Preparation method of polyethylene glycol terephthalate copolymer

Abstract

The invention discloses a preparation method of a polyethylene glycol terephthalate copolymer, and relates to the technical field of PET. A multi-stage esterification technology and a copolymerization modification technology are adopted, neopentyl glycol is introduced into a first-path esterification reaction to serve as a third monomer, a molecular chain end group of a second-path ester exchange reaction is more stable, the mixing proportion of reactants of the two paths is regulated, a composite additive and a catalyst are added into a condensation polymerization reaction, and the polymerization reaction is more stable. And process parameters such as temperature, pressure, time and the like of each reaction stage are accurately controlled, so that the polyethylene glycol terephthalate copolymer with high transparency and excellent heat resistance is prepared.

Description

Technical Field

[0001] The present invention relates to the technical field of PET, and specifically relates to a preparation method of polyethylene terephthalate copolymer. Background Art

[0002] Polyethylene terephthalate (PET) is an important engineering plastic with excellent mechanical properties, transparency and gas barrier properties, and is widely used in fields such as fibers, films and packaging containers. However, the molecular chain structure of PET is regular and easy to crystallize, resulting in insufficient transparency, and the molecular chain has poor rigidity and a low glass transition temperature, making it difficult to meet the high-temperature use conditions. To improve the properties of PET, a copolymerization modification method is usually adopted, introducing a third monomer such as 1,4-cyclohexanedimethanol (CHDM), isosorbide, etc., to disrupt the crystallization of PET and obtain an amorphous copolyester.

[0003] In the prior art, to further increase the glass transition temperature of the copolyester, a method of introducing rigid monomers such as 2,2,4,4-tetramethyl-1,3-cyclobutanediol (CBDO) is usually adopted. However, these rigid monomers have low reaction activity, are easy to form precipitates, affect the product molecular weight and transparency, and there are problems such as melt foaming. In addition, the conversion rate of the modified monomers introduced in the prior art is not high and it is difficult to be directly applied. Therefore, how to prepare a copolyester with high transparency and high glass transition temperature without introducing rigid monomers is still a technical problem to be solved urgently.

[0004] Chinese Patent CN114805771A discloses an amorphous copolyester with a high glass transition temperature, its preparation method and application. The preparation method is: carrying out an ester exchange reaction on dimethyl oxalate and 4,4'-dihydroxybicyclohexane in a molar ratio of (2-5):1. After obtaining the end group-modified monomer, mixing the end group-modified monomer, dibasic acid, diol and catalyst and carrying out a polymerization reaction to obtain an amorphous copolyester with a high glass transition temperature. The prepared copolyester has characteristics such as a high glass transition temperature and a high molecular weight. However, limited by the by-product residue of the ester exchange reaction and the intrinsic characteristics of the amorphous structure, the long-term heat resistance stability of the copolyester is still insufficient.

[0005] Chinese Patent CN114591496A discloses a preparation method of a rigid copolyester. This method adopts a stepwise esterification and stepwise polycondensation reaction. The prepared polyester chips have good hue and transparency and excellent thermodynamic properties. However, the existing catalyst system has a low activation efficiency for high melting point monomers, and the heat stabilizer is easily decomposed during high-temperature polycondensation, resulting in the crystallinity and heat stability not reaching the theoretical expectation.

[0006] In summary, the existing PET preparation methods have the following problems: 1) The transparency of PET is not high enough, affecting its applications in fields such as transparent packaging products; 2) PET has poor heat resistance and a low glass transition temperature, making it difficult to meet the requirements of high-temperature usage environments. Therefore, there is an urgent need to provide a new preparation method for polyethylene terephthalate copolymers to solve the above problems. Summary of the Invention

[0007] The technical problem to be solved by the present invention is: to overcome the deficiencies of the prior art and provide a preparation method for polyethylene terephthalate copolymers. By adopting a multi-stage esterification process and a copolymerization modification technology, a polyethylene terephthalate copolymer with high transparency and excellent heat resistance is prepared.

[0008] The technical solution of the present invention is as follows:

[0009] A preparation method for polyethylene terephthalate copolymers, comprising the following steps:

[0010] S1 First-stage esterification reaction: Mix a dibasic acid, a diol and neopentyl glycol and carry out an esterification reaction. Control the reaction temperature at 210 - 240 °C, the pressure at 0.15 - 0.35 MPa, and the reaction time at 1.5 - 2.5 h. After the water output reaches the theoretical value, a first reactant system is obtained;

[0011] S2 Second-stage esterification reaction: Mix a diester and a diol and carry out a transesterification reaction. Control the reaction temperature at 180 - 220 °C, the pressure at 0.15 - 0.3 MPa, and the reaction time at 1.5 - 3 h. When the viscosity of the reaction system reaches 150 - 250 mPa·s, a second reactant system is obtained;

