High-strength PET optical film and preparation method thereof

Abstract

The invention relates to the field of preparation of optical polyester materials, in particular to a high-strength PET optical film and a preparation method thereof. The basic mechanical property of the film-forming base material is improved by blending the PET base material and the copolyester, the excellent mechanical property of the base material is utilized to carry out gradient two-way stretching with a large stretching ratio, and the comprehensive property of the prepared film material is improved by combining an intermittent relaxation process; and the stretched membrane material is treated through plasma surface treatment, so that the surface adhesive force is improved, the surface cleanliness is optimized, the surface scattering rate is reduced, and the comprehensive performance of the membrane material is further improved.

Description

Technical Field

[0001] The present invention relates to the field of preparation of optical polyester materials, and particularly relates to a high-strength PET optical film and a preparation method thereof. Background Art

[0002] Optical grade polyethylene terephthalate (PET) film has become a core material in the fields of display, photovoltaic and optical devices due to its high light transmittance, low haze and excellent mechanical properties. However, with the rapid development of high-end display technologies (such as mini-LED, flexible OLED) and new photovoltaic modules (such as bifacial power generation modules), the deficiencies of traditional PET optical films in mechanical properties, optical stability and high-temperature tolerance have become increasingly prominent, specifically manifested as the following technical bottlenecks: The tensile strength of existing PET optical films is generally lower than 150 MPa, and the elongation at break is less than 80%, making it difficult to meet the requirements of flexible display devices for repeated bending and dynamic stretching of materials. To improve the mechanical properties, two main types of solutions are mainly adopted in the prior art: Dispersing inorganic nanoparticles in the PET matrix to enhance rigidity, but this will lead to a decrease in light transmittance (≤92%) and an increase in haze (>1.2%). At the same time, due to poor interfacial compatibility, stress concentration is easily caused, and the elongation at break is further reduced to less than 60%. By toughening modification with elastomers, although the toughness can be improved (elongation at break >120%), the polarity difference between the elastomer and PET leads to phase separation, the tensile strength is significantly reduced to 100 - 120 MPa, and the elastomer thermal degradation during the processing is likely to generate crystal points, affecting the optical uniformity.

[0003] In addition, traditional PET optical films enhance the mechanical strength by increasing the crystallinity (>35%) in the biaxial stretching process, but the high crystallinity will form light-scattering crystal boundaries, resulting in a decrease in light transmittance (>5%) and an increase in haze (>1.5%). In addition, in the prior art, to suppress the warping deformation caused by the relaxation of molecular chains at high temperatures (>120°C), a high degree of orientation stretching process is often used, but excessive orientation will cause the material to become brittle, and the elongation at break drops to less than 50%.

[0004] Although prior arts such as chain extension modification (CN 118620254 B) or introducing a metal layer (CN 222119124 U) can improve the mechanical properties, they sacrifice the optical transparency; and single processes such as solid-phase polycondensation or plasma treatment are difficult to achieve breakthroughs in comprehensive performance.

[0005] In summary, the existing PET optical film technology has systematic defects such as the mutual restriction between mechanical properties and optical properties, the lack of synergy between surface treatment and substrate modification, and the difficulty in balancing high temperature resistance and processing stability. Therefore, there is an urgent need for an innovative solution that synergistically strengthens through molecular design, optimization of the blending process, control of stretching orientation, and surface treatment to achieve a breakthrough improvement in the comprehensive performance of PET optical films. Summary of the Invention

[0006] Based on the problems summarized above, the present invention provides a high-strength PET optical film and a preparation method thereof. The main feature is to improve the basic mechanical properties of the film-forming substrate by blending the PET substrate with a copolyester, and utilize the excellent mechanical properties of the substrate to obtain better comprehensive properties in the subsequent biaxial stretching step. In addition, the plasma surface treatment in the post-treatment further optimizes the film material. The specific technical solutions are as follows: A preparation method of a high-strength PET optical film, comprising the following steps: S1: After drying the PET substrate and the copolyester, mix them according to the mass ratio, add an interfacial compatibilizer, and perform segmented temperature-controlled melt extrusion after premixing. The extruded strip is cooled by a water-cooling tank and then pelletized and dried; the preparation method of the copolyester includes: Add terephthalic acid and ethylene glycol as monomers for feeding, and the feeding molar ratio of terephthalic acid to ethylene glycol is 1:1.3; Under the set reaction conditions, perform an esterification reaction on terephthalic acid, ethylene glycol, and 0.028 wt% of the catalyst antimony glycolate; After the esterification reaction is completed, add 0.9 wt% of furoic acid as a capping monomer to carry out a capping copolymerization reaction; After the capping copolymerization reaction is completed, discharge the material, quench, and pelletize; S2: Perform pre-crystallization treatment on the pellets after blending in S1, and then perform temperature-segmented controlled melt extrusion. The extruded melt is attached to a chill roll by an electrostatic adsorption device, and an amorphous thick sheet is formed after casting. Then, the thick sheet is biaxially stretched, heat-set after stretching, and finally cooled; S3: Cut off the uneven parts at the edges of the film sheet obtained in S2, and then perform plasma surface treatment. After treatment, wind up and age.

[0007] Furthermore, under the set reaction conditions in the copolyester preparation method, it specifically includes: The preheating temperature of the reaction kettle is set to 120 °C, the stirring time of terephthalic acid, ethylene glycol, and the catalyst antimony glycolate is set to 5 minutes, the oil bath temperature of the reaction kettle jacket is set to 270 °C, and the pressure in the esterification kettle rises to 120 - 150 kPa; the time point at the end of the esterification reaction is set when the mass of the by-product collected reaches 98% of the theoretical yield.

