High-temperature-resistant, high-flatness, high-reflectivity biaxially-stretched polyester film and preparation method thereof

Abstract

The application provides a high-temperature-resistant, high-flatness and high-reflectivity biaxially-stretched polyester film, characterized in that the high-temperature-resistant, high-flatness and high-reflectivity biaxially-stretched polyester film is of ABA three-layer structure, the A layer comprises the following raw materials: polyester, inorganic particle master batch, the B layer comprises the following raw materials: polyester, inorganic particle master batch, incompatible resin master batch, nucleating agent and toughening agent, and the nucleating agent is prepared by reacting polystyrene-allyl alcohol copolymer with epoxy cage polysilsesquioxane. The nucleating agent is prepared by using polystyrene-allyl alcohol copolymer and epoxy cage polysilsesquioxane as raw materials through polycondensation reaction by means of emulsion method, and the polyester film using the nucleating agent has excellent weather resistance, and in particular, the size retention ability of the film is stronger after experiencing long time high temperature.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of polyester film, and particularly relates to a high-temperature-resistant, high-flatness and high-reflectivity biaxially-stretched polyester film and a preparation method thereof. BACKGROUND

[0002] The polyester film with high reflectivity has the advantages of large density and good flatness, and is suitable for large-size display. The polyester film is an important component of a backlight module, is located at the bottom of the backlight module, and mainly functions to reflect light leaked from a light-blocking plate back to a panel to enhance brightness.

[0003] At present, the preparation of high-reflectivity polyester film at home and abroad mostly adopts a micropore method. The micropore method is a method for improving reflectivity and reducing weight by using the difference in refractive index between a base resin of a film and a void. One of the principle methods for preparing a reflectivity polyester film by the micropore method is to obtain micropores (or voids) by adding incompatible components (organic or inorganic incompatible components), that is, adding incompatible components to the base resin of the film, and then stretching in a biaxial or uniaxial direction. When the stretching stress is greater than the adhesion between the base resin and the incompatible components, interface debonding occurs, micropores (or voids) are generated between the base resin and the incompatible components, and the reflectivity of the film increases with the increase in the number of micropores (or voids). For example, patent CN110837142B discloses a white reflective polyester film. The white reflective polyester film has an ABA three-layer structure, the A layer is a surface layer, and the B layer is an intermediate layer. The A layer comprises polyester chips, inorganic particle polyester master batches and toughening polyester master batches. The B layer comprises polyester chips, inorganic particle polyester master batches, incompatible resin master batches and toughening polyester master batches. Patent CN108773141B discloses a polyester reflective film, a preparation method and application thereof. The polyester reflective film has an AB double-layer film structure or an ABA three-layer film structure. The A layer film is prepared by mixing the following components in a mass ratio: 40-68 parts of polyethylene terephthalate, 7-35 parts of titanium dioxide master batch, 10-25 parts of barium sulfate master batch and 1-3 parts of organic silicon particle master batch. The B layer film is prepared by mixing the following components in a mass ratio: 65-90 parts of polyethylene terephthalate, 2-20 parts of titanium dioxide master batch and 3-15 parts of organic silicon particle master batch. The above methods both add exogenous incompatible components, including inorganic particles and organic incompatible particles, to obtain a polyester film with high reflectivity.

[0004] However, along with the rapid increase in the demand for display equipment, people have more and more diversified requirements for display performance. Lightweight, thinness, softness, high brightness and wide color gamut are mainstreams. In addition, there are special requirements for high strength, high weather resistance, moisture resistance and high temperature resistance.

[0005] The above technology replaces part of the inorganic particles with organic incompatible particles, which not only produces micropores and reduces the density of the film, but also reduces the adverse effects of precipitation, migration and local optical unevenness during long-term high-temperature use of the inorganic particle-containing film. However, the organic incompatible particles have large particle size and poor blending property with polyester, which not only does not play the role of nucleating agent, but also seriously reduces the crystallinity of the polyester, resulting in poor dimensional stability of the film, especially after long-term high-temperature treatment. These conditions are fatal defects in large-area display backlights.

[0006] Therefore, it is necessary to develop a polyester film with high temperature resistance, high flatness and high reflectivity to solve the above technical problems. SUMMARY

[0007] To solve the above technical problems, the present application provides a high-temperature-resistant, high-flatness, high-reflectivity biaxially oriented polyester film and a preparation method thereof. A nucleating agent is prepared by polycondensation reaction using polystyrene-allyl alcohol copolymer and epoxy-based cage-shaped polyhedral oligomeric silsesquioxane as raw materials. The polyester film using this nucleating agent has excellent weather resistance, especially after long-term high-temperature treatment, the dimensional stability of the film is strong.

