A biaxially oriented polyethylene naphthalate film, a preparation method and application thereof

Abstract

The present application relates to the technical field of film material preparation for electrical insulation, and particularly relates to a biaxially oriented polyethylene naphthalate film, a preparation method and application, the method is characterized in that: the first annealing is performed on the PEN cast sheet to fully relax the molecular chain to eliminate the internal stress and micro-orientation caused by the casting extrusion, then the biaxial stretching is performed on the PEN cast sheet after the first annealing to make the PEN molecular chain biaxial orientation and induce crystallization, and the heat setting after the stretching can prevent the film from shrinking during the temperature reduction, and the size stability of the film is maintained. Finally, the second annealing is performed by maintaining the tension state of the preliminary biaxial stretching film, the crystal structure of the PEN is further improved, the generation and migration of the carrier are significantly inhibited, and the dielectric strength and high-temperature stability of the film are improved. The method is simple, does not depend on additives and other modification methods, is fully compatible with the existing PET film production process, is convenient for realizing large-scale production, and solves the problems of the existing PEN insulation film preparation, such as great difficulty, and difficulty in meeting the service requirements of F-grade heat-resistant grade.

Description

Technical Field

[0001] This invention relates to the field of electrical insulation film material preparation technology, specifically to a biaxially oriented polyethylene naphthalate film, its preparation method, and its application. Background Technology

[0002] Electrical insulation films, as core supporting materials for electrical equipment, are widely used in motors, electrical appliances, electronic components, and other electrical equipment. They are primarily used for winding insulation, interlayer insulation, and structural insulation, and their performance directly affects the safety, operational stability, and service life of electrical equipment. With the development of electrical equipment towards higher power, miniaturization, and higher reliability, higher requirements are placed on insulating films in terms of heat resistance, insulation properties, mechanical strength, and dimensional stability. Developing high-performance insulating films with balanced overall performance, good process adaptability, and controllable cost has become a core issue of continuous focus in the field of electrical materials.

[0003] Currently, the most widely used thin film materials in the electrical insulation field mainly include two categories: biaxially oriented polyethylene terephthalate (BOPET) insulating film and biaxially oriented polyimide (BOPI) insulating film. BOPET insulating film boasts advantages such as a wide availability of raw materials, highly mature polymerization and extrusion casting processes, a high degree of domestic production of biaxial stretching equipment, and extremely low production costs per unit area. However, the relatively low aromatic ring content in the BOPET molecular backbone leads to significant thermal intensification of molecular chain segments at high temperatures. In thermal environments exceeding 100°C, the molecular chain segments of this material undergo significant relaxation, resulting in a decrease in cohesive energy and a rapid decline in mechanical modulus, making this film unsuitable for heat-resistant environments above Class F. Biaxially oriented polyimide insulating film, due to its high-density aromatic heterocyclic structure in its molecular backbone, possesses excellent thermal stability, with a long-term operating temperature reaching 180°C, making it irreplaceable in high-end equipment such as aerospace, nuclear power, and rail transit traction motors. However, the production route of polyimide film requires casting a polyamic acid precursor solution and completing the imidization ring-closure reaction at high temperature. This process is not only extremely time-consuming and energy-intensive, but also releases a large amount of solvent and byproducts that impose a heavy burden on the environment. In addition, the application of biaxial stretching process in the field of polyimide film is limited by the rheological properties of raw materials and the uniformity of imidization. Continuous and efficient industrial production is extremely difficult. Furthermore, the polyimide resin synthesis technology has long been monopolized by a few foreign companies, and the raw material price is extremely expensive, which limits its large-scale application in mid-to-high-end electrical equipment.

[0004] Polyethylene naphthalate (PEN), a novel high-performance polyester material characterized by the substitution of benzene rings for naphthalene rings, is superior to traditional PET resin in that it uses naphthalene rings instead of benzene rings. Due to its larger conjugated structure, PEN exhibits higher molecular rigidity and a more planar structure. Compared to traditional PET polyester insulating films, PEN insulating films demonstrate superior performance in multiple aspects, such as heat resistance, chemical resistance, UV radiation resistance, creep resistance, mechanical strength, electrical insulation properties, and dimensional stability. It is particularly suitable for insulation requirements under high temperature, high voltage, and long-term operating environments. Furthermore, its production method is very similar to PET, using cast extrusion and biaxial stretching. Therefore, the processing technology and production cost of PEN insulating films are far superior to those of polyimide insulating films, making it an excellent candidate for application in the F-grade insulating film market, which falls between PET and PI insulating films.

[0005] However, due to the increased rigidity of the PEN molecular chain and the strong interaction brought about by the naphthalene ring, its glass transition temperature, crystallization melting temperature, and melt viscosity are significantly higher than those of PET. This directly leads to a significant narrowing of the processing window for PEN. During biaxial stretching, the sensitivity to stretching temperature, stretching ratio, stretching rate, and heat setting conditions increases dramatically. Even small fluctuations in process parameters can cause significant changes in film orientation, crystal morphology, and residual stress, resulting in batch instability in the mechanical properties, thermal shrinkage rate, and dielectric strength of the finished film. Therefore, the industrial production of PEN insulating films, barrier films, and other films has not yet been completed in China.

[0006] To address the aforementioned processing challenges, the common approach in existing technologies is to introduce a large number of functional additives into PEN wafers. While this approach alleviates processing difficulties to some extent, the introduction of numerous non-matrix components inevitably creates non-uniform regions in the film's chemical composition and physical structure. These microscopic defects, under the influence of a high-voltage electric field, are highly susceptible to becoming the starting points for partial discharge and electrical tree breakdown, leading to significant deterioration of the film's core insulation properties, such as insulation resistivity, dielectric strength, and breakdown voltage. Furthermore, the addition of additives introduces impurities, reducing the film's insulation performance and service stability, and increasing the complexity of the fabrication process and production costs.