[0008] Furthermore, for the terminal copolymerization reaction, its start and end conditions are set as follows: the power of the stirring motor reaches 38 - 42 W.

[0009] Furthermore, the mixing by mass ratio in S1 is to mix the dried copolyester and the PET substrate in a mass ratio of 1:9 - 5:5. Furthermore, the interfacial compatibilizer in S1 is an epoxy chain extender added at 0.5 - 1.5 wt% of the total mass. Furthermore, the segmented temperature-controlled melt extrusion in S1 is specifically set as follows: feeding section: 230 - 240 °C, melting section: 240 - 250 °C, mixing section: 250 - 260 °C, die head section: 255 - 265 °C.

[0010] Furthermore, the temperature-segmented controlled melt extrusion in S2 has the temperature segments set as follows: feeding section: 240 - 250 °C, melting section: 260 - 270 °C, die head section: 275 - 280 °C.

[0011] Furthermore, the biaxial stretching in S2 includes longitudinal stretching, and the parameters of the longitudinal stretching are as follows: The segmented temperature is set as follows: preheating roller 80 °C, first-stage stretching temperature 90 °C, second-stage stretching temperature 110 °C. The stretching ratio is set as follows: total stretching ratio 3.5 - 4.5, where the first-stage stretching ratio is 1.5 - 2.0 and the second-stage stretching ratio is 2.0 - 2.5. The relaxation method is intermittent relaxation. It pauses for 5 seconds every 10% of stretching, and the internal stress is released through dynamic adjustment by the tension roller.

[0012] Furthermore, the biaxial stretching in S2 includes transverse stretching, and the parameters of the transverse stretching are as follows: The segmented temperature is set as follows: preheating section 90 °C, stretching section 110 °C, shaping section 120 °C. The stretching ratio is set as follows: total stretching ratio 4.0 - 5.0, where the initial stretching ratio is 1.5 - 2.0 and the final-stage stretching ratio is 2.5 - 3.0.

[0013] Furthermore, for the plasma surface treatment in S3, the specific parameter settings include: The treatment power is set to 150 W. The gas source used is an Ar / O 2 mixed gas source, and the gas ratio is Ar / O 2 volume ratio of 3:1. The treatment mode is set to be carried out in sequential stages of pulse mode / continuous mode.

[0014] A high-strength PET optical film is prepared by using any one of the above-mentioned preparation methods.

[0015] Compared with the prior art, the present invention has the following beneficial effects: (1) In the present invention, by blending a PET substrate with a furan dicarboxylic acid copolyester, the π-π stacking effect contained in the furan dicarboxylic acid copolyester and the hydrogen bonds between the segments formed after blending the PET substrate with the furan dicarboxylic acid copolyester are utilized to greatly enhance the comprehensive mechanical properties of the substrate formed after blending; (2) By utilizing the excellent comprehensive mechanical properties of the blended substrate, gradient biaxial stretching with a large draw ratio is carried out, and the comprehensive properties of the prepared film are improved by combining an intermittent relaxation process.

[0016] (3) In the present invention, the film after stretching is treated by plasma surface treatment, which improves the surface adhesion, optimizes the surface cleanliness, reduces the surface scattering rate, and further improves the comprehensive properties of the film. Description of the Drawings

[0017] Figure 1 is a flowchart of a preparation method of a high-strength PET optical film of the present invention; Figure 2 is the 1 1H NMR spectrum of the material after blending treatment of the present invention; Figure 3 is a line comparison chart of the mean variances of the comprehensive mechanical properties of the PET substrate, the copolyester, and the material after blending of the present invention; Figure 4 is a scatter comparison chart of the mean variances of the light transmittance of the PET substrate, the copolyester, and the material after blending of the present invention. Detailed Embodiments

[0018] The following embodiments further explain and illustrate the technical solutions of the present invention. It is particularly pointed out that each specific embodiment is a concretization and explanation of the technical solution, and should not be regarded as a limitation on the protection scope of the present invention. Those of ordinary skill in the art still have the right to modify the technical solutions of these embodiments and perform equivalent substitutions for some or all of the technical features, and these modifications or substitutions do not change the essence of the corresponding technical solutions and do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions described in the present invention.

[0019] The present invention provides a high-strength PET optical film and a preparation method thereof, as shown in the attached Figure 1 , and the specific preparation steps are as follows: 1. Construction of a PET / copolymer-modified polyester blending system: 1.1 Preparation of the copolymer-modified polyester blend (1) Add terephthalic acid and ethylene glycol as monomers for feeding, and the feeding molar ratio of terephthalic acid to ethylene glycol is 1:1.3.

[0020] (2) The esterification of terephthalic acid and ethylene glycol begins. The reaction kettle is preheated to 120 °C, and terephthalic acid, ethylene glycol, and 0.028 wt% of the catalyst antimony glycolate are stirred for 5 minutes. Keep the valves of the esterification and polycondensation towers open. Open the nitrogen valve to slowly release nitrogen until the pressure in the kettle rises to 120 - 150 kPa, then open the needle valve to release gas, repeating three times until all the oxygen in the kettle is exhausted. Close the polycondensation path valve, keep stirring all the time, and at the same time set the oil bath temperature of the reaction kettle jacket to 270 °C, and then adjust the pressure in the kettle to 120 - 150 kPa with nitrogen for esterification. As the temperature rises, the substances in the kettle vaporize, and the pressure in the kettle slowly increases. Keep the pressure around 350 kPa by exhausting gas through the needle valve. When by-products start to be discharged from the condenser connected to the top of the fractionating column, it indicates that the esterification has started, and the pressure in the kettle slowly decreases. When the collected mass of by-product water reaches 98% of the theoretical yield, the esterification reaction ends, and the temperature at the top of the column should be lower than 100 °C.

