A method for preparing a biaxially oriented polypropylene (BOPP) film based on self-assembled coating modification

Abstract

The application discloses a kind of based on self-assembly coating modification BOPP film preparation method, comprising the following steps: surface activation treatment is carried out to biaxially oriented polypropylene base film;Purulan is mixed and dispersed with hydroxylated two-dimensional boron nitride and crosslinking agent is added to obtain coating solution;Coating solution is coated on the surface of the surface activation treated base film;The base film after coating is dried and crosslinked to form composite coating.The application introduces shallow trap by constructing self-assembly layered structure and hinders carrier migration, effectively inhibits space charge accumulation and electric field distortion, significantly improves the high-temperature insulation performance and energy storage density of film.

Description

Technical Field

[0001] This invention relates to the field of dielectric material preparation technology, specifically to a method for preparing BOPP thin films based on self-assembled coating modification. Background Technology

[0002] Bis-axially oriented polypropylene (BOPP) film is widely used in dry-type DC power capacitors due to its low loss, high breakdown strength, and excellent self-healing properties. However, with the development of power equipment towards higher energy density, miniaturization, and high temperature resistance, traditional BOPP film faces severe challenges in high-temperature environments above 120°C. Under high temperature and high electric field coupling, space charge accumulation easily occurs inside the BOPP film, leading to local electric field distortion and subsequently causing insulation breakdown failure. Simultaneously, its relatively low glass transition temperature limits the upper limit of the capacitor's operating temperature, reducing system reliability and efficiency. While existing technologies modify BOPP films through inorganic doping, organic blending, or surface deposition, these methods often suffer from problems such as difficult filler dispersion, poor interfacial compatibility, or complex processes, making it difficult to significantly improve high-temperature insulation and energy storage performance while ensuring film thinning. Therefore, there is an urgent need to develop a BOPP film modification method that can effectively suppress charge transport at high temperatures, improve electric field distribution, and has a simple process. Summary of the Invention

[0003] The purpose of this invention is to provide a method for preparing BOPP films based on self-assembled coating modification. This invention achieves the construction of a composite coating with shallow trapping effect and layered barrier structure on the surface of a BOPP base film, thereby significantly improving the high-temperature insulation performance and energy storage capacity of the film.

[0004] The technical solution of this invention: A method for preparing BOPP film based on self-assembled coating modification, comprising the following steps: Step S1: Perform surface activation treatment on the biaxially oriented polypropylene base film; Step S2: Pullulan and hydroxylated two-dimensional boron nitride are mixed and dispersed, and a crosslinking agent is added to obtain a coating solution; wherein, the mass fraction of pullulan in the coating solution is 0.5~1%, and the mass fraction of hydroxylated two-dimensional boron nitride is 0.0625%~0.20%; Step S3: Apply the coating solution to the surface of the surface-activated biaxially oriented polypropylene base film; Step S4: Dry and crosslink the coated biaxially oriented polypropylene base film to form a composite coating on the surface of the biaxially oriented polypropylene base film.

[0005] In the above-mentioned method for preparing BOPP film based on self-assembled coating modification, the crosslinking agent is an aldehyde crosslinking agent; the molar ratio of the aldehyde crosslinking agent to the total molar number of hydroxyl groups on the pullulan chain is 1:15~25.

[0006] In the aforementioned method for preparing BOPP films based on self-assembled coating modification, the mass fraction of pullulan in the coating solution is 0.75%, the mass fraction of hydroxylated two-dimensional boron nitride is 0.125%, and the molar ratio of the aldehyde crosslinking agent to the total molar number of hydroxyl groups on the pullulan chain is 1:20.

[0007] In the aforementioned method for preparing BOPP film based on self-assembled coating modification, the surface activation treatment of the biaxially oriented polypropylene base film is performed by plasma treatment of the biaxially oriented polypropylene base film; the plasma treatment voltage is 3~8kV, the frequency is 5~10kHz, and the treatment time is 0.5~2 minutes.

[0008] The aforementioned method for preparing BOPP film based on self-assembled coating modification involves immersing the biaxially oriented polypropylene base film in the coating solution using a dip-coating method to achieve coating. After each dip-coating, the biaxially oriented polypropylene base film is rotated 180 degrees, and the coating process is repeated at least twice.

[0009] The aforementioned method for preparing BOPP films based on self-assembled coating modification specifically includes obtaining the coating solution as follows: The pullulan was dissolved in water and subjected to ultrasonic treatment to obtain a polymer solution; The hydroxylated two-dimensional boron nitride was dissolved in water for pre-dispersion and vigorous dispersion to obtain a nanosheet solution. The polymer solution is added to the nanosheet solution and subjected to ultrasonic treatment to obtain a mixed solution; The crosslinking agent and the crosslinking reaction catalyst are added to the mixed solution, and the mixture is stirred to obtain the coating solution.

[0010] In the aforementioned method for preparing BOPP films based on self-assembled coating modification, the pre-dispersion is performed using magnetic stirring at a speed of 300-800 rpm for 0.5-1.5 hours. The strong dispersion is achieved using an ultrasonic probe for 1-3 hours; The crosslinking catalyst is a hydrochloric acid solution, and the molar ratio of the hydrochloric acid solution to the crosslinking agent is 1:3~8.

[0011] In the aforementioned method for preparing BOPP film based on self-assembled coating modification, the drying and crosslinking treatment involves vertically suspending the coated biaxially oriented polypropylene base film in a vacuum oven and drying and crosslinking it at 35~45°C.

[0012] The aforementioned method for preparing BOPP films based on self-assembled coating modification includes the following steps in the preparation of hydroxylated two-dimensional boron nitride: Boron nitride nanopowder was placed in a tube furnace and heated to 800-1200℃ at a heating rate of 5-15℃ / min and held at that temperature for 0.5-1.5 hours. After cooling to room temperature, the mixture is washed and dried to obtain the hydroxylated two-dimensional boron nitride.

[0013] In the aforementioned method for preparing BOPP film based on self-assembled coating modification, the thickness of the biaxially oriented polypropylene base film is 3~8μm; The thickness of the composite coating is 250nm~350nm.

[0014] Compared with existing technologies, this invention constructs a composite coating containing pullulan and hydroxylated two-dimensional boron nitride on the surface of a BOPP base film. Utilizing the hydrogen bonding and cross-linking network between pullulan and hydroxylated two-dimensional boron nitride, a dense and uniform self-assembled layered structure is formed. In this structure, the hydroxylated two-dimensional boron nitride is oriented in the in-plane direction, effectively extending the migration path of charge carriers in the electric field direction and physically hindering the development of leakage current. Simultaneously, the functional groups on the surfaces of pullulan and hydroxylated two-dimensional boron nitride in the coating synergistically introduce a high density of shallow trap energy levels, enhancing the capture ability of electrons and holes, promoting rapid charge trapping and detrapping balance, and avoiding long-term charge accumulation and electric field distortion caused by deep traps. Furthermore, by employing a single-sided coating strategy combined with precisely controlled coating thickness, the high-temperature breakdown field strength and discharge energy density of the film are significantly improved while maximizing the preservation of the thin and light characteristics of the BOPP base film, meeting the application requirements of miniaturization and high reliability for dry DC capacitors. Attached Figure Description

