Polyethylene naphthalate polymer as well as preparation method and application thereof

Abstract

The invention relates to the technical field of polymer preparation, in particular to a polyethylene naphthalate polymer and a preparation method and application thereof.The preparation method comprises the steps that nanocellulose is subjected to acid hydrolysis treatment and then subjected to ball milling, and then modified nanocellulose is obtained through silane coupling agent modification treatment; 2, 6-dimethyl naphthalate, ethylene glycol, a bio-based catalyst and the modified nano cellulose are added into a polymerization kettle, after uniform dispersion in a system, heating is performed for transesterification, then a heat stabilizer is added, temperature and pressure are controlled for pre-polycondensation, and the modified nano cellulose is obtained. And finally, regulating and controlling the temperature and the pressure, and carrying out final polycondensation to obtain the polyethylene naphthalate polymer stock solution. The bio-based catalyst comprises citric acid and gallic acid. The polymer is free of heavy metal pollution, low in cost, excellent in strength performance, high in thermal deformation temperature and good in light transmittance. The polyethylene naphthalate polymer is suitable for the fields of flexible electronic packaging, new energy battery diaphragms and the like.

Description

Technical Field

[0001] The invention relates to a polyethylene naphthalate polymer and a preparation method and application thereof, belonging to the technical field of polymer preparation. Background Art

[0002] Traditional polyethylene naphthalate (PEN) production and modification technologies have numerous insurmountable drawbacks. In terms of catalyst systems, traditional processes generally employ heavy metal catalysts such as antimony (e.g., Sb2O3) or titanium. While these catalysts are highly active, they can result in heavy metal residues exceeding 200 ppm in the product, failing to meet food contact material safety standards and creating a significant environmental burden during post-processing. In contrast, this application innovatively employs a bio-based catalyst system comprised of citric acid and gallic acid, completely avoiding the problem of heavy metal contamination. Its catalytic efficiency is also two times higher than that of a single biocatalyst, further shortening the reaction time.

[0003] Traditional methods for selecting reinforcing materials rely on inorganic nanoparticles such as silica and carbon nanotubes. While these fillers can improve some of the material's mechanical properties, their high surface energy and tendency to agglomerate require the addition of expensive surface modifiers to achieve uniform dispersion. This not only increases production costs but also significantly reduces the material's light transmittance (typically <85%).

[0004] From the perspective of the polymerization process, traditional PEN synthesis needs to be maintained in a high-temperature environment above 280°C. This not only leads to high energy consumption (more than 3000kWh of electricity per ton of product), but also triggers side reactions such as molecular chain breakage, causing the product yellowing index to exceed 10, seriously affecting the product appearance.

[0005] Patent application CN102731920A proposes a typical antimony-catalyzed PEN synthesis scheme. While this technology achieves a tensile strength of 110 MPa by controlling the reaction time to within 6 hours through optimizing the catalyst dosage (Sb2O3, 0.3 wt%), its fundamental drawback lies in the unresolved issue of residual toxicity. Test data show that PEN produced using this process contains as much as 220 ppm of antimony residue.

[0006] Patent application publication number US2015 / 0259456A1 improves the flexibility of PEN through copolymerization. Researchers introduced 10 mol% adipic acid as a comonomer. While this successfully increased the material's elongation at break to over 80%, this modification came at the expense of heat resistance—the heat deformation temperature dropped from 158°C to 120°C, rendering the material completely useless for high-temperature applications. More seriously, copolymerization disrupted the regularity of the molecular chains, resulting in a decrease in crystallinity and a 30% reduction in oxygen barrier properties.

[0007] The patent application with publication number JP2018123277A explored the feasibility of carbon nanotube-enhanced PEN. This technology uses carboxylated multi-walled carbon nanotubes (1 wt%) as filler, although it gives the material 10 -3 S / cm, but its defects are also obvious: first, carbon nanotubes are very easy to agglomerate. Even with the expensive ultrasonic dispersion process, the transmittance of the composite material still drops sharply to 78%; second, the high market price of carbon nanotubes increases the cost of the enhanced PEN, making it unsuitable for industrialization.

[0008] Patent application WO2019 / 185432A1 utilizes biocatalysts for PEN synthesis. While this patent addresses environmental concerns, the low catalytic activity of tannic acid necessitates raising the reaction temperature to 290°C for polycondensation. This not only increases energy consumption but also causes thermal degradation of the nanocellulose filler at such high temperatures.

[0009] Therefore, it is of great value to develop a polyethylene naphthalate polymer that has no heavy metal pollution, low cost, excellent strength performance, high heat deformation temperature and good light transmittance. Summary of the Invention

[0010] The present invention addresses the deficiencies in the prior art and provides a polyethylene naphthalate polymer, a preparation method thereof, and an application thereof. The polymer has no heavy metal pollution, is low in cost, has excellent strength performance, a high heat deformation temperature, and good light transmittance.

[0011] The technical solution of the present invention to solve the above technical problems is as follows: a method for preparing a polyethylene naphthalate polymer, the preparation method comprising: S1, subjecting the nanocellulose to acid hydrolysis treatment and then ball milling, and then modifying it with a silane coupling agent to obtain modified nanocellulose; S2, adding dimethyl 2,6-naphthalene dicarboxylate, ethylene glycol, a bio-based catalyst and modified nanocellulose into a polymerization kettle, and after being uniformly dispersed in the system, heating to carry out an ester exchange reaction, then adding a heat stabilizer and controlling the temperature and pressure to carry out a pre-polycondensation, and finally regulating the temperature and pressure to carry out a final polycondensation to obtain the polyethylene naphthalate polymer stock solution; then cooling and solidifying in a coagulation bath, granulating, and drying to obtain the polyethylene naphthalate polymer; The bio-based catalyst includes citric acid and gallic acid.

[0012] Furthermore, the nanocellulose is at least one of chitosan nanocellulose, seaweed-based nanocellulose, and lignin nanocellulose; The heat stabilizer is at least one of triphenyl phosphite and pentaerythritol ester.

[0013] Furthermore, the molar ratio of citric acid to gallic acid in the bio-based catalyst is (2-4):1; In step S2, the bio-based catalyst is dissolved in ethylene glycol and then added into the reaction system for dispersion and reaction.