[0007] Therefore, there is an urgent need for a PEN film for electrical insulation that has high purity, excellent comprehensive performance, and is suitable for long-term service environments with F-class heat resistance (155℃ high temperature), while also taking into account process feasibility and economy. This is of great significance for achieving a breakthrough in the localization of PEN insulation film. Summary of the Invention

[0008] To address the problems of high difficulty in preparing PEN insulating films and the inability to meet the F-class heat resistance requirements in existing technologies, this invention provides a biaxially oriented polyethylene naphthalate film, its preparation method, and its application.

[0009] To achieve the above objectives, the present invention employs the following technical solution: This invention provides a method for preparing biaxially oriented polyethylene naphthalate (PEG) films, comprising: The PEN slices were pretreated and PEN castings were prepared. The PEN castings were annealed for the first time to obtain uniform and flat castings that were non-oriented and free of defects. Unoriented, defect-free, uniform, and flat cast sheets are sequentially subjected to biaxial stretching and heat setting to obtain a preliminary biaxially stretched film. Maintaining the initial biaxially stretched film under tension, a second annealing process is performed to obtain a biaxially stretched polyethylene naphthalate film.

[0010] Optionally, the method for preprocessing PEN slices and preparing PEN castings is as follows: The PEN slices were dried to reduce the moisture content to below 30 ppm, resulting in pretreated PEN slices. PEN castings were prepared by extrusion casting of pretreated PEN slices.

[0011] Optionally, the drying temperature is 150-200℃; the melt temperature for extrusion casting is 270-280℃, and the extrusion temperature is 280-290℃; during extrusion, the screw speed is 20-120 r / min. -1 The melt pressure is maintained at 10-30 MPa, and the casting linear velocity is 0.8-1.2 m / min. -1 .

[0012] Optionally, the temperature of the first annealing is 120-140℃, and the time of the first annealing is 4-8 h.

[0013] Optionally, the method of sequentially subjecting a non-oriented, defect-free, uniform, flat cast sheet to biaxial stretching and heat setting to obtain a preliminary biaxially stretched film is as follows: The uniform and flat casting sheet without orientation or defects is preheated by hot air heating and hot air circulation to obtain the preheated casting sheet. The preheated castings are subjected to synchronous or asynchronous biaxial stretching, and then heat-set at the biaxial stretching temperature to obtain a preliminary biaxially stretched film.

[0014] Optionally, the preheating temperature is 160-180℃, and the preheating time is 120-300 s; the biaxial stretching temperature is 150-170℃, and the deformation rate is controlled at 0.05-0.5 s during the biaxial stretching process. -1The transverse and longitudinal stretching ratios are 3-5 times, and the stretching temperature fluctuation is kept ≤±2℃ during the stretching process; the heat setting time is 120-240 s.

[0015] Optionally, the temperature of the second annealing is 160-200℃, and the time of the second annealing is 30-120 min.

[0016] Optionally, the PEN slices have a glass transition temperature ≥110℃, a crystalline melting temperature ≥260℃, and an intrinsic viscosity of 0.60-0.80 dL / g. -1 Number average molecular weight 18,000-22,000, melt flow index 4-8 cm⁻¹ 3 10min -1 .

[0017] This invention also provides a biaxially oriented polyethylene naphthalate film prepared by the above-described method. The biaxially oriented polyethylene naphthalate film has a thickness of 20-80 μm, with transverse and longitudinal elastic moduli exceeding 3.5 GPa and a total modulus exceeding 8 GPa. Its transverse and longitudinal tensile strengths both exceed 150 MPa and its total strength exceeds 320 MPa. The DC high-voltage breakdown field strength at 25°C is 480-680 MV m. -1 The volume resistivity is 10 16 -10 18 Ω cm, 10⁻¹⁰ 7 The dielectric constant is 2.6-3.5 in the Hz range, and the dielectric loss is less than 0.010; the DC high voltage breakdown field strength at 155℃ is 400-550 MV m. -1 The volume resistivity is 10 15 -10 17 Ω cm, 10⁻¹⁰ 7 The dielectric constant is 2.7-3.6 in the Hz range, and the dielectric loss is less than 0.020.

[0018] The above-mentioned biaxially oriented polyethylene naphthalate film is used in electrical insulation.

[0019] Compared with the prior art, the present invention has the following beneficial effects: This invention provides a method for preparing biaxially stretched polyethylene naphthalate (PEN) films. The method involves a first annealing of the PEN casting to fully relax the molecular chains, eliminating internal stress caused by casting and extrusion, and preventing stress concentration. This process also releases the micro-orientation of the molecular chains, resulting in a more uniform distribution of chain segments and improving the uniformity of the casting's microstructure. Furthermore, it reduces initial micro-defects, preventing micro-voids and pores generated during subsequent casting processes, thus minimizing unevenness and cracking issues in the subsequent stretching stage. The PEN casting after the first annealing is then biaxially stretched. Tensile stress induces biaxial orientation and crystallization of the PEN molecular chains. Specifically, the rigid naphthalene rings align along the stretching plane under stress, with the naphthalene ring plane parallel to the film surface. The chain segments are arranged in a biaxially ordered manner along the in-plane biaxial orientation, forming a biaxial orientation. Under tensile stress, rapid crystallization forms an α-stable triclinic crystal structure, creating a microfiber structure with alternating crystalline and amorphous regions. The amorphous chains are stretched and clamped within the crystalline regions, forming rigid amorphous regions. Chain segment movement is suppressed, free volume is reduced, and the amorphous chains are almost unresponsive to the electric field, reducing dielectric loss and increasing breakdown strength. Heat setting after stretching prevents film shrinkage due to cooling and maintains dimensional stability. A second annealing process, maintaining the initially biaxially stretched film under tension, further refines the crystalline structure of PEN, optimizes the ratio and distribution of crystalline and amorphous phases, reduces areas of imperfect crystallization, eliminates structural defects generated during stretching, releases tensile stress, reduces local electric field concentration, and makes the rigid amorphous regions more compact. This reduces chain segment mobility and free volume, significantly suppressing carrier generation and migration, thus comprehensively improving the dielectric strength and high-temperature stability of the film. This method avoids dependence on additives and eliminates the need for modification of the prepared insulating film. Its production process is completely similar to existing conventional polyester (PET) film production processes. Subsequent production does not require large-scale modifications to existing polyester film production equipment; existing production lines can be used directly, effectively controlling production costs and facilitating large-scale production and market promotion. Furthermore, the prepared electrical insulation PEN film maintains stable mechanical and electrical properties during long-term service, significantly extending the lifespan of electrical equipment, reducing maintenance frequency and replacement costs, lowering overall operating costs, and possessing high industrial application value.

