A method for the synthesis of polyethylene terephthalate

Abstract

The application discloses a synthesis method of polyethylene terephthalate, which utilizes terephthalic acid and ethylene glycol diacetate as polymerization raw materials, and obtains polyethylene terephthalate through de-acetate exchange polymerization under the action of a catalyst. In the method, the non-expected condensation mode (such as forming a diethylene glycol fragment) does not appear to affect the polymer performance; and because EG is not used, the tolerance to oxygen is stronger during the reaction process, and the produced PET will be more resistant to oxidation and yellowing. On the other hand, the boiling point of EGDA is lower than that of EG, so that EGDA is easier to separate from the system in theory, and the evaporated EGDA can be directly used for the next polymerization without other impurities.

Description

Technical Field

[0001] This invention relates to a novel method for synthesizing polyethylene terephthalate (PET). This method can use terephthalic acid and ethylene glycol diacetate as polymerization raw materials to obtain PET, which belongs to the field of polymer chemistry. Background Technology

[0002] Polyethylene terephthalate (PET) is a high-strength, high-modulus thermoplastic resin with many excellent mechanical properties. It is also resistant to chemical corrosion and has very low permeability to oxygen and water, making it widely used in various containers, food packaging materials, textile industry, and civil construction.

[0003] When PET polyester plastics were first being developed, the purification technology for terephthalic acid (TPA) was not mature enough, and the quality of TPA was difficult to meet the requirements for PET polyester production. Therefore, a method was developed to first convert TPA into easily purified dimethyl terephthalate (DMT) through esterification. The purified DMT was then transesterified with ethylene glycol (EG) to synthesize polyester. Since the breakthrough in TPA purification technology in the 1960s, the direct esterification method of TPA and EG has developed rapidly. Because the direct esterification method has many advantages over the transesterification method (lower raw material consumption, smaller EG recovery system, no methanol byproduct, safer production, shorter production process, and lower cost), newly built PET polyester production plants have shifted their synthesis lines to the direct polymerization method. Currently, more than 75% of the world's PET is produced using the direct esterification method.

[0004] The direct esterification method uses high-purity TPA as the polymerization raw material and EG (electromagnetic oxide) for esterification under nitrogen protection, followed by continuous polycondensation under high vacuum to form PET polyester. Because TPA has low solubility in EG, an excess of EG is often required, which greatly increases the chance of diethylene glycol (DEG) formation. In the polyester industry, the DEG content is a crucial control indicator affecting the quality of polyester products. The presence of DEG lowers the melting point and softening point of polyester, and reduces its oxidation and light resistance. Furthermore, the distilled EG is prone to side reactions, producing impurities such as DEG and 1,4-dioxane, making it impossible to directly reuse EG. Summary of the Invention

[0005] This invention aims to provide a novel method for synthesizing polyethylene terephthalate (PET), specifically using TPA and EGDA as polymerization raw materials. Under the action of a catalyst, PET polymer is obtained through a "deacetic acid" transesterification process. In this process, the ethylene glycol segments in EGDA are terminally capped with acetyl groups, eliminating free active hydroxyl species. Therefore, undesirable condensation pathways (such as the formation of diethylene glycol segments) that could affect polymer properties will not occur during polymerization. Furthermore, because EG is not used, the reaction exhibits greater resistance to oxygen, resulting in PET that is more oxidation-resistant and less prone to yellowing. On the other hand, EGDA has a lower boiling point than EG, so theoretically, EGDA is more easily removed from the system, and the distilled EGDA contains no other impurities and can be directly used in the next polymerization.

[0006] The present invention provides a method for synthesizing polyethylene terephthalate, comprising the following steps:

[0007] Terephthalic acid, ethylene glycol diacetate, and a catalyst are mixed in a polyester reactor. The air inside the reactor is replaced with an inert gas, and a certain pressure is maintained in the reactor. The temperature is then increased. The first stage of transesterification occurs at 240-280℃, with the pressure consistently controlled at around 4-10 atm. The progress of the transesterification reaction is evaluated by the yield of the byproduct acetic acid. After the transesterification reaction, the second stage of polycondensation occurs at 270-290℃ under a high vacuum of less than 500 Pa. The progress of the polycondensation reaction is judged by the yield of EGDA and the mechanical stirring power of the reactor. The polycondensation reaction usually lasts for 0.5-4 hours, yielding the desired PET polymer.