[0015] Figure 1 These are XPS characterization spectra of hydroxylated boron nitride nanosheets in embodiments of the present invention, wherein... Figure 1 (a) shows the XPS full spectra of BNNS and BNNS-OH. Figure 1 (b) shows the XRD patterns of BNNS and BNNS-OH; Figure 2 This is a schematic diagram of the FTIR characterization spectrum and morphology of hydroxylated boron nitride nanosheets in an embodiment of the present invention, wherein... Figure 2 (a) shows the FTIR spectra of BNNS and BNNS-OH. Figure 2 (b) shows the TEM and HRTEM images of BNNS-OH; Figure 3 These are AFM morphology images and cross-sectional height curves of hydroxylated boron nitride nanosheets in embodiments of the present invention, wherein... Figure 3 (a) is the AFM morphology of BNNS-OH. Figure 3 (b) is the cross-sectional height curve along the dashed line in (a); Figure 4These are cross-sectional morphology and surface element distribution diagrams of the modified BOPP film in this embodiment of the invention, wherein... Figure 4 (a) is a SEM image of the cross-section of the modified thin film. Figure 4 (b) is a SEM image of a pure BOPP surface. Figure 4 (c) is a SEM image of the modified film surface. Figure 4 (d) is the surface scan distribution of EDS elements on the modified thin film surface; Figure 5 These are the dielectric constant and loss frequency spectra of different BOPP films at 25°C and 120°C in embodiments of the present invention. Figure 5 (a) shows the test results at 25°C. Figure 5 (b) shows the test results at 120℃; Figure 6 This is a graph showing the change in conductivity of each BOPP film at 25°C with electric field strength in an embodiment of the present invention. Figure 7 These are electric field intensity and charge density distribution diagrams for BOPP and BOPP-1 samples in embodiments of the present invention, wherein... Figure 7 (a) shows the electric field intensity distribution of the BOPP sample at different polarization times. Figure 7 (b) shows the electric field intensity distribution of the BOPP-1 sample at different polarization times. Figure 7 (A) represents the charge density distribution of the BOPP sample at different polarization times. Figure 7 (B) represents the charge density distribution of the BOPP-1 sample at different polarization times; Figure 8 These are electric field intensity and charge density distribution diagrams for BOPP-2 and BOPP-3 samples in embodiments of the present invention, wherein... Figure 8 (a) shows the electric field intensity distribution of the BOPP-2 sample at different polarization times. Figure 8 (b) shows the electric field intensity distribution of the BOPP-3 sample at different polarization times. Figure 8 (A) represents the charge density distribution of the BOPP-2 sample at different polarization times. Figure 8 (B) shows the charge density distribution of the BOPP-3 sample at different polarization times; Figure 9 This is a graph showing the electric field intensity and charge density distribution of the BOPP-4 sample in an embodiment of the present invention. Figure 9 (a) shows the electric field intensity distribution of the BOPP-4 sample at different polarization times. Figure 9 (A) represents the charge density distribution of the BOPP-4 sample at different polarization times; Figure 10These are Weibull distribution diagrams of the breakdown field strength of BOPP and modified films at different temperatures in embodiments of the present invention, wherein... Figure 10 (a) is the Weibull distribution at 25°C. Figure 10 (b) is the Weibull distribution at 120℃; Figure 11 This is a diagram showing the trap energy level distribution of BOPP and modified films in the embodiments of the present invention; Figure 12 This is a molecular orbital energy level diagram of PP, pullulan, and BNNS-OH in the embodiments of the present invention; Figure 13 This is an electrostatic potential distribution diagram of PP, pullulan, and hydroxylated boron nitride in an embodiment of the present invention, wherein... Figure 13 (a) shows the electrostatic potential distribution of PP. Figure 13 (b) shows the electrostatic potential distribution of the Pullland. Figure 13 (c) represents the electrostatic potential distribution of BNNS-OH; Figure 14 These are simulation diagrams of the breakdown path and electric field distribution of the thin film before and after modification in this embodiment of the invention. Figure 14 (a) shows the breakdown path distribution of the film before modification. Figure 14 (b) shows the breakdown path distribution of the modified film. Figure 14 (c) shows the breakdown electric field distribution of the film before modification. Figure 14 (d) represents the breakdown electric field distribution of the modified film; Figure 15 This is a comparative diagram of the electrical properties and energy storage characteristics of the BOPP film in the embodiments of the present invention, wherein... Figure 15 (a) is the electric displacement-electric field (DE) hysteresis loop. Figure 15 (b) shows the curves of discharge energy density and efficiency as a function of electric field; Figure 16 This is a graph showing the discharge efficiency and uniformity test results of the BOPP film in an embodiment of the present invention. Figure 16 (a) is a bar chart comparing the discharge energy density and efficiency of each sample. Figure 16 (b) shows the uniformity test data of BOPP-3 film in different regions and a schematic diagram of 9-point sampling. Figure 17 This is a scatter plot comparing the cycle stability test and discharge energy density of the BOPP film in this embodiment of the invention. Figure 17 (a) shows the charge-discharge cycle stability curves of BOPP-3 at 300 kV / mm and 120 °C. Figure 17 (b) is a scatter plot comparing the discharge energy density of the present invention and reported BOPP-based high-temperature composite materials at efficiencies above 90%. Detailed Implementation

[0016] The present invention will be further described below with reference to the accompanying drawings and embodiments, but this should not be construed as limiting the present invention.

[0017] Example 1: This example provides a method for preparing BOPP film based on self-assembled coating modification. This method aims to solve the problem of deteriorated insulation performance of existing biaxially oriented polypropylene films under high temperature and high field conditions by functionalizing the surface of the base film and constructing a composite coating. Specifically, the preparation method includes the following steps: Step S1 involves surface activation treatment of the biaxially oriented polypropylene (BOP) base film. Specifically, surface activation treatment refers to altering the physicochemical state of the BOP base film surface through physical or chemical means, such as introducing polar functional groups or adjusting the surface microstructure, thereby improving the wettability and reactivity of the base film surface. This step is a prerequisite for ensuring that subsequent coatings can firmly adhere to the non-polar polypropylene substrate surface. It should be understood that the specific implementation of surface activation treatment is not limited to a particular method; any method that improves the interfacial compatibility of the base film surface is acceptable, thus allowing for the selection of different subsequent process routes.

[0018] Step S2 involves mixing and dispersing pullulan and hydroxylated two-dimensional boron nitride, and adding a crosslinking agent to obtain a coating solution. Specifically, this step constructs the core component system of the composite coating. Pullulan, as an organic matrix, not only provides film-forming ability, but its polar hydroxyl groups also interact strongly with the functional groups on the surface of hydroxylated two-dimensional boron nitride, promoting the uniform dispersion and orientation of nanosheets within the matrix. Simultaneously, the added crosslinking agent forms chemical bonds or physical entanglements between the polymer molecular chains and between the polymer and nanosheets, resulting in a dense and stable self-assembled structure during subsequent curing. This synergistic design based on hydroxyl interactions and crosslinking networks is key to achieving excellent insulation performance and mechanical stability in the coating, and it is independent of specific chemical substances, exhibiting broad applicability.

[0019] Step S3 involves coating the coating solution onto the surface of the surface-activated biaxially oriented polypropylene base film. Specifically, the coating process involves transferring the uniform and stable coating solution prepared in step S2 onto the base film surface after treatment in step S1, forming a continuous and uniform liquid film. Because the base film surface has been activated, the coating solution can spread better and penetrate into the microstructure of the base film surface, enhancing the interfacial adhesion.