[0008] A high-temperature-resistant, high-flatness, high-reflectivity biaxially oriented polyester film, the high-temperature-resistant, high-flatness, high-reflectivity biaxially oriented polyester film is a three-layer structure of ABA, the A layer comprises the following raw materials: polyester, inorganic particle master batch, the B layer comprises the following raw materials: polyester, inorganic particle master batch, incompatible resin master batch, nucleating agent, toughening agent, the nucleating agent is prepared by reaction of polystyrene-allyl alcohol copolymer and epoxy-based cage-shaped polyhedral oligomeric silsesquioxane.

[0009] Further, the A layer comprises the following raw materials by weight: 60-80 parts of polyester, 20-40 parts of inorganic particle master batch, the B layer comprises the following raw materials by weight: 10-60 parts of polyester, 10-70 parts of inorganic particle master batch, 10-20 parts of incompatible resin master batch, 0.3-0.5 parts of nucleating agent, 1-3 parts of toughening agent, the molar ratio of polystyrene-allyl alcohol copolymer and epoxy-based cage-shaped polyhedral oligomeric silsesquioxane is 1:3-5. Preferably, the B layer comprises the following raw materials by weight: 50-60 parts of polyester, 10-20 parts of inorganic particle master batch, 10-20 parts of incompatible resin master batch, 0.3-0.5 parts of nucleating agent, 1-3 parts of toughening agent.

[0010] The epoxy-based cage-shaped polyhedral oligomeric silsesquioxane is selected from one or a combination of two or more of octa-epoxy cyclohexyl ethyl cage-shaped polyhedral oligomeric silsesquioxane and octa-glycidyl ether propyl POSS.

[0011] The polystyrene-allyl alcohol copolymer has a hydroxyl content of 6-8wt%, and a weight average molecular weight of 2000-3000.

[0012] The nucleating agent is prepared by a method comprising the following steps:

[0013] The epoxy cage polysilsesquioxane, polystyrene-allyl alcohol copolymer, catalyst, and co-solvent are dispersed into water, stirred to form a stable emulsion, heated and kept at a constant temperature for reaction, and then freeze-dried after the reaction to obtain the nucleating agent.

[0014] The temperature is raised to 80-100℃, and the reaction time is 3-5h. The catalyst is a tertiary amine catalyst, including one or a combination of two of triethylamine and benzyl dimethyl amine. The amount of the catalyst is 1-1.5wt% of the total mass of the epoxy cage polysilsesquioxane and polystyrene-allyl alcohol copolymer. The co-solvent is acetone, and the mass of the co-solvent accounts for 20-30% of the total mass of the co-solvent and water. The mass of the epoxy cage polysilsesquioxane and polystyrene-allyl alcohol copolymer accounts for 10-20wt% of the mass of the emulsion. The stirring speed is 800-1500rpm.

[0015] The polyester has an intrinsic viscosity of 0.6-0.75dL / g, and is selected from one or a combination of two or more of polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate.

[0016] The preparation raw material of the inorganic particle master batch comprises 40-70wt% base material polyester resin, 30-70wt% inorganic particles (preferably 30-50wt% inorganic particles), and 1-5wt% dispersant A. The average particle size of the inorganic particles is 15-2000nm (preferably 150-250nm), and is selected from one or a combination of two or more of barium sulfate, calcium carbonate, silicon dioxide, titanium dioxide, aluminum oxide, clay, and kaolin. The base material polyester resin is selected from one or a combination of two or more of polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate, and has an intrinsic viscosity of 0.6-0.75dL / g. The dispersant A is selected from one or a combination of two of zinc stearate, ethylene bis-stearamide, erucic amide, solid paraffin, liquid paraffin, polyethylene wax, and oxidized polyethylene wax. The preparation method of the inorganic particle master batch is not particularly limited, and a conventional melt co-extrusion can be used.

