High-temperature-resistant and penetration-resistant polyester film as well as preparation method and application thereof

Abstract

The invention discloses a high-temperature-resistant and penetration-resistant polyester film as well as a preparation method and application thereof, belongs to the technical field of high polymer materials, and aims to solve the problems that an existing polyester film is sharply increased in dielectric loss, relatively low in glass-transition temperature, poor in dimensional stability and low in cost due to chain end dipole relaxation in a high-temperature and high-frequency environment. And therefore, the application of the capacitor in the field of high-end capacitors is limited. According to the invention, end group functionalization is carried out on a PET raw material through anhydride, a chain end hydroxyl group is converted into a high-activity carboxyl group, and a specially designed modified functional unit containing epoxy cyclohexyl isobutyl POSS and 4-hydroxyethyl benzocyclobutene is accurately covalently grafted to the tail end of a PET molecular chain. And finally, forming a stable network structure by biaxial stretching and initiating a cross-linking reaction in a heat setting stage. The polyester film prepared by the invention has extremely low high-temperature dielectric loss, higher temperature-resistant grade and excellent electric penetration strength, and can be used as a substrate film of a high-performance capacitor film.

Description

Technical Field

[0001] This invention belongs to the field of polymer materials technology, specifically relating to a high-temperature resistant and penetration-resistant polyester film, its preparation method, and its application. Background Technology

[0002] Polyethylene terephthalate (PET), as an engineering plastic with excellent comprehensive properties, is widely used to prepare biaxially oriented polyester (BOPET) films due to its good mechanical properties, electrical insulation, chemical stability, and cost advantages. These films are then used as the basic dielectric material in film capacitors. However, with the rapid development of power electronics, 5G communications, new energy vehicles, and advanced rail transportation, increasingly stringent requirements are being placed on the operating temperature, frequency, and electric field strength of capacitors. The application of traditional BOPET films faces severe challenges, and their performance bottlenecks are mainly reflected in the following aspects:

[0003] (1) Sharp increase in dielectric loss at high temperature: One of the main failure mechanisms of capacitor films at high temperatures is the intensified movement of molecular chain segments, which leads to a sharp increase in dipole relaxation loss. The ends of PET molecular chains usually contain highly polar terminal hydroxyl and terminal carboxyl groups. Under the action of alternating electric fields and high temperatures, the mobility of these chain end groups is activated, and their dipole orientation cannot keep up with the frequency of electric field changes, resulting in significant dipole relaxation, which is macroscopically manifested as a sharp increase in the dielectric loss tangent (tanδ). This not only causes huge energy loss, but also triggers thermal breakdown, leading to catastrophic failure of the capacitor.

[0004] (2) Insufficient temperature resistance and dimensional stability: The glass transition temperature (Tg) of PET is usually around 80℃, which limits its long-term safe operating temperature. When the ambient temperature approaches or exceeds Tg, the amorphous regions of the PET molecular chains begin to move extensively, resulting in a decrease in the mechanical properties of the film and an increase in the thermal shrinkage rate, which seriously affects the capacitance stability and service life of the capacitor.

[0005] To improve the performance of polymer dielectric materials, physical blending modification is commonly employed. Existing technology CN116444799A discloses a POSS-based polyphosphazene and a PET / POSS-based polyphosphazene composite material. The technical solution involves first synthesizing a macromolecular flame retardant containing a polyhedral oligomeric silsesquioxane (POSS) and polyphosphazene structure, and then physically blending it with PET using a mixer. This approach is essentially an additive technology aimed at addressing the flame retardancy issue of PET. However, this physical blending method has inherent drawbacks: First, the additive and the PET matrix are only bonded by weak van der Waals forces, making compatibility a significant challenge. Additive molecules easily aggregate in the matrix, forming unevenly dispersed microregions. These microregions often become "electrical weaknesses" where the electric field is concentrated, potentially reducing the film's electrical penetration strength. Second, the introduction of additives cannot fundamentally suppress the movement of the PET molecular chains themselves, especially the movement of the terminal dipole groups, thus limiting its effectiveness in reducing high-temperature dielectric loss.

[0006] Therefore, there is an urgent need in this field for a new technical solution that can precisely suppress the key factors that lead to high-temperature dielectric loss from the molecular structure level, while significantly improving the temperature resistance and electrical penetration strength of PET films to meet the needs of next-generation high-performance capacitors. Summary of the Invention

[0007] The purpose of this invention is to provide a high-temperature resistant and penetration-resistant polyester film, its preparation method and application, so as to solve the technical problems of poor high-temperature dielectric properties and low temperature resistance of PET films in the prior art.