[0012] S3 Preparation of a composite additive: Add a crystallization promoter to a carrier, stir, then add a nucleating agent, stir and disperse ultrasonically to obtain a composite additive; wherein, the mass ratio of the crystallization promoter to the nucleating agent is 1:(1 - 2.5);

[0013] S4 Mixing and Polycondensation: Pump the first reactant system and the second reactant system into the polycondensation reactor through metering pumps for mixing. After preheating to 200 - 220 °C, add a composite additive and a catalyst, and carry out catalytic polycondensation for 3 - 6 h. Among them, the mass ratio of the first reactant system to the second reactant system is 1:(0.1 - 0.5). The catalytic polycondensation reaction adopts stepped temperature rise control. In the first 1 - 2 h, the temperature is raised to 240 - 260 °C at a rate of 20 - 30 °C / h to quickly activate the catalyst, promote the growth of oligomer chains, shorten the reaction induction period, and then the temperature is raised to 260 - 290 °C at a rate of 5 - 10 °C / h to slow down the temperature rise rate and avoid thermal degradation of molecular chains or side reactions (such as acetaldehyde formation) caused by local overheating. The catalytic polycondensation reaction adopts two-stage pressure control. The pressure in the first 1 - 2 h is maintained at 200 - 500 Pa, and then the pressure is reduced to ≤40 Pa. Control the metering pumps so that the feed flow rate of the first reactant system is 85 - 90 kg / h, and the feed flow rate of the second reactant system is 9 - 42.5 kg / h.

[0014] S5 Discharging: When the intrinsic viscosity of the polycondensation product is 0.75 - 0.78 dl / g, it is melt-extruded through a twin-screw extruder and underwater pelletized to obtain a polyethylene terephthalate copolymer.

[0015] Preferably, in step S1, the dibasic acid is terephthalic acid and the diol is ethylene glycol; in step S2, the diester is dimethyl terephthalate and the diol is ethylene glycol.

[0016] Preferably, in step S1, the molar ratio of the dibasic acid, diol to neopentyl glycol is 1:(1.15 - 1.4):(0.15 - 0.45).

[0017] Preferably, in step S2, the molar ratio of the diester to the diol is 1:(1.15 - 1.2).

[0018] Preferably, in step S3, the nucleating agent is sorbitol, dibenzylidene sorbitol or dimethyl dibenzylidene sorbitol; the crystallization promoter is polyethylene glycol dibenzoate, triphenyl phosphate, phthalate, amide ester or imide ester; the carrier is ethylene glycol or a polyester with M w = 1000 - 2000 g / mol.

[0019] Preferably, in step S3, the mass ratio of the total mass of the nucleating agent and the crystallization promoter to the mass of the carrier is 1:(10 - 15).

[0020] Preferably, in step S3, the rotation speed of the first stirring is 50 - 100 rpm, the stirring time is 10 - 15 min; the rotation speed of the second stirring is 200 - 300 rpm, the stirring time is 20 - 30 min; the ultrasonic frequency is 20 - 40 kHz, and the dispersion time is 10 - 15 min.

[0021] Preferably, in step S4, the mass of the composite additive accounts for 0.15-2.5% of the total mass of the first reactant system, the second reactant system, the composite additive, and the catalyst.

[0022] Preferably, in step S4, the catalyst is antimony glycolate, antimony trioxide, tetrabutyl titanate, tetraisopropyl titanate, or germanium dioxide.

[0023] Preferably, in step S4, the mass of the catalyst accounts for 100-500 ppm of the total mass of the first reactant system, the second reactant system, the composite additive, and the catalyst.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] 1. The present invention adopts a multi-stage esterification process and a copolymerization modification technology. In the first-stage esterification reaction, neopentyl glycol is introduced as a third monomer, the molecular chain end groups in the second-stage transesterification reaction are more stable, the mixing ratio of the two-way reactants is regulated, a composite additive and a catalyst are added in the polycondensation reaction, and the process parameters such as temperature, pressure, and time in each reaction stage are precisely controlled, thereby preparing a polyethylene terephthalate copolymer with high transparency and excellent heat resistance.

[0026] 2. The present invention adopts a synergistic design of a two-way esterification reaction. In the first-stage esterification reaction, by introducing neopentyl glycol as a third monomer, the regularity of the PET molecular chain is destroyed, excessive crystallization is inhibited, and thus its transparency is improved. In the second-stage esterification reaction, a diester and a diol are subjected to a transesterification reaction to generate oligomers, providing active sites for the subsequent polycondensation reaction. Finally, by regulating the ratio of the end groups (-COOH / -OCH3) of the products of the two-way esterification reactions, the residual end carboxyl groups are reduced, the influence of side reactions on the color value is reduced, and the kinetic balance of the polycondensation reaction is achieved, ensuring the uniform growth of the molecular chain.