[0014] Furthermore, the amount of the nanocellulose added is 1%-5% of the mass of the dimethyl 2,6-naphthalene dicarboxylate; The amount of the bio-based catalyst added is 0.1%-0.12% of the mass of the dimethyl 2,6-naphthalene dicarboxylate; The amount of the heat stabilizer added is 0.2%-0.5% of the mass of the dimethyl 2,6-naphthalene dicarboxylate.

[0015] Furthermore, in step S1, the nanocellulose is added to a sulfuric acid aqueous solution for acid hydrolysis, solid-liquid separation is performed to obtain an acid-hydrolyzed nanofiber bundle, which is then ball-milled and modified by adding a silane coupling agent; The mass concentration of sulfuric acid in the sulfuric acid aqueous solution is 40%-60%, and the acid hydrolysis treatment time is 30-60 minutes.

[0016] Furthermore, in step S2, the transesterification reaction temperature is 200-220° C., the reaction time is 2-3 h, and the transesterification reaction is carried out at normal pressure and under microwave conditions, with a microwave power of 300-500 W.

[0017] Furthermore, during the transesterification reaction, methanol in the system is evaporated, and when the amount of methanol distilled in the transesterification reaction stage reaches 85%-95% of the theoretical mass value, the system enters the pre-condensation process.

[0018] Furthermore, in step S2, the pre-polycondensation reaction temperature is 250-260°C, the reaction time is 1-2h, and the vacuum degree is ≤50Pa; In step S2, the final polycondensation reaction temperature is 230-240°C, and the reaction is carried out until the intrinsic viscosity reaches ≥0.8 dL / g and the vacuum degree reaches ≤50 Pa.

[0019] The invention also discloses a polyethylene naphthalate polymer, which is prepared according to the preparation method of the invention.

[0020] The invention also discloses an application of a polyethylene naphthalate polymer, wherein the polyethylene naphthalate polymer is used for preparing a PEN film.

[0021] The beneficial effects of the present invention are: In the preparation method of polyethylene naphthalate polymer described in the present invention, a citric acid / gallic acid composite bio-based catalyst is used to completely replace the traditional antimony-based catalyst, achieving zero heavy metal residue (detection limit <1ppm), and no toxic substances are discharged during the production process, solving the industry's long-standing environmental compliance problem. The preparation method described in the present invention uses a bio-based catalyst to eliminate heavy metal pollution at the source, and the product can be directly used in the field of food packaging. In addition, by using a citric acid / gallic acid composite catalytic system, the present invention reduces the optimal reaction temperature to the range of 240-260°C while maintaining biodegradability, thereby protecting the structural integrity of the nanocellulose and controlling the total reaction time to within 7 hours, effectively improving production efficiency and reducing energy consumption.

[0022] The preparation method described herein uses acid-hydrolyzed nanocellulose as the reinforcing phase. Its abundant surface hydroxyl groups form a hydrogen-bonding network with the PEN matrix, enabling uniform dispersion of nanofiber bundles without the need for any dispersant. Furthermore, the nanofiber structure forms a light-scattering-inhibiting network within the matrix, maintaining the composite material's transmittance above 90% while reducing filler costs by 40%. By in-situ reinforcing the nanocellulose to form a three-dimensional network, the material achieves an increase in elongation at break to 65% while maintaining a heat distortion temperature above 160°C, without altering the PEN molecular structure. This results in a material with a tensile strength of up to 142 MPa and a heat distortion temperature of 160°C while maintaining high transmittance (≥90%), surpassing existing modification solutions in overall performance.

[0023] The preparation method described in this invention combines a gradient temperature control process (200-260°C) with microwave-assisted technology to precisely control the reaction temperature field, reducing total energy consumption by over 20% while minimizing side reactions and keeping the yellowing index of the final product below 5. Furthermore, the entire preparation process shortens reaction time and reduces the cost of nanocellulose raw materials, offering significant advantages for industrial application.

[0024] In summary, in the preparation method of the polyethylene naphthalate polymer described in the present invention: at the molecular design level, the carboxyl group of the bio-based catalyst forms a coordination bond with the hydroxyl group of the nanocellulose. This unique interface binding mechanism improves both the catalytic efficiency and the enhancement effect; at the process level, the introduction of microwave-assisted technology reduces the activation energy of the polycondensation reaction, and with the appropriate temperature, the entire polymer preparation process is more controllable; at the application level, by precisely controlling the content of nanocellulose, the material properties can be optimized in a targeted manner. These breakthroughs give the polyethylene naphthalate polymer described in the present invention a broader industrialization prospect. The polyethylene naphthalate polymer has a tensile strength ≥120MPa, a heat deformation temperature ≥160°C, and an oxygen permeability <1 (cc•mil / m 2•day), light transmittance ≥90%, with better mechanical and barrier properties, suitable for flexible electronic packaging, new energy battery separators and other fields. DETAILED DESCRIPTION

[0025] The present invention can be implemented in many other ways than those described herein, and those skilled in the art can make similar improvements without violating the scope of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed.

[0026] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art to which the present invention belongs. The terms used are only for describing specific embodiments and are not intended to limit the present invention.

[0027] A method for preparing a polyethylene naphthalate polymer, comprising: S1, subjecting the nanocellulose to acid hydrolysis treatment and then ball milling, and then modifying it with a silane coupling agent to obtain modified nanocellulose; S2, adding dimethyl 2,6-naphthalene dicarboxylate (NDC), ethylene glycol (EG), a bio-based catalyst, and modified nanocellulose into a polymerization kettle, uniformly dispersing the NDC in the system, heating to carry out an ester exchange reaction, then adding a heat stabilizer and controlling the temperature and pressure to carry out a pre-polycondensation, and finally regulating the temperature and pressure to carry out a final polycondensation to obtain the polyethylene naphthalate polymer stock solution; then cooling and solidifying the NDC in a coagulation bath, granulating, and drying to obtain the polyethylene naphthalate polymer; The bio-based catalyst includes citric acid and gallic acid.

[0028] Specifically, the nanocellulose is at least one of chitosan nanocellulose (Nanjing Tianlu Nanotechnology Co., Ltd.), seaweed-based nanocellulose (Nanjing Tianlu Nanotechnology Co., Ltd.), and lignin nanocellulose (Nanjing Tianlu Nanotechnology Co., Ltd.); The heat stabilizer is at least one of triphenyl phosphite and pentaerythritol ester.

[0029] More specifically, the mass ratio of dimethyl 2,6-naphthalene dicarboxylate (NDC) to ethylene glycol (EG) in the embodiments of the present invention is 1:(1-2).