[0020] By drying PEN chips to remove residual moisture adsorbed inside, the hydrolysis, chain breakage, and molecular weight reduction of polyester caused by moisture during subsequent high-temperature melt extrusion are avoided, ensuring stable melt viscosity. The drying temperature is 150-200℃, which can thoroughly dry the chips while avoiding side reactions such as thermal oxidation. The thoroughly dried PEN melt has more uniform flowability, reducing defects such as bubbles, crystal points, and film breakage during extrusion, and significantly improving the stability and thickness uniformity of the cast film. The process is simple and controllable, with moderate energy consumption, making it suitable for continuous industrial production.

[0021] Cast extrusion ensures complete melting of the chips, preventing the formation of unmelted particles and guaranteeing uniform and stable melt viscosity. The set temperature and rate range for extrusion meets the requirements for flow molding while minimizing defects such as yellowing and chain breakage. Furthermore, the melt viscosity is moderate, the extrusion pressure is stable, and the output is uniform. The combination of moderately high-temperature extrusion and gradual cooling results in lower internal stress during melt cooling, which is beneficial for subsequent biaxial stretching and heat setting, improving dimensional stability and mechanical strength. This extrusion process is simple and convenient to operate, highly controllable, and less prone to screen clogging and film breakage, making it suitable for long-term, stable production of high-purity PEN castings.

[0022] In the casting annealing process, the temperature of the first annealing is 120-140℃, and the time of the first annealing is 4-8 hours. The casting is annealed in an environment higher than the glass transition temperature and lower than the crystallization temperature, which allows the molecular chains to fully relax to eliminate the internal stress caused by casting and extrusion and avoid stress concentration; it also releases the micro-orientation of the molecular chains, making the chain segments more uniformly distributed and improving the uniformity of the microstructure of the casting; it reduces initial micro-defects and closes the tiny voids and pores generated during the casting process, reducing unevenness and cracking problems in the subsequent stretching stage from the source.

[0023] In the biaxial stretching process, preheating ensures uniform internal temperature of the cast sheet, improves its flexibility, reduces the resistance to movement of PEN molecular chains, promotes full orientation and crystallization of molecular chains during stretching, and avoids stress concentration, film breakage, and uneven thickness during stretching, thus ensuring film thickness accuracy and surface smoothness. By controlling the stretching temperature, stretching rate, and stretching ratio, the preheated cast sheet is subjected to synchronous or asynchronous biaxial stretching, ensuring uniform stretching tension and avoiding defects during the process.

[0024] In the thin film annealing process, the temperature of the second annealing is 160-200℃, and the time of the second annealing is 30-120min. After stretching, the film is kept under tension and annealed within the crystallization temperature range to further improve the crystalline structure of PEN, optimize the ratio and distribution of crystalline and amorphous phases, reduce the imperfect crystallization area, eliminate structural defects generated during stretching, release tensile stress, reduce local electric field concentration, make the rigid amorphous region more compact, almost freeze the chain segment movement, further reduce the free volume, significantly suppress the generation and migration of charge carriers, and comprehensively improve the dielectric strength and high temperature stability of the thin film.

[0025] This invention also provides a biaxially oriented polyethylene naphthalate (PET) film prepared using the above-described method. Testing shows that this PET film exhibits significantly superior heat resistance, thermo-oxidative stability, and high-temperature dimensional stability compared to conventional electrical insulation PET films. Its long-term operating temperature can stably reach 155°C, meeting the long-term insulation requirements of Class F. It is not prone to warping or deformation, effectively avoiding insulation failure due to high-temperature aging, and is suitable for use in electrical equipment under high-temperature conditions. It possesses outstanding mechanical strength, with transverse and longitudinal elastic moduli exceeding 3.5 GPa and total modulus exceeding 8 GPa. Its transverse and longitudinal tensile strengths both exceed 150 MPa, and its total strength exceeds 320 MPa, without cracks or damage. During the processing of electrical components, such as wrapping, lamination, and stamping, it is less prone to defects such as breakage, wrinkling, and pinholes, effectively improving processing yield. Simultaneously, it enhances the structural stability and reliability of the components during long-term use, reducing the risk of failure due to processing damage. Its DC breakdown voltage can reach 400 MVm. -1 The volume resistivity is as high as 10. 15 Ω cm. At 25-155℃, 10-10 7 Under Hz testing conditions, the dielectric constant is between 2.6 and 3.6, and the dielectric loss is less than 0.02. Even under long-term service in high-temperature and high-humidity environments, the key insulation performance parameters remain stable, making it suitable for scenarios with extremely high requirements for insulation reliability, such as new energy motors, high-frequency transformers, and high-voltage capacitors, ensuring the safe and stable operation of electrical equipment.

