compared to straight-chain structures (such as ethylene glycol), further reducing the probability of generating color-causing impurities. Therefore, the use of higher CHDM copolymerization chain segment content combined with BHET raw materials effectively reduces the possibility of color degradation of recycled PETG and reduces the occurrence of yellowing.

[0072] The recycled PETG chip of Example 3 was tested for DSC ( Figure 4 ), and compared with the DSC graph of the PET standard sample ( Figure 5 ).

[0073] ① Glass transition temperature (Tg) difference: The glass transition temperature (Tg) of the PET standard sample is 75.25℃, and the introduction of CHDM copolymer structure makes the regularity of the molecular chain decrease and the flexibility increase, and the Tg is usually slightly higher than that of PET, and the glass transition temperature in the DSC graph of PETG is 78.98℃ ② Melting point (Tm) and melting peak characteristic difference: PET (semi-crystalline polyester) has a clear and sharp melting peak (Tm), and the melting temperature is 241.3℃, which represents the melting of the crystalline structure. PETG (amorphous polyester) usually shows no obvious melting peak in the DSC graph of PETG due to the CHDM copolymer unit disturbing the regularity of the chain segment arrangement.

[0074] ③ Difference in cold crystallization (Tc) peak: PET usually has a clear cold crystallization peak after glass transition due to its strong crystallization ability; PETG usually does not have a clear cold crystallization peak due to the CHDM copolymer unit greatly inhibiting the crystallization ability, even if there is, it is very weak and not easy to detect. Therefore, the DSC graph of the PET standard sample has a clear cold crystallization peak (161.42℃), and the DSC graph of PETG has no clear cold crystallization process.

[0075] In summary, PET is a highly regular semi-crystalline polymer with strong crystallinity, and therefore shows clear melting and crystallization characteristics in DSC. PETG greatly reduces the chain regularity and crystallization ability due to the introduction of amorphous CHDM copolymer units, so it shows typical amorphous polymer characteristics in DSC, only showing clear glass transition, no clear crystallization and melting behavior.

[0076] It should be understood that although the present specification is described in terms of embodiments, not every embodiment contains only one independent technical solution, and the description of the specification is only for the sake of clarity, and those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can be appropriately combined to form other embodiments that those skilled in the art can understand.

[0077] The series of detailed descriptions listed above are only specific descriptions of the feasible embodiments of the present application, and are not intended to limit the protection scope of the present application, and any equivalent embodiments or changes made without departing from the spirit of the present application should be included in the protection scope of the present application.

Claims

1. A method for synthesizing PETG, characterized in that, The steps include: Transesterification: bis(hydroxyethyl) terephthalate) and 1,4-cyclohexanedimethanol are transesterified under the catalysis of a transesterification catalyst to obtain a transesterification product; Pre-polycondensation: a reaction system is constructed by combining bis(hydroxyethyl) terephthalate, the transesterification product obtained in the transesterification step, a polymerization catalyst, and a stabilizer. The reaction system is heated to 270-273°C and the pressure is reduced to ≤10 kPa for pre-polycondensation. After the pre-condensation is completed, the reaction system is heated to 273~275℃, and the pressure is further reduced to ≤1kPa to continue the condensation; Final polycondensation: After the intermediate polycondensation is completed, the reaction system is heated to 280~283℃ and the pressure is reduced to ≤100Pa for final polycondensation to obtain PETG.

2. The synthetic method of PETG according to claim 1, wherein Before the transesterification step, an alcoholysis step is also included: The recovered polyethylene terephthalate and ethylene glycol are heated and alcoholyzed under the catalysis of an alcoholysis catalyst to obtain bis(hydroxyethyl) terephthalate; The bis(hydroxyethyl) terephthalate in the transesterification step and the pre-polycondensation step is obtained by alcoholysis of recycled polyethylene terephthalate.

3. The synthetic method of PETG according to claim 2, wherein The alcoholysis catalyst is one or a combination of two or more of potassium carbonate, zinc oxide and zinc acetate.

4. The synthetic method of PETG according to claim 3, wherein The mass ratio of recovered polyethylene terephthalate to alcoholysis catalyst is 1:(0.001~0.003).

5. The synthetic method of PETG according to claim 2, wherein In the alcoholysis step, the recovered polyethylene terephthalate is alcoholyzed at a temperature of 180-220° C. for 2-3 hours.

6. The synthetic method of PETG according to claim 1, wherein The transesterification catalyst is a titanium catalyst; the polymerization catalyst is one or both of a titanium catalyst and an antimony catalyst; The titanium catalyst is one or a composite of two or more of tetrabutyl titanate, isopropyl titanate, modified titanium dioxide, titanium-metal binuclear catalyst, and modified titanium alkoxide; The antimony catalyst is one of antimony trioxide, antimony glycol, and antimony acetate, or a composite of two or more.

7. The synthetic method of PETG according to claim 6, wherein In the transesterification step, the reaction temperature of bis(hydroxyethyl) terephthalate and 1,4-cyclohexanedimethanol is 220-240° C., the reaction pressure is ≤100 Pa, and the reaction time is 2-3 h.

8. The synthetic method of PETG according to claim 2, wherein Before the pre-condensation step, a pre-mixing step is also included: The bis(hydroxyethyl) terephthalate obtained by alcoholysis of the recovered polyethylene terephthalate and the transesterification product obtained by transesterification are mixed at 150-180° C. for 30-60 minutes.

9. The synthetic method of PETG according to claim 2, wherein After the alcoholysis step, the product obtained by the alcoholysis is filtered sequentially through a filter device with a precision of 20-40 μm and 0.3-0.5 μm.

10. The synthetic method of PETG according to claim 1, wherein The stabilizer in the pre-polycondensation step is one or two or more of triethyl phosphate, triphenyl phosphate, triphenyl phosphite, and trinonyl phosphite.

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