[0020] Step S4 involves drying and crosslinking the coated biaxially oriented polypropylene (BOP) base film to form a composite coating on its surface. Specifically, this drying and crosslinking process involves vertically suspending the coated BOP base film in a vacuum oven and drying and crosslinking it at 35-45°C. The drying and crosslinking process is a simultaneous or stepwise physicochemical process. On one hand, the coating is cured and shaped by removing the solvent; on the other hand, under thermal or other energy excitation, the crosslinking agent initiates a crosslinking reaction between pullulan and hydroxylated two-dimensional boron nitride, locking the pre-assembled structure formed in step S2 into a stable three-dimensional network. The resulting composite coating on the surface of the BOP base film not only retains the layered blocking properties of hydroxylated two-dimensional boron nitride but also introduces abundant shallow trap energy levels through the crosslinking network, effectively suppressing carrier migration and improving the electric field distribution. This method, through a surface modification strategy, significantly improves the high-temperature insulation performance of the film while maximizing the preservation of the original thin and light advantages of the base film, meeting the application requirements of miniaturization and high reliability for dry-type DC capacitors.

[0021] Furthermore, pullulan is chosen as the organic matrix in this invention because its molecular chain is rich in polar hydroxyl groups, which not only possess excellent water solubility and film-forming ability, but also interact strongly with the surface of inorganic fillers through hydrogen bonding, promoting interfacial compatibility. Hydroxylated two-dimensional boron nitride, as a typical two-dimensional insulating nanomaterial, has extremely high thermal conductivity. Its layered structure can be oriented in the in-plane direction within the coating, physically extending the carrier migration path. The use of aldehyde crosslinking agents allows for acetalization reactions with the hydroxyl groups on the polysaccharide polymer chains, forming a dense three-dimensional crosslinked network during drying. This firmly locks the two-dimensional nanosheets within the polymer matrix, preventing displacement or aggregation under an electric field, thereby ensuring the long-term stability of the composite coating structure.

[0022] In a preferred embodiment, the pullulan has a mass fraction of 0.5-1%, and the hydroxylated two-dimensional boron nitride has a mass fraction of 0.0625%-0.20%. The aldehyde crosslinking agent is glutaraldehyde, and the molar ratio of glutaraldehyde to the total molar number of hydroxyl groups on the pullulan chain is 1:(15-25). The 0.5-1% pullulan concentration ensures a suitable viscosity in the solution, which is beneficial for the uniformity of the liquid film during dip-coating and provides sufficient film-forming material to cover defects on the base film surface. The glutaraldehyde to hydroxyl group molar ratio of 1:(15-25) ensures sufficient crosslinking density while maintaining a certain degree of flexibility in the coating, preventing the coating from becoming brittle or peeling off from the base film due to excessive crosslinking. The mass fraction of hydroxylated two-dimensional boron nitride is 0.0625%~0.20%. This concentration range is chosen based on a clear physical mechanism: when the content of hydroxylated two-dimensional boron nitride is below 0.0625%, the density of shallow trap sites introduced into the coating is insufficient, making it difficult to effectively capture high-energy electrons and suppress space charge accumulation, thus limiting the improvement in insulation performance; while when the content exceeds 0.20%, excessive nanosheets are prone to stacking and agglomeration in the limited polymer matrix, forming microstructural defects and electric field concentration points, which become weak points leading to breakdown. The concentration within this range allows for a balance between uniform dispersion of nanosheets and self-assembled layered structures within the coating, with the shallow trap density reaching its peak and the deep trap density significantly reduced, thus exhibiting excellent insulation and energy storage performance under high temperature and high field conditions. It should be understood that in this embodiment, the aldehyde crosslinking agent, in addition to glutaraldehyde, may also include glyoxal, adipaldehyde, octanedialdehyde, etc.

[0023] In step S1, the biaxially oriented polypropylene (BOP) base film undergoes surface activation treatment, including plasma treatment. Specifically, plasma treatment is a key pre-processing step for enhancing the surface energy of non-polar polypropylene. The plasma treatment voltage is 3-8 kV, the frequency is 5-10 kHz, and the treatment time is 0.5-2 minutes. This specific process parameter window allows for efficient etching of a micro-rough structure and introduction of oxygen-containing polar functional groups onto the surface of the BOP base film without damaging its mechanical properties. This significantly improves the hydrophilicity and wettability of the base film surface, thus providing the necessary interfacial physicochemical basis for the firm adhesion of subsequent coatings. If the treatment voltage is too low or the time is too short, the surface modification will be insufficient, and the coating will easily peel off. If the voltage is too high or the time is too long, it may lead to excessive etching or even thermal damage to the base film surface, deteriorating the film's insulation performance.

[0024] In step S2, the preparation process of hydroxylated two-dimensional boron nitride involves placing boron nitride nanopowder in a tube furnace, heating it to 800-1200°C at a heating rate of 5-15°C / min, holding it at that temperature for 0.5-1.5 hours, cooling it to room temperature, and then cleaning and drying it to obtain hydroxylated two-dimensional boron nitride. In the accompanying drawings of this invention, PP refers to polypropylene, BNNS refers to the original two-dimensional boron nitride nanosheets, and BNNS-OH refers to the two-dimensional boron nitride nanosheets modified by high-temperature hydroxylation (i.e., hydroxylated two-dimensional boron nitride). Specifically, this high-temperature heat treatment step is a key preliminary step for imparting functional groups to the surface of boron nitride nanosheets. At a high temperature of 1000°C, boron atoms on the surface of the boron nitride nanosheets react with water vapor or trace amounts of oxygen in the atmosphere, generating hydroxyl functional groups in situ. Figure 2 The Fourier transform infrared (FTIR) spectrum of (a) shows that the processed BNNS-OH is at 3428 cm⁻¹. -1 A distinct OH stretching vibration peak was observed, confirming the successful introduction of surface hydroxyl groups. This provides the necessary chemically active sites for subsequent hydrogen bond network formation with the pullulan matrix. Simultaneously, the precisely controlled heat treatment process maintains the high crystallinity of the nanosheet phase while achieving surface modification. Figure 1 As shown in the X-ray diffraction (XRD) pattern of (b), BNNS-OH retains the characteristic diffraction peaks of the (002) and (100) crystal planes consistent with the original BNNS. Only a new diffraction peak belonging to B(OH)3 appears at about 27.9°, indicating that the high-temperature treatment did not destroy its inherent crystal structure, thus ensuring that the excellent insulation and thermal conductivity of the filler itself are preserved. If this step is omitted or the temperature is insufficient, the nanosheet surface will lack sufficient polar groups, resulting in poor dispersion and weak interfacial bonding in the polymer matrix, and thus failing to achieve the expected modification effect.