[0027] 3. The present invention adopts a multi-stage esterification process and a copolymerization modification technology. By regulating the products of the esterification reaction and the transesterification reaction, the occurrence of side reactions can be effectively reduced, the transparency of the polyethylene terephthalate copolymer is improved, the light transmittance ≥ 96%, the haze ≤ 2%, the product is prevented from being atomized or whitened during processing or use and losing transparency, and the transparency requirements for the forming and processing of transparent packaging products with a thickness ≥ 1 mm can be met. At the same time, the glass transition temperature of the polyethylene terephthalate copolymer can be increased, its heat resistance can be enhanced, the glass transition temperature > 82 °C, and the application requirements under high-temperature environments can be met.

[0028] 4. The present invention uses sorbitol nucleating agents, ester crystallization promoters, and carriers to prepare composite additives. Among them, the sorbitol nucleating agents form a hydrogen bond network through the rigid aromatic ring structure and hydroxyl groups in the molecule, forming uniform heterogeneous nucleation sites in the polymer melt, inducing the ordered arrangement of polymer molecular chains, and forming fine and uniform crystals; while the ester crystallization promoters achieve controllable crystallization under high-temperature processing through polar interactions and thermal stability advantages; the carriers form hydrogen bonds or van der Waals forces with the hydroxyl groups of the nucleating agents and the ester groups of the crystallization promoters through their polar groups, breaking the agglomeration of composite additive particles during ultrasonic dispersion to form a nanoscale dispersion liquid, ensuring the uniform distribution of the composite additives in subsequent high-temperature polycondensation. Finally, the sorbitol nucleating agents and the ester crystallization promoters act synergistically to form an efficient crystallization path of "nucleation first and then expansion", controlling the crystallization in the "semicrystalline state" and taking into account both light transmittance and heat resistance.

[0029] 5. During the polycondensation reaction process of the present invention, a stepped process control is adopted. That is, on the one hand, a stepped temperature increase control is adopted to promote chain growth and accelerate the removal of small molecules. On the other hand, a two-stage pressure control is adopted to avoid violent boiling and deeply remove volatiles. At the same time, combined with the control of the feed flow rate, it is ensured that the residence time distribution of the materials is uniform, which can effectively inhibit the occurrence of side reactions, thereby improving the stability of the intrinsic viscosity and being suitable for industrial production. Detailed implementation mode

[0030] In order to enable those skilled in the art to better understand the technical solutions in the present invention, the following will clearly and completely describe the technical solutions of the present invention in conjunction with the embodiments of the present invention.

[0031] Example 1

[0032] The preparation method of the polyethylene terephthalate copolymer in this example includes the following steps:

[0033] S1 First-stage esterification reaction: Mix terephthalic acid, ethylene glycol, and neopentyl glycol in a molar ratio of 1:1.15:0.15, and react under the conditions of 210°C and 0.15 MPa for 2.5 h. After the water output reaches the theoretical value, obtain the first reactant system;

[0034] S2 Second-stage esterification reaction: Mix dimethyl terephthalate and ethylene glycol in a molar ratio of 1:1.15, and react under the conditions of 180°C and 0.15 MPa for 2 h. Stop the reaction when the viscosity of the reaction system reaches 150 mPa·s to obtain the second reactant system;

[0035] Preparation of S3 composite additive: Mix poly(ethylene glycol) dibenzoate and sorbitol in a mass ratio of 1:1, and add the mixture to ethylene glycol for stirring and ultrasonic treatment. Among them, the total mass of poly(ethylene glycol) dibenzoate and sorbitol is in a mass ratio of 1:10 to the mass of ethylene glycol. The first stirring speed is 50 rpm, and the stirring time is 10 min. The second stirring speed is 200 rpm, and the stirring time is 20 min. The ultrasonic frequency is 20 kHz, and the dispersion time is 10 min to obtain the composite additive.

[0036] S4 Mixing and polycondensation: Pump the first reactant system and the second reactant system with a mass ratio of 1:0.5 into the polycondensation reactor through a metering pump for mixing. After preheating to 200 °C, add 0.15% of the composite additive and 100 ppm of ethylene glycol antimony based on the total mass of the reaction system, and carry out catalytic polycondensation reaction for 6 h. Among them, in the first 2 h of the catalytic polycondensation reaction, the temperature is raised to 250 °C at a rate of 25 °C / h, and in the next 4 h, the temperature is raised to 270 °C at a rate of 5 °C / h. And control the pressure in the first 2 h of the catalytic polycondensation reaction to be maintained at 500 Pa (gauge pressure), and then reduce the pressure to 40 Pa (absolute pressure). Control the metering pump to make the feeding flow rate of the first reactant system 85 kg / h and the feeding flow rate of the second reactant system 42.5 kg / h.

[0037] S5 Discharging: When the intrinsic viscosity of the polycondensation product is 0.75 dl / g, melt-extrude it through a twin-screw extruder and cut it underwater to obtain polyethylene terephthalate copolymer.