[0008] The specific technical solution is as follows:

[0009] A high-temperature resistant and penetration-resistant polyester film is disclosed. The polyester film is composed of modified polyester, the molecular structure of which includes a polyester backbone and modified functional units covalently bonded to the ends of the polyester backbone. The modified functional units are composed of cage-like silsesquioxane structural units and thermotropic crosslinkable structural units. The modified functional units form a crosslinking network through the reaction of the thermotropic crosslinkable structural units. The polyester is polyethylene terephthalate. The cage-like silsesquioxane structural units are polyhedral oligomeric silsesquioxanes. The thermotropic crosslinkable structural units contain benzocyclobutene groups. The benzocyclobutene groups are linked to the polyhedral oligomeric silsesquioxanes via ether bonds.

[0010] Furthermore, the connection structure between the polyester main chain and the modified functional unit is as follows: the terminal hydroxyl groups of the polyester main chain are modified with trimellitic anhydride to form aromatic dicarboxylic acid groups, and the modified functional unit is introduced to the terminal position of the polyester main chain through the aromatic dicarboxylic acid groups, so that at most one modified functional unit is introduced on a single polyester molecular chain.

[0011] Furthermore, the carboxyl groups at the ends of the polyester main chain that are not connected to the modified functional unit are capped with phenyl glycidyl ether.

[0012] Furthermore, the method for preparing the high-temperature resistant and penetration-resistant polyester film includes the following steps:

[0013] S1: Preparation of thermo-crosslinkable structural units: Grignard reagents were prepared using 4-bromobenzocyclobutene as raw material, and protected ethanol side chains were introduced. After deprotection, thermo-crosslinkable structural units were obtained.

[0014] S2: Preparation of modified functional units: Using an epoxy ring-opening reaction, the thermo-crosslinkable structural unit is connected to a polyhedral oligomeric silsesquioxane molecule to obtain a modified functional unit.

[0015] S3: Polyester modification: Polyester is dissolved in a solvent, trimellitic anhydride is added to react, and active carboxyl groups are introduced at the end of the polyester main chain to obtain the modified polyester.

[0016] S4: Crosslinking film formation: The modified polyester is subjected to esterification reaction with the modified functional unit, the content of the modified functional unit is controlled to be 4.0-8.0 wt%, and the residual carboxyl groups are capped to obtain the modified polyester; the modified polyester is made into a film by biaxial stretching process, and heat-treated at a preset crosslinking temperature during heat setting to initiate the crosslinking reaction of the thermo-crosslinkable structural unit and form a crosslinked network.

[0017] Furthermore, the protected ethanol side chain in step S1 is 2-(2-bromoethoxy)tetrahydropyran; the thermo-crosslinkable structural unit is 4-hydroxyethylbenzocyclobutene.

[0018] Furthermore, the polyhedral oligomeric silsesquioxane molecule mentioned in step S2 is epoxycyclohexylisobutyl POSS.

[0019] Furthermore, the epoxy ring-opening reaction described in step S2 is specifically carried out by reacting at 80°C for 12 hours in the presence of sodium hydroxide.

[0020] Furthermore, the end-capping treatment in step S4 involves adding phenyl glycidyl ether and triethylamine, and reacting at 80°C for 3 hours.

[0021] Furthermore, in step S4, the biaxial stretching process has the following characteristics: the transverse stretching temperature and the longitudinal stretching temperature are both 100-110℃, and the stretching ratio is 2.5-3.5 times; the heat treatment temperature is 220-250℃, and the time is 2 minutes.

[0022] Furthermore, the high-temperature resistant and penetration-resistant polyester film is applied to the base film of the capacitor film.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] (1) Significantly reduced dielectric loss at high temperatures: By anchoring the POSS cage with huge steric hindrance at the PET chain end, the movement of polar groups at the chain end is physically restricted, fundamentally suppressing dipole relaxation at high temperatures. At the same time, the network formed by BCB crosslinking further restricts the movement of the entire molecular chain. The synergistic effect enables the film to maintain extremely low dielectric loss even at temperatures as high as 150°C.

[0025] (2) Significantly improved temperature resistance and dimensional stability: The thermosetting three-dimensional network formed by BCB crosslinking significantly improves the material's Tg and heat distortion temperature. Even at temperatures much higher than the Tg of pure PET, the film still maintains excellent dimensional stability and mechanical strength, without softening or severe thermal shrinkage.

[0026] (3) Excellent electrical penetration resistance: POSS nanoparticles are uniformly dispersed in the system through covalent bonds, avoiding the agglomeration problem caused by physical blending and eliminating electrical weaknesses. At the same time, the POSS cage fills the free volume between the chains, improving the compactness of the material and effectively enhancing the DC breakdown field strength and electrical penetration resistance of the film. Attached Figure Description

[0027] Figure 1 This is a flowchart illustrating the preparation process of a high-temperature resistant and penetration-resistant polyester film according to the present invention.

[0028] Figure 2 This is a synthetic route diagram of a high-temperature resistant and penetration-resistant polyester according to the present invention;

[0029] Figure 3 The dielectric performance test results are for the embodiments and comparative samples of this invention;

[0030] Figure 4 The results of DC breakdown field strength tests for the embodiments and comparative samples of this invention are shown below.