[0017] The preparation raw material of the incompatible resin master batch comprises 40-70 wt% base material polyester resin, 30-50 wt% incompatible resin, 3-5 wt% dispersant B. The average particle size of the incompatible resin is 1-5 μm, the incompatible resin is selected from one or a combination of two or more of silicone resin, polymethylpentene, polystyrene, polyethylene, polypropylene, polybutylene, polymethylstyrene, further, the incompatible resin is polypropylene; the base material polyester resin is selected from one or a combination of two or more of polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, and the intrinsic viscosity is 0.6-0.75 dL / g; the dispersant B is selected from one or a combination of two of polystyrene-allyl alcohol copolymer, maleic anhydride grafted polypropylene, maleic anhydride grafted polyethylene, further, the dispersant B is polystyrene-allyl alcohol copolymer. The inventor unexpectedly found that when the incompatible resin is polypropylene and the dispersant B is polystyrene-allyl alcohol copolymer, the high-temperature dimensional deformation resistance of the polyester film is the best. The preparation method of the incompatible resin master batch is not particularly limited, and conventional twin-screw melt co-extrusion granulation can be used, and the extrusion temperature is 230-300°C.

[0018] The toughening agent is fumed silica.

[0019] The thickness of the ABA three-layer structure is 50-250 μm, and the thickness ratio of the ABA three-layer structure is 5-20:60-90:5-20.

[0020] The application also provides a preparation method of the high-temperature-resistant, high-flatness, high-reflectivity biaxially stretched polyester film, comprising the following steps:

[0021] S1 granulation: uniformly mix the raw materials of A and B layers respectively and perform twin-screw mixing and granulation to obtain raw material particles of A and B layers;

[0022] S2 sheet casting: use a three-layer co-extrusion process to prepare a composite thick sheet from the raw material particles of A and B layers obtained in step S1, and then quench the sheet on a cold drum;

[0023] S3 film formation: perform longitudinal stretching, transverse stretching, heat setting, cooling, winding and packaging on the sheet obtained in step S2 to obtain the high-temperature-resistant, high-flatness, high-reflectivity biaxially stretched polyester film.

[0024] The temperature for the twin-screw mixing and granulation in step S1 is 230-300°C;

[0025] The die temperature in the three-layer co-extrusion process in step S2 is 285-295°C, the cold drum temperature is 20-40°C, and the cold drum rotation speed is 30-100 m / min;

[0026] The longitudinal stretching ratio of the longitudinal stretching of step S3 is 2-4 times, the stretching ratio of the transverse stretching is 3-5 times, and the heat setting temperature is 150-240℃.

[0027] Compared with the prior art, the present application has the following advantages:

[0028] The present application uses an emulsion method to prepare a nucleating agent from polystyrene-allyl alcohol copolymer and an epoxy-based cage-shaped poly-silsesquioxane through a condensation reaction. The polyester film using this nucleating agent has excellent weather resistance, especially after long-term high-temperature treatment, the film has strong size retention ability.

[0029] The polyester film of the present application has low density, high reflectivity, and good flatness and stiffness during large-size product processing and application. DETAILED DESCRIPTION

[0030] To make the purpose, technical solutions and advantages of the present application clearer, the technical solutions of the present application will be described in detail below. The following examples facilitate better understanding of the present application, but do not limit the present application. The experimental methods in the following examples are conventional methods, unless otherwise specified.

[0031] Polyethylene terephthalate was purchased from DuPont, with intrinsic viscosity of 0.65 dL / g;

[0032] Rutile titanium dioxide CY-T200 was purchased from Zhejiang Jiupeng New Material Co., Ltd., with average particle size of 200 nm;

[0033] Polystyrene-allyl alcohol copolymer SAA-101 was purchased from LyondellBasell, with weight average molecular weight of 2500 and hydroxyl content of 7.7 wt%.

[0034] Polystyrene-allyl alcohol copolymer SAA-100 was purchased from LyondellBasell, with weight average molecular weight of 3000 and hydroxyl content of 6.4 wt%.

[0035] Maleic anhydride grafted polyethylene 573P was purchased from Honeywell.

[0036] Polypropylene T1701 was purchased from Sinopec Yanshan Petrochemical Co., Ltd.

[0037] Silicone resin powder Silok-5593 was purchased from Silok, with average particle size of 3.5 μm.

[0038] Preparation of inorganic particle master batch

[0039] Preparation example a

[0040] 50 parts of rutile titanium dioxide CY-T200, 47 parts of polyethylene terephthalate, 3 parts of ethylene bis-stearamide were mixed uniformly and added into a twin-screw extruder for extrusion granulation, the temperature of each zone of the twin-screw extruder was as follows: the temperature of zone I was 70-120°C, the temperature of zone II was 130-200°C, the temperature of zone III was 200-250°C, the temperature of zone IV was 200-250°C, the temperature of zone V was 200-250°C, the temperature of the die head was 200-250°C, and the residence time of the material in the twin-screw was 4.5 minutes.