[0038] Example 2

[0039] The preparation method of the polyethylene terephthalate copolymer in this example includes the following steps:

[0040] S1 First-stage esterification reaction: Mix terephthalic acid, ethylene glycol and neopentyl glycol in a molar ratio of 1:1.3:0.3, and react at 230 °C and 0.25 MPa for 2 h. After the water output reaches the theoretical value, obtain the first reactant system.

[0041] S2 Second-stage esterification reaction: Mix dimethyl terephthalate and ethylene glycol in a molar ratio of 1:1.18, and react at 200 °C and 0.22 MPa for 2 h. Stop the reaction when the viscosity of the reaction system reaches 200 mPa·s to obtain the second reactant system.

[0042] S3 Preparation of composite additive: Mix triphenyl phosphate and dibenzylidene sorbitol in a mass ratio of 1:1.5, and add the mixture to polyethylene terephthalate (M wStirring and sonication are carried out in (with a molecular weight of 1000 g / mol); among them, the total mass ratio of triphenyl phosphate and dibenzylidene sorbitol to the mass of the polyester is 1:12, the rotation speed of the first stirring is 80 rpm, and the stirring time is 12 min; the rotation speed of the second stirring is 250 rpm, and the stirring time is 25 min; the ultrasonic frequency is 30 kHz, and the dispersion time is 12 min to obtain the composite additive;

[0043] S4 Mixing and polycondensation: The mass ratio of the first reactant system to the second reactant system of 1:0.1 is pumped into the polycondensation reactor by a metering pump for mixing. After preheating to 210 °C, 1.5% of the composite additive and 300 ppm of tetrabutyl titanate based on the total mass of the reaction system are added, and catalytic polycondensation reaction is carried out for 5 h; among them, in the first 1.5 h of the catalytic polycondensation reaction, the temperature is raised to 255 °C at a rate of 30 °C / h, and in the subsequent 3.5 h, the temperature is raised to 283 °C at a rate of 8 °C / h; and the pressure in the first 1.5 h of the catalytic polycondensation reaction is maintained at 350 Pa (gauge pressure), and then the pressure is reduced to 30 Pa (absolute pressure); the metering pump is controlled to make the feeding flow rate of the first reactant system 90 kg / h and the feeding flow rate of the second reactant system 9 kg / h;

[0044] S5 Discharging: When the intrinsic viscosity of the polycondensation product is 0.78 dl / g, it is melt-extruded by a twin-screw extruder and underwater pelletized to obtain the polyethylene terephthalate copolymer.

[0045] Example 3

[0046] The preparation method of the polyethylene terephthalate copolymer in this example includes the following steps:

[0047] S1 First-stage esterification reaction: Terephthalic acid, ethylene glycol and neopentyl glycol are mixed in a molar ratio of 1:1.4:0.45 and reacted at 240 °C and 0.35 MPa for 1.5 h. After the water output reaches the theoretical value, the first reactant system is obtained;

[0048] S2 Second-stage esterification reaction: Dimethyl terephthalate and ethylene glycol are mixed in a molar ratio of 1:1.2 and reacted at 220 °C and 0.3 MPa for 3 h. When the viscosity of the reaction system reaches 250 mPa·s, the reaction is stopped to obtain the second reactant system;

[0049] S3 Preparation of composite additive: Phthalate and dimethyl dibenzylidene sorbitol are mixed in a mass ratio of 1:2.5. After mixing, polybutylene terephthalate (M wStirring and sonication are carried out in (with a molecular weight of 2000 g / mol); wherein, the total mass ratio of phthalate and dimethyl dibenzylidene sorbitol to the mass of polyester is 1:15, the first stirring speed is 100 rpm, and the stirring time is 15 min; the second stirring speed is 300 rpm, and the stirring time is 30 min; the ultrasonic frequency is 40 kHz, and the dispersion time is 15 min to obtain a composite additive;

[0050] S4 Mixing and polycondensation: The mass ratio of the first reactant system to the second reactant system of 1:0.17 is pumped into the polycondensation reactor through a metering pump for mixing. After preheating to 220 °C, 2.5% of the composite additive and 500 ppm of germanium dioxide based on the total mass of the reaction system are added, and catalytic polycondensation reaction is carried out for 3 h; wherein, in the first 1 h of the catalytic polycondensation reaction, the temperature is raised to 240 °C at a rate of 20 °C / h, and in the subsequent 2 h, the temperature is raised to 260 °C at a rate of 10 °C / h; and the pressure in the first 1 h of the catalytic polycondensation reaction is maintained at 200 Pa (gauge pressure), and then the pressure is reduced to 20 Pa (absolute pressure); the metering pump is controlled to make the feeding flow rate of the first reactant system 86 kg / h and the feeding flow rate of the second reactant system 15 kg / h;

[0051] S5 Discharging: When the intrinsic viscosity of the polycondensation product is 0.76 dl / g, it is melt-extruded through a twin-screw extruder and underwater pelletized to obtain a polyethylene terephthalate copolymer.