[0031] Figure 5 The results show the thermal dimensional stability test results of the embodiments and comparative samples of this invention. Detailed Implementation

[0032] The following embodiments further explain and illustrate the technical solutions of the present invention. It should be specifically noted that each specific embodiment is a concretization and explanation of the technical solution and should not be considered as a limitation on the scope of protection of the present invention. Those skilled in the art still have the right to modify the technical solutions of these embodiments and make 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, nor do they cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions described in the present invention.

[0033] This invention proposes a high-temperature and penetration-resistant polyester film, its preparation method, and its applications. The core strategy is "terminal-group targeted grafting and in-situ crosslinking." The technical concept lies in precisely chemically repairing and enhancing the functional properties of the chain ends where the weakest links in PET are not altered, without changing the excellent main chain structure of PET. (See attached image.) Figure 1 The attached diagram shows the preparation process of the high-temperature resistant and penetration-resistant polyester film of this invention. Figure 2 The following is a synthetic route diagram for the high-temperature resistant and penetration-resistant polyester of this invention. The specific preparation steps are as follows:

[0034] S1. Preparation of 4-hydroxyethylbenzocyclobutene

[0035] Reagents and solvents: 4-bromobenzocyclobutene (BCB-Br); magnesium powder (Mg); iodine; 2-(2-bromoethoxy)tetrahydropyran; cuprous iodide (CuI); p-toluenesulfonic acid monohydrate; anhydrous tetrahydrofuran (THF); methanol.

[0036] (1) Preparation of BCB-MgBr

[0037] Reaction conditions and procedures: 1.2 mol equivalents (eq) of Mg were placed in a reaction flask and evacuated and purged with nitrogen three times. Anhydrous THF was added to bring the mixture concentration to 10 mL / g, and one iodine crystal was added. 1.0 eq of BCB-Br was dissolved in anhydrous THF and added dropwise to the Mg suspension, controlling the system temperature to not exceed 35°C. After the addition was complete, the mixture was refluxed and stirred at 65°C for 2 hours. The system turned grayish-brown, yielding a BCB-MgBr THF solution.

[0038] (2) Introduction of protected ethanol side chains

[0039] Reagents and solvents: 2-(2-bromoethoxy)tetrahydropyran; anhydrous THF; saturated ammonium chloride solution.

[0040] Reaction conditions and procedures: The above BCB-MgBr solution was cooled to 0-5℃. 1.1 eq of 2-(2-bromoethoxy)tetrahydropyran was dissolved in a small amount of anhydrous THF and slowly added dropwise to the reaction system. After the addition was complete, 0.05 eq of CuI was added to the system as a catalyst, and nitrogen protection was maintained. The temperature was raised to room temperature, and stirring was continued for 5 hours. After the reaction was complete, a saturated ammonium chloride solution was slowly added for quenching. The mixture was extracted with dichloromethane, and the organic phases were combined. The organic phase was dried over anhydrous magnesium sulfate, filtered, and rotary evaporated to obtain the intermediate BCB-CH2-CH2-O-THP.

[0041] (3) Deprotection

[0042] The above intermediate was dissolved in methanol. 0.05 eq p-toluenesulfonic acid monohydrate was added. The mixture was stirred at 30°C for 3 hours to complete THP deprotection. After the reaction was complete, the mixture was neutralized with saturated sodium bicarbonate solution. Extraction, drying, rotary evaporation, and purification by silica gel column chromatography were performed to finally obtain the target product, 4-hydroxyethylbenzocyclobutene (BCB-CH2-CH2-OH).

[0043] Reaction process:

[0044]

[0045] S2. Preparation of modified functional units

[0046] Using an epoxy ring-opening reaction, the BCB module prepared in the first step is precisely linked to an epoxy-containing POSS molecule to form a stable ether bond and a secondary hydroxyl group.

[0047] Reagents and solvents: epoxycyclohexylisobutyl POSS; sodium hydroxide (NaOH); anhydrous toluene.

[0048] Reaction steps: Equimolar amounts of epoxycyclohexylisobutyl POSS and BCB-CH2-CH2-OH were dissolved in anhydrous toluene. A catalytic amount of NaOH was added. Under nitrogen protection, the mixture was heated to 80°C and stirred for 12 hours. After the reaction was complete, the solvent was removed by rotary evaporation, and the modified functional unit BCB-POSS-OH was obtained by precipitation in acetonitrile, followed by filtration and washing.

[0049] Reaction process:

[0050]

[0051] S3. Polyester modification

[0052] Reagents and solvents: PET; trimellitic anhydride; anhydrous N-methylpyrrolidone (NMP); triethylamine (TEA).