[0025] In step S3, the coating solution is applied to the surface of the surface-activated biaxially oriented polypropylene (BOP) base film, including immersing the BOP base film in the coating solution using a dip-coating method. As a preferred embodiment, during the dip-coating process, the BOP base film is rotated 180 degrees after each dip, and the coating process is repeated at least twice. This operational feature is a key technical means to overcome the inherent defects of the traditional dip-coating method and achieve uniform modification on one side. Specifically, in the conventional vertical dip-coating process, due to gravity, the coating solution often accumulates at the lower end of the film, resulting in uneven coating thickness distribution along the longitudinal direction, which in turn causes local electric field distortion. This embodiment introduces a 180-degree rotation operation between each dip, alternating the upper and lower ends of the film. The periodic reversal of the gravity direction effectively counteracts the gravity accumulation effect of the liquid film, ensuring uniform spreading and leveling of the coating solution on the base film surface. Meanwhile, repeating the coating cycle at least twice further ensures the continuity and density of the coating, ultimately forming a uniform composite coating with a thickness controlled within the range of 250nm to 350nm on one side of the biaxially oriented polypropylene base film. This single-sided uniform coating strategy not only meets the stringent requirements of dry DC capacitors for thinner films, but also avoids the interlayer interface problems that may be caused by double-sided coating, thus preserving the original excellent performance of the base film to the maximum extent.

[0026] In step S4, the coated biaxially oriented polypropylene (BOPP) base film undergoes a drying and crosslinking treatment to form a composite coating on its surface. This includes vertically suspending the coated BOPP base film in a vacuum oven and drying and crosslinking it under vacuum drying conditions of 35-45°C. In this embodiment, it represents an optimized balance between the heat-sensitive characteristics of the BOPP base film and the crosslinking kinetics of the coating. On one hand, this temperature provides sufficient activation energy for the acetalization reaction between glutaraldehyde and pullulan hydroxyl groups, enabling the coating to simultaneously complete crosslinking and curing during solvent evaporation, forming a dense three-dimensional network structure. On the other hand, this temperature is far below the glass transition temperature and melting point of BOPP, effectively preventing thermal shrinkage, changes in crystallinity, or deterioration of mechanical properties of the base film at high temperatures. The vacuum environment helps accelerate the removal of solvent molecules, reducing the formation of bubbles or pore defects within the coating. Furthermore, the preparation method in this embodiment only modifies one side of the BOPP base film. This design choice is based on the practical needs of miniaturizing dry DC capacitors. Compared to traditional double-sided coating processes, single-sided modification strategies significantly improve the high-temperature breakdown field strength and energy storage density of the film while minimizing the proportion of inactive coating materials. This retains the original advantages of BOPP base film in terms of lightness, thinness, and high volumetric capacity, and avoids problems such as increased overall film thickness and winding difficulties caused by double-sided coating, thus having higher engineering application value.

[0027] In this embodiment, the thickness of the biaxially oriented polypropylene (BOPP) base film is 3–8 μm; the thickness of the composite coating is 250 nm–350 nm. Specifically, this limitation of size parameters reflects the synergistic design of microstructure and macroscopic performance. This embodiment preferably uses a base film thickness of 5.8 μm, which is the mainstream specification for BOPP films used in current power capacitors. Based on this, an ultrathin composite coating of 250 nm–350 nm is constructed. This ensures that the coating can completely cover the microscopic defects on the base film surface and provide a sufficient density of shallow trap sites, while avoiding negative effects such as interfacial stress concentration, decreased flexibility, or increased dielectric loss that may occur with excessively thick coatings. Within this thickness range, a clear, tight, and non-delaminating interfacial bond is formed between the composite coating and the BOPP base film, with uniform coating thickness, verifying the effectiveness of the aforementioned dip-coating and spin-coating processes. This precise matching between the nanoscale coating and the micron-scale base film is an important structural basis for achieving both high insulation strength and high energy storage density in modified films under high temperature and high field conditions.

[0028] Example 2: This example provides a method for preparing BOPP film based on self-assembled coating modification, based on Example 1, specifically including the following steps: Step S1 involves performing plasma surface activation treatment on the biaxially oriented polypropylene (BOPP) base film. The activation treatment involves plasma treatment of the BOPP base film, preferably with a voltage of 6 kV, a frequency of 8 kHz, and a treatment time of 1 minute.

[0029] Step S2 involves mixing and dispersing pullulan and hydroxylated two-dimensional boron nitride, and adding a crosslinking agent to obtain the coating solution. The specific steps are as follows: Step S21 involves dissolving pullulan in water and sonicating it to obtain a polymer solution. In this embodiment, 0.75g of pullulan powder is slowly added to 50ml of deionized water and sonicated for 1 hour until the solution becomes clear and transparent. This step aims to ensure that the pullulan is fully dissolved at the molecular level, avoiding the introduction of microscopic defects in subsequent coatings due to the presence of undissolved particles or polymer clusters, thus laying the foundation for the formation of a dense and uniform organic matrix.

[0030] Step S22 involves dissolving hydroxylated two-dimensional boron nitride in water for pre-dispersion and vigorous dispersion to obtain a nanosheet solution. This is the core process for achieving excellent insulation performance of the coating. In this embodiment, the preparation process of hydroxylated two-dimensional boron nitride nanosheets involves placing 5g of boron nitride nanopowder in a crystal tube of a tube furnace, heating it to 1000℃ at a heating rate of 10℃ / min, and holding it at that temperature for 1 hour; after cooling to room temperature, it is then cleaned and dried to obtain hydroxylated two-dimensional boron nitride nanosheets.

[0031] Hydroxylated two-dimensional boron nitride nanosheets were added to 50 ml of deionized water and pre-dispersed at 500 rpm for 1 hour using magnetic stirring to initially wet the nanosheet powder and remove macroscopic agglomerations. Subsequently, strong dispersion was performed using an ultrasonic probe for 2 hours to obtain a uniform and stable nanosheet suspension. The pre-dispersion step is crucial for eliminating dry powder agglomerates, while strong dispersion utilizes the high shear force generated by ultrasonic cavitation to exfoliate multilayered nanosheets into single-layer or few-layer structures. Only by obtaining highly dispersed single-layer or few-layer nanosheets can effective layered barrier structures be formed through self-assembly during subsequent film formation, thereby extending the carrier migration path and suppressing leakage current. If the pre-dispersion step is omitted or the strong dispersion time is shortened, the nanosheets will not be fully dissociated, resulting in a large number of agglomerates remaining inside the coating.

[0032] Step S23 involves adding the polymer solution to the nanosheet solution and then sonicating it to obtain a mixed solution. In this embodiment, the pullulan solution obtained in step S21 is slowly added dropwise to the hydroxylated two-dimensional boron nitride suspension obtained in step S22, followed by brief sonication. This "post-mixing" strategy, combined with brief sonication, can promote hydrogen bonding between the polymer molecular chains and the hydroxyl groups on the nanosheet surface without damaging the already dispersed nanosheet structure. This enhances the compatibility of the two-phase interface and prevents secondary aggregation caused by excessively high local concentrations due to direct mixing.

[0033] Step S24: Add a crosslinking agent and a crosslinking reaction catalyst to the mixed solution, and stir to obtain a coating solution. In this embodiment, glutaraldehyde is added to the mixed solution obtained in step S23 as a crosslinking agent, and hydrochloric acid solution is added simultaneously as a crosslinking reaction catalyst. The molar ratio of the hydrochloric acid solution to the crosslinking agent is 1:3~8. In this embodiment, the molar ratio of hydrochloric acid solution to glutaraldehyde is 1:5, and then the mixture is stirred continuously at room temperature for 2 hours to achieve uniform mixing. An acidic environment can effectively catalyze the acetalization reaction of glutaraldehyde with the hydroxyl groups on the pullulan chain, initiating the pre-crosslinking process in the solution stage, which helps to quickly lock the orientation and arrangement structure of the nanosheets during subsequent drying and film formation, forming a stable three-dimensional network.