[0008] The molar ratio of terephthalic acid and ethylene glycol diacetate is 1:1 to 1:2, preferably 1:1.2.

[0009] The catalyst comprises one or a combination of several of the following: stannous oxalate, antimony trioxide, antimony acetate, antimony glycolate, boron trifluoride diethyl ether, tetrabutyl titanate, and bis(trifluoromethanesulfonyl)imide. Preferably, the catalyst is antimony trioxide, stannous oxalate, or antimony acetate, and most preferably, antimony acetate.

[0010] The system also includes the stabilizer triethyl phosphoroacetate, preferably added in an amount of 0.1 wt%, based on the mass of terephthalic acid.

[0011] The mass ratio of the catalyst to terephthalic acid is 1:100000 to 1:100, preferably 1:0000 to 1:1000, and most preferably 1:2000.

[0012] The pressure inside the reactor for the first stage of transesterification reaction is 4-10 atm, and the temperature inside the reactor is 240-280℃. Preferably, the pressure inside the reactor is 8 atm and the temperature is 260-280℃.

[0013] The temperature of the reactor for the second stage of polycondensation is 270-290℃, and the vacuum degree is less than 500Pa. The preferred temperature is 280℃ and the vacuum degree is less than 200Pa.

[0014] This invention presents a novel PET polymerization method using different polymerizing raw materials (terephthalic acid and ethylene glycol diacetate), yielding polymers with molecular weights exceeding 15,000. This method avoids undesirable condensation pathways (such as the formation of diethylene glycol fragments) that could affect polymer properties; furthermore, because EG is not used, the reaction exhibits greater tolerance to oxygen, resulting in PET that is more resistant to oxidation and yellowing. On the other hand, EGDA has a lower boiling point than EG, therefore theoretically EGDA is more easily removed from the system, and the distilled EGDA contains no other impurities and can be directly used in the next polymerization. Moreover, this method provides a new route and approach for PET synthesis. Attached Figure Description

[0015] Figure 1 The 1H NMR spectra of PET oligomers and polymers obtained according to the present invention in CF3COOD are shown. 1 HNMR).

[0016] Figure 2 These are DSC images of PET obtained using the present invention (Example 6) and PET obtained using ethylene glycol (Example 10) as a substrate.

[0017] Figure 3 This describes the effect of different catalysts on the polymerization results. Detailed Implementation

[0018] The present invention will be further illustrated 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.

[0019] This invention discloses a novel synthetic route for polyethylene terephthalate (PET), the core technology of which lies in using ethylene glycol diacetate as one of the polymerization principles. During the depolymerization process, the polymer structure is characterized by nuclear magnetic resonance (NMR), the molecular weight is determined by gel permeation chromatography (GPC), and the thermal properties are characterized by differential scanning calorimetry (DSC).

[0020] Examples 1-6: Effect of different catalysts on the molecular weight of polyethylene terephthalate

[0021] Terephthalic acid (100 g), ethylene glycol diacetate (105 g, 1.2 eq), and catalyst (50 mg, 0.05 wt.%) were mixed in a 500 mL titanium-lined polyester reactor. The air in the reactor was then purged with nitrogen, and the final pressure in the polyester reactor was maintained at 8 atm. The transesterification reaction occurred at 260–280 °C under a pressure controlled at approximately 8 atm. The progress of the reaction was evaluated by the yield of the byproduct acetic acid; the transesterification reaction was completed after approximately 4 hours. Subsequently, a polycondensation reaction was initiated at 280 °C under a high vacuum of less than 200 Pa. The polycondensation reaction lasted for 3 hours to obtain the desired polymer. The number-average molecular weight (Mn), weight-average molecular weight (Mw), and polydispersity index (PDI) of the polymer are shown in [reference needed]. Figure 3 The effect of different catalysts on the polymerization results is shown in the figure. Figure 3 .

