A high-strength PET optical film and its preparation method

Abstract

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. In the present invention, the basic mechanical properties of the film-forming substrate are improved by blending a PET substrate with a copolyester, and a gradient biaxial stretching with a large draw ratio is carried out by utilizing the excellent mechanical properties of the substrate. The comprehensive properties of the prepared film are improved by combining an intermittent relaxation process; 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

Technical Field

[0001] The present invention relates to the field of preparation of optical polyester materials, and particularly 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:

[0003] 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, the existing technologies mainly adopt two types of solutions:

[0004] By dispersing inorganic nanoparticles in the PET matrix to enhance rigidity, but it 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 crystal points are easily generated due to thermal degradation of the elastomer during the processing, affecting the optical uniformity.

[0005] In addition, traditional PET optical films enhance the mechanical strength by increasing the crystallinity (>35%) in the biaxial stretching process, but 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 existing technology, 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%.

[0006] Existing technologies such as chain extension modification (CN 118620254 B) or introducing a metal layer (CN 222119124 U) can improve the mechanical properties, but sacrifice the optical transparency; while single processes such as solid-phase polycondensation or plasma treatment are difficult to achieve breakthroughs in comprehensive performance.

[0007] 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

[0008] Based on the problems summarized above, the present invention provides a high-strength PET optical film and its preparation method. The main feature is to improve the basic mechanical properties of the film-forming substrate by blending a PET substrate with a copolyester, and utilize the excellent mechanical properties of the substrate to obtain better comprehensive performance in the subsequent biaxial stretching step. In addition, the film material is further optimized by post-treatment of plasma surface treatment. The specific technical solutions are as follows:

[0009] A preparation method of a high-strength PET optical film, comprising the following steps:

[0010] 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, segmented temperature-controlled melt extrusion is carried out. The extruded strip is cooled in a water-cooling tank and then pelletized and dried; the preparation method of the copolyester includes:

[0011] 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;

[0012] Under the set reaction conditions, carry out an esterification reaction on terephthalic acid, ethylene glycol and 0.028 wt% of the catalyst antimony glycolate;

[0013] After the esterification reaction is completed, add 0.9 wt% of furan carboxylic acid as a capping monomer to carry out a capping copolymerization reaction;

[0014] After the capping copolymerization reaction is completed, carry out discharging, quenching and pelletizing;

[0015] S2: Carry out pre-crystallization treatment on the pellets after blending in S1, and then carry out temperature-segmented controlled melt extrusion. The extruded melt is attached to a rapid-cooling roll through 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;

[0016] S3: Cut off the uneven parts at the edges of the film sheet obtained in S2, and then carry out plasma surface treatment. After treatment, carry out winding and aging.

[0017] Further, the set reaction conditions in the copolyester preparation method specifically include:

[0018] The preheating temperature of the reaction kettle is set at 120 °C, the stirring time of terephthalic acid, ethylene glycol and the catalyst antimony glycolate is set at 5 minutes, the oil bath temperature of the jacket of the reaction kettle is set at 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 collected mass of the by - product reaches 98% of the theoretical yield.

[0019] Further, for the end - capped copolymerization reaction, its start and end conditions are set as: the power of the stirring motor reaches 38 - 42 W.

[0020] Further, 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.

[0021] Further, the interfacial compatibilizer in S1 is to add an epoxy - based chain extender with a total mass of 0.5 - 1.5 wt%.

[0022] Further, the segmented temperature - controlled melt extrusion in S1 is specifically set as: the feeding section: 230 - 240 °C, the melting section: 240 - 250 °C, the mixing section: 250 - 260 °C, and the die head section: 255 - 265 °C.

[0023] Further, the temperature - segmented controlled melt extrusion in S2 has the temperature segments set as: the feeding section: 240 - 250 °C, the melting section: 260 - 270 °C, and the die head section: 275 - 280 °C.

[0024] Further, the biaxial stretching in S2 includes longitudinal stretching, and the parameters of the longitudinal stretching include:

[0025] The segmented temperature is set as: the pre - heating roller at 80 °C, the first - stage stretching temperature at 90 °C, and the second - stage stretching temperature at 110 °C.

[0026] The stretching ratio is set as: the total stretching ratio is 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.