[0052] Example 4

[0053] The preparation method of the polyethylene terephthalate copolymer in this example includes the following steps:

[0054] S1 First-stage esterification reaction: Terephthalic acid, ethylene glycol and neopentyl glycol are mixed in a molar ratio of 1:1.25:0.35 and reacted at 225 °C and 0.3 MPa for 1.5 h. After the water output reaches the theoretical value, the first reactant system is obtained;

[0055] S2 Second-stage esterification reaction: Dimethyl terephthalate and ethylene glycol are mixed in a molar ratio of 1:1.16 and reacted at 210 °C and 0.2 MPa for 2.5 h. When the viscosity of the reaction system reaches 220 mPa·s, the reaction is stopped to obtain the second reactant system;

[0056] S3 Preparation of composite additive: The amide ester and sorbitol are mixed in a mass ratio of 1:2. After mixing, they are added to ethylene glycol for stirring and sonication; wherein, the total mass ratio of the amide ester and sorbitol to the mass of ethylene glycol is 1:13, the first stirring speed is 70 rpm, and the stirring time is 13 min; the second stirring speed is 270 rpm, and the stirring time is 22 min; the ultrasonic frequency is 35 kHz, and the dispersion time is 13 min to obtain a composite additive;

[0057] S4 Mixing and Polycondensation: The mass ratio of the first reactant system to the second reactant system is 1:0.29, which is pumped into the polycondensation reactor through a metering pump for mixing. After preheating to 200 °C, a composite additive accounting for 1% of the total mass of the reaction system and 250 ppm of antimony trioxide are added, and a catalytic polycondensation reaction is carried out for 4.5 h. Among them, in the first 2 h of the catalytic polycondensation reaction, the temperature is raised to 240 °C at a rate of 20 °C / h, and in the subsequent 2.5 h, the temperature is raised to 265 °C at a rate of 10 °C / h. And the pressure in the first 2 h of the catalytic polycondensation reaction is maintained at 300 Pa (gauge pressure), and then the pressure is reduced to 35 Pa (absolute pressure). The feeding flow rate of the first reactant system is controlled by the metering pump to be 87 kg / h, and the feeding flow rate of the second reactant system is 25 kg / h.

[0058] S5 Discharging: When the intrinsic viscosity of the polycondensation product reaches 0.77 dl / g, it is melt-extruded through a twin-screw extruder and underwater pelletized to obtain a polyethylene terephthalate copolymer.

[0059] Example 5

[0060] The preparation method of the polyethylene terephthalate copolymer in this example includes the following steps:

[0061] S1 First-stage Esterification Reaction: Terephthalic acid, ethylene glycol and neopentyl glycol are mixed in a molar ratio of 1:1.2:0.25 and reacted at 235 °C and 0.2 MPa for 1.8 h. After the water output reaches the theoretical value, the first reactant system is obtained.

[0062] S2 Second-stage Esterification Reaction: Dimethyl terephthalate and ethylene glycol are mixed in a molar ratio of 1:1.17 and reacted at 190 °C and 0.25 MPa for 2.2 h. When the viscosity of the reaction system reaches 180 mPa·s, the reaction is stopped to obtain the second reactant system.

[0063] S3 Preparation of Composite Additive: Imidoester and dimethyl dibenzylidene sorbitol are mixed in a mass ratio of 1:1.8, and after mixing, they are added to poly(propylene terephthalate) (M w = 1500 g / mol) for stirring and ultrasonic treatment. Among them, the total mass of imidoester and dimethyl dibenzylidene sorbitol is in a mass ratio of 1:14 to the mass of ethylene glycol. The first stirring speed is 60 rpm and the stirring time is 14 min. The second stirring speed is 230 rpm and the stirring time is 28 min. The ultrasonic frequency is 25 kHz and the dispersion time is 14 min to obtain the composite additive.

[0064] S4 Mixing and Polycondensation: The mass ratio of the first reactant system to the second reactant system is 1:0.4, which is pumped into the polycondensation reactor by a metering pump for mixing. After preheating to 220 °C, a composite additive accounting for 1.8% of the total mass of the reaction system and 350 ppm of tetra-isopropyl titanate are added, and catalytic polycondensation reaction is carried out for 5 h. Among them, in the first 2 h of the catalytic polycondensation reaction, the temperature is raised to 260 °C at a rate of 20 °C / h, and in the subsequent 3 h, the temperature is raised to 290 °C at a rate of 10 °C / h. And the pressure in the first 1 h of the catalytic polycondensation reaction is maintained at 250 Pa (gauge pressure), and then the pressure is reduced to 25 Pa (absolute pressure). The metering pump is controlled to make the feeding flow rate of the first reactant system be 88 kg / h and the feeding flow rate of the second reactant system be 35 kg / h.

[0065] S5 Discharging: When the intrinsic viscosity of the polycondensation product is 0.78 dl / g, it is melt-extruded by a twin-screw extruder and underwater pelletized to obtain a polyethylene terephthalate copolymer.