[0053] Reaction Procedure: PET (1.0 eq) of terminal hydroxyl groups was added to a reaction flask. Anhydrous NMP was added to bring the concentration of the mixture to 10-15 mL / g PET. The mixture was stirred at 150°C until the PET was fully dissolved, and nitrogen gas was purged. 1.2 eq of trimellitic anhydride was added to the system in portions, maintaining a nitrogen atmosphere. 0.1 eq of TEA was added as a catalyst. The system temperature was maintained, and stirring was continued for 6 hours. During the reaction, the anhydride ring of trimellitic anhydride was ring-opened by the terminal hydroxyl groups of PET, forming a monoesterified trimellitic acid structure while retaining two free carboxyl groups. After the reaction, the reaction solution was cooled to below 60°C. The reaction solution was slowly poured into deionized water to precipitate the product. The precipitate was collected by filtration and repeatedly washed with deionized water to remove residual solvent and free trimellitic anhydride. The product was then vacuum dried at 70°C to constant weight to obtain the modified polyester (PET-CH2-O-CO-Ph-(COOH)2).

[0054] Reaction process:

[0055]

[0056] S4. Thin Film Preparation

[0057] Modified polyester is obtained by covalently grafting the carboxyl groups at the ends of modified PET with the hydroxyl groups in the modified functional units through an esterification reaction. Subsequently, in-situ crosslinking is achieved through film forming and heat setting to form a capacitor film substrate film with high temperature resistance, penetration resistance, and high dielectric strength.

[0058] Reagents and solvents: Stannous isooctanoate; Phenyl glycidyl ether; TEA; NMP.

[0059] (1) Esterification and capping: PET-CH2-O-CO-Ph-(COOH)2 was added to a reaction flask, with a carboxyl molar amount of 1.0 eq. NMP was added to make the mixture concentration 10 mL / g. The mixture was heated to 150 °C and nitrogen gas was introduced. The mixture was stirred until the PET was completely dissolved. A measured amount of BCB-POSS-OH was added in batches to ensure that the content of modified functional units in the final modified polyester film was 4.0-8.0 wt%. 0.025 eq of stannous isooctanoate was added, and the system temperature was maintained. The mixture was stirred for 8 hours. After the reaction was completed, the solution was cooled to 80 °C. 0.9 eq of phenyl glycidyl ether and 0.04 eq of TEA were slowly added dropwise with stirring. After the addition was completed, the reaction was continued at this temperature for 3 hours to allow the remaining carboxyl groups to react fully. The purpose of this step was to cap the remaining carboxyl groups, improve the thermal stability of PET during melt processing, and avoid acid-catalyzed degradation. After the reaction was completed, the solution was slowly added dropwise to 10 times its volume of cold deionized water with stirring to precipitate the precipitate. The mixture was stirred for 30 minutes. The solid product was collected by vacuum filtration and washed repeatedly with a large amount of deionized water several times to remove residual solvent, unreacted phenyl glycidyl ether, and other small molecule impurities. The obtained solid product was placed in a vacuum drying oven and dried at 70°C to constant weight to finally obtain modified polyester PET-g-POSS-BCB.

[0060] Reaction process:

[0061]

[0062] (2) Crosslinking film formation: The dried PET-g-POSS-BCB is melt-formed. Then, biaxial stretching is performed: the transverse and longitudinal stretching temperatures are 100-110℃, and the stretching ratio is 2.5-3.5 times. After stretching, the film is kept at 220-250℃ for 2 minutes. During this process, the BCB groups undergo a thermally induced ring-opening reaction to generate an active o-quinone dimethylene intermediate, which then undergoes an intermolecular addition reaction to form a stable covalent crosslinked network structure. After crosslinking is completed, the film is cooled to room temperature to obtain a modified polyester film with high temperature resistance, penetration resistance, and excellent dielectric properties.

[0063] Table 1. Reagents used in the following examples.

[0064]

[0065] Example 1

[0066] 1. Preparation of 4-hydroxyethylbenzocyclobutene

[0067] (1) Preparation of Grignard reagent

[0068] In a 50 mL dry three-necked flask, add Mg (0.15 g, 6.17 mmol), and evacuate and purge with nitrogen three times. Add 10 mL of anhydrous THF and one iodine grain as an initiator. Dissolve BCB-Br (0.94 g, 5.14 mmol) in 5 mL of anhydrous THF to prepare a solution, which is then placed in a constant-pressure dropping funnel. Slowly add the BCB-Br solution dropwise to the Mg suspension, controlling the system temperature to not exceed 35 °C using an ice-water bath. After the addition is complete, remove the ice bath and reflux and stir in a 65 °C oil bath for 2 hours to obtain a BCB-MgBr THF solution.