[0041] Preparation of incompatible resin master batch

[0042] Preparation Example b1

[0043] 50 parts of polypropylene T1701, 45 parts of polyethylene terephthalate, 5 parts of polystyrene-allyl alcohol copolymer SAA-100 were mixed uniformly and added into a twin-screw extruder for extrusion granulation, the temperature of each zone of the twin-screw extruder was as follows: the temperature of zone I was 240°C; the temperature of zone II was 245°C; the temperature of zone III was 250°C; the temperature of zone IV was 255°C; the temperature of zone V was 260°C; the temperature of zone VI was 260°C; the temperature of zone VII was 260°C; the temperature of zone VIII was 260°C; the temperature of zone IX was 260°C; the temperature of the die head was 250°C, the vacuum degree was -0.08 Pa, and the residence time of the material in the twin-screw was 4.5 minutes.

[0044] Preparation Example b2 SAA-100 helps the dispersion of incompatible resins and improves reflectivity

[0045] The rest was the same as Preparation Example b1, except that the extrusion raw material was 50 parts of polypropylene T1701, 47 parts of polyethylene terephthalate, and 3 parts of polystyrene-allyl alcohol copolymer SAA-100.

[0046] Preparation Example b3

[0047] The rest was the same as Preparation Example b1, except that the extrusion raw material was 30 parts of polypropylene T1701, 65 parts of polyethylene terephthalate, and 5 parts of polystyrene-allyl alcohol copolymer SAA-100.

[0048] Preparation Example b4

[0049] The rest was the same as Preparation Example b1, except that equal amounts of polystyrene-allyl alcohol copolymer SAA-101 were used instead of polystyrene-allyl alcohol copolymer SAA-100.

[0050] Preparation Example b5

[0051] The rest was the same as Preparation Example b1, except that equal amounts of silicone powder Silok-5593 were used instead of polypropylene T1701.

[0052] Preparation Example b6

[0053] The rest is the same as Preparation Example Bl, except that equal mass of maleic anhydride grafted polyethylene 573P is used instead of polystyrene-allyl alcohol copolymer SAA-100.

[0054] Preparation of nucleating agent

[0055] Preparation Example Cl

[0056] 0.001 mol of octaepoxy cyclohexyl ethyl cage polysilsesquioxane, 0.003 mol of polystyrene-allyl alcohol copolymer SAA-100, 0.11 g of triethylamine, 18 g of acetone were dispersed into 72 g of water to form a stable emulsion by stirring at a rotation speed of 800 rpm, and the temperature was raised to 100°C and kept constant for 3 h of reaction. After the reaction was completed, freeze-drying was performed to obtain the nucleating agent. The average particle size was 79 nm.

[0057] Preparation Example C2

[0058] The rest is the same as Preparation Example Cl, except that the amount of polystyrene-allyl alcohol copolymer SAA-100 is 0.005 mol. The average particle size was 98 nm.

[0059] Preparation Example C3

[0060] The rest is the same as Preparation Example Cl, except that equal molar amount of polystyrene-allyl alcohol copolymer SAA-101 is used instead of SAA-100. The average particle size was 76 nm.

[0061] Example 1

[0062] A layer raw material: 60 parts of polyethylene terephthalate, 40 parts of preparation example a inorganic particle master batch.

[0063] B layer raw material: 50 parts of polyethylene terephthalate, 20 parts of preparation example a inorganic particle master batch, 20 parts of preparation example Bl incompatible resin master batch, 0.5 parts of preparation example Cl nucleating agent, 3 parts of Degussa R972 fumed silica.