[0034] In this embodiment, the mass fraction of pullulan in the prepared coating solution is 0.75%, the mass fraction of hydroxylated two-dimensional boron nitride is 0.125%, and the molar ratio of glutaraldehyde to the total molar number of hydroxyl groups on the pullulan chain is 1:20.

[0035] Step S3: Using the dip-coating method in Example 1, the coating solution is coated onto the surface of the surface-activated biaxially oriented polypropylene base film.

[0036] Step S4: The coated biaxially oriented polypropylene (BOP) base film is subjected to a drying and crosslinking treatment to form a composite coating on the surface of the BOP base film. In this embodiment, the drying and crosslinking treatment involves vertically suspending the coated BOP base film in a vacuum oven and drying and crosslinking it at 40°C.

[0037] In this embodiment, the biaxially oriented polypropylene film is a commercial power capacitor polypropylene film produced by Ningbo Da Dongnan Wanxiang Technology Co., Ltd.; BNNS powder is purchased from Jiangsu Xianfeng Nanomaterials Technology Co., Ltd., with a sheet diameter of 1~5μm and a thickness of 2~4nm; pullulan powder, glutaraldehyde (50%) and hydrochloric acid solution (37%) are all purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.

[0038] Example 3: This example provides a method for preparing BOPP films based on self-assembled coating modification. The difference from Example 2 lies in the mass fraction of hydroxylated two-dimensional boron nitride. In this example, the mass fraction of hydroxylated two-dimensional boron nitride in the coating solution is 0%.

[0039] Example 4: This example provides a method for preparing BOPP films based on self-assembled coating modification. The difference from Example 2 lies in the different mass fraction of hydroxylated two-dimensional boron nitride. In this example, the mass fraction of hydroxylated two-dimensional boron nitride in the coating solution is 0.0625%.

[0040] Example 5: This example provides a method for preparing BOPP films based on self-assembled coating modification. The difference from Example 2 lies in the mass fraction of hydroxylated two-dimensional boron nitride. In this example, the mass fraction of hydroxylated two-dimensional boron nitride in the coating solution is 0.25%.

[0041] Example 6: This example provides a method for preparing BOPP films based on self-assembled coating modification. The difference from Example 2 lies in the mass fraction of pullulan, the mass fraction of hydroxylated two-dimensional boron nitride, and the total molar ratio of glutaraldehyde to the total number of hydroxyl groups on the pullulan chain. In this example, the mass fraction of pullulan in the coating solution is 0.5%, the mass fraction of hydroxylated two-dimensional boron nitride is 0.0625%, and the molar ratio of glutaraldehyde to the total number of hydroxyl groups on the pullulan chain is 1:15.

[0042] Example 7: This example provides a method for preparing BOPP films based on self-assembled coating modification. The difference from Example 2 lies in the different mass fraction of hydroxylated two-dimensional boron nitride. In this example, the mass fraction of pullulan in the coating solution is 1%, the mass fraction of hydroxylated two-dimensional boron nitride is 0.20%, and the molar ratio of glutaraldehyde to the total molar number of hydroxyl groups on the pullulan chain is 1:25.

[0043] Example 8: This example provides a method for preparing BOPP film based on self-assembled coating modification. The difference from Example 2 lies in the voltage, frequency, and treatment time of the plasma treatment method for the biaxially oriented polypropylene base film. In this example, the plasma treatment voltage is 3kV, the frequency is 5kHz, and the treatment time is 0.5 minutes.

[0044] Example 9: This example provides a method for preparing BOPP film based on self-assembled coating modification. The difference from Example 2 lies in the voltage, frequency, and treatment time of the plasma treatment method for the biaxially oriented polypropylene base film. In this example, the plasma treatment voltage is 8kV, the frequency is 10kHz, and the treatment time is 2 minutes.

[0045] Example 10: This example provides a method for preparing BOPP films based on self-assembled coating modification. The difference between this example and Example 2 lies in the different process of preparing hydroxylated two-dimensional boron nitride. In this example, boron nitride nanopowder is placed in a tube furnace, heated to 800°C at a heating rate of 5°C / min, and held at that temperature for 0.5 hours.

[0046] Example 11: This example provides a method for preparing BOPP films based on self-assembled coating modification. The difference between this example and Example 2 lies in the different process of preparing hydroxylated two-dimensional boron nitride. In this example, boron nitride nanopowder is placed in a tube furnace and heated to 1200°C at a heating rate of 15°C / min and held at that temperature for 1.5 hours.

[0047] Example 12: This example conducts comprehensive physicochemical characterization, electrical performance testing, and microscopic mechanism analysis on the hydroxylated two-dimensional boron nitride-doped pullulan-coated modified BOPP films prepared in Examples 2-5 to verify the effectiveness of the technical solution of the present invention in improving the high-temperature insulation and energy storage performance of the films. Specifically, the sample of Example 2 is named BOPP-3, the sample of Example 3 is named BOPP-1, the sample of Example 4 is named BOPP-2, the sample of Example 5 is named BOPP-4, and the unmodified pure biaxially oriented polypropylene film is used as the control group (BOPP).

[0048] For characterization and testing, a Nicolet iS50 Fourier transform infrared spectrometer was used to measure the FTIR spectra of the doped material; a SmartLab X-ray diffractometer and an Axis Supra X-ray photoelectron spectrometer were used for diffraction and energy dispersive spectroscopy; a Bruker Dimension Icon atomic force microscope (AFM) was used to test the thickness and morphology of the doped material; an EMUC7 ultrathin cryostat was used to slice the film; a JSM-IT800 field emission scanning electron microscope was used to observe the surface morphology and cross-sectional characteristics of the film; and an HT-7700 transmission electron microscope (TEM) was used to study the structural characteristics of the doped material. DC breakdown characteristics were tested using a ball-and-plate electrode method. During the test, the sample was completely immersed in an oil bath, and the voltage was uniformly increased at a rate of 100 V / s at two temperatures: 25℃ and 120℃. Each sample was tested 15 times. Dielectric properties were measured using a Novocontrol Concept 40 broadband wide-temperature dielectric spectrometer, with the test frequency set between 0.1 and 10. 5 The applied voltage was 1V. Trapping characteristics were characterized by a surface potential decay test system, with the thin film surface potential continuously recorded for 40 minutes using an electrometer. Volumetric conductivity was measured using the three-electrode method with applied field strengths of 100, 120, 140, and 160 kV / mm. A Keithley 6517B ammeter was used to collect test results, and the average value of the last minute was taken as the corresponding conduction current. Space charge testing was performed using a thermal pulse method (TPM) space charge testing platform. The thin film was cut into 5cm × 5cm pieces and coated with double-sided electrodes with a diameter of 5mm. First, an applied electric field of 10 kV / mm was applied to obtain the thermal pulse current frequency domain signal. Then, an electric field of 150 kV / mm was applied. The internal electric field and space charge distribution of the thin film were obtained through methods such as amplitude-frequency characteristic correction of the amplifier signal, scale transformation, and Poisson processing. The total sampling time for each laser pulse was 10 ms, and each signal group averaged 50 times.

[0049] First, the microstructure of the filler and the film was characterized.