[0069] (2) Introduction of protected ethanol side chains

[0070] The Grignard reagent solution was cooled to 0°C. 2-(2-bromoethoxy)tetrahydropyran (1.18 g, 5.65 mmol) was dissolved in 5 mL of anhydrous THF and slowly added dropwise to the reaction system. After the addition was complete, CuI (0.05 g, 0.26 mmol) was added, and the mixture was allowed to warm naturally to room temperature, with stirring continued for 5 hours. After the reaction was complete, 20 mL of saturated ammonium chloride solution was slowly added dropwise under ice bath to quench the reaction. Extraction was performed using dichloromethane (3 × 20 mL). The organic phases were combined, dried over anhydrous magnesium sulfate, filtered, and the solvent was removed by rotary evaporation to obtain the intermediate BCB-CH2-CH2-O-THP.

[0071] (3) Deprotection yields BCB-CH2-CH2-OH

[0072] The crude product obtained in the previous step was dissolved in 20 mL of methanol. A catalytic amount of p-toluenesulfonic acid monohydrate (0.049 g, 0.257 mmol) was added. The mixture was stirred at 30 °C for 3 hours. The reaction was monitored by TLC. After the reaction was complete, saturated sodium bicarbonate solution was added to neutralize the solution. Most of the methanol was removed by rotary evaporation, followed by extraction with ethyl acetate (3 × 20 mL). The organic phases were combined, dried over anhydrous magnesium sulfate, and rotary evaporated to obtain the crude product. The crude product was purified by silica gel column chromatography to give BCB-CH2-CH2-OH.

[0073] 2. Preparation of modified functional units

[0074] In a 50 mL reaction flask, 0.54 g (0.538 mmol) of epoxycyclohexylisobutyl POSS and 0.08 g (0.538 mmol) of BCB-CH2-CH2-OH obtained in step 1 were added. 15 mL of anhydrous toluene was added to dissolve the POSS. 0.001 g (0.027 mmol) of NaOH was added. Under nitrogen protection, the mixture was heated to 80 °C and stirred for 12 hours. After the reaction was complete, it was cooled to room temperature. Toluene was removed by rotary evaporation to obtain a white crude solid. The crude solid was dissolved in 10 mL of THF and then poured into 50 mL of acetonitrile with vigorous stirring to precipitate. The white solid was collected by suction filtration, washed with a small amount of cold acetonitrile, and dried under vacuum to obtain pure BCB-POSS-OH.

[0075] 3. Polyester modification

[0076] In a 250 mL three-necked flask, 15.0 g (0.6 mmol) of PET was added, followed by 120 mL of anhydrous NMP. Under nitrogen protection, the mixture was heated to 150 °C and stirred until the PET was fully dissolved. Triterpenoid anhydride (0.28 g, 1.44 mmol) was added in portions. TEA (0.006 g, 0.06 mmol) was then added. The system temperature was maintained, and the mixture was stirred continuously for 6 hours. After the reaction was complete, the reaction solution was cooled to 60 °C and slowly poured into 1 L of deionized water with stirring to precipitate the solid. The white solid was collected by filtration and washed repeatedly with deionized water (3 × 100 mL) and methanol (2 × 50 mL). The solid was dried under vacuum at 70 °C to constant weight to obtain PET-CH2-O-CO-Ph-(COOH)2.

[0077] 4. Thin Film Preparation

[0078] (1) Esterification and end-capping

[0079] In a 250 mL reaction flask, add 10 g of PET-CH2-O-CO-Ph-(COOH)2 obtained in step S3, add 100 mL of anhydrous NMP, heat to 150 °C and purge with nitrogen, stirring until completely dissolved. Add BCB-POSS-OH (0.44 g, 0.4 mmol) in portions. Add stannous isooctanoate (0.016 g, 0.04 mmol). Maintain the system temperature and continue stirring for 8 hours. After the reaction is complete, cool the solution to 80 °C, and slowly add phenyl glycidyl ether (0.21 g, 1.4 mmol) and TEA (0.0065 g, 0.064 mmol) dropwise with stirring. After the addition is complete, continue the reaction at this temperature for 3 hours. After cooling the final reaction solution, slowly add 1 L of cold deionized water dropwise with vigorous stirring to precipitate. Collect the white solid product by suction filtration, wash repeatedly with plenty of deionized water, and finally wash with methanol. The modified polyester PET-g-POSS-BCB was finally obtained by vacuum drying at 70℃ to constant weight.

[0080] (2) Cross-linking to form a film

[0081] 2.0 g of dried PET-g-POSS-BCB was placed into a mold in a flatbed hot press. Preheating at 260°C for 5 minutes allowed for complete melting, followed by applying 10 MPa pressure and holding for 3 minutes to obtain a uniform film. The film was transferred to a biaxial stretching apparatus and stretched three times in both the transverse and longitudinal directions at 105°C. The stretched film was fixed onto a metal frame and placed in an oven at 235°C for heat setting for 2 minutes. This process completes the crosslinking of the BCB groups. The oven was then closed, and the film was allowed to cool to room temperature. The resulting modified polyester film, containing 4.2 wt% POSS-BCB groups, was obtained.

[0082] Example 2

[0083] Similar to Example 1, except that the amount of BCB-POSS-OH in step 4 is increased to 0.85 g (0.78 mmol), and the target modified polyester film is prepared with a POSS-BCB group content of 7.8 wt%.