[0064] S1 granulation: the raw materials of A and B layers were respectively mixed uniformly and subjected to double screw extrusion granulation to obtain raw material particles of A and B layers; the temperature of each zone of the double screw extruder was as follows: first zone 245°C; second zone 250°C; third zone 255°C; fourth zone 260°C; fifth zone 265°C; sixth zone 270°C; seventh zone 265°C; eighth zone 260°C; ninth zone 255°C; the temperature of the die was 255°C, the vacuum degree was -0.08 Pa, and the residence time of the material in the double screw was 3.5 minutes;

[0065] S2 casting sheet: the raw material particles of A, B layers obtained in step S1 were used to prepare a composite thick sheet by three-layer co-extrusion process, and then the casting sheet was quenched on a cold drum; wherein the die temperature in the three-layer co-extrusion process was 285°C, the cold drum temperature was 20°C, and the cold drum rotation speed was 75 m / min;

[0066] S3 film formation: the casting sheet obtained in step S2 was subjected to longitudinal stretching with a longitudinal stretching ratio of 3.5 times, transverse stretching with a transverse stretching ratio of 4.5 times, heat setting at a heat setting temperature of 180°C, cooling, winding and packaging to obtain the high-temperature-resistant, high-flatness, high-reflectivity biaxially stretched polyester film. The film thickness was 235 mm, and the thickness ratio of the ABA three-layer structure was 20:60:20.

[0067] Examples 2-6

[0068] The rest was the same as Example 1, except that the incompatible resin master batch was prepared as b2-b6, respectively.

[0069] Examples 7-8

[0070] The rest was the same as Example 1, except that the nucleating agent was prepared as preparation examples c2-c3, respectively.

[0071] Example 9

[0072] The rest was the same as Example 1, except that the amount of nucleating agent prepared in preparation example c1 in the B layer raw material was 0.3 parts.

[0073] Example 10

[0074] The rest was the same as Example 1, except that,

[0075] A layer raw material: 60 parts of polyester, 40 parts of inorganic particle master batch prepared in preparation example a.

[0076] B layer raw material: 60 parts of polyester, 10 parts of inorganic particle master batch prepared in preparation example a, 10 parts of incompatible resin master batch prepared in preparation example b1, 0.5 parts of nucleating agent prepared in preparation example c1, 3 parts of Degussa R972 fumed silica.

[0077] Comparative Example 1

[0078] The rest was the same as Example 1, except that no nucleating agent was added to the B layer. There was no effect on reflectivity

[0079] The polyester films prepared in the above examples and comparative examples were tested for the following properties:

[0080] Reflectance: According to GB / T 3979-2008, the reflectance of the polyester film to the light of 400-700 nm wavelength was tested on a spectrophotometer, and the reflectance was the weighted average of the reflectance of every 10 nm wavelength of 400-700 nm, and the weight corresponded to the energy distribution curve of D65 light source.

[0081] Film density: According to GB / T 1033.1-2008 standard, a FA / JA series electronic balance was used, and the sample of 100 mm x 100 mm was taken. The thickness of the sample at nine different points around the sample was tested by a micrometer, and the length of the four edges of the sample was measured by a steel ruler. According to the formula "density = mass / volume", the density of the film was calculated.

[0082] Thermal shrinkage: the dimensional stability of the sample at high temperature was measured by comparing the size change of the sample before and after heating. The sample was prepared according to GB / T 12027-2004, and the size of the sample was 120 mm x 120 mm. The length of the marked length was 100 mm x 100 mm. The thermal shrinkage of the marked length was measured at 85℃ for 30 min. The time from taking out the sample to the end of the test was not more than 2 min. The length and width were each measured at 3 points to take the average value. The sample size was measured at the same position before and after heating, and the shrinkage before and after heating was calculated.

[0083] Thermal aging: the polyester film prepared by the examples and the comparative examples was aged in an aging oven at 85℃ for 500 h. The length and width were each measured at 3 points to take the average value. The sample size was measured at the same position before and after heating, and the shrinkage before and after thermal aging was calculated.

[0084] Table 1

[0085]

[0086] It can be seen from the reflectance test results in Table 1 that using polystyrene-allyl alcohol copolymer as the dispersant of incompatible resin polypropylene has obvious effect of improving the reflectance of the film and the anti-high-temperature dimensional deformation ability. It can be seen from the thermal shrinkage performance test results that the nucleating agent not only has the effect of improving the thermal shrinkage performance at 85℃ / 30min, but also can significantly improve the thermal shrinkage performance after aging.