[0030] Specifically, the molar ratio of citric acid to gallic acid in the bio-based catalyst is (2-4):1; In step S2, the bio-based catalyst is dissolved in ethylene glycol to form a catalyst solution, which is then added to the reaction system for dispersion and reaction.

[0031] More specifically, in the embodiment of the present invention, the ratio of the bio-based catalyst to ethylene glycol in the catalyst solution is (1-1.2) g:500 mL.

[0032] More specifically, citric acid and gallic acid are dissolved in ethylene glycol and stirred at 60-80° C. until completely dissolved.

[0033] Specifically, the amount of the nanocellulose added is 1%-5% of the mass of the dimethyl 2,6-naphthalene dicarboxylate; The amount of the bio-based catalyst added is 0.1%-0.12% of the mass of the dimethyl 2,6-naphthalene dicarboxylate; The amount of the heat stabilizer added is 0.2%-0.5% of the mass of the dimethyl 2,6-naphthalene dicarboxylate.

[0034] Specifically, in step S1, nanocellulose is added to a sulfuric acid aqueous solution for acid hydrolysis, solid-liquid separation is performed to obtain acid-hydrolyzed nanofiber bundles, which are then ball-milled and modified by adding a silane coupling agent; The mass concentration of sulfuric acid in the sulfuric acid aqueous solution is 40%-60%, and the acid hydrolysis treatment time is 30-60 minutes.

[0035] More specifically, the ball milling conditions in the embodiment of the present invention are: ball diameter 5 mm, rotation speed 400 rpm, time 2-3 h. The ball milling method improves the dispersion of nanocellulose, reduces agglomeration, and improves the uniformity of the composite material.

[0036] More specifically, the silane coupling agent used in the embodiment of the present invention is KH-550, and the mass ratio of the silane coupling agent to the nanocellulose is 1:10.

[0037] More specifically, after adding dimethyl 2,6-naphthalene dicarboxylate (NDC), ethylene glycol (EG), a bio-based catalyst and modified nanocellulose into a polymerization kettle, the materials are dispersed by stirring or ultrasonication at 80°C for 30-40 minutes, and then the temperature is raised to carry out the transesterification reaction.

[0038] Specifically, in step S2, the transesterification reaction temperature is 200-220° C., the reaction time is 2-3 h, and the transesterification reaction is carried out at normal pressure and under microwave conditions, with a microwave power of 300-500 W.

[0039] Specifically, the methanol in the system is evaporated during the transesterification reaction, and when the amount of methanol distilled during the transesterification reaction reaches 85%-95% of the theoretical mass value, the system enters the pre-condensation process.

[0040] The transesterification reaction principle is as follows: ; Among them, NDC is dimethyl 2,6-naphthalene dicarboxylate and EG is ethylene glycol.

[0041] Specifically, in step S2, the pre-polycondensation reaction temperature is 250-260°C, the reaction time is 1-2h, and the vacuum degree is ≤50Pa; In step S2, the final polycondensation reaction temperature is 230-240°C, and the reaction is carried out until the intrinsic viscosity reaches ≥0.8 dL / g and the vacuum degree reaches ≤50 Pa.

[0042] More specifically, the polyethylene naphthalate polymer has an intrinsic viscosity of 0.8-1.0 dL / g and a yellowness index of ≤8.

[0043] More specifically, in the embodiment of the present invention, when the polyethylene naphthalate polymer is prepared and cooled and solidified, the coagulation bath is pure water at room temperature.

[0044] A polyethylene naphthalate polymer is prepared according to the preparation method of the present invention.

[0045] The invention discloses an application of a polyethylene naphthalate polymer, wherein the polyethylene naphthalate polymer is used for preparing a PEN film.

[0046] Example 1 S1. Modification of nanocellulose: 30 g of chitosan nanocellulose was added to a 60% sulfuric acid aqueous solution for acid hydrolysis for 30 min. After centrifugation, it was ball milled under the following conditions: ball diameter 5 mm, rotation speed 400 rpm, and ball milling time 2 h. Then, a silane coupling agent was added and mixed evenly to obtain modified nanocellulose.

[0047] S2. Preparation of polymer: 1000 g of dimethyl 2,6-naphthalene dicarboxylate (NDC) and 400 g of ethylene glycol (EG) were added to a polymerization reactor. A bio-based catalyst consisting of 0.8 g of citric acid and 0.4 g of gallic acid (dissolved in 500 ml of ethylene glycol solution) was added. The modified nanocellulose prepared in step S1 was added and ultrasonically dispersed at 80°C for 40 min.

[0048] The temperature was raised to 220°C for a transesterification reaction for 2.5 hours, assisted by a 400W intermittent microwave. The temperature was then raised to 260°C for pre-polycondensation, with the vacuum gradually reduced to 50 Pa. 2g of triphenyl phosphite was added and allowed to react for 1.5 hours. Finally, the temperature was lowered to 240°C for final polycondensation, maintaining a vacuum of ≤10 Pa until the reaction was complete, yielding a polyethylene naphthalate (PEN) polymer stock solution. This solution was then cooled and solidified in a coagulation bath, pelletized in a pelletizer, and dried to obtain PEN polymer chips for performance testing.

[0049] Example 2 S1. Modification of nanocellulose: 10 g of chitosan nanocellulose was added to a 40% sulfuric acid aqueous solution for acid hydrolysis for 60 min. After centrifugation, the mixture was ball milled under the following conditions: ball diameter 5 mm, rotation speed 400 rpm, and ball milling time 3 h. Then, a silane coupling agent was added and mixed evenly to obtain modified nanocellulose.

[0050] S2. Preparation of polymer: 1000 g of dimethyl 2,6-naphthalene dicarboxylate (NDC) and 400 g of ethylene glycol (EG) were added to a polymerization reactor. A bio-based catalyst consisting of 0.96 g of citric acid and 0.24 g of gallic acid (dissolved in 500 ml of ethylene glycol solution) was added. The modified nanocellulose prepared in step S1 was added and ultrasonically dispersed at 80°C for 40 min.

[0051] The temperature was raised to 220°C for a transesterification reaction for 2.5 hours, assisted by a 400W intermittent microwave. The temperature was then raised to 260°C for pre-polycondensation, with the vacuum gradually reduced to 50 Pa. 5g of pentaerythritol was added and allowed to react for 1.5 hours. Finally, the temperature was lowered to 235°C for final polycondensation, maintaining a vacuum of ≤10 Pa until the reaction was complete, yielding a polyethylene naphthalate (PEN) polymer stock solution. The solution was then cooled and solidified in a coagulation bath, pelletized in a pelletizer, and dried to obtain PEN polymer chips for performance testing.