[0084] Comparative Example 1

[0085] Unmodified raw PET chips were processed using the same process as step 4 (2) of Example 1, with the heat setting temperature set to the conventional 240°C, and finally BOPET films were prepared.

[0086] Comparative Example 2

[0087] 10g of unmodified raw PET chips were directly and physically mixed with 0.44g of BCB-POSS-OH monomer. The mixture was then hot-pressed and biaxially stretched to form a film according to the process parameters in step 4 (2) of Example 1, and finally BOPET / POSS film was prepared.

[0088] Comparative Example 3

[0089] Similar to Example 1, an excess of BCB-POSS-OH (1.0 g, 0.92 mmol) was added to prepare a PET-g-POSS-BCB film, wherein the content of POSS-BCB groups was 10 wt%.

[0090] Comparative Example 4

[0091] Similar to Example 1, except that steps 1 and 2 are skipped, and epoxy cyclohexyl isobutyl POSS is directly grafted onto the slightly functionalized PET to finally obtain PET-g-POSS.

[0092] Performance testing:

[0093] 1. Dielectric property testing

[0094] Refer to GB / T 1409-2006 "Recommended methods for measuring the permittivity and dielectric loss factor of electrical insulation materials at power frequency, audio frequency, and high frequency (including meter wave wavelength)".

[0095] Test objective: To evaluate the dielectric loss characteristics of thin film materials under different temperature conditions, and to characterize the energy loss level and dielectric stability of the materials in high-temperature environments.

[0096] Instruments and methods: broadband dielectric impedance spectrometer; using the parallel plate capacitor method.

[0097] Sample preparation: Cut circular samples with a diameter of approximately 20-30 mm from the prepared film, and prepare at least 3 parallel samples. The sample surface should be flat, clean, and free of pinholes or scratches. To ensure good electrical contact, a circular electrode with a diameter slightly smaller than the sample size should be vacuum-deposited at the center of both the upper and lower surfaces of the sample.

[0098] Specific testing steps: Place the sample with electrodes in the test fixture, ensuring good contact between the electrodes and the instrument probe. Under a fixed test frequency of 1kHz, perform a variable temperature test on the sample: the sample is heated from 30℃ to 170℃ at a heating rate of 2-3℃ / min. Record the dielectric loss tangent (tanδ) values ​​of the thin film sample at different temperatures to evaluate the dielectric loss variation trend of the material within the operating temperature range.

[0099] 2. Thermal performance testing (DSC)

[0100] Refer to GB / T 19466.2-2004 "Differential Scanning Calorimetry (DSC) for Plastics - Part 2: Determination of Glass Transition Temperature"

[0101] Test objective: To determine the upper limit of the material's operating temperature. The higher the temperature (Tg), the better the mechanical properties and dimensional stability at high temperatures.

[0102] Instruments and methods: Differential scanning calorimeter (DSC).

[0103] Sample preparation: Cut a small sample of 5-10 mg from the film and place it in an aluminum crucible.

[0104] Specific test procedures: Under a nitrogen atmosphere, the temperature is increased from 30℃ to 280℃ at a rate of 10℃ / min to eliminate thermal history. The temperature is then decreased to 30℃ at a rate of 20℃ / min. The temperature is then increased again to 280℃ at a rate of 10℃ / min, and the temperature gradient (Tg) is determined from the second heating curve.

[0105] 3. DC breakdown field strength test

[0106] Refer to GB / T 1408.1-2016 "Insulating Materials - Electrical Strength Tests - Part 1: Power Frequency Tests"

[0107] Test objective: To measure the ability of a material to resist electrical breakdown under a strong electric field, which is the most critical safety indicator for insulating materials.

[0108] Instruments and methods: High voltage DC breakdown tester. A ball-plate electrode system was used.

[0109] Sample preparation: Cut 100mm × 100mm square samples. To obtain statistically reliable results, at least 15-20 test samples should be prepared for each example / comparative example.

[0110] Specific test procedures: Measure the thickness at at least 5 points randomly in the center region of the sample using a micrometer and take the average value. Place the sample between the electrodes, immersing the entire test system in the insulating liquid to prevent surface flashover and partial discharge. Apply a DC voltage at a uniform ramp rate of 500V / s until the sample breaks down, and record the breakdown voltage (Vb). High-temperature test: Place the electrode system containing the sample in an oven, heat to 150℃ and hold for 20 minutes, then perform the ramp-up breakdown test.

[0111] Data processing: Breakdown field strength Eb = Vb / d, in kV / mm; where d is the average thickness of the sample, in mm. Each sample group is tested no less than 10 times, and the average value is taken as the final test result.

[0112] 4. Thermal dimensional stability test

[0113] Test objective: To evaluate the ability of a thin film to maintain its original dimensions at high temperatures, which is crucial for applications in precision electronic components.