[0021] (3) After the esterification stage ends, close the esterification path valve and open the polycondensation path valve and the vacuum pump. Carry out the polycondensation reaction at an oil bath temperature of 270 °C and an absolute pressure below 50 Pa. When the power of the stirring motor reaches 38 - 42 W, close the vacuum pump. Add 0.9 wt% of furan carboxylic acid, the end-capping monomer, to the feeding hopper connected to the kettle body, and seal the feeding hopper and conduct five nitrogen replacements. Then boost the pressure in the feeding hopper to atmospheric pressure with nitrogen, open the valve connecting the hopper and the kettle body, close the valve after the material is sucked into the kettle, and stir for 15 minutes after the kettle body is sealed again. When the power of the stirring motor reaches 38 - 42 W again, close the vacuum pump and the stirring paddle, and let in nitrogen to normal pressure to prepare for discharging.

[0022] (4) Let in nitrogen into the kettle to normal pressure, open the discharge hole at the bottom of the reaction kettle to start discharging, and the melt flows into the prepared water tank to be quenched into amorphous fine strips; then granulate the strip-shaped polyester through a pelletizer.

[0023] In the above steps, the reaction process of terephthalic acid, ethylene glycol, and furan carboxylic acid is as follows: During the reaction-bonding process, both the end-capping effect after the reaction of furoic acid with the polyester and the residual unreacted small-molecule furoic acid have a plasticizing effect on the material. The combined action of end-group plasticization and direct small-molecule plasticization significantly increases the elongation at break of the material. Due to the presence of conjugated groups in the furoic acid unit, there is a π-π stacking effect. The π-π stacking occurs between the end-group furoic acid units and can achieve the equivalent effect of increasing the molecular weight through non-covalent interactions; although the plasticizing effect greatly improves the elongation at break and ductility of the material during stretching, it will reduce the overall rigidity and tensile strength of the material; at this time, through the π-π stacking effect, with its reversible breakage and recombination during stretching, it can maintain a certain tensile strength without sacrificing the elongation at break, compensating for the shortcoming of insufficient strength caused by the plasticizing effect. However, compared with the commercial PET material without copolymer modification, its tensile strength is still insufficient, and the performance between materials needs to be complemented through subsequent blending. Due to the addition of furoic acid, the color depth of the finally formed material increases for the copolymer polyester, which has some effects on the optical properties of the copolymer polyester.

[0024] 1.2 Construction of the blend system (1) Dry the PET substrate in a forced-air drying oven at 80 °C for 12 hours, and dry the copolymer polyester at 60 °C for 6 - 8 hours.

[0025] (2) Mix the dried copolymer polyester and PET substrate in a mass ratio of 1:9 - 5:5, and add 0.5 - 1.5 wt% of the epoxy-based chain extender based on the total mass to promote interfacial compatibility. Use a high-speed mixer to premix all the raw materials at a rotation speed of 300 - 500 rpm for 5 - 10 minutes to ensure uniform dispersion.

[0026] (3) Use a co-rotating twin-screw extruder (aspect ratio ≥ 40:1) for segmented temperature-controlled melt extrusion. The temperature segments are as follows: Feeding section: 230 - 240 °C Melting section: 240 - 250 °C Mixing section: 250 - 260 °C Die head section: 255 - 265 °C During this period, control the screw rotation speed to remain at 200 - 300 rpm to ensure sufficient shear force but avoid degradation; control the residence time of the material at 3 - 5 minutes.

[0027] (4) Cool the extruded strip in a water cooling tank, control the water temperature at 25 - 30 °C, cut into pellets after cooling, and finally dry at 80 °C for 4 hours.

[0028] In the above steps, by blending the copolymer polyester and the PET substrate, the mechanical properties and optical properties of the blended polyester are balanced, as shown in the appendix Figure 2 , which is the1 1H NMR spectrum. As can be seen from the figure, The aromatic protons of the benzene ring of the PET characteristic signal appear at 7.5 - 8.5 ppm (multiplet), with a sharp peak shape and a large integral area, indicating the integrity of the PET main chain structure; The methylene group (-OCH2CH2O-) of the ethylene glycol segment is located at 4.3 - 4.7 ppm (quartet), and the change in the peak shape reflects the mobility of the segment; The α-position protons of the furan ring in the characteristic signal of the furan carboxylic acid copolyester are at 7.0 - 7.5 ppm (multiplet), overlapping with the PET aromatic peak, indicating the occurrence of π-π stacking; the weak peak of the protons adjacent to the ester group appears at 8.0 - 8.3 ppm, and its peak position shifts when hydrogen bonding is formed with the PET ester group (~8.1 ppm); The peak of the ethylene glycol segment of PET in the spectrum (4.3 - 4.7 ppm) changes, indicating that the polar groups of the furan carboxylic acid copolyester form hydrogen bonds with the PET segment, enhancing the intermolecular force and improving the tensile strength; The furan ring proton peak (7.0 - 7.5 ppm) partially overlaps with the PET aromatic peak, indicating that physical crosslinking points are formed through π-π stacking, which can inhibit crack propagation, improve impact toughness, and increase the elongation at break of the material; The aromatic proton peak of PET (7.5 - 8.5 ppm) shifts slightly to a lower field, indicating that the strong electron-withdrawing group (ester group) of the furan carboxylic acid copolyester induces a decrease in the electron cloud density of the PET benzene ring, improving the compatibility between the two phases; with good compatibility between the two phases, the material has a homogeneous structure, can retain high light transmittance, and reduces the impact of the copolyester itself on the optical properties.