[0027] The relaxation method adopts intermittent relaxation, pausing for 5 seconds every 10% of stretching, and dynamically adjusting the release of internal stress through the tension roller.

[0028] Further, the biaxial stretching in S2 includes transverse stretching, and the parameters of the transverse stretching include:

[0029] The segmented temperature is set as: the pre - heating section at 90 °C, the stretching section at 110 °C, and the shaping section at 120 °C.

[0030] The stretching ratio is set as: the total stretching ratio is 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.

[0031] Further, for the plasma surface treatment described in S3, the specific parameter settings include:

[0032] The treatment power is set to 150 W;

[0033] The gas source used is an Ar / O2 mixed gas source, and the gas ratio is 3:1 in terms of the volume ratio of Ar / O2;

[0034] The treatment mode is set to be carried out in successive stages of pulse mode / continuous mode.

[0035] A high-strength PET optical film is obtained by using any one of the above-described preparation methods.

[0036] Compared with the prior art, the present invention has the following beneficial effects:

[0037] (1) By blending the PET substrate with the furan dicarboxylic acid copolyester, the present invention utilizes 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, thereby greatly enhancing the comprehensive mechanical properties of the substrate formed after blending;

[0038] (2) By utilizing the excellent comprehensive mechanical properties of the blended substrate, the present invention performs gradient biaxial stretching with a large draw ratio and combines an intermittent relaxation process to improve the comprehensive properties of the prepared film material.

[0039] (3) By using plasma surface treatment to treat the film material after stretching, the present invention improves the surface adhesion, optimizes the surface cleanliness, reduces the surface scattering rate, and further enhances the comprehensive properties of the film material. Description of the Drawings

[0040] Figure 1 is a flowchart of a preparation method of a high-strength PET optical film according to the present invention;

[0041] Figure 2 is of the material after blending treatment according to the present invention 1 1H NMR spectrum;

[0042] Figure 3 is a line comparison chart of the mean variance of the comprehensive mechanical properties of the PET substrate, the copolyester, and the material after blending according to the present invention;

[0043] Figure 4 is a scatter comparison chart of the mean variance of the light transmittance of the PET substrate, the copolyester, and the material after blending according to the present invention. Detailed Embodiments

[0044] The following embodiments further explain and illustrate the technical solutions of the present invention. It is specifically pointed out that each specific implementation manner 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 replacements on some or all of the technical features, and these modifications or replacements 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.

[0045] The present invention provides a high-strength PET optical film and a preparation method thereof. As shown in the appendix Figure 1 , the specific preparation steps are as follows:

[0046] 1. Construction of the PET / copolymerized modified polyester blend system:

[0047] 1.1 Preparation of the copolymerized modified polyester blend

[0048] (1) Add terephthalic acid and ethylene glycol as monomers for feeding. The feeding molar ratio of terephthalic acid to ethylene glycol is 1:1.3.

[0049] (2) Start the esterification of terephthalic acid and ethylene glycol. Preheat the reaction kettle at a temperature of 120°C, stir terephthalic acid, ethylene glycol and 0.028 wt% of the catalyst antimony glycol 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, and repeat three times until all the oxygen in the kettle is exhausted; close the polycondensation passage valve, always keep stirring, 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 through nitrogen for esterification; as the temperature rises, the substances in the kettle vaporize, and the pressure in the kettle slowly rises. Keep the pressure at about 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 fractionation tower, it indicates that the esterification has started, and the pressure in the kettle slowly drops. When the collected mass of by-product water reaches 98% of the theoretical yield, end the esterification reaction. At this time, the temperature at the top of the tower should be lower than 100°C.

[0050] (3)After the esterification stage is completed, close the valve of the esterification passage and open the valves of the polycondensation passage and the vacuum pump. Conduct 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, turn off the vacuum pump, add 0.9 wt% of furan carboxylic acid, which is the end-capping monomer, into the feeding hopper connected to the kettle body, and seal the feeding hopper and conduct five nitrogen replacements; then raise the pressure in the feeding hopper to atmospheric pressure by introducing 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, turn off the vacuum pump and the stirring paddle, introduce nitrogen to normal pressure, and prepare for discharging.

[0051] (4)Introduce 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.