[0026] The above-mentioned biaxially oriented polyethylene naphthalate (PET) film is used in electrical insulation. Because the PET film can stably withstand a long-term operating temperature of 155℃, it fully covers the heat resistance requirements of Class F insulation, effectively reducing the risk of short-circuit faults caused by insulation deformation. Since the PET film has an elastic modulus of over 8 GPa and a tensile strength exceeding 320 MPa, it is less prone to micro- or macro-defects such as cracks, wrinkles, pinholes, or edge damage during processing stages such as slot molding for motor slot insulation, winding and binding of electromagnetic wires, lamination of laminates, and stamping and cutting of capacitors. This significantly improves processing yield and production efficiency, while avoiding localized electric field concentrations or weak points in insulation caused by processing damage, ensuring the structural integrity and electrical reliability of the device during long-term use. More importantly, even under the combined effects of high temperature and high humidity, the key insulation parameters of the film can remain stable over a long period of time, making the film safe to be adapted to application scenarios with extremely high requirements for insulation reliability, such as new energy drive motors, photovoltaic inverters, and rail transit traction systems, thereby comprehensively improving the safety margin and full life cycle reliability of electrical equipment. Attached Figure Description

[0027] Figure 1 This is a schematic flowchart of a method for preparing biaxially oriented polyethylene naphthalate film according to the present invention.

[0028] Figure 2 This is a macroscopic view of the biaxially oriented polyethylene naphthalate film for electrical insulation prepared in Example 1 of the present invention.

[0029] Figure 3 The image shows a WAXS diagram of the biaxially oriented polyethylene naphthalate film for electrical insulation prepared in Example 1 of this invention.

[0030] Figure 4 The image shows the Weibull distribution of DC breakdown voltage at different temperatures for the biaxially oriented polyethylene naphthalate film for electrical insulation prepared in Example 1 of the present invention. Detailed Implementation

[0031] To enable those skilled in the art to understand the features and effects of the present invention, the terms and expressions used in the specification and claims are explained and defined in general below. Unless otherwise specified, all technical and scientific terms used herein have the ordinary meaning understood by those skilled in the art regarding the present invention, and in case of conflict, the definitions in this specification shall prevail.

[0032] The theories or mechanisms described and disclosed herein, whether right or wrong, should not in any way limit the scope of the invention, that is, the contents of the invention can be implemented without being limited by any particular theory or mechanism.

[0033] In this document, all features defined by numerical ranges or percentage ranges, such as numerical values, quantities, contents, and concentrations, are for the sake of brevity and convenience only. Accordingly, descriptions of numerical ranges or percentage ranges should be considered as covering and specifically disclosing all possible sub-ranges and individual numerical values ​​(including integers and fractions) within those ranges.

[0034] In this article, unless otherwise specified, “contains,” “includes,” “containing,” “has,” or similar terms cover the meanings of “composed of” and “mainly composed of,” for example, “A contains a” covers the meanings of “A contains a and others” and “A contains only a.”

[0035] For the sake of brevity, not all possible combinations of the technical features in each implementation scheme or embodiment are described herein. Therefore, as long as there is no contradiction in the combination of these technical features, the technical features in each implementation scheme or embodiment can be combined arbitrarily, and all possible combinations should be considered within the scope of this specification.

[0036] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0037] The following examples use instruments and equipment conventional in the art. Experimental methods in the following examples, unless otherwise specified, are generally performed under conventional conditions or as recommended by the manufacturer. All raw materials used in the following examples are conventional commercially available products with specifications conventional in the art. In this specification and the following examples, unless otherwise specified, "%" refers to weight percentage, "parts" refers to parts by weight, and "ratio" refers to weight proportion.

[0038] The present invention will be further described in detail below with reference to specific embodiments. These descriptions are for explanation purposes only and are not intended to limit the scope of the invention.

[0039] See Figure 1 This invention provides a method for preparing biaxially oriented polyethylene naphthalate (PEG) films, comprising: S1: Pre-treat the PEN slices and prepare PEN castings, specifically as follows: The PEN slices were dried at 150-200℃ for 8-12 h to reduce the moisture content of the PEN slices to below 30 ppm, thus obtaining pretreated PEN slices. The pretreated PEN chips are placed in the extruder hopper and fully melted at 270-280℃. They are then extruded at a temperature range of 280-290℃ and gradually cooled by casting rollers at 100-110℃ and traction rollers at 20-40℃ before being wound up.

[0040] Preferably, the PEN slices have a glass transition temperature ≥110℃, a crystalline melting temperature ≥260℃, and an intrinsic viscosity of 0.60-0.80 dL / g. -1 Number average molecular weight 18,000-22,000, melt flow index 4-8 cm⁻¹ 3 10min -1 (Test conditions: 1.2 kg f, 300℃). One of the following drying methods was selected: vacuum chamber, fluidized bed, or drying tower.

[0041] The PEN chips, after melting, must be filtered through a filter screen of 1000 mesh or higher, and the extrusion screw speed is 20-120 r / min. -1 The melt pressure is maintained at 10-30 MPa, and the casting linear velocity is 0.8-1.2 m / min. -1The extrusion motor torque is ≥180 Nm, and the winding motor torque is ≥1.2 Nm. The prepared PEN castings are uniform and transparent, with no longitudinal or transverse striations, crystal points, or other defects on the surface, and the casting thickness is 200-1000 μm.

[0042] S2: The PEN casting is subjected to a first annealing to obtain a uniform and flat casting without orientation or defects, specifically: In a relaxed, flat state, the PEN casting is subjected to a first annealing at 120-140℃ for 4-8 hours. This annealing temperature is higher than the glass transition temperature of PEN but lower than the crystallization temperature of PEN, which allows the molecular chains to fully relax, thereby eliminating the micro-orientation and internal stress caused by casting, compensating for the micropores inside the casting to prevent them from expanding into defects during stretching, and maintaining the amorphous and non-crystalline state of the casting.