[0029] As shown in the appendix Figure 3 and appendix Figure 4 , through the comparison of the comprehensive mechanical properties and optical properties of the PET substrate, copolyester, and PET blend copolyester, combined with the above spectral analysis, the positive effects on the material after blending treatment can be confirmed. Among them, the mechanical properties include tensile strength and elongation at break, and the tests are carried out with reference to the national standard GB / T 1040-2006 "Determination of Tensile Properties of Plastics". The amorphous film after hot pressing and quenching is cut into dumbbell-shaped specimens with a size of 25×4 mm 2 and then the tensile properties are tested using a universal tensile testing machine at a speed of 10 mm / min. The optical properties include the test of light transmittance, which is carried out with reference to the national standard GB / T 2410-2008 "Determination of Light Transmittance and Haze of Transparent Plastics". The specimen is an amorphous film after hot pressing and quenching, cut into a size of 50×50 mm, and after cleaning the surface, it is tested using a light transmittance and haze analyzer.

[0030] 2. Gradient biaxial stretching (1)Perform pre-crystallization treatment on the granulated product after blending. Set the temperature at 150 - 170 °C, the treatment time at 3.5 - 4 hours, and control the moisture content to drop to a certain level to prevent hydrolysis during melt extrusion.

[0031] (2)Use a Barrier single-screw extruder (length-diameter ratio ≥ 30:1) for melt extrusion. Control the temperature in sections, and set the temperature for each section as follows: Feeding section: 240 - 250 °C Melting section: 260 - 270 °C Die head section: 275 - 280 °C During the extrusion process, the melt eliminates the temperature gradient through a static mixer, and the die head adopts a coat-hanger structure; The extruded melt is attached to a quenching roll through an electrostatic adsorption device. The quenching roll is cooled with 30 °C circulating water. After casting, an amorphous thick sheet is formed, and its thickness is 12 - 16 times that of the final finished film.

[0032] (3)First, longitudinally stretch the amorphous thick sheet. Temperature control: Preheating roll 80 °C → First-stage stretching temperature 90 °C → Second-stage stretching temperature 110 °C; Set the total stretching ratio at 3.5 - 4.5. Among them, the first-stage stretching ratio is 1.5 - 2.0, and the second-stage stretching ratio is 2.0 - 2.5; The relaxation method adopts intermittent relaxation. Stop for 5 seconds every 10% stretching, and dynamically adjust and release the internal stress through a tension roll; After longitudinal stretching, perform transverse stretching. Temperature control: Preheating section 90 °C → Stretching section 110 °C → Setting section 120 °C; Set the total stretching ratio at 4.0 - 5.0. Among them, the initial stretching ratio is 1.5 - 2.0, and the final-stage stretching ratio is 2.5 - 3.0; After transverse stretching, perform setting and cooling. Set the heat setting temperature at 230 - 240 °C, and keep the temperature in the cooling section at 50 - 60 °C.

[0033] In the above steps, during the biaxial stretching stage, the traditional PET biaxial stretching process usually performs initial stretching in the range of 95 - 100 °C above the glass transition temperature (Tg ≈ 75 °C) but below the crystallization temperature (Tc ≈ 120 °C). In the present invention, the temperature of the first stage is reduced to 80 - 90 °C, making the PET molecular chains in a semi-rigid state. At this temperature, the local movement ability of the molecular chains is limited, and during stretching, mainly chain segment orientation occurs rather than large-scale slippage, thereby forming a high-density orientation nucleus in the initial stage. This orientation nucleus serves as a "template" for subsequent stretching, which can inhibit the generation of disordered crystallization. In the second stage, the temperature is raised to 110 - 120 °C. At this time, the molecular chain activity increases, and the formed orientation nuclei guide the surrounding molecular chains to be arranged orderly along the stretching direction. At the same time, local stress concentration is eliminated through thermal activation. This stage promotes the perfection of the crystal region and forms a microcrystalline structure with uniform size.

[0034] The setting ratio of the longitudinal stretching ratio (3.5 - 4.0) to the transverse stretching ratio (4.0 - 5.0) is larger than that of the traditional PET biaxial stretching process. This is mainly because the blend substrate prepared in the previous step has excellent mechanical properties. Especially, the elongation at break is greatly improved compared with the ordinary PET substrate. By setting a larger stretching ratio, the plastic properties of the blend substrate can be fully exerted. In addition, the asymmetric stretching ratio design in the longitudinal and transverse directions constructs a gradient orientation structure inside the film. In the surface layer region, due to the higher temperature in contact with the stretching roller, the molecular chains are highly oriented in the longitudinal direction, forming a dense arrangement layer. In the middle transition layer, affected by the biaxial stretching resultant force, the molecular chains show a 45° cross orientation, forming an energy dissipation network. In the inner layer region, partial transverse orientation is retained to provide toughness reserve through the molecular chain rebound. In this way, the stress is gradually transmitted and dispersed through the gradient layer, while the crack propagation is inhibited.