[0052] In the above steps, the reaction processes of terephthalic acid, ethylene glycol, and furan carboxylic acid are as follows:

[0053]

[0054] During the reaction combination process, both the end-capping effect after the reaction of furan carboxylic acid with polyester and the residual unreacted furan carboxylic acid small molecules play a plasticizing effect on the material. The combined action of end-group plasticization and direct small molecule plasticization greatly increases the elongation at break of the material. Due to the presence of conjugated groups in the furan carboxylic acid unit, there is a π-π stacking effect. The π-π stacking occurs between the end-group furan carboxylic acid units, which can achieve the same 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 the stretching process, 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 the stretching process, it can maintain a certain tensile strength without losing the elongation at break, making up 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 subsequent blending is required to complement the properties between materials. Due to the addition of furan carboxylic acid in the copolymerized polyester alone, the color depth of the finally formed material increases, which has some effects on the optical properties of the copolymerized polyester.

[0055] 1.2 Construction of the blend system

[0056] (1)Dry the PET substrate in a blast drying oven at 80 °C for 12 hours, and dry the copolymerized polyester at 60 °C for 6 - 8 hours.

[0057] (2) Mix the dried copolyester and PET substrate in a mass ratio of 1:9 to 5:5, and add 0.5 - 1.5 wt% of epoxy chain extender based on the total mass to promote interfacial compatibility. Use a high-speed mixer to premix all raw materials at a rotation speed of 300 - 500 rpm for 5 - 10 minutes to ensure uniform dispersion.

[0058] (3) Use a co-rotating twin-screw extruder (length-diameter ratio ≥ 40:1) for segmented temperature-controlled melt extrusion. The temperature segments are as follows:

[0059] Feeding section: 230 - 240 °C

[0060] Melting section: 240 - 250 °C

[0061] Mixing section: 250 - 260 °C

[0062] Die head section: 255 - 265 °C

[0063] 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.

[0064] (4) Cool the extruded strip in a water cooling tank with the water temperature controlled at 25 - 30 °C, pelletize after cooling, and finally dry at 80 °C for 4 hours.

[0065] In the above steps, by blending the copolyester and PET substrate, the mechanical properties and optical properties of the blended polyester are balanced. As shown in the Figure 2 , which is the 1 1H NMR spectrum of the blended PET. As can be seen from the figure,

[0066] The aromatic protons of the benzene ring in 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;

[0067] 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;

[0068] The α-position protons of the furan ring in the furan carboxylic acid copolyester characteristic signal are at 7.0 - 7.5 ppm (multiplet), overlapping with the PET aromatic peak, indicating the occurrence of π-π stacking interaction; the ortho-position protons of the ester group appear as a weak peak at 8.0 - 8.3 ppm, which is the peak position shift when forming a hydrogen bond with the PET ester group (~8.1 ppm);

[0069] The peak of ethylene glycol segment of PET (4.3 - 4.7 ppm) in the spectrum has changed, indicating that the polar groups of the furan carboxylic acid copolyester form hydrogen bonds with the PET segments, enhancing the intermolecular force and improving the tensile strength;

[0070] The proton peak of the furan ring (7.0 - 7.5 ppm) overlaps partially with the aromatic peak of PET, 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;

[0071] The aromatic proton peak of PET (7.5 - 8.5 ppm) shifts slightly to the low 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 influence of the copolyester itself on the optical properties.

[0072] As shown in Figure 3 and Figure 4 , through the comparison of the comprehensive mechanical properties and optical properties of PET substrates, copolyesters, and PET blend polyesters, combined with the above spectral analysis, the positive effects on the materials 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 films after hot pressing and quenching are 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, with a tensile 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 specimens are amorphous films after hot pressing and quenching, cut into a size of 50×50 mm, and after cleaning the surface, they are tested using a light transmittance and haze analyzer.

[0073] 2. Gradient biaxial stretching

[0074] (1) Pre-crystallization treatment is carried out on the granulated blend, with the temperature set at 150 - 170 °C and the treatment time of 3.5 - 4 hours, controlling the moisture content to a certain level to prevent hydrolysis during melt extrusion.