[0043] S3: The uniform, flat, unoriented, and defect-free cast sheet is sequentially subjected to biaxial stretching and heat setting to obtain a preliminary biaxially stretched film, specifically: The uniform, flat, non-oriented, and defect-free PEN castings are first preheated at 160-180℃ for 120-300 seconds using a hot air stream with hot air circulation to ensure uniform temperature across all areas of the casting. An infrared thermometer is used to monitor the overall temperature of the casting. After preheating, the PEN castings are then subjected to synchronous or asynchronous bidirectional stretching at 150-170℃, with the deformation rate controlled at 0.05-0.5 s⁻¹. -1 The stretch ratio (MD) in both the longitudinal and transverse directions is 3-5 times, and the stretch ratio (TD) in the transverse direction is 3-5 times. During the stretching process, the temperature fluctuation should be kept ≤±2℃, and the stretching tension should be uniform. The stretched film is then heat-set in its original temperature environment for 120-240 seconds, maintaining uniform temperature within the oven with a temperature fluctuation of ≤±2℃.

[0044] S4: Maintaining the initial tension of the biaxially stretched film, perform a second annealing to obtain a biaxially stretched polyethylene naphthalate film, specifically: Maintaining the initial biaxially stretched film under tension, a second annealing is performed at a temperature of 160-200℃ for 30-120 minutes. This annealing temperature is within the crystallization temperature range of PEN, so that the molecular chains inside the film are fully oriented to perfect the condensed structure, eliminate defects, release tensile stress, and obtain a PEN film for electrical insulation with excellent performance.

[0045] This invention also provides a biaxially oriented polyethylene naphthalate (PEG) film prepared by the above-described method. The PEG film has a crystallinity of 20%-40%, a thickness of 20-80 μm, and exhibits transverse and longitudinal elastic moduli exceeding 3.5 GPa, a total modulus exceeding 8 GPa, transverse and longitudinal tensile strengths exceeding 150 MPa, and a total strength exceeding 320 MPa. Its DC high-voltage breakdown field strength at 25°C is 480-680 MV m. -1 The volume resistivity is 10 16 -10 18 Ω cm, 10⁻¹⁰ 7 The dielectric constant is 2.6-3.5 in the Hz range, and the dielectric loss is less than 0.010; the DC high voltage breakdown field strength at 155℃ is 400-550 MV m. -1 The volume resistivity is 10 15 -10 17 Ω cm, 10⁻¹⁰ 7 The dielectric constant is 2.7-3.6 in the Hz range, and the dielectric loss is less than 0.020.

[0046] The above-mentioned biaxially oriented polyethylene naphthalate (PET) film is used in electrical insulation. Because the PET film can stably withstand a long-term operating temperature of 155℃, it fully covers the heat resistance requirements of Class F insulation, effectively reducing the risk of short-circuit faults caused by insulation deformation. Since the PET film has a transverse and longitudinal elastic modulus exceeding 3.5 GPa and a total modulus exceeding 8 GPa, and a transverse and longitudinal tensile strength exceeding 150 MPa and a total strength exceeding 320 MPa, it is less prone to micro- or macro-defects such as cracks, wrinkles, pinholes, or edge damage during processing stages such as slot forming for motor slot insulation, winding and binding of electromagnetic wires, lamination of laminates, and stamping and cutting of capacitors. This significantly improves processing yield and production efficiency, while avoiding localized electric field concentrations or weak points in insulation caused by processing damage, ensuring the structural integrity and electrical reliability of the device during long-term use. More importantly, even under the combined effects of high temperature and high humidity, the key insulation parameters of the film can remain stable over a long period of time, making the film safe to be adapted to application scenarios with extremely high requirements for insulation reliability, such as new energy drive motors, photovoltaic inverters, and rail transit traction systems, thereby comprehensively improving the safety margin and full life cycle reliability of electrical equipment.

[0047] The following are the methods for evaluating the performance of raw materials and final products: Refer to GB / T 13542.2-2021 "Films for Electrical Insulation - Part 2: Test Methods" The glass transition temperature, melting temperature, and crystallinity were measured using differential scanning calorimetry (DSC). Thermal stability was assessed by determining the 5% mass loss temperature using thermogravimetric analysis (TGA).

[0048] The intrinsic viscosity was determined using a phenol / tetrachloroethane (1:1) solution as a solvent and an Ubbelohde viscometer. The number-average molecular weight of PEN slices was determined by gel permeation chromatography (mobile phase: hexafluoroisopropanol). The elastic modulus and tensile strength of the insulating film were tested using a universal testing machine. The DC breakdown voltage of the insulating film was tested using a high-voltage DC power supply. The volume resistivity of the insulating film was tested using a volume surface resistivity meter. The dielectric constant and dielectric loss of the insulating film were tested using a broadband dielectric impedance spectrometer.

[0049] The shrinkage rate of the insulating film was tested using a forced-air drying oven at 150℃ for 30 minutes.

[0050] The technical solution of the present invention will be described in detail below through specific embodiments: Example 1 Select a intrinsic viscosity of 0.68 dL / g -1 Melt flow index 5 cm 3 10 min -1 PEN chips were vacuum dried at 160℃ for 8 h to completely remove moisture, and then extruded at 285℃ to form 500 μm thick PEN castings. The castings were annealed at 120℃ for 8 h. The castings were then placed in a biaxial stretching machine, preheated at 160℃ for 120 s, and simultaneously biaxially stretched at 160℃ with a transverse and longitudinal stretch ratio of 4. The stretching process was isothermal, and the film deformation rate was 0.06 s⁻¹. -1 Heat set at 160℃ for 120 seconds. Anneal at 160℃ for 120 minutes to obtain PEN film for electrical insulation.