[0035] In the longitudinal stretching stage, after stretching 10% each time, pause for 5 seconds. During this period, keep the clamp distance constant but release the tension to 30% of the initial value. In this process, during the pause stage, the molecular chains undergo local retraction, releasing the conformational entropy loss accumulated during the stretching process. According to the Maxwell model, setting a 5s residence time can make about 35% of the stress relaxation completed, effectively reducing the residual stress. The reduction of the internal stress reduces the phase difference caused by stress birefringence, reducing the haze and increasing the light transmittance. In addition, due to the gradient orientation structure inhibiting the molecular chain junction orientation at high temperatures, the heat distortion temperature is increased.

[0036] 3. Post-treatment (1) Cut off the uneven part at the edge, and then perform surface treatment using plasma. The treatment power is set to 150W, and the gas source used is an Ar / O 2 mixed gas source, and the gas ratio is Ar / O 2 volume ratio of 3:1. The treatment mode is set to be carried out in the pulse mode / continuous mode in sequence. After treatment, the surface adhesion is improved, the surface cleanliness is optimized, and the surface scattering rate is reduced; (2) Carry out winding and aging. The winding tension is controlled at 10 - 15 N / m, and aged at room temperature for 48 hours to eliminate the residual stress.

[0037] Example 1 A method for preparing a high-strength PET optical film is as follows: S1: After drying the PET substrate and the copolyester, the copolyester and the PET substrate are mixed in a mass ratio of 3:7, and an epoxy-based chain extender with a total mass of 1wt% is added. Premix for 5 - 10 minutes at a rotation speed of 300 - 500 rpm, and then use a co-rotating twin-screw extruder for segmented temperature-controlled melt extrusion. The temperature segments are as follows: Feeding section: 230 - 240 °C Melting section: 240 - 250 °C Mixing section: 250 - 260 °C Die head section: 255 - 265 °C During this period, control the screw speed to remain at 200 - 300 rpm. Finally, pelletize the extruded strip after cooling it in a water cooling tank, and dry it at 80 °C for 4 hours.

[0038] S2: Pre-crystallize the pellets after blending at 150 - 170 °C for 3.5 - 4 hours, and then perform melt extrusion with temperature segmented control. The temperature setting for each section is as follows: Feeding section: 240 - 250 °C Melting section: 260 - 270 °C Die head section: 275 - 280 °C The extruded melt is attached to a quenching roll through an electrostatic adsorption device, and an amorphous thick sheet is formed after casting. Then, perform biaxial stretching on the thick sheet. The longitudinal stretching parameters include: preheating roll at 80 °C → first stretching temperature at 90 °C → second stretching temperature at 110 °C, set the total stretching ratio to 4, where the first stretching ratio is 1.8 and the second stretching ratio is 2.2; the relaxation method adopts intermittent relaxation, and stay for 5 seconds every 10% stretching; the transverse stretching parameters include: preheating section at 90 °C → stretching section at 110 °C → shaping section at 120 °C, set the total stretching ratio to 4.5, where the initial stretching ratio is 1.8 and the final stretching ratio is 2.7; after stretching, perform heat setting at 230 - 240 °C and cooling at 50 - 60 °C.

[0039] S3: Cut off the uneven parts at the edges of the film obtained in S2, and then perform surface treatment using plasma. Set the treatment power to 150W, and use the gas source as Ar / O 2 Mixed gas source, the gas ratio is Ar / O 2 Volume ratio of 3:1, and the treatment mode is set to be carried out in successive stages of pulse mode / continuous mode; after treatment, perform winding and aging. Control the winding tension at 10 - 15 N / m and age at room temperature for 48 hours.

[0040] Example 2 A method for preparing a high-strength PET optical film is as follows: S1: After drying the PET substrate and copolyester, mix the copolyester and the PET substrate in a mass ratio of 1:9, and add 0.5 wt% of an epoxy-based chain extender based on the total mass. Premix at a speed of 300 - 500 rpm for 5 - 10 minutes, and then use a co-rotating twin-screw extruder to perform melt extrusion with segmented temperature control. The temperature segmentation is as follows: Feeding section: 230 - 240 °C Melting section: 240 - 250 °C Mixing section: 250 - 260 °C Head section: 255 - 265 °C During this period, control the screw speed to remain at 200 - 300 rpm. Finally, pelletize the extruded strip after cooling it in a water cooling tank, and dry it at 80 °C for 4 hours.

[0041] S2: Pre-crystallize the pellets after blending at 150 - 170 °C for 3.5 - 4 hours, and then perform melt extrusion with temperature segmented control. The temperature settings for each segment are as follows: Feeding section: 240 - 250 °C Melting section: 260 - 270 °C Die head section: 275 - 280 °C The extruded melt is attached to a quenching roll through an electrostatic adsorption device, and an amorphous thick sheet is formed after casting. Then, the thick sheet is biaxially stretched. The longitudinal stretching parameters include: preheating roll at 80 °C → first stretching temperature at 90 °C → second stretching temperature at 110 °C, with a total stretching ratio of 3.5 set. Among them, the first stretching ratio is 1.5 and the second stretching ratio is 2.0; the relaxation method adopts intermittent relaxation, with a 5 - second pause for every 10% stretching; the transverse stretching parameters include: preheating section at 90 °C → stretching section at 110 °C → sizing section at 120 °C, with a total stretching ratio of 4.0 set. Among them, the initial stretching ratio is 1.5 and the final stretching ratio is 2.5; after stretching, heat setting is performed at 230 - 240 °C and cooling is carried out at 50 - 60 °C.