[0075] (2) Use a Barrier type single-screw extruder (length-diameter ratio ≥ 30:1) for melt extrusion, with temperature segmented control, and the temperature setting for each segment:

[0076] Feeding section: 240 - 250 °C

[0077] Melting section: 260 - 270 °C

[0078] Die head section: 275 - 280 °C

[0079] During the extrusion process, the melt eliminates the temperature gradient through a static mixer, and the die head adopts a coat-hanger structure;

[0080] The extruded melt is attached to the chill roll through an electrostatic adsorption device. The chill roll is cooled with circulating water at 30°C, and an amorphous thick sheet is formed after casting. The thickness is 12 - 16 times that of the final finished film.

[0081] (3)First, longitudinally stretch the amorphous thick sheet,

[0082] Temperature control: preheating roll at 80°C → first-stage stretching temperature at 90°C → second-stage stretching temperature at 110°C; set the total stretching ratio to 3.5 - 4.5, among which, 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, staying for 5 seconds every 10% stretching, and dynamically adjusting the release of internal stress through the tension roll;

[0083] After longitudinal stretching, conduct transverse stretching,

[0084] Temperature control: preheating section at 90°C → stretching section at 110°C → shaping section at 120°C; set the total stretching ratio to 4.0 - 5.0, among which, the initial stretching ratio is 1.5 - 2.0, and the final-stage stretching ratio is 2.5 - 3.0;

[0085] After transverse stretching, conduct shaping and cooling. The heat setting temperature is set at 230 - 240°C, and the temperature in the cooling section is maintained at 50 - 60°C.

[0086] In the above steps, during the biaxial stretching stage, the traditional PET biaxial stretching process usually conducts 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 slip, thus 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.

[0087] 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, part of the transverse orientation is retained, providing toughness reserve through the molecular chain rebound. In this way, the stress is gradually transmitted and dispersed through the gradient layer, while inhibiting the crack propagation.

[0088] 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 orientation of molecular chain knots at high temperatures, the heat distortion temperature is increased.

[0089] 3. Post-treatment

[0090] (1) Cut off the uneven part at the edge, and then perform surface treatment with plasma. The treatment power is set to 150W, the gas source used is an Ar / O2 mixed gas source, the gas ratio is the volume ratio of Ar / O2 3:1, and 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;

[0091] (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.

[0092] Example 1

[0093] A method for preparing a high-strength PET optical film is as follows:

[0094] 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:

[0095] Feeding section: 230 - 240 °C

[0096] Melting section: 240 - 250 °C

[0097] Mixing section: 250 - 260 °C

[0098] Die head section: 255 - 265 °C

[0099] During this period, the screw speed is controlled to remain 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.

[0100] S2: The pellets after blending are pre - crystallized at 150 - 170 °C for 3.5 - 4 hours, and then melt - extruded. The temperature is controlled in sections, and the temperature setting for each section is as follows:

[0101] Feeding section: 240 - 250 °C

[0102] Melting section: 260 - 270 °C

[0103] Die head section: 275 - 280 °C

[0104] 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: pre - heating roll at 80 °C → first - stage stretching temperature at 90 °C → second - stage stretching temperature at 110 °C, with a total stretching ratio of 4. Among them, the first - stage stretching ratio is 1.8, and the second - stage stretching ratio is 2.2; the relaxation method adopts intermittent relaxation, and it stays for 5 seconds every 10% of stretching; the transverse stretching parameters include: pre - heating section at 90 °C → stretching section at 110 °C → shaping section at 120 °C, with a total stretching ratio of 4.5. Among them, the initial stretching ratio is 1.8, and the final - stage stretching ratio is 2.7; after stretching, it is heat - set at 230 - 240 °C and cooled at 50 - 60 °C.

[0105] 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 to 150W, the gas source used is an Ar / O2 mixed gas source, the gas ratio is Ar / O2 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, it is wound up and aged. The winding tension is controlled at 10 - 15 N / m, and it is aged at room temperature for 48 hours.