[0051] Example 2 Select a intrinsic viscosity of 0.68 dL / g -1 Melt flow index 5 cm 3 10 min -1 PEN chips were vacuum dried at 160℃ for 8 hours to completely remove moisture, and then extruded at 285℃ to form 500μm thick PEN wafers. The wafers were annealed at 120℃ for 8 hours. The wafers were then placed in a biaxial stretching machine, preheated at 160℃ for 120 seconds, and simultaneously biaxially stretched at 160℃ with a transverse and longitudinal stretch ratio of 4. The stretching process was maintained at an isothermal temperature, and the film deformation rate was 0.2 s⁻¹.-1 Heat set at 160℃ for 120 seconds. Anneal at 160℃ for 120 minutes to obtain PEN film for electrical insulation.

[0052] Example 3 Select a intrinsic viscosity of 0.68 dL / g -1 Melt flow index 5 cm 3 10 min -1 PEN chips were vacuum dried at 160℃ for 8 hours to completely remove moisture, and then extruded at 285℃ to form 500 μm thick PEN wafers. The wafers were annealed at 120℃ for 8 hours. The wafers were then placed in a biaxial stretching machine, preheated at 160℃ for 120 seconds, and simultaneously biaxially stretched at 160℃ with a transverse and longitudinal stretch ratio of 5. The stretching process was maintained at an isothermal temperature, and the film deformation rate was 0.15 s⁻¹. -1 Heat set at 160℃ for 120 seconds. Anneal at 150℃ for 120 minutes to obtain PEN film for electrical insulation.

[0053] Example 4 Select a intrinsic viscosity of 0.68 dL / g -1 Melt flow index 5 cm 3 10 min -1 PEN chips were vacuum dried at 160℃ for 8 h to completely remove moisture, and then extruded at 285℃ to form 500 μm thick PEN castings. The castings were annealed at 120℃ for 4 h. The castings were then placed in a biaxial stretching machine, preheated at 160℃ for 120 s, and then asynchronously biaxially stretched at 160℃ with a transverse and longitudinal stretch ratio of four. The stretching process was maintained at an isothermal temperature, and the film deformation rate was 0.06 s⁻¹. -1 Heat set at 160℃ for 120 seconds. Anneal at 160℃ for 120 minutes to obtain PEN film for electrical insulation.

[0054] Example 5 Select a intrinsic viscosity of 0.68 dL / g -1 Melt flow index 5 cm 3 10 min -1 PEN chips were vacuum dried at 160℃ for 8 h to completely remove moisture, and then extruded at 285℃ to form 500 μm thick PEN castings. The castings were annealed at 120℃ for 8 h. The castings were then placed in a biaxial stretching machine, preheated at 160℃ for 120 s, and then asynchronously biaxially stretched at 160℃ with a transverse and longitudinal stretch ratio of four. The stretching process was maintained at an isothermal temperature, and the film deformation rate was 0.2 s⁻¹. -1 Heat set at 160℃ for 120 seconds. Anneal at 160℃ for 120 minutes to obtain PEN film for electrical insulation.

[0055] Example 6 Select a intrinsic viscosity of 0.68 dL / g -1 Melt flow index 5 cm 3 10 min -1 PEN chips were vacuum dried at 160℃ for 8 h to completely remove moisture, and then extruded at 285℃ to form 500 μm thick PEN castings. The castings were annealed at 120℃ for 8 h. The castings were then placed in a biaxial stretching machine, preheated at 160℃ for 120 s, and simultaneously biaxially stretched at 160℃ with a transverse and longitudinal stretch ratio of 4. The stretching process was isothermal, and the film deformation rate was 0.06 s⁻¹. -1 Heat set at 160℃ for 120 seconds. Anneal at 180℃ for 30 minutes to obtain PEN film for electrical insulation.

[0056] Example 7 Select a intrinsic viscosity of 0.68 dL / g -1 Melt flow index 5 cm 3 10 min -1 PEN chips were vacuum dried at 160℃ for 8 h to completely remove moisture, and then extruded at 285℃ to form 500 μm thick PEN castings. The castings were annealed at 120℃ for 8 h. The castings were then placed in a biaxial stretching machine, preheated at 160℃ for 120 s, and simultaneously biaxially stretched at 155℃ with a transverse and longitudinal stretch ratio of 4. The stretching process was isothermal, and the film deformation rate was 0.06 s⁻¹. -1 Heat set at 155℃ for 120 seconds. Anneal at 160℃ for 120 minutes to obtain PEN film for electrical insulation.

[0057] The key performance parameters of the PEN insulating film obtained in each embodiment are shown in the table below: Example 1 2 3 4 5 6 7 Film thickness (μm) 31 31 20 33 33 31 31 Transverse and longitudinal elastic modulus (GPa) 10.0 9.9 9.5 11.5 10.2 10.3 10.3 Transverse and longitudinal tensile strength (MPa) 470 407 411 414 387 392 468 <![CDATA[Breakdown strength at 155 °C (MV m -1 )]]> 480 448 428 505 453 483 537 Volume resistivity at 155 ℃ (Ω cm) <![CDATA[1.15×10 16 ]]> <![CDATA[0.39×10 16 ]]> <![CDATA[1.02×10 16 ]]> <![CDATA[0.82×10 16 ]]> <![CDATA[0.25×10 16 ]]> <![CDATA[0.16×10 16 ]]> <![CDATA[0.33×10 16 ]]> The performance data of the PEN insulating film obtained through Examples 1-7 show that, through a double annealing strategy and optimized pre-drying extrusion process, the present invention achieves PEN insulating films with transverse and longitudinal elastic moduli exceeding 3.5 GPa, total modulus exceeding 8 GPa, transverse and longitudinal tensile strength exceeding 150 MPa, total strength exceeding 320 MPa, crystallinity of 25%-35%, and heat shrinkage rate below 1.0% (150 ℃ 30 min). The breakdown field strength under DC high voltage at 155 ℃ is 400-550 MV m. -1 The volume resistivity is 10 15 -10 17 Ω cm. This ensures its high application value in the field of high-temperature electrical insulation.