Claims

1. A high temperature resistant, high flatness, high reflectivity biaxially oriented polyester film, characterized in that, The high-temperature-resistant, high-flatness, high-reflectivity biaxially stretched polyester film has an ABA three-layer structure, the A layer comprises the following raw materials in parts by weight: 60-80 parts of polyester, 20-40 parts of inorganic particle master batch; the B layer comprises the following raw materials in parts by weight: 10-60 parts of polyester, 10-70 parts of inorganic particle master batch, 10-20 parts of incompatible resin master batch, 0.3-0.5 parts of nucleating agent, 1-3 parts of toughening agent; The nucleating agent is prepared by polycondensation reaction of polystyrene-allyl alcohol copolymer and epoxy cage-shaped polyhedral oligomeric silsesquioxane; the molar ratio of the polystyrene-allyl alcohol copolymer and the epoxy cage-shaped polyhedral oligomeric silsesquioxane is 1:3-5; the epoxy cage-shaped polyhedral oligomeric silsesquioxane is selected from one or a combination of more than two of octa-epoxy cyclohexyl ethyl cage-shaped polyhedral oligomeric silsesquioxane and octa-glycidyl ether oxygen propyl POSS; the polystyrene-allyl alcohol copolymer has a hydroxyl content of 6-8 wt% and a weight average molecular weight of 2000-3000; The preparation raw materials of the incompatible resin master batch comprise 40-70 wt% of base polyester resin, 30-50 wt% of incompatible resin, and 3-5 wt% of dispersant B; the incompatible resin is polypropylene; the base polyester resin is selected from one or a combination of more than two of polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate; the base polyester resin has an intrinsic viscosity of 0.6-0.75 dL / g; and the dispersant B is polystyrene-allyl alcohol copolymer.

2. The high-temperature-resistant, high-flatness, high-reflectivity biaxially-stretched polyester film according to claim 1, characterized in that, The nucleating agent is prepared by a method comprising the following steps: The epoxy cage-shaped polyhedral oligomeric silsesquioxane, polystyrene-allyl alcohol copolymer, catalyst, and cosolvent are dispersed into water, stirred to form a stable emulsion, heated and kept at a constant temperature for reaction, and then freeze-dried after the reaction to obtain the nucleating agent.

3. The high-temperature-resistant, high-flatness, high-reflectivity biaxially-stretched polyester film according to claim 2, characterized in that, The temperature is raised to 80-100℃, and the reaction time is 3-5 h; the catalyst is a tertiary amine catalyst; the amount of the catalyst is 1-1.5 wt% of the total mass of the epoxy cage-shaped polyhedral oligomeric silsesquioxane and polystyrene-allyl alcohol copolymer; the cosolvent is acetone, and the mass of the cosolvent accounts for 20-30% of the total mass of the cosolvent and water; and the mass of the epoxy cage-shaped polyhedral oligomeric silsesquioxane and polystyrene-allyl alcohol copolymer accounts for 10-20 wt% of the mass of the emulsion.

4. The high-temperature-resistant, high-flatness, high-reflectivity biaxially-stretched polyester film according to claim 3, characterized in that, The tertiary amine catalyst is selected from one or a combination of more than two of triethylamine and benzyl dimethyl amine.

5. The high-temperature-resistant, high-flatness, high-reflectivity biaxially-stretched polyester film according to claim 1, characterized in that, The preparation raw materials of the inorganic particle master batch comprise 40-70 wt% of base polyester resin, 30-70 wt% of inorganic particles, and 1-5 wt% of dispersant A; the average particle size of the inorganic particles is 15-2000 nm, and the inorganic particles are selected from one or a combination of more than two of barium sulfate, calcium carbonate, silicon dioxide, titanium dioxide, aluminum oxide, clay, and kaolin.

6. The high-temperature-resistant, high-flatness, high-reflectivity biaxially-stretched polyester film according to claim 1, characterized in that, The average particle size of the incompatible resin is 1-5 μm.

7. The high-temperature-resistant, high-flatness, high-reflectivity biaxially-stretched polyester film according to claim 1, characterized in that, The thickness of the ABA three-layer structure is 50-250 μm, and the thickness ratio of the ABA three-layer structure is 5-20:60-90:5-20.

8. A method for preparing the high-temperature-resistant, high-flatness, high-reflectivity biaxially-stretched polyester film according to any one of claims 1-7, comprising the following steps: S1: granulation: uniformly mixing and twin-screw mixing the raw materials of layers A and B respectively to obtain raw material particles of layers A and B; S2: sheet casting: using a three-layer co-extrusion process to prepare a composite thick sheet from the raw material particles of layers A and B obtained in step S1, and then quenching the sheet on a cold drum; S3: film forming: longitudinally stretching, transversely stretching, heat setting, cooling, winding and packaging the sheet obtained in step S2 to obtain the high-temperature-resistant, high-flatness, high-reflectivity biaxially-stretched polyester film.

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