[0106] Example 2

[0107] A method for preparing a high - strength PET optical film is as follows:

[0108] S1: After drying the PET substrate and the copolyester, the copolyester and the PET substrate are mixed at a mass ratio of 1:9, and an epoxy chain extender with a total mass of 0.5 wt% is added. The mixture is premixed at a rotational speed of 300 - 500 rpm for 5 - 10 minutes, and then a co-rotating twin-screw extruder is used for segmented temperature-controlled melt extrusion. The temperature segments are as follows:

[0109] Feeding section: 230 - 240 °C

[0110] Melting section: 240 - 250 °C

[0111] Mixing section: 250 - 260 °C

[0112] Die head section: 255 - 265 °C

[0113] During this period, the screw rotational speed is controlled to remain at 200 - 300 rpm. Finally, the extruded strip is cooled by a water cooling tank and pelletized, and dried at 80 °C for 4 hours.

[0114] S2: The pellets after blending are subjected to pre-crystallization treatment at 150 - 170 °C for 3.5 - 4 hours, and then melt extrusion is carried out with segmented temperature control. The temperature setting for each segment is as follows:

[0115] Feeding section: 240 - 250 °C

[0116] Melting section: 260 - 270 °C

[0117] Die head section: 275 - 280 °C

[0118] 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. 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, where the first stretching ratio is 1.5 and the second stretching ratio is 2.0; the relaxation method is 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, where the initial stretching ratio is 1.5 and the final stretching ratio is 2.5; after stretching, heat setting is carried out at 230 - 240 °C, and cooling is carried out at 50 - 60 °C.

[0119] 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 to 150 W, the gas source used is an Ar / O2 mixed gas source, the gas ratio is an Ar / O2 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.

[0120] Example 3

[0121] A method for preparing a high-strength PET optical film is as follows:

[0122] S1: After drying the PET substrate and the copolyester, the copolyester and the PET substrate are mixed in a mass ratio of 2:8, and an epoxy chain extender with a total mass of 0.8 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 segmented temperature-controlled melt extrusion. The temperature segments are as follows:

[0123] Feeding section: 230 - 240 °C

[0124] Melting section: 240 - 250 °C

[0125] Mixing section: 250 - 260 °C

[0126] Die head section: 255 - 265 °C

[0127] During this period, control the screw rotation speed to remain at 200 - 300 rpm. Finally, cool the extruded strip through a water cooling tank, pelletize it, and dry it at 80 °C for 4 hours.

[0128] S2: Pre-crystallize the blended pellets at 150 - 170 °C for 3.5 - 4 hours, and then perform melt extrusion with segmented temperature control. The temperature setting for each section is as follows:

[0129] Feeding section: 240 - 250 °C

[0130] Melting section: 260 - 270 °C

[0131] Die head section: 275 - 280 °C

[0132] The extruded melt is attached to a quenching roll through an electrostatic adsorption device, and after casting, an amorphous thick sheet is formed. 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 3.8 set. Among them, the first stretching ratio is 1.6, and the second stretching ratio is 2.2; 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 total stretching ratio of 4.2 set. Among them, the initial stretching ratio is 1.7, and the final stretching ratio is 2.6; after stretching, heat setting is performed at 230 - 240 °C, and cooling is performed at 50 - 60 °C.

[0133] S3: Cut off the uneven part of the edge of the diaphragm obtained in S2, and then perform surface treatment using plasma. The treatment power is set to 150 W, the gas source used is an Ar / O2 mixed gas source, the gas ratio is 3:1 in terms of the volume ratio of Ar / O2, and the treatment mode is set to be carried out in successive stages of pulse mode / continuous mode; after the 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.

[0134] Example 4

[0135] A method for preparing a high-strength PET optical film is as follows:

[0136] S1: After drying the PET substrate and the copolyester, the copolyester and the PET substrate are mixed in a mass ratio of 4:6, and an epoxy 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 segmented temperature-controlled melt extrusion. The temperature segmentation is as follows:

[0137] Feeding section: 230 - 240 °C

[0138] Melting section: 240 - 250 °C

[0139] Mixing section: 250 - 260 °C

[0140] Die head section: 255 - 265 °C

[0141] During this period, the screw rotation speed is controlled to remain at 200 - 300 rpm. Finally, the extruded strip is cooled by a water cooling tank and then pelletized, and dried at 80 °C for 4 hours.