[0050] like Figure 1-3 As shown, Figure 1 The X-ray photoelectron spectroscopy (XPS) results of (a) show that the XPS spectrum of BNNS shows two strong N1s and B1s peaks at 397.5 eV and 190.3 eV, and two weak C1s and O1s peaks at approximately 284.2 eV and 532.8 eV. Compared with the original BNNS, the C1s and O1s peaks in the XPS spectrum of BNNS-OH are more significant, and the O element content is increased from trace to 16.7%, which confirms that the high-temperature hydroxylation treatment successfully introduced oxygen-containing functional groups on the nanosheet surface. Figure 1The X-ray diffraction (XRD) pattern of (b) shows that the XRD pattern of BNNS has diffraction peaks at 26.6°, 41.7°, 44.0°, 50.2°, 55.1° and 76.0°, corresponding to the (002), (100), (102) and (004) crystal planes, respectively. The XRD pattern of BNNS-OH is similar to that of BNNS, retaining the above characteristic crystal plane diffraction peaks, indicating that the high-temperature treatment did not destroy its bulk crystal structure; the two showed a difference at 27.9°, the new peak being attributed to (010)B(OH)3 diffraction, indicating the formation of hydroxyl surface groups. Figure 2 In the Fourier transform infrared (FTIR) spectrum of (a), BNNS is at 1386 cm⁻¹. -1 and 806cm -1 Characteristic absorption peaks are observed at 3428 cm⁻¹, which are related to the stretching and bending vibrations of BN, respectively. Compared with BNNS, BNNS-OH shows a peak at 3428 cm⁻¹. -1 The presence of a stronger OH stretching vibration peak indicates that BNNS was successfully surface-hydroxylated, which is consistent with the XPS and XRD results. Figure 2 The transmission electron microscope (TEM) and high resolution transmission electron microscope (HRTEM) images in (b) show that BNNS-OH has clear parallel lattice fringes with a lattice spacing of 0.33 nm, corresponding to the (002) crystal plane of BNNS, indicating that its high crystallinity is maintained. Figure 3 Atomic force microscopy (AFM) results in (a) confirmed that the BNNS-OH sheets were separated from each other, with an average diameter of approximately 4 μm and a thickness distribution between 2 and 4 nm. Figure 3 The step heights marked on the AFM section height curves in (b) are 2nm, 3.1nm and 3.6nm, respectively, which have extremely high aspect ratios. This two-dimensional sheet structure is conducive to forming an effective layered barrier in the coating.

[0051] Regarding the morphology and elemental distribution of thin films, such as Figure 4 As shown, Figure 4 The cross-sectional scanning electron microscope (SEM) image of (a) shows that the thickness of the composite coating on the surface of the modified BOPP film is about 303 nm, and the interface between the coating and the BOPP base film is tight without delamination or pore defects. This is due to the synergistic effect of plasma surface treatment and spin-coating impregnation process, which successfully achieved the uniform construction and firm adhesion of the nanoscale composite coating on the surface of the non-polar base film, laying a solid structural foundation for the subsequent improvement of the high-temperature insulation performance of the film. Figure 4 (b) shows that the surface of the pure BOPP film is relatively smooth, but it has inherent defects generated during the processing; while Figure 4(c) shows that the modified film surface is covered with a uniform coating. Although dispersed granular protrusions are visible, the overall structure is continuous and dense, effectively modifying the defects on the base film surface. When the content of hydroxylated two-dimensional boron nitride is high and the dispersion process is not properly controlled, obvious nanosheet stacking and agglomeration marks appear on the coating surface. These agglomerates not only disrupt the continuity of the coating but also induce severe electric field distortion under high electric fields, becoming weak points that lead to breakdown and significantly deteriorating the insulation performance of the film. In contrast, the coating prepared following the "pre-dispersion, strong dispersion, and post-mixing" timing logic of this embodiment maintains good dispersion and surface smoothness even at high filler contents. This fully demonstrates that this specific timing logic is a prerequisite for achieving uniform dispersion of nanosheets and self-assembled layered structures. Figure 4 The energy-dispersive X-ray spectroscopy (EDS) surface scan results (d) show that the four elements B, C, N, and O are uniformly distributed on the film surface. Among them, B and N elements come from BNNS-OH, O element comes from pullulan and BNNS-OH, and C element comes from pullulan and BOPP base film, which proves the good dispersion of coating components at the microscale.

[0052] Dielectric properties are a key factor determining the energy storage characteristics of a capacitor. For example... Figure 5 As shown, at 25℃ and 120℃, the dielectric constant of the modified film increased with increasing BNNS-OH content. This is mainly attributed to two mechanisms: first, the heterogeneous interfaces formed between the coating and the base film, and between the filler and the matrix, induce Maxwell-Wagner-Sillars (MWS) interfacial polarization; second, the polar hydroxyl groups on the pullulan molecular chain and the functional groups on the BNNS-OH surface provide additional dipole moment contributions. Meanwhile, the dielectric constant of the modified film decreased slightly with increasing testing frequency, due to the hysteresis of the interfacial polarization response at high frequencies and the difficulty of the polar groups to synchronize with changes in the external electric field. Notably, when the temperature increased from 25℃ to 120℃, the dielectric constant of all samples decreased slightly. This is because the increased thermal motion of the pullulan macromolecular chain at high temperatures hindered the ordered orientation of the dipoles along the electric field direction. Nevertheless, all modified films maintained low dielectric loss over a wide temperature and frequency range, thanks to the self-assembled layered structure of BNNS-OH in the pullulan matrix, which effectively limited long-range charge migration and suppressed leakage conduction loss.

[0053] Electrical conductivity directly affects the charge / discharge efficiency and thermal stability of thin films. For example... Figure 6As shown, at 25℃, the volumetric conductivity of all samples increased with the electric field strength from 100 kV / mm to 160 kV / mm. At the high field strength of 160 kV / mm, the conductivity of the modified film first decreased and then increased with increasing BNNS-OH content. Among them, the BOPP-3 sample (0.125 wt% BNNS-OH) had the lowest conductivity, only 3.8 × 10⁻⁶. -15 S / m, relatively pure BOPP film (5.4×10 -15 The conductivity (S / m) decreased by 29.6%. This indicates that an appropriate amount of BNNS-OH synergistically improved the surface morphology with the pullulan coating, and the horizontally arranged nanosheets effectively prolonged the carrier migration path, hindering the development of leakage current. However, when the BNNS-OH content increased to 0.25 wt% (BOPP-4), the conductivity rebounded to 7.1 × 10⁻⁶. -15 The S / m is even higher than that of pure BOPP. This is because the excessive filler agglomerates, introduces structural defects and causes local electric field distortion, which in turn promotes charge transport.