[0066] Comparative Example 1

[0067] The difference from Example 1 is that step S2 is not carried out, and in step S4, the second reactant system is not added to the polycondensation reactor.

[0068] Comparative Example 2

[0069] The difference from Example 1 is that step S3 is not carried out, and in step S4, the composite additive is not added to the polycondensation reactor.

[0070] Comparative Example 3

[0071] The difference from Example 1 is that in step S4, the catalytic polycondensation reaction is carried out at 270 °C and 40 Pa for 6 h throughout the process.

[0072] Comparative Example 4

[0073] The difference from Example 1 is that in step S1, neopentyl glycol is not added.

[0074] Comparative Example 5

[0075] The difference from Example 1 is that in step S2, the viscosity of the reaction system is 100 mPa·s.

[0076] Comparative Example 6

[0077] The difference from Example 1 is that in step S2, the viscosity of the reaction system is 300 mPa·s.

[0078] Comparative Example 7

[0079] The difference from Example 1 is that in step S3, the mass ratio of polyglycol dibenzoate to sorbitol is 1:0.8.

[0080] Comparative Example 8

[0081] The difference from Example 1 is that in step S3, the mass ratio of dipentaerythritol dibenzoate to sorbitol is 1:3.

[0082] Comparative Example 9

[0083] The difference from Example 1 is that in step S3, the nucleating agent is talcum powder and the crystallization accelerator is dioctyl phthalate.

[0084] Comparative Example 10

[0085] The difference from Example 1 is that in step S3, instead of adding the carrier ethylene glycol, the nucleating agent and the crystallization accelerator are directly added to the polycondensation reactor in step S4.

[0086] Comparative Example 11

[0087] The difference from Example 1 is that in step S4, the mass ratio of the first reactant system to the second reactant system is 1:0.05.

[0088] Comparative Example 12

[0089] The difference from Example 1 is that in step S4, the mass ratio of the first reactant system to the second reactant system is 1:0.7.

[0090] Comparative Example 13

[0091] The difference from Example 1 is that in step S, when the intrinsic viscosity of the polycondensate is 0.72 dl / g, it is melt extruded through a twin-screw extruder and underwater pelletized to obtain a polyethylene terephthalate copolymer.

[0092] Comparative Example 14

[0093] The difference from Example 1 is that in step S5, when the intrinsic viscosity of the polycondensate is 0.82 dl / g, it is melt extruded through a twin-screw extruder and underwater pelletized to obtain a polyethylene terephthalate copolymer.

[0094] Comparative Example 15

[0095] The difference from Example 1 is that in step S4, the metering pump is controlled to make the feeding flow rate of the first reactant system 70 kg / h.

[0096] Comparative Example 16

[0097] The difference from Example 1 is that in step S4, the metering pump is controlled to make the feeding flow rate of the first reactant system 95 kg / h.

[0098] Comparative Example 17

[0099] The difference from Example 1 is that in step S4, the metering pump is controlled to make the feeding flow rate of the second reactant system 4 kg / h.

[0100] Comparative Example 18

[0101] The difference from Example 1 is that in step S4, the metering pump is controlled to make the feeding flow rate of the second reactant system 50 kg / h.

[0102] The performance of the polyethylene terephthalate copolymers prepared in Examples 1-5 and Comparative Examples 1-18 was tested. The test methods are as follows:

[0103] Transmittance: Tested with reference to "ASTM D1003 Standard Test Method for Haze and Light Transmittance of Transparent Plastics". The larger the transmittance, the higher the transparency of the product.

[0104] Haze: Tested with reference to "ASTM D1003 Standard Test Method for Haze and Light Transmittance of Transparent Plastics". The smaller the haze, the higher the transparency of the product.

[0105] Glass transition temperature T g : Tested with reference to "ASTM E1356-21 Standard Test Method for Determining Glass Transition Temperature by Differential Scanning Calorimetry (DSC)". The larger the glass transition temperature, the better the heat resistance of the product.

[0106] Intrinsic viscosity: Tested with reference to "ASTM D4603 Standard Test Method for Determining the Intrinsic Viscosity of Poly(ethylene terephthalate) (PET) Using a Glass Capillary Viscometer". The larger the intrinsic viscosity, the better the mechanical properties and heat resistance of the product, but too high an intrinsic viscosity is not conducive to the processing and application of the material.

[0107] The test results are shown in Table 1-2:

[0108] Table 1 Performance test results of polyethylene terephthalate copolymers prepared in Examples 1-5

[0109] Test Items Example 1 Example 2 Example 3 Example 4 Example 5 Transmittance (%) 96.5 96.2 96 96.1 96.3 Haze (%) 1.8 1.6 2 1.65 1.83 <![CDATA[T g (℃)]]> 83.31 84.35 82.4 85.2 84.1 Intrinsic Viscosity (dl / g) 0.75 0.78 0.76 0.77 0.78

[0110] Table 2 Performance test results of polyethylene terephthalate copolymers prepared in Comparative Examples 1-18

[0111]

[0112] As can be seen from Table 1, the transmittance of the polyethylene terephthalate copolymers prepared in Examples 1-5 is ≥96%, the haze is ≤2%, Tg > 82°C, and the intrinsic viscosity is ≥0.75 dl / g.