[0142] S2: Perform pre-crystallization treatment on the granulated product after blending at 150 - 170 °C for 3.5 - 4 hours, and then carry out melt extrusion with segmented temperature control. The temperature setting for each section is as follows:

[0143] Feeding section: 240 - 250 °C

[0144] Melting section: 260 - 270 °C

[0145] Die head section: 275 - 280 °C

[0146] The extruded melt is attached to the chill roll through an electrostatic adsorption device, and after casting, an amorphous thick sheet is formed. Then, the thick sheet is subjected to biaxial stretching. The longitudinal stretching parameters include: preheating roll at 80 °C → first-stage stretching temperature at 90 °C → second-stage stretching temperature at 110 °C. The total stretching ratio is set to 4.3, where the first-stage stretching ratio is 1.9 and the second-stage stretching ratio is 2.4. The relaxation method is 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 → shaping section at 120 °C. The total stretching ratio is set to 4.8, where 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.

[0147] 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 to 150 W, the gas source used is an Ar / O2 mixed gas source, the gas ratio is the volume ratio of Ar / O2 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.

[0148] Example 5

[0149] A method for preparing a high-strength PET optical film is as follows:

[0150] 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. 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 segmented temperature-controlled melt extrusion. The temperature segments are as follows:

[0151] Feeding section: 230 - 240 °C

[0152] Melting section: 240 - 250 °C

[0153] Mixing section: 250 - 260 °C

[0154] Head section: 255 - 265 °C

[0155] During this period, the screw rotation speed is controlled to remain at 200 - 300 rpm. Finally, the extruded strip is cooled in a water-cooling tank and pelletized, and dried at 80 °C for 4 hours.

[0156] S2: Carry out pre-crystallization treatment on the granulated product after blending at 150 - 170 °C for 3.5 - 4 hours, and then carry out melt extrusion with segmented temperature control. The temperature of each section is set as follows:

[0157] Feeding section: 240 - 250 °C

[0158] Melting section: 260 - 270 °C

[0159] Die head section: 275 - 280 °C

[0160] The extruded melt is attached to the rapid cooling roller through an electrostatic adsorption device, and an amorphous thick sheet is formed after casting, and then the thick sheet is subjected to biaxial stretching; the longitudinal stretching parameters include: preheating roller at 80 °C → first-stage stretching temperature at 90 °C → second-stage stretching temperature at 110 °C, setting a total stretching ratio of 4.5, where the first-stage stretching ratio is 2.0 and the second-stage stretching ratio is 2.5; the relaxation method adopts intermittent relaxation, staying 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, setting a total stretching ratio of 5.0, where 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.

[0161] 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 to 150 W, the gas source used is an Ar / O2 mixed gas source, the gas ratio is the volume ratio of Ar / O2 of 3:1, and the treatment mode is set to be carried out in the order 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.

[0162] Comparative Example 1

[0163] A method for preparing a high-strength PET optical film is as follows:

[0164] Referring to the preparation steps of Example 1, the difference is that in step S1, the PET substrate is not blended with the copolyester, and is only directly connected to step S2 after drying until the preparation is completed.

[0165] Comparative Example 2

[0166] A method for preparing a high-strength PET optical film is as follows:

[0167] Referring to the preparation steps of Example 1, the difference is that the biaxial stretching in step S2 adopts a non-segmented stretching method. The specific parameters include: for longitudinal stretching, preheating roller at 80 °C → stretching temperature at 110 °C, setting a stretching ratio of 4; the relaxation method adopts intermittent relaxation, staying for 5 seconds every 10% stretching; for transverse stretching, preheating section at 90 °C → stretching section at 120 °C, setting a stretching ratio of 4.5.

[0168] Comparative Example 3

[0169] A method for preparing a high-strength PET optical film is as follows:

[0170] Referring to the preparation steps of Example 1, the difference is that the intermittent relaxation step is cancelled in the longitudinal stretching stage of step S2.

[0171] Comparative Example 4

[0172] A method for preparing a high-strength PET optical film is as follows:

[0173] Referring to the preparation steps of Example 1, the difference is that the step of plasma surface treatment is not set in step S3.