[0052] Example 3 S1. Modification of nanocellulose: 50 g of lignin nanocellulose was added to a 50% sulfuric acid aqueous solution for acid hydrolysis for 50 min. After centrifugation, it was ball milled under the following conditions: ball diameter 5 mm, rotation speed 400 rpm, and ball milling time 2 h. Then, a silane coupling agent was added and mixed evenly to obtain modified nanocellulose.

[0053] S2. Preparation of polymer: 1000 g of dimethyl 2,6-naphthalene dicarboxylate (NDC) and 400 g of ethylene glycol (EG) were added to a polymerization reactor. A bio-based catalyst consisting of 0.9 g of citric acid and 0.3 g of gallic acid (dissolved in 500 ml of ethylene glycol solution) was added. The modified nanocellulose prepared in step S1 was added and ultrasonically dispersed at 80°C for 60 min.

[0054] The transesterification reaction was carried out at 220°C for 2 hours, assisted by a 400W intermittent microwave. The temperature was then raised to 260°C for pre-polycondensation. The vacuum level was gradually reduced to 50 Pa, and 3g of triphenyl phosphite and 2g of pentaerythritol were added for 1.5 hours. Finally, the temperature was lowered to 240°C for final polycondensation, maintaining a vacuum level of ≤10 Pa until the reaction was complete, yielding a polyethylene naphthalate (PEN) polymer stock solution. The solution was then cooled and solidified in a coagulation bath, pelletized in a pelletizer, and dried to obtain PEN polymer chips for performance testing.

[0055] Example 4 S1. Modification of nanocellulose: 20 g of seaweed-based nanocellulose was added to a 40% sulfuric acid aqueous solution for acid hydrolysis for 60 min. After centrifugation, the mixture was ball milled under the following conditions: ball diameter 5 mm, rotation speed 400 rpm, and ball milling time 3 h. Then, a silane coupling agent was added and mixed evenly to obtain modified nanocellulose.

[0056] S2. Preparation of polymer: 1000 g of dimethyl 2,6-naphthalene dicarboxylate (NDC) and 400 g of ethylene glycol (EG) were added to a polymerization reactor. A bio-based catalyst consisting of 0.8 g of citric acid and 0.4 g of gallic acid (dissolved in 500 ml of ethylene glycol solution) was added. The modified nanocellulose prepared in step S1 was added and ultrasonically dispersed at 80°C for 60 min.

[0057] The temperature was raised to 220°C for a transesterification reaction for 2 hours, assisted by a 400W intermittent microwave. The temperature was then raised to 260°C for pre-polycondensation. The vacuum level was gradually reduced to 50 Pa, and 4g of triphenyl phosphite was added for a further 2 hours. Finally, the temperature was lowered to 240°C for final polycondensation, maintaining a vacuum level of ≤10 Pa until the reaction was complete. This yielded a polyethylene naphthalate (PEN) polymer stock solution. The solution was then cooled and solidified in a coagulation bath, pelletized in a pelletizer, and dried to obtain PEN polymer chips for performance testing.

[0058] Example 5 S1. Modification of nanocellulose: 40 g of lignin nanocellulose was added to a 60% sulfuric acid aqueous solution for acid hydrolysis for 30 min. After centrifugation, it was ball milled under the following conditions: ball diameter 5 mm, rotation speed 400 rpm, and ball milling time 2 h. Then, a silane coupling agent was added and mixed evenly to obtain modified nanocellulose.

[0059] S2. Preparation of polymer: 1000 g of dimethyl 2,6-naphthalene dicarboxylate (NDC) and 400 g of ethylene glycol (EG) were added to a polymerization reactor. A bio-based catalyst consisting of 0.8 g of citric acid and 0.4 g of gallic acid (dissolved in 500 ml of ethylene glycol solution) was added. The modified nanocellulose prepared in step S1 was added and ultrasonically dispersed at 80°C for 40 min.

[0060] The temperature was raised to 220°C for a transesterification reaction for 2.5 hours, assisted by a 400W intermittent microwave. The temperature was then raised to 260°C for pre-polycondensation. The vacuum level was gradually reduced to 50 Pa, and 2g of triphenyl phosphite and 1g of pentaerythritol were added for 1.5 hours. Finally, the temperature was lowered to 230°C for final polycondensation, maintaining a vacuum level of ≤10 Pa until the reaction was complete, yielding a polyethylene naphthalate (PEN) polymer stock solution. The solution was then cooled and solidified in a coagulation bath, pelletized in a pelletizer, and dried to obtain PEN polymer chips for performance testing.

[0061] Example 6 S1. Modification of nanocellulose: 30 g of lignin nanocellulose was added to a 60% sulfuric acid aqueous solution for acid hydrolysis for 30 min. After centrifugation, the mixture was ball milled under the following conditions: ball diameter 5 mm, rotation speed 400 rpm, and ball milling time 2 h. Then, a silane coupling agent was added and mixed evenly to obtain modified nanocellulose.

[0062] S2. Preparation of polymer: 1000 g of dimethyl 2,6-naphthalene dicarboxylate (NDC) and 400 g of ethylene glycol (EG) were added to a polymerization reactor. A bio-based catalyst consisting of 0.9 g of citric acid and 0.3 g of gallic acid (dissolved in 500 ml of ethylene glycol solution) was added. The modified nanocellulose prepared in step S1 was added and ultrasonically dispersed at 80°C for 40 min.

[0063] The transesterification reaction was carried out at 220°C for 3 hours, assisted by a 300W intermittent microwave. The temperature was then raised to 260°C for pre-polycondensation. The vacuum level was gradually reduced to 50 Pa, and 2g of triphenyl phosphite and 2g of pentaerythritol were added for 1.5 hours. Finally, the temperature was lowered to 240°C for final polycondensation, maintaining a vacuum level of ≤10 Pa until the reaction was complete, yielding a polyethylene naphthalate (PEN) polymer stock solution. The solution was then cooled and solidified in a coagulation bath, pelletized in a pelletizer, and dried to obtain PEN polymer chips for performance testing.