[0054] The space charge distribution characteristics reveal the microscopic origin of the electric field distortion inside the thin film. Figure 7 These are the electric field intensity and charge density distribution diagrams for BOPP and BOPP-1 samples, where... Figure 7 (a) shows the electric field intensity distribution of the BOPP sample at different polarization times. Figure 7 (b) shows the electric field intensity distribution of the BOPP-1 sample at different polarization times. Figure 7 (A) represents the charge density distribution of the BOPP sample at different polarization times. Figure 7 (B) represents the charge density distribution of the BOPP-1 sample at different polarization times; Figure 8 These are the electric field intensity and charge density distribution diagrams for BOPP-2 and BOPP-3 samples, where... Figure 8 (a) shows the electric field intensity distribution of the BOPP-2 sample at different polarization times. Figure 8 (b) shows the electric field intensity distribution of the BOPP-3 sample at different polarization times. Figure 8 (A) represents the charge density distribution of the BOPP-2 sample at different polarization times. Figure 8 (B) shows the charge density distribution of the BOPP-3 sample at different polarization times; Figure 9 This is a graph showing the electric field intensity and charge density distribution of the BOPP-4 sample. Figure 9 (a) shows the electric field intensity distribution of the BOPP-4 sample at different polarization times. Figure 9(A) shows the charge density distribution of the BOPP-4 sample at different polarization times. It can be seen that after polarization for 60 minutes under a 150 kV / mm DC field, the electric field on the anode side of the pure BOPP film is severely distorted, and a large amount of negative charge accumulates in the bulk. This is due to the charge imbalance caused by the difference in the mobility of positive and negative charges. In contrast, the charge density inside the modified films (especially BOPP-3) is significantly reduced, and the electric field distribution is more uniform. This is because the shallow traps introduced by the coating enhance the charge trapping ability, and the physical barrier of the two-dimensional nanosheets inhibits charge injection and migration. However, for the BOPP-4 sample, the surface defects caused by filler agglomeration become charge injection channels, resulting in an increase in positive polarity charge density, which migrates into the bulk over time, deteriorating the internal electric field distribution. This is consistent with the conductivity test results.

[0055] Breakdown field strength is a core indicator for evaluating the withstand voltage capability of insulating materials. Based on the Weibull distribution function (… Where F(x) is the probability of breakdown of the electrical insulation material; x is the breakdown field strength of the electrical insulation material; α is a scale parameter, representing the breakdown field strength when the breakdown probability is 63.2%; and β is a shape parameter, representing the dispersion of the breakdown field strength. Statistical analysis of the breakdown data is performed, and the results are as follows: Figure 10 As shown, where Figure 10 (a) is the Weibull distribution at 25°C. Figure 10 (b) shows the Weibull distribution at 120℃. At 25℃, the characteristic breakdown field strength (α parameter) of the BOPP-3 sample reached 777.2 kV / mm, an increase of 11.8% compared to pure BOPP (694.8 kV / mm). At 120℃, the breakdown field strength of pure BOPP dropped sharply to 521.6 kV / mm, while the BOPP-3 sample remained at 616.8 kV / mm, an increase of 18.2%, demonstrating excellent high-temperature insulation stability. However, the breakdown field strength of the BOPP-4 sample at 120℃ dropped to 577.5 kV / mm, a decrease of 6.4% compared to BOPP-3, further confirming the negative impact of excessive filler agglomeration on insulation performance. The change in the shape parameter β also reflects the dispersion of breakdown data; the higher β value of the modified film indicates more reliable insulation performance.

[0056] Trapping properties serve as a bridge connecting microstructure and macroscopic insulation properties. The trap energy level E of the thin film sample... t and density N(E) t The calculation is as follows: ; ; In the formula, k B Boltzmann constant; T is absolute temperature; v ATE Escape frequency; It is the vacuum permittivity; denoted as the relative permittivity; e is the electron charge; L is the sample thickness; and φ is the sample surface potential.

[0057] The obtained trap energy levels and density distributions are as follows: Figure 11 As shown in Table 1.

[0058] Table 1

[0059] The results show that coating modification mainly alters the trap density rather than the trap depth. Compared to pure BOPP, the shallow trap density (0.93 eV) of the BOPP-3 sample decreased from 5.56 × 10⁻⁶. 22 eV -1 ·m -3 Increased to 8.33×10 22 eV -1 ·m -3 The increase reached 52.2%; while the deep trap density (1.03 eV) increased from 2.15 × 10⁻⁶. 23 eV -1 ·m -3 Reduced to 1.90×10 23 eV -1 ·m -3 The decrease was 12.6%. This optimized distribution of "increasing shallow traps and decreasing deep traps" is crucial: high-density shallow traps can promote the rapid entry and exit balance of charges, preventing charges from accumulating in deep traps to form space charge packets, thereby suppressing electric field distortion and improving breakdown strength. In the BOPP-4 sample, the density of shallow traps increased by 17.9%, but the density of deep traps increased instead, which is due to the deep localized states introduced by agglomeration defects.

[0060] To further explore the mechanism of coating modification, structural models of polypropylene, pullulan, and BNNS-OH were established in this embodiment. The band structure and density of states distribution of the three materials were obtained using quantum chemical calculations, theoretically explaining the optimization mechanism of insulation performance by the coating modification process. Considering the speed and accuracy of the calculations, the B3LYP hybrid functional and 6-31G(d) basis set from density functional theory (DFT) were applied to obtain the electronic band structure, electronic density of states spectrum, and potential distribution of PP, pullulan, and BNNS-OH. Figure 12Band distribution diagrams for different molecular chain structures are presented. It can be seen that the PP molecular chain has the largest band gap, with its highest occupied molecular orbital (HOMO) at -7.68 eV and its lowest unoccupied molecular orbital (LUMO) at 2.01 eV, resulting in an energy level difference of 9.69 eV. This means that the most energy is required to excite electrons from the valence band to the conduction band, which may be the reason for charge accumulation inside the film. Compared with PP, BNNS-OH and pullulan have lower LUMO levels, at -0.74 and -0.52 eV respectively, thus exhibiting strong electron adsorption capabilities. Meanwhile, BNNS-OH and pullulan have higher HOMO levels, at -6.8 and -6.78 eV respectively, enhancing their hole-trapping ability. Quantitative molecular surface analysis results (see...) Figure 13 The results show that the carbon chain backbone region of PP exhibits a positive electrostatic potential, while the overall molecular surface exhibits a negative electrostatic potential. Furthermore, as a nonpolar dielectric, the electrostatic potential of the PP molecular chain is relatively small. However, under no electric field conditions, the carbon, boron, and nitrogen backbone regions of BNNS-OH and pullulan molecules exhibit a positive electrostatic potential, while the area near the hydroxyl groups exhibits a negative electrostatic potential. These potentials are significantly higher than those of PP, indicating the presence of strong positive and negative electrostatic potentials in these regions, which are conducive to charge attraction. The pullulan coating introduces new energy levels into the band gap, increasing the HOMO level and decreasing the LUMO level. The higher LUMO levels are unoccupied, while the levels below the HOMO are completely filled with electrons. This indicates that the introduction of the pullulan matrix significantly reduces the band gap. Under an applied electric field, valence band electrons at higher HOMO levels are more easily excited to the conduction band, thus lowering the potential barrier required for electron transitions. Simultaneously, the lower LUMO levels are more likely to form traps that can capture migrating electrons, exhibiting typical shallow trap characteristics. During charge transport, positive and negative charges constantly undergo trapping and escaping. Each jump dissipates energy, making it difficult for charge kinetic energy to accumulate and reducing the number of effective electrons that can participate in ionization collisions, thus suppressing electron avalanches. Furthermore, high-energy electrons accelerated in an electric field are more likely to collide with molecules with higher electron affinity. According to Koopmans' theorem, electron affinity can be approximated as the negative of the lowest unoccupied molecular orbital (LUMO) energy. Therefore, PP has a negative electron affinity, while the coating material has a positive one, indicating that the coating has a stronger attraction for high-energy electrons than PP. Simultaneously, BNNS-OH and pullulan molecules, due to their narrower band gaps, can effectively dissipate the energy of high-energy electrons, making it difficult for them to accumulate enough energy to trigger ionization collisions, thereby mitigating the destructive effect on the molecular chain.