[0113] As can be seen from Table 2, compared with Example 1, in Comparative Example 1, due to the absence of the two-way esterification reaction and the lack of flexible chain segments in the dimethyl terephthalate (DMT) esterification path, the regularity of the molecular chains of the finally prepared product decreased, and the distribution of the crystalline phase was uneven, resulting in a 7.2% decrease in light transmittance and an 8.31 °C decrease in Tg. In Comparative Example 2, the composite additive was not used, resulting in an increase in the crystal size of the finally prepared product, the haze increased to 4.1%, and the imperfect crystallization caused a 6.31 °C decrease in Tg. In Comparative Example 3, during the catalytic polycondensation reaction, the temperature and pressure were not controlled in stages, and the intense polycondensation process led to a broadening of the molecular weight distribution of the finally prepared product, the intrinsic viscosity decreased to 0.7 dl / g, the amount of thermal degradation products increased, and the light transmittance decreased to 87.6%. In Comparative Example 4, since neopentyl glycol, a comonomer, was not added in the first-way esterification reaction, the chain segment rigidity of the finally prepared product increased, Tg decreased by 6.51 °C, the intermolecular force weakened, and the haze increased to 4.8%. In Comparative Example 5, due to the too low viscosity of the transesterification reaction system in the second-way esterification reaction and insufficient molecular weight of the prepolymer, the intrinsic viscosity of the finally prepared product was only 0.71 dl / g, the reaction activity in the polycondensation stage decreased, and the light transmittance decreased by 3.1%. On the contrary, in Comparative Example 6, due to the too high viscosity of the transesterification reaction system in the second-way esterification reaction, mass transfer in the system was blocked, resulting in side reactions, and the haze of the finally prepared product increased to 6.8%, local overheating generated gel particles, and the light transmittance decreased to 88.9%. In Comparative Example 7, due to the too high content of the crystallization accelerator in the composite additive, the crystallization rate was too fast, forming large spherulites, resulting in the haze of the finally prepared product increasing to 4.5%, phase separation leading to a decrease in mechanical properties, and Tg decreasing by 6.81 °C. On the contrary, in Comparative Example 8, due to the too high content of the nucleating agent in the composite additive, the nucleation points were too dense, hindering the growth of the crystal region, and the haze of the finally prepared product increased to 4.3%. In Comparative Example 9, a small molecule plasticizer was used as the crystallization accelerator, which decomposed at the polycondensation temperature (>250 °C), and the used nucleating agent was an inorganic nucleating agent with poor dispersibility and easy agglomeration, resulting in an 8.5% decrease in the light transmittance of the product and a 0.7% increase in the haze. In Comparative Example 10, no carrier was used when preparing the composite additive, and the nucleating agent agglomerated after being directly added to the polycondensation reactor, resulting in uneven dispersion, affecting the generation of heterogeneous nucleation points for nucleation, and reducing the light transmittance of the product by 10% and increasing the haze to 3.2%.

[0114] Meanwhile, as can also be seen from Table 2, in Comparative Example 11, due to the too low content of the second reactant system during the catalytic polycondensation process, the lack of flexible chain segments led to an increase in brittleness, resulting in the Tg of the finally prepared product dropping to 74.8 °C, the molecular weight distribution becoming wider, and the haze rising to 5.8%. On the contrary, in Comparative Example 12, due to the too high content of the second reactant system during the catalytic polycondensation process, the regularity of the main chain was damaged, resulting in the light transmittance of the finally prepared product decreasing to 86.2%, the crystallinity decreasing significantly, and the haze rising to 7.5%. In Comparative Example 13, due to the too low intrinsic viscosity of the polycondensation product, the insufficient molecular chain length, the decrease in mechanical strength, and the high end group content leading to poor thermal stability, the Tg of the finally prepared product dropped to 79.1 °C. On the contrary, in Comparative Example 14, due to the too high intrinsic viscosity of the polycondensation product, the serious entanglement of molecular chains, the deterioration of processing performance, and the unstable melt flow, the haze of the finally prepared product rose to 8.1%. In Comparative Example 15, due to the too low flow rate of the first reactant system during the catalytic polycondensation reaction process, local overheating occurred, resulting in the light transmittance of the finally prepared product decreasing to 90.1%. On the contrary, in Comparative Example 16, due to the too high flow rate of the first reactant system during the catalytic polycondensation reaction process, the insufficient mixing of materials occurred, resulting in the light transmittance of the finally prepared product decreasing to 87.8%. In Comparative Example 17, due to the too slow flow rate of the second reactant system during the catalytic polycondensation reaction process, the balance of the two-phase structure was damaged, resulting in the Tg of the finally prepared product decreasing by 10.11 °C. On the contrary, in Comparative Example 18, due to the too fast flow rate of the second reactant system during the catalytic polycondensation reaction process, the balance of the two-phase structure was damaged, resulting in the Tg of the finally prepared product decreasing by 7.01 °C.