[0064] Example 7 S1. Modification of nanocellulose: 20 g of seaweed-based nanocellulose was added to a 40% sulfuric acid aqueous solution for acid hydrolysis for 60 min. After centrifugation, the mixture was ball milled under the following conditions: ball diameter 5 mm, rotation speed 400 rpm, and ball milling time 2 h. Then, a silane coupling agent was added and mixed evenly to obtain modified nanocellulose.

[0065] S2. Preparation of polymer: 1000 g of dimethyl 2,6-naphthalene dicarboxylate (NDC) and 400 g of ethylene glycol (EG) were added to a polymerization reactor. A bio-based catalyst consisting of 0.96 g of citric acid and 0.24 g of gallic acid (dissolved in 500 ml of ethylene glycol solution) was added. The modified nanocellulose prepared in step S1 was added and ultrasonically dispersed at 80°C for 40 min.

[0066] The temperature was raised to 220°C for a transesterification reaction for 2.5 hours, assisted by a 400W intermittent microwave. The temperature was then raised to 260°C for pre-polycondensation, with the vacuum gradually reduced to 50 Pa. 3g of pentaerythritol was added and allowed to react for 1.5 hours. Finally, the temperature was lowered to 240°C for final polycondensation, maintaining a vacuum of ≤10 Pa until the reaction was complete, yielding a polyethylene naphthalate (PEN) polymer stock solution. This solution was then cooled and solidified in a coagulation bath, pelletized in a pelletizer, and dried to obtain PEN polymer chips for performance testing.

[0067] Example 8 S1. Modification of nanocellulose: 40 g of lignin nanocellulose was added to a 60% sulfuric acid aqueous solution for acid hydrolysis for 35 minutes. After centrifugation, it was ball milled under the following conditions: ball diameter 5 mm, rotation speed 400 rpm, and ball milling time 3 hours. Then, a silane coupling agent was added and mixed evenly to obtain modified nanocellulose.

[0068] S2. Preparation of polymer: 1000 g of dimethyl 2,6-naphthalene dicarboxylate (NDC) and 400 g of ethylene glycol (EG) were added to a polymerization reactor. A bio-based catalyst consisting of 0.8 g of citric acid and 0.2 g of gallic acid (dissolved in 500 ml of ethylene glycol solution) was added. The modified nanocellulose prepared in step S1 was added and ultrasonically dispersed at 80°C for 40 min.

[0069] The temperature was raised to 200°C for a transesterification reaction for 3 hours, assisted by a 500W intermittent microwave. The temperature was then raised to 250°C for pre-polycondensation. The vacuum level was gradually reduced to 50 Pa, and 1g of triphenyl phosphite and 2g of pentaerythritol were added for a further 2 hours. Finally, the temperature was lowered to 230°C for final polycondensation, maintaining a vacuum level of ≤10 Pa until the reaction was complete. This yielded a polyethylene naphthalate (PEN) polymer stock solution. The solution was then cooled and solidified in a coagulation bath, pelletized in a pelletizer, and dried to obtain PEN polymer chips for performance testing.

[0070] Comparative Example 1 Conventional antimony trioxide is used as a catalyst to prepare polyethylene naphthalate polymer. The specific preparation process is as follows: A polymerization reactor was charged with 1000g of dimethyl 2,6-naphthalene dicarboxylate and 400g of ethylene glycol. 0.3g of antimony trioxide (0.3wt%) was added as a catalyst (dissolved in 500ml of ethylene glycol solution). No nanocellulose or bio-based catalyst was added. The reaction was heated to 220°C for 2.5 hours for transesterification. The temperature was then raised to 275°C for pre-polycondensation. The vacuum level was gradually reduced to 50 Pa, and 3g of triphenyl phosphite was added for 1.5 hours. Finally, the temperature was lowered to 255°C for final polycondensation, maintaining a vacuum level of ≤10 Pa until the reaction was complete. This yielded a polyethylene naphthalate (PEN) polymer solution. The solution was then cooled and solidified in a coagulation bath, pelletized in a pelletizer, and dried to obtain PEN polymer chips for performance testing.

[0071] Comparative Example 2 The same method as in Example 1 was used to prepare polyethylene naphthalate polymer, except that nanocellulose was not added in this comparative example 2. The specific preparation process was as follows: 1000 g of dimethyl 2,6-naphthalene dicarboxylate (NDC) and 400 g of ethylene glycol (EG) were added to a polymerization kettle, and a bio-based catalyst consisting of 0.8 g of citric acid and 0.4 g of gallic acid (dissolved in 500 ml of ethylene glycol solution) was added, and ultrasonic dispersion was performed at 80°C for 40 minutes.

[0072] The temperature was raised to 220°C for a transesterification reaction for 2.5 hours, assisted by a 400W intermittent microwave. The temperature was then raised to 260°C for pre-polycondensation, with the vacuum gradually reduced to 50 Pa. 2g of triphenyl phosphite was added and allowed to react for 1.5 hours. Finally, the temperature was lowered to 240°C for final polycondensation, maintaining a vacuum of ≤10 Pa until the reaction was complete, yielding a polyethylene naphthalate (PEN) polymer stock solution. This solution was then cooled and solidified in a coagulation bath, pelletized in a pelletizer, and dried to obtain PEN polymer chips for performance testing.

[0073] Comparative Example 3 The polyethylene naphthalate polymer was prepared by the same method as in Example 1, except that the nanocellulose was enlarged in this comparative example 3, and the amount of the nanocellulose added in this comparative example 3 was 7% of the mass of the dimethyl 2,6-naphthalate. The specific preparation process was as follows: S1. Modification of nanocellulose: 70 g of chitosan nanocellulose was added to a 60% sulfuric acid aqueous solution for acid hydrolysis for 30 min. After centrifugation, it was ball milled under the following conditions: ball diameter 5 mm, rotation speed 400 rpm, and ball milling time 2 h. Then, a silane coupling agent was added and mixed evenly to obtain modified nanocellulose.

[0074] S2. Preparation of polymer: 1000 g of dimethyl 2,6-naphthalene dicarboxylate (NDC) and 400 g of ethylene glycol (EG) were added to a polymerization reactor. A bio-based catalyst consisting of 0.8 g of citric acid and 0.4 g of gallic acid (dissolved in 500 ml of ethylene glycol solution) was added. The modified nanocellulose prepared in step S1 was added and ultrasonically dispersed at 80°C for 60 min.