[0042] S3: Cut off the uneven parts at the edges of the film sheet obtained in S2, and then perform surface treatment using plasma. The treatment power is set at 150 W, and the gas source used is Ar / O 2 Mixed gas source, and the gas ratio is Ar / O 2 Volume ratio of 3:1, and the treatment mode is set to be carried out in sequential stages of pulse mode / continuous mode; after treatment, winding and aging are performed. The winding tension is controlled at 10 - 15 N / m, and aging is carried out at room temperature for 48 hours.

[0043] Example 3 A method for preparing a high-strength PET optical film is as follows: S1: After drying the PET substrate and copolyester, mix the copolyester and PET substrate in a mass ratio of 2:8, and add an epoxy chain extender with a total mass of 0.8 wt%. Premix at a speed of 300 - 500 rpm for 5 - 10 minutes, and then use a co-rotating twin-screw extruder for melt extrusion with segmented temperature control. The temperature segments are as follows: Feeding section: 230 - 240 °C Melting section: 240 - 250 °C Mixing section: 250 - 260 °C Head section: 255 - 265 °C During the process, the rotation speed of the screw is controlled to be maintained at 200 - 300 rpm. Finally, the extruded strip is cooled in a water cooling tank and then pelletized, and dried at 80°C for 4 hours.

[0044] S2: The pellets after blending are pre-crystallized at 150 - 170°C for 3.5 - 4 hours, and then melt-extruded with temperature segmented control. The temperature settings for each segment are as follows: Feeding section: 240 - 250°C Melting section: 260 - 270°C Die head section: 275 - 280°C The extruded melt is attached to a quenching roll through an electrostatic adsorption device, and an amorphous thick sheet is formed after casting. Then, the thick sheet is biaxially stretched. The longitudinal stretching parameters include: preheating roll at 80°C → first-stage stretching temperature at 90°C → second-stage stretching temperature at 110°C, with a total stretching ratio of 3.8 set. Among them, the first-stage stretching ratio is 1.6, and the second-stage stretching ratio is 2.2; the relaxation method adopts intermittent relaxation, with a 5-second pause for every 10% stretching; the transverse stretching parameters include: preheating section at 90°C → stretching section at 110°C → sizing section at 120°C, with a total stretching ratio of 4.2 set. Among them, the initial stretching ratio is 1.7, and the final-stage stretching ratio is 2.6; after stretching, heat setting is carried out at 230 - 240°C, and cooling is carried out at 50 - 60°C.

[0045] S3: The uneven parts at the edges of the film obtained in S2 are cut off, and then surface treatment is carried out using plasma. The treatment power is set at 150W, and the gas source used is Ar / O 2 mixed gas source, and the gas ratio is Ar / O 2 volume ratio of 3:1, and the treatment mode is set to be carried out in sequential stages of pulse mode / continuous mode; after treatment, winding and aging are carried out. The winding tension is controlled at 10 - 15 N / m, and aging is carried out at room temperature for 48 hours.

[0046] Example 4 A method for preparing a high-strength PET optical film is as follows: S1: After drying the PET substrate and copolyester, the copolyester and the PET substrate are mixed in a mass ratio of 4:6, and an epoxy-based chain extender with a total mass of 1.3 wt% is added. Premixing is carried out at a rotation speed of 300 - 500 rpm for 5 - 10 minutes, and then a co-rotating twin-screw extruder is used for melt-extrusion with segmented temperature control. The temperature segments are as follows: Feeding section: 230 - 240°C Melting section: 240 - 250°C Mixing section: 250 - 260°C Die head section: 255 - 265°C During the process, the rotation speed of the screw is controlled to be maintained at 200 - 300 rpm. Finally, the extruded strip is cooled in a water cooling tank and then pelletized, and dried at 80°C for 4 hours.

[0047] S2: Pre-crystallize the granulated product after blending at 150 - 170 °C for 3.5 - 4 hours, then conduct melt extrusion with temperature segmented control. The temperature settings for each segment are as follows: Feeding section: 240 - 250 °C Melting section: 260 - 270 °C Die head section: 275 - 280 °C The extruded melt is attached to a quenching roller through an electrostatic adsorption device, and an amorphous thick sheet is formed after casting. Then, the thick sheet is subjected to biaxial stretching. The longitudinal stretching parameters include: preheating roller at 80 °C → first stretching temperature at 90 °C → second stretching temperature at 110 °C, with a total stretching ratio of 4.3 set. Among them, the first stretching ratio is 1.9, and the second stretching ratio is 2.4. The relaxation method adopts intermittent relaxation, with a 5 - second pause for every 10% stretching. The transverse stretching parameters include: preheating section at 90 °C → stretching section at 110 °C → shaping section at 120 °C, with a total stretching ratio of 4.8 set. Among them, the initial stretching ratio is 1.9, and the final stretching ratio is 2.9. After stretching, heat setting is carried out at 230 - 240 °C, and cooling is carried out at 50 - 60 °C.

[0048] S3: Cut off the uneven parts at the edges of the film obtained in S2, and then perform surface treatment using plasma. The treatment power is set at 150 W, and the gas source used is an Ar / O 2 mixed gas source, and the gas ratio is Ar / O 2 volume ratio of 3:1, and the treatment mode is set to be carried out in successive stages of pulse mode / continuous mode; after treatment, winding and aging are carried out. The winding tension is controlled at 10 - 15 N / m, and aging is carried out at room temperature for 48 hours.