[0061] Phase-field simulation further demonstrates the effect of coating on the breakdown path. Figure 14 These are simulation diagrams of the breakdown path and electric field distribution of the thin film before and after modification in this embodiment of the invention. Figure 14 (a) shows the breakdown path distribution of the film before modification. Figure 14 (b) shows the breakdown path distribution of the modified film. Figure 14 (c) shows the breakdown electric field distribution of the film before modification. Figure 14 (d) represents the breakdown electric field distribution of the modified film. In the unmodified film, the breakdown path develops longitudinally in a single dendritic pattern, with the electric field highly concentrated at the tip of the breakdown channel. In the modified film, however, the two-dimensional BNNS-OH filler forces the breakdown path to extend laterally and branch, significantly increasing the number of branches. This path complexity not only consumes more electrical energy but also makes the electric field distribution more uniform, reducing the local electric field concentration effect, thus macroscopically manifesting as an increase in the breakdown field strength. The simulation results are in high agreement with the experimentally measured breakdown performance and trap characteristics, verifying the effectiveness of the synergistic mechanism of "layered barrier and shallow trap regulation".

[0062] Finally, the energy storage performance of the modified thin film under high temperature and high field conditions was comprehensively evaluated. Figure 15 This is a comparative diagram of the electrical properties and energy storage characteristics of the BOPP film in the embodiments of the present invention, wherein... Figure 15 (a) is the electric displacement-electric field (DE) hysteresis loop. Figure 15 (b) shows the curves of discharge energy density and efficiency as a function of electric field; Figure 16 This is a graph showing the discharge efficiency and uniformity test results of the BOPP film in an embodiment of the present invention. Figure 16 (a) is a bar chart comparing the discharge energy density and efficiency of each sample. Figure 16 (b) shows the uniformity test data of BOPP-3 film in different regions and a schematic diagram of 9-point sampling. Figure 17 This is a scatter plot comparing the cycle stability test and discharge energy density of the BOPP film in this embodiment of the invention. Figure 17 (a) shows the charge-discharge cycle stability curves of BOPP-3 at 300 kV / mm and 120 °C. Figure 17 (b) is a scatter plot comparing the discharge energy density of the present invention and previously reported BOPP-based high-temperature composite materials at efficiencies above 90%. Figure 15 As shown in (a), the electric displacement-electric field (DE) loop of the modified film is narrower and higher than that of pure BOPP, indicating that it effectively reduces energy loss while improving polarization intensity. At 120℃ and 600 kV / mm, the discharge energy density (Ue) of the BOPP-3 sample reaches 7.88 J / cm². 3 The charge / discharge efficiency (η) is as high as 90.1%, which is 120% and 26.1 percentage points higher than that of pure BOPP, respectively. Figure 15 (b) and Figure 16 (a)). This performance far exceeds that of most reported BOPP-based high-temperature composites ( Figure 17 (b)). Furthermore, the consistency of the energy storage performance of the BOPP-3 film in different regions was verified through a 9-point sampling test. Figure 16 (b)), and after 10,000 charge-discharge cycles at 120℃ and 300kV / mm, the discharge energy density and charge-discharge efficiency show almost no degradation. Figure 17 (a) demonstrates that the modified film exhibits excellent uniformity and long-term reliability under actual working conditions.

[0063] In summary, this invention successfully achieved a significant simultaneous improvement in the high-temperature insulation and energy storage performance of BOPP films by constructing a BNNS-OH doped pullulan self-assembled coating, providing a practical solution for next-generation dry DC capacitor dielectric materials.

[0064] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for preparing BOPP film based on self-assembled coating modification, characterized in that, Includes the following steps: Step S1: Perform surface activation treatment on the biaxially oriented polypropylene base film; Step S2: Pullulan and hydroxylated two-dimensional boron nitride are mixed and dispersed, and a crosslinking agent is added to obtain a coating solution; wherein, the mass fraction of pullulan in the coating solution is 0.5~1%, and the mass fraction of hydroxylated two-dimensional boron nitride is 0.0625%~0.20%; Step S3: Apply the coating solution to the surface of the surface-activated biaxially oriented polypropylene base film; Step S4: Dry and crosslink the coated biaxially oriented polypropylene base film to form a composite coating on the surface of the biaxially oriented polypropylene base film.

2. The method for preparing BOPP film based on self-assembled coating modification according to claim 1, characterized in that, The crosslinking agent is an aldehyde crosslinking agent; the molar ratio of the aldehyde crosslinking agent to the total molar number of hydroxyl groups on the pullulan chain is 1:15~25.

3. The method for preparing BOPP film based on self-assembled coating modification according to claim 2, characterized in that, The coating solution contains 0.75% pullulan by mass and 0.125% hydroxylated two-dimensional boron nitride by mass; the molar ratio of the aldehyde crosslinking agent to the total number of hydroxyl groups on the pullulan chain is 1:

20.

4. The method for preparing BOPP film based on self-assembled coating modification according to claim 1, characterized in that, The surface activation treatment of the biaxially oriented polypropylene base film is to perform plasma treatment on the biaxially oriented polypropylene base film; the plasma treatment voltage is 3~8kV, the frequency is 5~10kHz, and the treatment time is 0.5~2 minutes.

5. The method for preparing BOPP film based on self-assembled coating modification according to claim 1, characterized in that, The biaxially oriented polypropylene base film is immersed in the coating solution using a dip-coating method to achieve coating. After each dip-coating, the biaxially oriented polypropylene base film is rotated 180 degrees, and the coating process is repeated at least twice.

6. The method for preparing BOPP film based on self-assembled coating modification according to claim 1, characterized in that, The preparation of the coating solution specifically includes: The pullulan was dissolved in water and subjected to ultrasonic treatment to obtain a polymer solution; The hydroxylated two-dimensional boron nitride was dissolved in water for pre-dispersion and vigorous dispersion to obtain a nanosheet solution. The polymer solution is added to the nanosheet solution and subjected to ultrasonic treatment to obtain a mixed solution; The crosslinking agent and the crosslinking reaction catalyst are added to the mixed solution, and the mixture is stirred to obtain the coating solution.

7. The method for preparing BOPP film based on self-assembled coating modification according to claim 6, characterized in that, The pre-dispersion is performed using magnetic stirring at a speed of 300-800 rpm for 0.5-1.5 hours. The strong dispersion is achieved using an ultrasonic probe for 1-3 hours; The crosslinking catalyst is a hydrochloric acid solution, and the molar ratio of the hydrochloric acid solution to the crosslinking agent is 1:3~8.

8. The method for preparing BOPP film based on self-assembled coating modification according to claim 1, characterized in that, The drying and crosslinking process involves vertically suspending the coated biaxially oriented polypropylene base film in a vacuum oven and drying and crosslinking it at 35-45°C.

9. The method for preparing BOPP film based on self-assembled coating modification according to claim 1, characterized in that, The preparation process of the hydroxylated two-dimensional boron nitride includes: Boron nitride nanopowder was placed in a tube furnace and heated to 800-1200℃ at a heating rate of 5-15℃ / min and held at that temperature for 0.5-1.5 hours. After cooling to room temperature, the mixture is washed and dried to obtain the hydroxylated two-dimensional boron nitride.

10. The method for preparing BOPP film based on self-assembled coating modification according to claim 1, characterized in that, The thickness of the biaxially oriented polypropylene base film is 3~8μm; The thickness of the composite coating is 250nm~350nm.

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