[0075] The temperature was raised to 220°C for a transesterification reaction for 2.5 hours, assisted by a 400W intermittent microwave. The temperature was then raised to 260°C for pre-polycondensation, with the vacuum gradually reduced to 50 Pa. 2g of triphenyl phosphite was added and allowed to react for 1.5 hours. Finally, the temperature was lowered to 240°C for final polycondensation, maintaining a vacuum of ≤10 Pa until the reaction was complete, yielding a polyethylene naphthalate (PEN) polymer stock solution. This solution was then cooled and solidified in a coagulation bath, pelletized in a pelletizer, and dried to obtain PEN polymer chips for performance testing.

[0076] Comparative Example 4 The same method as in Example 1 was used to prepare polyethylene naphthalate polymer, except that the concentration of the sulfuric acid aqueous solution during the acid hydrolysis treatment was increased in this comparative example 4. The specific preparation process is as follows: S1. Modification of nanocellulose: 30 g of chitosan nanocellulose was added to an 80% sulfuric acid aqueous solution for acid hydrolysis for 30 min. After centrifugation, the mixture was ball milled under the following conditions: ball diameter 5 mm, rotation speed 400 rpm, and ball milling time 2 h. Then, a silane coupling agent was added and mixed evenly to obtain modified nanocellulose.

[0077] S2. Preparation of polymer: 1000 g of dimethyl 2,6-naphthalene dicarboxylate (NDC) and 400 g of ethylene glycol (EG) were added to a polymerization reactor. A bio-based catalyst consisting of 0.8 g of citric acid and 0.4 g of gallic acid (dissolved in 500 ml of ethylene glycol solution) was added. The modified nanocellulose prepared in step S1 was added and ultrasonically dispersed at 80°C for 40 min.

[0078] The temperature was raised to 220°C for a transesterification reaction for 2.5 hours, assisted by a 400W intermittent microwave. The temperature was then raised to 260°C for pre-polycondensation, with the vacuum gradually reduced to 50 Pa. 2g of triphenyl phosphite was added and allowed to react for 1.5 hours. Finally, the temperature was lowered to 240°C for final polycondensation, maintaining a vacuum of ≤10 Pa until the reaction was complete, yielding a polyethylene naphthalate (PEN) polymer stock solution. This solution was then cooled and solidified in a coagulation bath, pelletized in a pelletizer, and dried to obtain PEN polymer chips for performance testing.

[0079] Comparative Example 5 The same method as in Example 1 was used to prepare polyethylene naphthalate polymer, except that the concentration of the sulfuric acid aqueous solution during the acid hydrolysis treatment was reduced in this comparative example 5. The specific preparation process is as follows: S1. Modification of nanocellulose: 30 g of chitosan nanocellulose was added to a 20% sulfuric acid aqueous solution for acid hydrolysis for 30 min. After centrifugation, it was ball milled under the following conditions: ball diameter 5 mm, rotation speed 400 rpm, and ball milling time 2 h. Then, a silane coupling agent was added and mixed evenly to obtain modified nanocellulose.

[0080] S2. Preparation of polymer: 1000 g of dimethyl 2,6-naphthalene dicarboxylate (NDC) and 400 g of ethylene glycol (EG) were added to a polymerization reactor. A bio-based catalyst consisting of 0.8 g of citric acid and 0.4 g of gallic acid (dissolved in 500 ml of ethylene glycol solution) was added. The modified nanocellulose prepared in step S1 was added and ultrasonically dispersed at 80°C for 40 min.

[0081] The temperature was raised to 220°C for a transesterification reaction for 2.5 hours, assisted by a 400W intermittent microwave. The temperature was then raised to 260°C for pre-polycondensation, with the vacuum gradually reduced to 50 Pa. 2g of triphenyl phosphite was added and allowed to react for 1.5 hours. Finally, the temperature was lowered to 240°C for final polycondensation, maintaining a vacuum of ≤10 Pa until the reaction was complete, yielding a polyethylene naphthalate (PEN) polymer stock solution. This solution was then cooled and solidified in a coagulation bath, pelletized in a pelletizer, and dried to obtain PEN polymer chips for performance testing.

[0082] Comparative Example 6 The same method as in Example 1 was used to prepare polyethylene naphthalate polymer, except that the bio-based catalyst in this comparative example 6 contained only citric acid. The specific preparation process was as follows: S1. Modification of nanocellulose: 30 g of chitosan nanocellulose was added to a 60% sulfuric acid aqueous solution for acid hydrolysis for 30 min. After centrifugation, it was ball milled under the following conditions: ball diameter 5 mm, rotation speed 400 rpm, and ball milling time 2 h. Then, a silane coupling agent was added and mixed evenly to obtain modified nanocellulose.

[0083] S2. Preparation of polymer: 1000 g of dimethyl 2,6-naphthalene dicarboxylate (NDC) and 400 g of ethylene glycol (EG) were added to a polymerization reactor, along with a bio-based catalyst consisting of 1.2 g of citric acid (dissolved in 500 ml of ethylene glycol solution) and the modified nanocellulose prepared in step S1. The mixture was ultrasonically dispersed at 80°C for 40 min.

[0084] The temperature was raised to 220°C for a transesterification reaction for 2.5 hours, assisted by a 400W intermittent microwave. The temperature was then raised to 260°C for pre-polycondensation, with the vacuum gradually reduced to 50 Pa. 2g of triphenyl phosphite was added and allowed to react for 1.5 hours. Finally, the temperature was lowered to 240°C for final polycondensation, maintaining a vacuum of ≤10 Pa until the reaction was complete, yielding a polyethylene naphthalate (PEN) polymer stock solution. This solution was then cooled and solidified in a coagulation bath, pelletized in a pelletizer, and dried to obtain PEN polymer chips for performance testing.

[0085] Comparative Example 7 The same method as in Example 1 was used to prepare polyethylene naphthalate polymer, except that in Comparative Example 7, only gallic acid was used in the bio-based catalyst. The specific preparation process was as follows: S1. Modification of nanocellulose: 30 g of chitosan nanocellulose was added to a 60% sulfuric acid aqueous solution for acid hydrolysis for 30 min. After centrifugation, it was ball milled under the following conditions: ball diameter 5 mm, rotation speed 400 rpm, and ball milling time 2 h. Then, a silane coupling agent was added and mixed evenly to obtain modified nanocellulose.

[0086] S2. Preparation of polymer: 1000 g of dimethyl 2,6-naphthalene dicarboxylate (NDC) and 400 g of ethylene glycol (EG) were added to a polymerization reactor, along with 1.2 g of a bio-based catalyst composed of gallic acid (dissolved in 500 ml of ethylene glycol solution) and the modified nanocellulose prepared in step S1. The mixture was ultrasonically dispersed at 80°C for 40 min.