[0022] Experimental results show that antimony trioxide is the most effective polymerization catalyst. The resulting product not only has a slightly yellowish color, but more importantly, the degree of polymerization exceeds 15,000, which meets the requirements for the degree of polymerization of fiber-grade PET. In particular, the addition of 0.1 wt% of the stabilizer triethyl phosphoroacetate not only makes the PET whiter, but also increases the molecular weight of the PET to a certain extent.

[0023] Examples 7-9: Effect of TPA and EGDA feed ratio on polymerization reaction

[0024] Terephthalic acid (100 g), ethylene glycol diacetate of varying masses, antimony trioxide (50 mg, 0.05 wt.%), and triethyl phosphoroacetate (100 mg, 0.1 wt.%) were mixed in a 500 mL titanium-lined polyester reactor. The air in the reactor was then purged with nitrogen, and the final pressure in the polyester reactor was maintained at 8 atm. The transesterification reaction occurred at 260–280 °C under a pressure controlled at approximately 8 atm. The progress of the reaction was evaluated by the yield of the byproduct acetic acid; the transesterification reaction was completed after approximately 4 hours. Subsequently, a polycondensation reaction was initiated at 280 °C under a high vacuum of less than 200 Pa. The polycondensation reaction lasted for 3 hours to obtain the desired polymer. The number-average molecular weight (Mn), weight-average molecular weight (Mw), and polydispersity index (PDI) of the polymer are shown in Table 1 below.

[0025] Table 1. Effect of TPA and EGDA feed ratio on polymerization reaction

[0026]

[0027] Experimental results show that when the amount of EGDA is 1.02 eq, the molecular weight of the final PET polymer is not high due to insufficient EGDA content. Furthermore, when the amount of EGDA is increased to 1.5 eq, the molecular weight of the PET polymer does not increase significantly. Therefore, the polymerization effect of PET is best only when the molar ratio of EGDA to TPA is 1.2.

[0028] Example 10:

[0029] Terephthalic acid (100 g), ethylene glycol (75 g, 1.2 eq), antimony trioxide (50 mg, 0.05 wt.%), and triethyl phosphoroacetate (100 mg, 0.1 wt.%) were mixed in a 500 mL titanium-lined polyester reactor. The air in the reactor was then purged with nitrogen, and the final pressure in the polyester reactor was maintained at 8 atm. The transesterification reaction occurred at 260–280 °C under a pressure controlled at approximately 8 atm. The progress of the reaction was evaluated by the yield of the byproduct acetic acid; the transesterification reaction was completed after approximately 4 hours. Subsequently, a polycondensation reaction was initiated at 280 °C under a high vacuum of less than 200 Pa. The polycondensation reaction lasted for 3 hours to obtain the desired polymer. The number-average molecular weight of the obtained polymer was 37014, the weight-average molecular weight was 67002, and the polydispersity index was 1.81.

Claims

1. A method for synthesizing polyethylene terephthalate, characterized in that: Terephthalic acid, ethylene glycol diacetate, and catalyst are mixed in a polyester reactor. The air inside the reactor is replaced with an inert gas, and a certain pressure is maintained in the polyester reactor. Then, the temperature is raised. The first stage of transesterification occurs at 240-280℃. After the transesterification reaction is completed, the second stage of polycondensation occurs at 270-290℃ for 0.5-4 hours to obtain the desired PET polymer. The molar ratio of terephthalic acid to ethylene glycol diacetate is 1:1 to 1:2; The catalyst is selected from one or a combination of antimony trioxide, stannous oxalate, and antimony acetate. The reaction system also includes the stabilizer triethyl phosphoroacetate; During the first stage of the transesterification reaction, the pressure inside the reactor is controlled at 4-10 atm and the temperature inside the reactor is 240-280℃. During the second stage of polycondensation reaction, the reactor temperature is 270-290℃ and the vacuum degree is less than 500 Pa.

2. The synthesis method according to claim 1, characterized in that: The mass ratio of the catalyst to terephthalic acid is from 1:100000 to 1:100.

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