Claims

1. A method for preparing a polyethylene terephthalate copolymer, characterized in that, It includes the following steps: S1 First - stage esterification reaction: Mix a dibasic acid, a diol and neopentyl glycol and carry out an esterification reaction. Control the reaction temperature at 210 - 240 °C, the pressure at 0.15 - 0.35 MPa, and the reaction time at 1.5 - 2.5 h. After the water output reaches the theoretical value, obtain the first reactant system; S2 Second - stage esterification reaction: Mix a diester and a diol and carry out a transesterification reaction. Control the reaction temperature at 180 - 220 °C, the pressure at 0.15 - 0.3 MPa, and the reaction time at 1.5 - 3 h. When the viscosity of the reaction system reaches 150 - 250 mPa·s, obtain the second reactant system; S3 Preparation of composite additive: Add a crystallization promoter to a carrier, stir, then add a nucleating agent, stir and disperse ultrasonically to obtain a composite additive; wherein, the mass ratio of the crystallization promoter to the nucleating agent is 1:(1 - 2.5); S4 Mixing and polycondensation: Pump the first reactant system and the second reactant system into a polycondensation reactor through metering pumps for mixing. After pre - heating to 200 - 220 °C, add the composite additive and a catalyst, and carry out a catalytic polycondensation reaction for 3 - 6 h; wherein, the mass ratio of the first reactant system to the second reactant system is 1:(0.1 - 0.5); The catalytic polycondensation reaction adopts stepped temperature control. In the first 1 - 2 h, heat up at a rate of 20 - 30 °C / h to 240 - 260 °C, and then heat up at a rate of 5 - 10 °C / h to 260 - 290 °C; The catalytic polycondensation reaction adopts two - stage pressure control. The pressure in the first 1 - 2 h is maintained at 200 - 500 Pa, and then the pressure is reduced to ≤40 Pa; Control the metering pumps to make the feeding flow rate of the first reactant system 85 - 90 kg / h and the feeding flow rate of the second reactant system 9 - 42.5 kg / h; S5 Discharging: When the intrinsic viscosity of the polycondensation product is 0.75 - 0.78 dl / g, extrude it through a twin - screw extruder by melting, and cut it underwater to obtain a polyethylene terephthalate copolymer.

2. The preparation method of the polyethylene terephthalate copolymer according to claim 1, characterized in that, In step S1, the dibasic acid is terephthalic acid and the diol is ethylene glycol; in step S2, the diester is dimethyl terephthalate and the diol is ethylene glycol.

3. The method for preparing the polyethylene terephthalate copolymer according to claim 1, characterized in that, In step S1, the molar ratio of the dibasic acid, the diol and neopentyl glycol is 1:(1.15 - 1.4):(0.15 - 0.45).

4. The preparation method of the polyethylene terephthalate copolymer according to claim 1, characterized in that, In step S2, the molar ratio of the diester to the diol is 1:(1.15 - 1.2).

5. The method for preparing the polyethylene terephthalate copolymer according to claim 1, characterized in that, In step S3, the nucleating agent is sorbitol, dibenzylidene sorbitol or dimethyl dibenzylidene sorbitol; the crystallization accelerator is polyethylene glycol dibenzoate, triphenyl phosphate, phthalate, amide ester or imide ester; and the carrier is ethylene glycol or a polyester with M w = 1000 - 2000 g / mol.

6. The preparation method of the polyethylene terephthalate copolymer according to claim 1, characterized in that, In step S3, the mass ratio of the total mass of the nucleating agent and the crystallization promoter to the mass of the carrier is 1:(10 - 15).

7. The preparation method of the polyethylene terephthalate copolymer according to claim 1, characterized in that, In step S3, the rotation speed of the first stirring is 50 - 100 rpm, and the stirring time is 10 - 15 min; the rotation speed of the second stirring is 200 - 300 rpm, and the stirring time is 20 - 30 min; the ultrasonic frequency is 20 - 40 kHz, and the dispersion time is 10 - 15 min.

8. The method for preparing the polyethylene terephthalate copolymer according to claim 1, wherein In step S4, the mass of the composite additive accounts for 0.15 - 2.5% of the total mass of the first reactant system, the second reactant system, the composite additive and the catalyst.

9. The preparation method of the polyethylene terephthalate copolymer according to claim 1, characterized in that, In step S4, the catalyst is antimony glycolate, antimony trioxide, tetrabutyl titanate, tetraisopropyl titanate or germanium dioxide.

10. The method for preparing the polyethylene terephthalate copolymer according to claim 1, wherein In step S4, the mass of the catalyst accounts for 100 - 500 ppm of the total mass of the first reactant system, the second reactant system, the composite additive, and the catalyst.

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