[0087] The temperature was raised to 220°C for a transesterification reaction for 2.5 hours, assisted by a 400W intermittent microwave. The temperature was then raised to 260°C for pre-polycondensation, with the vacuum gradually reduced to 50 Pa. 2g of triphenyl phosphite was added and allowed to react for 1.5 hours. Finally, the temperature was lowered to 240°C for final polycondensation, maintaining a vacuum of ≤10 Pa until the reaction was complete, yielding a polyethylene naphthalate (PEN) polymer stock solution. This solution was then cooled and solidified in a coagulation bath, pelletized in a pelletizer, and dried to obtain PEN polymer chips for performance testing.

[0088] Comparative Example 8 The same method as in Example 1 was used to prepare polyethylene naphthalate polymer, except that the mass ratio of citric acid to gallic acid in the bio-based catalyst of Comparative Example 8 was 1:1 (not within the ratio range defined in the present invention). The specific preparation process was as follows: S1. Modification of nanocellulose: 30 g of chitosan nanocellulose was added to a 60% sulfuric acid aqueous solution for acid hydrolysis for 30 min. After centrifugation, it was ball milled under the following conditions: ball diameter 5 mm, rotation speed 400 rpm, and ball milling time 2 h. Then, a silane coupling agent was added and mixed evenly to obtain modified nanocellulose.

[0089] S2. Preparation of polymer: 1000 g of dimethyl 2,6-naphthalene dicarboxylate (NDC) and 400 g of ethylene glycol (EG) were added to a polymerization reactor. A bio-based catalyst consisting of 0.6 g of citric acid and 0.6 g of gallic acid (dissolved in 500 ml of ethylene glycol solution) was added. The modified nanocellulose prepared in step S1 was added and ultrasonically dispersed at 80°C for 40 min.

[0090] The temperature was raised to 220°C for a transesterification reaction for 2.5 hours, assisted by a 400W intermittent microwave. The temperature was then raised to 260°C for pre-polycondensation, with the vacuum gradually reduced to 50 Pa. 2g of triphenyl phosphite was added and allowed to react for 1.5 hours. Finally, the temperature was lowered to 240°C for final polycondensation, maintaining a vacuum of ≤10 Pa until the reaction was complete, yielding a polyethylene naphthalate (PEN) polymer stock solution. This solution was then cooled and solidified in a coagulation bath, pelletized in a pelletizer, and dried to obtain PEN polymer chips for performance testing.

[0091] Comparative Example 9 The same method as in Example 1 was used to prepare polyethylene naphthalate polymer, except that the temperature of the final polycondensation was increased in this comparative example 9. The specific preparation process was as follows: S1. Modification of nanocellulose: 30 g of chitosan nanocellulose was added to a 60% sulfuric acid aqueous solution for acid hydrolysis for 30 min. After centrifugation, it was ball milled under the following conditions: ball diameter 5 mm, rotation speed 400 rpm, and ball milling time 2 h. Then, a silane coupling agent was added and mixed evenly to obtain modified nanocellulose.

[0092] S2. Preparation of polymer: 1000 g of dimethyl 2,6-naphthalene dicarboxylate (NDC) and 400 g of ethylene glycol (EG) were added to a polymerization reactor. A bio-based catalyst consisting of 0.8 g of citric acid and 0.4 g of gallic acid (dissolved in 500 ml of ethylene glycol solution) was added. The modified nanocellulose prepared in step S1 was added and ultrasonically dispersed at 80°C for 40 min.

[0093] The temperature was raised to 220°C for a transesterification reaction for 2.5 hours, assisted by a 400W intermittent microwave. The temperature was then raised to 260°C for pre-polycondensation, with the vacuum gradually reduced to 50 Pa. 2g of triphenyl phosphite was added and allowed to react for 1.5 hours. Finally, the temperature was lowered to 270°C for final polycondensation, maintaining a vacuum of ≤10 Pa until the reaction was complete, yielding a polyethylene naphthalate (PEN) polymer stock solution. This solution was then cooled and solidified in a coagulation bath, pelletized in a pelletizer, and dried to obtain PEN polymer chips for performance testing.

[0094] The polyethylene naphthalate polymer (PEN) slices prepared in the above examples and comparative examples (the PEN slices used in the performance test were square particles with a side length of 2 mm) were subjected to performance tests. The specific test results are shown in Table 1 below. The test methods involved are as follows: Tensile strength: universal material testing machine Instron 5967, ASTM D638.

[0095] Heat Deflection Temperature: Ceast HDT-3 VICAT, ASTM D648.

[0096] Transmittance: UV-visible spectrophotometer PerkinElmer Lambda 950, ASTM D1003.

[0097] Oxygen transmission rate: MOCON OX-TRAN 2 / 21 coulometric sensor method, ASTM D3985.

[0098] Table 1 Performance test results

[0099] The data in the table above demonstrate that, in Examples 1-8, the carboxyl groups of the bio-based catalysts form coordination bonds with the hydroxyl groups of the nanocellulose. This unique interfacial bonding mechanism simultaneously improves catalytic efficiency and enhances the performance. It also reduces reaction temperature and energy consumption, and by adjusting the amount of nanocellulose added, it allows for targeted optimization of material properties. The resulting polyethylene naphthalate polymer exhibits a tensile strength of ≥120 MPa, a heat distortion temperature of ≥160°C, an oxygen transmission rate of <1 (cc·mil / m²·day), and a light transmittance of ≥90%.

[0100] From the comparison of the experimental results of Comparative Example 1 and Example 1, it can be seen that the performance of the polyethylene naphthalate polymer prepared by the preparation method of the present invention is significantly better than that of the polyethylene naphthalate polymer prepared using conventional antimony trioxide as a catalyst.

[0101] From the comparison of the experimental results of Comparative Example 2 and Example 1, it can be seen that if nanocellulose is not added during the preparation of the polyethylene naphthalate polymer, the hydrogen bond network cannot be formed due to the lack of nanocellulose, resulting in a decrease in tensile strength and heat deformation temperature. At the same time, due to the lack of scattering inhibition, the transmittance of the network is slightly reduced and the oxygen barrier property is slightly poor.