[0174] For the PET optical films prepared in Comprehensive Examples 1-5 and Comparative Examples 1-4, their comprehensive properties were compared horizontally. Among them, the mechanical properties include tensile strength and elongation at break, and the tests were carried out with reference to the national standard GB / T 1040-2006 "Determination of Tensile Properties of Plastics". The surface hardness was tested with reference to the national standard GB / T 6739-2006 "Paints and Varnishes - Determination of Film Hardness by the Pencil Method". The optical properties include the tests of light transmittance and haze, and the tests were carried out with reference to the national standard GB / T 2410-2008 "Determination of Light Transmittance and Haze of Transparent Plastics". The clarity part was tested with reference to the national standard GB / T 25273 "Method for Determining Haze of Films for Liquid Crystal Displays". The temperature resistance properties include the thermal shrinkage rate, which was tested 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 was 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:

[0175] Table 1 Comparison Table of Mechanical Properties of Examples 1-5 and Comparative Examples 1-4

[0176]

[0177] Table 2 Comparison Table of Optical Properties of Examples 1-5 and Comparative Examples 1-4

[0178]

[0179] Table 3 Comparison Table of Temperature Resistance Properties of Examples 1-5 and Comparative Examples 1-4

[0180]

[0181] As can be seen from the above comparison results, 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 greatly 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, thus 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 is not used for surface treatment 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 preparation method of a high-strength PET optical film, characterized in that, It includes the following steps: S1: After drying the PET substrate and the copolyester, mix them according to the mass ratio, add an interfacial compatibilizer, and after premixing, carry out segmented temperature-controlled melt extrusion. The extruded strip is cooled by a water-cooling tank and then pelletized and dried; for the copolyester, the preparation method 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, carry out 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 furan carboxylic acid as a capping monomer to carry out a capping copolymerization reaction; After the capping copolymerization reaction is completed, carry out discharging, quenching and pelletizing; S2: Carry out pre-crystallization treatment on the pellets after blending in S1, then carry out temperature-segmented controlled melt extrusion. The extruded melt is attached to a quenching roller by an electrostatic adsorption device, and an amorphous thick sheet is formed after casting. Then, the thick sheet is subjected to biaxial stretching, heat setting after stretching is completed, and finally cooling; The said biaxial stretching includes longitudinal stretching, and the parameters of the longitudinal stretching include: The segmented temperature is set as: preheating roller 80 °C, the first-stage stretching temperature 90 °C, the second-stage stretching temperature 110 °C; The stretching ratio is set as: the total stretching ratio is 3.5 - 4.5, among which, 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% of stretching, and dynamically adjust and release the internal stress through a tension roller; The said biaxial stretching includes transverse stretching, and the parameters of the transverse stretching include: The segmented temperature is set as: preheating section 90 °C, stretching section 110 °C, shaping section 120 °C; The stretching ratio is set as: the total stretching ratio is 4.0 - 5.0, among which, the initial stretching ratio is 1.5 - 2.0, and the final-stage stretching ratio is 2.5 - 3.0; S3: Cut off the uneven parts at the edges of the film sheet obtained in S2, then carry out plasma surface treatment, and after treatment, carry out winding and aging.

2. The preparation method of a high-strength PET optical film according to claim 1, wherein, Under the set reaction conditions in S1, specifically include: The preheating temperature of the reaction kettle is set at 120 °C, the stirring time of terephthalic acid, ethylene glycol and the catalyst antimony glycolate is set at 5 minutes, the oil bath temperature of the reaction kettle jacket is set at 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.

3. The preparation method of a high-strength PET optical film as claimed in claim 1, characterized in that, For the capping copolymerization reaction in S1, the start and end conditions are set as: the power of the stirring motor reaches 38 - 42 W.

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

5. The preparation method of a high-strength PET optical film according to claim 1, characterized in that, The temperature-segmented controlled melt extrusion described in S2 has the temperature segmented as follows: feeding section: 240 - 250 °C, melting section: 260 - 270 °C, die head section: 275 - 280 °C.

6. The preparation method of a high-strength PET optical film as described in claim 1, wherein, The plasma surface treatment described in S3 has the specific parameter settings including: The treatment power is set to 150 W; The gas source used is an Ar / O2 mixed gas source, and the gas ratio is the volume ratio of Ar / O2 3:1; The treatment mode is set to be carried out in the sequence of pulse mode / continuous mode.

7. A high-strength PET optical film, characterized in that, Prepared by the preparation method described in any one of claims 1 - 6.

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