[0049] Example 5 A method for preparing a high-strength PET optical film is as follows: S1: After drying the PET substrate and the copolyester, the copolyester and the PET substrate are mixed in a mass ratio of 5:5, and an epoxy-based chain extender with a total mass of 1.5 wt% is added. Premix at a rotation speed of 300 - 500 rpm for 5 - 10 minutes, and then use a co-rotating twin-screw extruder for melt extrusion with segmented temperature control. The temperature segments are as follows: Feeding section: 230 - 240 °C Melting section: 240 - 250 °C Mixing section: 250 - 260 °C Die head section: 255 - 265 °C During this period, control the screw rotation speed to remain at 200 - 300 rpm. Finally, cool the extruded strip through a water-cooling tank, cut it into pellets, and dry it at 80 °C for 4 hours.

[0050] S2: Precrystallize the granulated product after blending at 150 - 170 °C for 3.5 - 4 hours, then perform melt extrusion with temperature segmented control, and the temperature setting for each segment is as follows: Feeding section: 240 - 250 °C Melting section: 260 - 270 °C Die head section: 275 - 280 °C The extruded melt is attached to a chill roll through an electrostatic adsorption device, and an amorphous thick sheet is formed after casting. Then, the thick sheet is biaxially stretched. The longitudinal stretching parameters include: preheating roll at 80 °C → first-stage stretching temperature at 90 °C → second-stage stretching temperature at 110 °C, with a set total stretching ratio of 4.5. Among them, the first-stage stretching ratio is 2.0, and the second-stage stretching ratio is 2.5; the relaxation method adopts intermittent relaxation, with a 5-second pause for every 10% of stretching; the transverse stretching parameters include: preheating section at 90 °C → stretching section at 110 °C → sizing section at 120 °C, with a set total stretching ratio of 5.0. Among them, the initial stretching ratio is 2.0, and the final-stage stretching ratio is 3.0; after stretching, heat setting is carried out at 230 - 240 °C, and cooling is carried out at 50 - 60 °C.

[0051] S3: Cut off the uneven parts at the edges of the film sheet obtained in S2, and then perform surface treatment using plasma. The treatment power is set at 150 W, and the gas source used is an Ar / O 2 mixed gas source, and the gas ratio is Ar / O 2 volume ratio of 3:1, and the treatment mode is set to be carried out in successive stages of pulse mode / continuous mode; after treatment, winding and aging are carried out. The winding tension is controlled at 10 - 15 N / m, and aging is carried out at room temperature for 48 hours.

[0052] Comparative Example 1 A method for preparing a high-strength PET optical film is as follows: Refer to the preparation steps of Example 1, except that in step S1, the PET substrate is not blended with the copolyester, and is directly connected to step S2 after drying until the preparation is completed.

[0053] Comparative Example 2 A method for preparing a high-strength PET optical film is as follows: Refer to the preparation steps of Example 1, except that in step S2, the biaxial stretching adopts a non-segmented stretching method. The specific parameters include: for longitudinal stretching, preheating roll at 80 °C → stretching temperature at 110 °C, with a set stretching ratio of 4; the relaxation method adopts intermittent relaxation, with a 5-second pause for every 10% of stretching; for transverse stretching, preheating section at 90 °C → stretching section at 120 °C, with a set stretching ratio of 4.5.

[0054] Comparative Example 3 A method for preparing a high-strength PET optical film is as follows: Refer to the preparation steps of Example 1, except that in step S2, the intermittent relaxation step is cancelled in the longitudinal stretching stage.

[0055] Comparative Example 4 A method for preparing a high-strength PET optical film is as follows: Referring to the preparation steps of Example 1, the difference is that the step of plasma surface treatment is not set in step S3.

[0056] For the PET optical films prepared by integrating Examples 1-5 and Comparative Examples 1-4, their comprehensive performances are compared horizontally. Among them, the mechanical properties include tensile strength and elongation at break, and the tests are carried out with reference to the national standard GB / T 1040-2006 "Determination of Tensile Properties of Plastics"; the surface hardness is tested with reference to the national standard GB / T 6739-2006 "Determination of Film Hardness by Pencil Method for Paints and Varnishes"; the optical properties include the tests of light transmittance and haze, and the tests are carried out with reference to the national standard GB / T 2410-2008 "Determination of Light Transmittance and Haze of Transparent Plastics"; the clarity part is tested with reference to the national standard GB / T 25273 "Method for Determining Haze of Films for Liquid Crystal Displays"; the temperature resistance performance includes the test of thermal shrinkage rate with reference to the national standard GB / T 27584-2011 "Optical Functional Films - Polyethylene Terephthalate (PET) Films - Determination of Dimensional Changes after Heating", and the high and low temperature resistance performance is tested with reference to the national standard GB / T7141-2008 "Test Method for Thermal Aging of Plastics". The specific test comparison results are as follows: Table 1 Comparison Table of Mechanical Properties of Examples 1-5 and Comparative Examples 1-4

[0057] Table 2 Comparison Table of Optical Properties of Examples 1-5 and Comparative Examples 1-4

[0058] Table 3 Comparison Table of Temperature Resistance Performance of Examples 1-5 and Comparative Examples 1-4