[0102] Comparing the experimental results of Comparative Example 3 and Example 1, it can be seen that if too much nanocellulose is added during the preparation of polyethylene naphthalate polymer, the nanocellulose will not be evenly dispersed in the polymer, ultimately resulting in a significant decrease in the strength properties of the polymer. Therefore, using the nanocellulose dosage specified in the present invention is more conducive to obtaining a polyethylene naphthalate polymer product with excellent performance.

[0103] From the comparison of the experimental results of Comparative Example 4 and Example 1, it can be seen that when modifying nanocellulose, increasing the concentration of the sulfuric acid aqueous solution during the acid hydrolysis treatment will lead to excessive hydrolysis reaction that destroys the cellulose structure and slightly reduces the strength of the product, but the thermal stability can still be retained.

[0104] From the comparison of the experimental results of Comparative Example 5 and Example 1, it can be seen that when modifying nanocellulose, reducing the concentration of the sulfuric acid aqueous solution during the acid hydrolysis treatment will lead to insufficient acid hydrolysis treatment of the nanocellulose, resulting in poor dispersibility of the nanocellulose, and ultimately affecting the strength properties of the polyethylene naphthalate polymer.

[0105] From the comparison of the experimental results of Comparative Example 6 and Example 1, it can be seen that if only citric acid is included in the bio-based catalyst, the catalytic efficiency of the system will be low, the tensile strength and heat deformation temperature of the polymer will be greatly reduced, and the performance and application of the polymer will be seriously affected.

[0106] From the comparison of the experimental results of Comparative Example 7 and Example 1, it can be seen that if the bio-based catalyst contains only gallic acid, the system is prone to a small amount of degradation at this polymerization temperature due to the slightly higher catalytic activity of gallic acid, resulting in a slight decrease in the tensile strength of the polymer.

[0107] Comparing the experimental results of Comparative Example 8 and Example 1, it can be seen that if the ratio of gallic acid to citric acid in the bio-based catalyst is outside the specified range of the present invention, the catalytic effect and interfacial bonding are weakened, resulting in suboptimal polymer properties. Therefore, using the bio-based catalyst specified in the present invention is more conducive to obtaining polyethylene naphthalate polymer with excellent overall properties.

[0108] From the comparison of the experimental results of Comparative Example 9 and Example 1, it can be seen that if the final polycondensation temperature is increased, the high temperature causes partial degradation of the molecular chains, and the tensile strength and transmittance decrease. However, because the high temperature promotes crystallization, the heat deformation temperature increases slightly.

[0109] The technical features of the above-described embodiments can be combined arbitrarily. To make the description concise, not all possible combinations of the technical features in the above-described embodiments are exhaustively listed. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0110] For those skilled in the art, several variations and improvements may be made without departing from the scope of the present invention, which all fall within the scope of protection of the present invention. The scope of protection of the present invention shall be based on the appended claims.

Claims

1. A method for preparing a polyethylene naphthalate polymer, characterized in that: The preparation method is: S1, subjecting the nanocellulose to acid hydrolysis treatment and then ball milling, and then modifying it with a silane coupling agent to obtain modified nanocellulose; S2, adding dimethyl 2,6-naphthalene dicarboxylate, ethylene glycol, a bio-based catalyst and modified nanocellulose into a polymerization kettle, and after being uniformly dispersed in the system, heating to carry out an ester exchange reaction, then adding a heat stabilizer and controlling the temperature and pressure to carry out a pre-polycondensation, and finally regulating the temperature and pressure to carry out a final polycondensation to obtain a polyethylene naphthalate polymer stock solution; then cooling and solidifying in a coagulation bath, granulating, and drying to obtain the polyethylene naphthalate polymer; The bio-based catalyst includes citric acid and gallic acid.

2. The method for preparing a polyethylene naphthalate polymer according to claim 1, wherein: The nanocellulose is at least one of chitosan nanocellulose, seaweed-based nanocellulose, and lignin nanocellulose; The heat stabilizer is at least one of triphenyl phosphite and pentaerythritol ester.

3. The method for preparing a polyethylene naphthalate polymer according to claim 1, wherein: The molar ratio of citric acid to gallic acid in the bio-based catalyst is (2-4):1; In step S2, the bio-based catalyst is dissolved in ethylene glycol and then added into the reaction system for dispersion and reaction.

4. The method for preparing a polyethylene naphthalate polymer according to claim 1, wherein: The amount of the nanocellulose added is 1%-5% of the mass of the dimethyl 2,6-naphthalene dicarboxylate; The amount of the bio-based catalyst added is 0.1%-0.12% of the mass of the dimethyl 2,6-naphthalene dicarboxylate; The amount of the heat stabilizer added is 0.2%-0.5% of the mass of the dimethyl 2,6-naphthalene dicarboxylate.

5. The method for preparing a polyethylene naphthalate polymer according to claim 1, wherein: In step S1, nanocellulose is added to a sulfuric acid aqueous solution for acid hydrolysis, solid-liquid separation is performed to obtain acid-hydrolyzed nanofiber bundles, which are then ball-milled and modified by adding a silane coupling agent; The mass concentration of sulfuric acid in the sulfuric acid aqueous solution is 40%-60%, and the acid hydrolysis treatment time is 30-60 minutes.

6. The method for preparing a polyethylene naphthalate polymer according to claim 1, wherein: In step S2, the transesterification reaction temperature is 200-220° C., the reaction time is 2-3 h, and the transesterification reaction is carried out at normal pressure and under microwave conditions with a microwave power of 300-500 W.

7. The method for preparing a polyethylene naphthalate polymer according to claim 1, wherein: During the transesterification reaction, methanol in the system is evaporated. When the amount of methanol distilled in the transesterification reaction stage reaches 85%-95% of the theoretical mass value, the system enters the pre-condensation process.

8. The method for preparing a polyethylene naphthalate polymer according to claim 1, wherein: In step S2, the pre-polycondensation reaction temperature is 250-260° C., the reaction time is 1-2 hours, and the vacuum degree is ≤50 Pa; In step S2, the final polycondensation reaction temperature is 230-240°C, and the reaction is carried out until the intrinsic viscosity reaches ≥0.8 dL / g and the vacuum degree reaches ≤50 Pa.

9. A polyethylene naphthalate polymer, characterized in that The polyethylene naphthalate polymer is prepared according to the preparation method according to any one of claims 1 to 8.

10. A use of a polyethylene naphthalate polymer according to claim 9, characterized in that: The polyethylene naphthalate polymer is used to prepare PEN film.

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