[0114] Instruments and methods: Precision forced-air drying oven, high-precision ruler or two-dimensional image measuring instrument.

[0115] Sample preparation: Cut a 100mm×100mm square sample and accurately mark the measurement baseline on the sample.

[0116] Specific test procedures: At room temperature, accurately measure the initial length L0 of the sample in the longitudinal (MD) and transverse (TD) directions. Suspend or lay the sample flat on talc powder and place it in an oven preheated to 150°C for 30 minutes. Remove the sample and cool it to room temperature under standard conditions. Measure the length L1 of the sample again in the MD and TD directions. Calculate the heat shrinkage rate: (L0-L1) / L0×100%.

[0117] Table 2 shows the thermal and dielectric performance tests of each embodiment and comparative sample.

[0118]

[0119] Table 3. DC breakdown field strength and thermo-dimensional stability tests for each embodiment and comparative sample.

[0120]

[0121] Analysis of the comparison results in Tables 2 and 3:

[0122] (1) The Tg values ​​of unmodified Comparative Example 1 and Comparative Example 2 were 78.5℃ and 77.2℃, respectively, which are within the typical range of conventional BOPET. The Tg value of Comparative Example 2 was slightly lower, indicating that when BCB-POSS monomers were introduced only through physical blending, they not only failed to effectively constrain the PET molecular chains, but also weakened the regularity of chain segment stacking to some extent due to insufficient interfacial compatibility. In contrast, the Tg value of Comparative Example 4 increased to 82.3℃, indicating that the covalent grafting of POSS restricted the movement of PET molecular chains to some extent, but the increase was limited, indicating that simply introducing the rigid cage structure of POSS was not enough to significantly suppress the chain segment relaxation behavior at high temperatures. The Tg values ​​of Example 1 and Example 2 were further increased to 88.6℃ and 93.4℃, respectively, which were significantly higher than all the comparative example samples. This indicates that a stable cross-linked network structure was successfully constructed in the PET matrix through covalent grafting of BCB-POSS and subsequent in-situ thermal cross-linking, which produced a significant physical and chemical dual constraint effect on the movement of molecular chains. In Comparative Example 3, when the POSS-BCB content was increased to 10 wt%, the Tg was only 89.1 °C, which was lower than that of Example 2. This indicates that an excessively high crosslinking unit content may lead to uneven local network structure or internal stress accumulation, thereby weakening the effective restriction on the movement of the overall molecular chain. This further verifies that there is a reasonable optimal introduction range for POSS-BCB in this invention.

[0123] (2) In the low temperature range of 30-80℃, the differences in tanδ among the samples are relatively small. However, when the temperature rises above 120℃, especially when it approaches or exceeds Tg, the differences between the samples rapidly increase, as shown in the attached figure. Figure 3 The tanδ values ​​of Comparative Examples 1 and 2 increased sharply at 150℃ and 170℃, reflecting the intense molecular chain movement and significantly increased dipole relaxation loss in conventional BOPET and physical blend systems at high temperatures. The tanδ value of Comparative Example 4 decreased relatively at high temperatures, but still reached 14.0 × 10⁻⁶ at 170℃. -3This indicates that relying solely on the introduction of POSS is insufficient to fundamentally suppress high-temperature dielectric losses. In contrast, Examples 1 and 2 exhibited significantly lower tanδ values ​​across the entire test temperature range, with the advantage being particularly pronounced in the high-temperature range. This demonstrates that the covalent network formed by in-situ crosslinking of BCB can effectively suppress segment relaxation and dipole polarization losses at high temperatures. Comparative Example 3 showed a significant rebound in tanδ at 150℃ and 170℃, further illustrating that excessively high crosslinking unit content can introduce structural defects or localized stress concentrations, thereby weakening the stability of dielectric properties.

[0124] (3) The DC breakdown field strength of the unmodified Comparative Example 1 at room temperature and 150℃ was 335kV / mm and 185kV / mm, respectively, showing the typical characteristic of a significant decrease in the high-temperature breakdown strength of BOPET, as shown in the attached figure. Figure 4 The breakdown field strength of Comparative Example 2 was further reduced, indicating that the physical blending method not only failed to enhance insulation performance, but may also have created electric field distortion regions due to interface defects and phase separation. In Comparative Example 4, the breakdown field strength of PET-g-POSS was improved at both room temperature and high temperature, indicating that covalent grafting of POSS helps improve the structural density and electric field uniformity of the material, but the improvement is still limited. The breakdown field strengths of Examples 1 and 2 at room temperature and high temperature were significantly better than all the comparative examples. Furthermore, the thermal shrinkage rates of the example samples at 150℃ for 30 min were significantly reduced, with the MD and TD shrinkage rates of Example 2 being much lower than those of Comparative Example 1, as shown in the attached figure. Figure 4 , 5 This indicates that the cross-linked network effectively locks the orientation structure at high temperatures, thereby significantly improving the thermal dimensional stability of the film. The rebound in TD shrinkage rate in Comparative Example 3 further confirms that excessive cross-linking introduces uneven internal stress release, leading to a decrease in dimensional stability.