[0058] See Figure 2 and Figure 3 As can be seen from the macroscopic image and microscopic WAXS diagram of the thin film prepared in Example 1, the obtained thin film is uniform and transparent, without internal defects, and has good uniformity, which ensures the stability of the thin film in all aspects.

[0059] See Figure 4 DC breakdown experiments were conducted on the thin film prepared in Example 1 under different temperature conditions, and it was found that its breakdown strength reached 600 MV m. -1 Furthermore, it also exceeds 480 MV m at a high temperature of 155℃. -1 This further demonstrates that the biaxially oriented polyethylene naphthalate film for electrical insulation prepared by this invention has the characteristics of high breakdown strength and good high temperature resistance.

[0060] Comparative Example 1 Select a intrinsic viscosity of 0.68 dL / g -1 Melt flow index 5 cm 3 10 min -1 PEN chips were extruded at 285℃ without removing moisture to form 500 μm thick PEN castings. The castings were annealed at 120℃ for 8 h. The castings were then placed in a biaxial stretching machine, preheated at 160℃ for 120 s, and simultaneously biaxially stretched at 160℃ with a transverse and longitudinal draw ratio of four. Isothermal stretching was maintained throughout the process, and the film deformation rate was 0.06 s⁻¹. -1 Heat set at 160℃ for 120 seconds. Anneal at 160℃ for 120 minutes to obtain PEN film for electrical insulation.

[0061] Comparative Example 2 Select a intrinsic viscosity of 0.68 dL / g -1 Melt flow index 5 cm 3 10 min -1 PEN chips were vacuum dried at 160℃ for 8 h to completely remove moisture, and then extruded at 285℃ to form 500 μm thick PEN castings. The castings were annealed at 120℃ for 8 h. The castings were then placed in a biaxial stretching machine, preheated at 160℃ for 120 s, and simultaneously biaxially stretched at 140℃ with a transverse and longitudinal stretch ratio of 4. Isothermal stretching was maintained throughout the process, and the film deformation rate was 0.06 s⁻¹. -1 Heat set at 160℃ for 120 seconds. Anneal at 160℃ for 120 minutes to obtain PEN film for electrical insulation.

[0062] Comparative Example 3 Select a intrinsic viscosity of 0.68 dL / g -1 Melt flow index 5 cm 310 min -1 PEN chips were vacuum dried at 160℃ for 8 hours to completely remove moisture, and then extruded at 285℃ to form 500μm thick PEN castings. The castings were annealed at 120℃ for 8 hours. The castings were then placed in a biaxial stretching machine, preheated at 160℃ for 120 seconds, and simultaneously biaxially stretched at 190℃ with a transverse and longitudinal stretch ratio of four. Isothermal stretching was maintained throughout the process, and the film deformation rate was 0.2 s⁻¹. -1 Heat set at 160℃ for 120 seconds. Anneal at 160℃ for 120 minutes to obtain PEN film for electrical insulation.

[0063] Comparative Example 4 Select a intrinsic viscosity of 0.68 dL / g -1 Melt flow index 5 cm3 10 min -1 PEN chips were vacuum dried at 160℃ for 8 h to completely remove moisture, and then extruded at 285℃ to form 500 μm thick PEN castings. The castings were not annealed. The castings were placed in a biaxial stretching machine, preheated at 160℃ for 120 s, and then simultaneously biaxially stretched at 160℃ with a transverse and longitudinal stretch ratio of four. Isothermal stretching was maintained during the process, and the film deformation rate was 0.2 s⁻¹. -1 Heat set at 160℃ for 120 seconds. The film is not annealed.

[0064] Comparative Example 5 Select a intrinsic viscosity of 0.68 dL / g -1 Melt flow index 5 cm 3 10 min -1 PEN chips were vacuum dried at 160℃ for 8 h to completely remove moisture, and then extruded at 285℃ to form 500 μm thick PEN castings. The castings were not annealed. The castings were placed in a biaxial stretching machine, preheated at 160℃ for 120 s, and then simultaneously biaxially stretched at 160℃ with a transverse and longitudinal stretch ratio of four. Isothermal stretching was maintained during the process, and the film deformation rate was 0.2 s⁻¹. -1 Heat set at 160℃ for 120 seconds. Anneal at 160℃ for 120 minutes to obtain PEN film for electrical insulation.

[0065] Comparative Example 6 Select a intrinsic viscosity of 0.68 dL / g -1 Melt flow index 5 cm 3 10 min -1PEN chips were vacuum dried at 160℃ for 8 h to completely remove moisture, and then extruded at 285℃ to form 500 μm thick PEN castings. The castings were annealed at 120℃ for 8 h. The castings were then placed in a biaxial stretching machine, preheated at 160℃ for 120 s, and simultaneously biaxially stretched at 160℃ with a transverse and longitudinal stretch ratio of 4. The stretching process was maintained at an isothermal temperature, and the film deformation rate was 0.2 s⁻¹. -1 Heat set at 160℃ for 120 seconds. The film is not annealed.