[0059] From the above comparison results, it can be seen that in Comparative Example 1, there is no blending with copolyester, resulting in lower mechanical properties. Theoretically, the impact on other properties is relatively small, but from the results, other properties are also significantly affected. This is because in Comparative Example 1 without blending treatment, the elongation at break before the biaxial stretching process is lower than that of the blended substrate, resulting in its inability to well adapt to the process with a large stretching ratio during the biaxial stretching process, thus affecting its overall performance; in Comparative Example 2, the segmented temperature stretching is not adopted, resulting in poor orientation of molecular chains during stretching, more local stress concentration phenomena, and poor uniformity of the microcrystalline structure size, thereby affecting its various properties; in Comparative Example 3, the intermittent relaxation step in the longitudinal stretching stage is cancelled, resulting in incomplete elimination of residual internal stress, which has a great impact on the overall optical properties of the formed film material; in Comparative Example 4, plasma surface treatment is not used in the post-treatment stage, affecting the surface cleanliness of the film material, resulting in a high surface scattering rate, and also affecting the elimination of surface stress of the film material, thus having a certain impact on various properties.

Claims

1. A method for preparing a high-strength PET optical film, characterized in that: The steps include: S1: After drying the PET substrate and the copolyester, they are mixed according to a mass ratio, and an interfacial compatibilizer is added. After premixing, they are melt-extruded in sections with controlled temperature. The extruded material strips are cooled in a water cooling tank, pelletized, and dried. The copolyester preparation method comprises: Terephthalic acid and ethylene glycol were added as monomers, and the molar ratio of terephthalic acid to ethylene glycol was 1:1.3; Under the set reaction conditions, terephthalic acid, ethylene glycol and 0.028wt% of the catalyst antimony glycol were subjected to esterification reaction; After the esterification reaction is completed, 0.9 wt% of furan carboxylic acid as the end-capping monomer is added to carry out end-capping copolymerization reaction; After the end-capping copolymerization reaction is completed, discharge, quenching and granulation are carried out; S2: Pre-crystallization treatment is performed on the granules after S1 blending, and then temperature-stage controlled melt extrusion is performed, and the extruded melt is attached to a rapid cooling roller through an electrostatic adsorption device, and an amorphous thick sheet is formed after sheet casting, and then the thick sheet is biaxially stretched, heat-set after the stretching is completed, and finally cooled; S3: The uneven edge portion of the membrane obtained in S2 is cut off, and then the surface is treated with plasma, and then rolled up and aged.

2. The method for preparing a high-strength PET optical film according to claim 2, characterized in that: The set reaction conditions described in S1 specifically include: The preheating temperature of the reactor was set at 120°C, the stirring time of terephthalic acid, ethylene glycol and catalyst antimony glycol was set at 5 minutes, the temperature of the reactor jacket oil bath was set at 270°C, and the pressure in the esterification reactor was raised to 120-150 kPa; the time point at which the esterification reaction ended was set when the mass of the collected by-products reached 98% of the theoretical yield.

3. The method for preparing a high-strength PET optical film according to claim 2, characterized in that: The starting and ending conditions of the end-capping copolymerization reaction described in S1 are set as follows: the stirring motor power reaches 38 to 42W.

4. The method for preparing a high-strength PET optical film according to claim 1, characterized in that: The mixing according to the mass ratio described in S1 is to mix the dried copolyester and the PET substrate in a mass ratio of 1:9 to 5:5; The interfacial compatibilizer described in S1 is an epoxy chain extender added in an amount of 0.5 to 1.5 wt% of the total mass; The segmented temperature-controlled melt extrusion described in S1 is specifically set as follows: feed section: 230-240°C, melting section: 240-250°C, mixing section: 250-260°C, and die section: 255-265°C.

5. The method for preparing a high-strength PET optical film according to claim 1, characterized in that: The temperature section controlled melt extrusion described in S2 has the following temperature sections set as follows: feed section: 240-250°C, melting section: 260-270°C, die section: 275-280°C.

6. The method for preparing a high-strength PET optical film according to claim 1, characterized in that: The biaxial stretching described in S2 includes longitudinal stretching, and the parameters of the longitudinal stretching include: The segment temperature settings are: 80°C for preheating roller, 90°C for the first stretching stage, and 110°C for the second stretching stage; The stretching ratio is set as follows: the total stretching ratio is 3.5 to 4.5, wherein the first stretching ratio is 1.5 to 2.0, and the second stretching ratio is 2.0 to 2.5; The relaxation method adopts intermittent relaxation, staying for 5 seconds every time the stretching is 10%, and the internal stress is released by dynamic adjustment of the tension roller.

7. The method for preparing a high-strength PET optical film according to claim 1, characterized in that: The biaxial stretching described in S2 includes transverse stretching, and the parameters of the transverse stretching include: The segment temperatures are set as follows: 90°C for preheating, 110°C for stretching, and 120°C for shaping; The stretching ratio is set as follows: total stretching ratio 4.0-5.0, wherein the initial stretching ratio is 1.5-2.0, and the final stretching ratio is 2.5-3.

0.

8. The method for preparing a high-strength PET optical film according to claim 1, characterized in that: The specific parameter settings of the plasma surface treatment described in S3 include: The processing power is set to 150W; The gas source used is Ar / O2 mixed gas source, and the gas ratio is Ar / O2 volume ratio 3:1; The processing mode is set to pulse mode / continuous mode in stages.

9. A high-strength PET optical film, characterized in that: The preparation method is described in any one of claims 1 to 9.

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