[0125] In summary, by introducing a BCB-POSS structure at the end of the PET molecular chain and performing in-situ thermal crosslinking after film formation, this invention successfully constructs a polyester film system that combines high-temperature structural stability, low dielectric loss, and high breakdown field strength. Compared with traditional BOPET and physical blend systems, this film exhibits significantly superior comprehensive electrical properties and dimensional stability under high-temperature conditions, making it particularly suitable for capacitor film substrate applications in high-temperature, high-electric-field environments.

Claims

1. A high-temperature resistant and penetration-resistant polyester film, said polyester film being composed of modified polyester, wherein the molecular structure of said modified polyester comprises a polyester backbone and modified functional units covalently bonded to the ends of said polyester backbone, characterized in that, The modified functional unit is composed of cage-like silsesquioxane structural units and thermo-crosslinkable structural units; the modified functional units form a crosslinking network through the reaction of the thermo-crosslinkable structural units; the polyester is polyethylene terephthalate; the cage-like silsesquioxane structural unit is a polyhedral oligomeric silsesquioxane; the thermo-crosslinkable structural unit contains benzocyclobutene groups; the benzocyclobutene groups are linked to the polyhedral oligomeric silsesquioxane through ether bonds.

2. The high-temperature resistant and penetration-resistant polyester film as described in claim 1, characterized in that, The connection structure between the polyester backbone and the modified functional unit is as follows: the terminal hydroxyl groups of the polyester backbone are modified with trimellitic anhydride to form aromatic dicarboxylic acid groups, and the modified functional unit is introduced to the terminal position of the polyester backbone through the aromatic dicarboxylic acid groups, so that at most one modified functional unit is introduced on a single polyester molecular chain.

3. The high-temperature resistant and penetration-resistant polyester film as described in claim 1, characterized in that, The carboxyl groups at the ends of the polyester main chain that are not connected to the modified functional unit are capped with phenyl glycidyl ether.

4. A method for preparing a high-temperature resistant and penetration-resistant polyester film according to any one of claims 1-3, characterized in that, Includes the following steps: S1: Preparation of thermo-crosslinkable structural units: Grignard reagents were prepared using 4-bromobenzocyclobutene as raw material, and protected ethanol side chains were introduced. After deprotection, thermo-crosslinkable structural units were obtained. S2: Preparation of modified functional units: Using an epoxy ring-opening reaction, the thermo-crosslinkable structural unit is connected to a polyhedral oligomeric silsesquioxane molecule to obtain a modified functional unit. S3: Polyester modification: Polyester is dissolved in a solvent, trimellitic anhydride is added to react, and active carboxyl groups are introduced at the end of the polyester main chain to obtain the modified polyester. S4: Crosslinking film formation: The modified polyester is subjected to esterification reaction with the modified functional unit, the content of the modified functional unit is controlled to be 4.0-8.0 wt%, and the residual carboxyl groups are capped to obtain the modified polyester; the modified polyester is made into a film by biaxial stretching process, and heat-treated at a preset crosslinking temperature during heat setting to initiate the crosslinking reaction of the thermo-crosslinkable structural unit and form a crosslinked network.

5. The method for preparing a high-temperature resistant and penetration-resistant polyester film as described in claim 4, characterized in that, The protected ethanol side chain in step S1 is 2-(2-bromoethoxy)tetrahydropyran; the thermo-crosslinkable structural unit is 4-hydroxyethylbenzocyclobutene.

6. The method for preparing a high-temperature resistant and penetration-resistant polyester film as described in claim 4, characterized in that, The polyhedral oligomeric silsesquioxane molecule mentioned in step S2 is epoxycyclohexylisobutyl POSS.

7. The method for preparing a high-temperature resistant and penetration-resistant polyester film as described in claim 4, characterized in that, The epoxy ring-opening reaction described in step S2 is specifically carried out at 80°C for 12 hours in the presence of sodium hydroxide.

8. The method for preparing a high-temperature resistant and penetration-resistant polyester film as described in claim 4, characterized in that, The end-capping treatment involves adding phenyl glycidyl ether and triethylamine, and reacting at 80°C for 3 hours.

9. The method for preparing a high-temperature resistant and penetration-resistant polyester film as described in claim 4, characterized in that, The biaxial stretching process described in step S4: the transverse stretching temperature and the longitudinal stretching temperature are both 100-110℃, and the stretching ratio is 2.5-3.5 times; the heat treatment temperature is 220-250℃, and the time is 2 minutes.

10. The application of the high-temperature resistant and penetration-resistant polyester film according to any one of claims 1-3, characterized in that, The high-temperature resistant and penetration-resistant polyester film is used as the base film for capacitor films.

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