[0066] The key performance parameters of the PEN insulating films obtained from each comparative example are shown in the table below: Comparative Example 1 2 3 4 5 6 Film thickness (μm) / / 27 31 31 31 Transverse and longitudinal elastic modulus (GPa) / / 7.9 8.6 8.7 8.9 Transverse and longitudinal tensile strength (MPa) / / 271 314 308 367 <![CDATA[155 °C breakdown strength (MV m -1 )]]> / / 396 417 419 433 Volume resistivity at 155 ℃ (Ω cm) / / <![CDATA[0.08×10 16 ]]> <![CDATA[0.11×10 16 ]]> <![CDATA[0.23×10 16 ]]> <![CDATA[0.13×10 16 ]]> As shown in the table above: In Comparative Example 1, the lack of drying treatment before casting exceeds the scope of the claims. The PEN casting produced by casting is extremely brittle and cannot be bent or stretched. This is attributed to the sensitivity of PEN resin to moisture during high-temperature melting. Moisture causes hydrolytic breakage of the PEN molecular chains, resulting in a sharp decrease in molecular weight and a significant deterioration in the strength and toughness of the casting. In Comparative Example 2, the stretching temperature exceeds the scope of the claims. The excessively low stretching temperature leads to low film flexibility. During the stretching process, when the stretching ratio approaches 3, the clamps cannot provide sufficient tensile stress, causing the film to detach. In Comparative Example 3, the film stretching temperature is too high, exceeding the scope of the claims. The film exhibits whitening and brittleness, and its mechanical and dielectric properties are severely deteriorated. In Comparative Example 4, no annealing treatment is performed on the casting and film, exceeding the scope of the claims. The film's thermal shrinkage rate is as high as 12%, indicating severe thermal shrinkage, and its mechanical and dielectric properties are also somewhat deteriorated. This method is not suitable for high-temperature electrical insulation environments. In Comparative Example 5, no annealing treatment is performed on the casting, exceeding the scope of the claims. The poor uniformity of the film leads to deterioration in mechanical properties and reduced toughness. Comparative Example 6 does not undergo film annealing, which exceeds the scope of the claims. The film thermal shrinkage rate is close to 12%, which is severe and unsuitable for high-temperature electrical insulation environments.

[0067] The above description is merely a preferred embodiment of the present invention and is not intended to limit the technical solution of the present invention in any way. Those skilled in the art should understand that, without departing from the spirit and principles of the present invention, the technical solution can be modified and replaced in several simple ways, and these modifications and replacements are all within the scope of protection covered by the claims.

Claims

1. A method for preparing a biaxially oriented polyethylene naphthalate film, characterized in that, include: The PEN slices were pretreated and PEN castings were prepared. The PEN castings were annealed for the first time to obtain uniform and flat castings that were non-oriented and free of defects. Unoriented, defect-free, uniform, and flat cast sheets are sequentially subjected to biaxial stretching and heat setting to obtain a preliminary biaxially stretched film. Maintaining the initial biaxially stretched film under tension, a second annealing process is performed to obtain a biaxially stretched polyethylene naphthalate film.

2. The method for preparing biaxially oriented polyethylene naphthalate film according to claim 1, characterized in that, The method for preprocessing PEN slices and preparing PEN castings is as follows: The PEN slices were dried to reduce the moisture content to below 30 ppm, resulting in pretreated PEN slices. PEN castings were prepared by extrusion casting of pretreated PEN slices.

3. The method for preparing biaxially oriented polyethylene naphthalate film according to claim 2, characterized in that, The drying temperature is 150-200℃; the melting temperature for extrusion casting is 270-280℃, and the extrusion temperature is 280-290℃; during extrusion, the screw speed is 20-120 rpm. -1 The melt pressure is maintained at 10-30 MPa, and the casting linear velocity is 0.8-1.2 mm / min. -1 .

4. The method for preparing biaxially oriented polyethylene naphthalate film according to claim 1, characterized in that, The temperature of the first annealing is 120-140℃, and the time of the first annealing is 4-8 hours.

5. The method for preparing biaxially oriented polyethylene naphthalate film according to claim 1, characterized in that, The method for obtaining a preliminary biaxially stretched film by sequentially subjecting a uniform, flat, non-oriented, and defect-free cast sheet to biaxial stretching and heat setting is as follows: The uniform and flat casting sheet without orientation or defects is preheated by hot air heating and hot air circulation to obtain the preheated casting sheet. The preheated castings are subjected to synchronous or asynchronous biaxial stretching, and then heat-set at the biaxial stretching temperature to obtain a preliminary biaxially stretched film.

6. The method for preparing biaxially oriented polyethylene naphthalate film according to claim 5, characterized in that, The preheating temperature is 160-180℃, and the preheating time is 120-300 s; the biaxial stretching temperature is 150-170℃, and the deformation rate is controlled at 0.05-0.5 s during the biaxial stretching process. -1 The transverse and longitudinal stretching ratios are 3-5 times, and the stretching temperature fluctuation is kept ≤±2℃ during the stretching process; the heat setting time is 120-240 s.

7. The method for preparing biaxially oriented polyethylene naphthalate film according to claim 1, characterized in that, The second annealing temperature is 160-200℃, and the second annealing time is 30-120 min.

8. The method for preparing biaxially oriented polyethylene naphthalate film according to any one of claims 1-7, characterized in that, The PEN chips have a glass transition temperature ≥110℃, a crystalline melting temperature ≥260℃, and an intrinsic viscosity of 0.60-0.80 dL / g. -1 Number average molecular weight 18,000-22,000, melt flow index 4-8 cm⁻¹ 3 10 min -1 .

9. A biaxially oriented polyethylene naphthalate film prepared by the preparation method according to any one of claims 1-8, characterized in that, The biaxially oriented polyethylene naphthalate film has a thickness of 20-80 μm, with transverse and longitudinal elastic moduli exceeding 3.5 GPa and total modulus exceeding 8 GPa. Its transverse and longitudinal tensile strengths both exceed 150 MPa and total strength exceeds 320 MPa. The DC high-voltage breakdown field strength at 25°C is 480-680 MV m. -1 The volume resistivity is 10 16 -10 18 Ω cm, 10⁻¹⁰ 7 The dielectric constant is 2.6-3.5 in the Hz range, and the dielectric loss is less than 0.010; the DC high voltage breakdown field strength at 155℃ is 400-550 MV m. -1 The volume resistivity is 10 15 -10 17 Ω cm, 10⁻¹⁰ 7 The dielectric constant is 2.7-3.6 in the Hz range, and the dielectric loss is less than 0.

020.

10. The application of the biaxially oriented polyethylene naphthalate film according to claim 9 in electrical insulation.

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