Biaxially oriented polypropylene composite film for capacitor and preparation method of biaxially oriented polypropylene composite film

Abstract

The invention relates to a biaxially oriented polypropylene composite film for a capacitor and a preparation method of the biaxially oriented polypropylene composite film. The biaxially oriented polypropylene composite film is of an ABA three-layer structure; wherein the layer A is prepared from the following raw materials: PP, a nucleating agent and a composite filler of MAH-g-PP and nanoparticles; the layer B is prepared from the following raw materials: PET master batch, modified silicon dioxide and a brush polymer. According to the preparation method, the three-layer structure composite film is prepared, the layers are firmly combined through covalent bonds, hydrogen-bond interaction and the like, and the interlayer peel strength is remarkably improved; a nano particle filler with a high dielectric constant is introduced into the A-layer structure, so that the dielectric strength of the film is improved, and meanwhile, the film has relatively high anti-breakdown strength; and the layer B effectively improves the mechanical stability of the film material, and has higher mechanical property and material rigidity compared with the traditional PP film.

Description

Technical Field

[0001] This invention relates to the field of capacitor film technology, specifically to a biaxially oriented polypropylene composite film for capacitors and its preparation method. Background Technology

[0002] As one of the three major passive components, capacitors are widely used in various electronic products. Among them, film capacitors, benefiting from their high voltage withstand capability, good temperature characteristics, and long lifespan, have significant advantages and broad prospects in new energy fields such as new energy vehicles, photovoltaics, and wind power generation. With the advancement of technology, the performance requirements for film capacitors are becoming increasingly stringent. Since the capacitor film is the core material of film capacitors, optimizing and improving the performance of capacitor components by enhancing the performance of the capacitor film material is currently one of the main research directions in this field.

[0003] Biaxially oriented polypropylene (BOPP) film has become the preferred dielectric material for manufacturing metallized film capacitors due to its excellent dielectric properties, high insulation strength, and good self-healing characteristics. As the electronics and power industry moves towards miniaturization and high reliability, the requirements for capacitors are increasing, necessitating capacitor films that are ultra-thin while also possessing higher withstand voltage, lower loss factor, and better high-temperature stability.

[0004] Conventional BOPP capacitor films have a single-layer homogeneous structure. PP is a semi-crystalline material with poor mechanical properties and a low glass transition temperature. Performance improvements are often achieved through purifying raw materials and optimizing the stretching process, which is approaching a technological bottleneck. Simply reducing the thickness leads to a decrease in dielectric strength and an increase in defect rate; while problems such as high dielectric loss and thermal shrinkage at high temperatures also limit its application in high-end fields. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention prepares a polypropylene composite membrane with an A / B / A three-layer structure through material combination and structural design to comprehensively improve performance.

[0006] To achieve the above objectives, the present invention provides the following technical solution: A biaxially oriented polypropylene composite film for capacitors, the composite film having an ABA three-layer structure; wherein, the A layer material includes PP, a nucleating agent, a composite filler of MAH-g-PP and nanoparticles; the B layer material includes PET masterbatch, modified silica, and a brushed polymer; The composite filler of MAH-g-PP and nanoparticles is obtained by feeding MAH-g-PP and nanoparticles into an extruder, adding stearic acid lubricant and polyvinyl alcohol, and then extruding and granulating. The brush polymer is obtained by free radical copolymerization of 2-isopropenyl-2-oxazoline and ethyl 2-bromomethacrylate to obtain a polymer backbone, and then by reacting bromine and ethylenediamine to introduce brush polymer chains under the polymer backbone.

[0007] Further, the components of the A layer raw material by weight are: 100 parts PP, 0.5-2 parts nucleating agent, and 12-25 parts composite filler of MAH-g-PP and nanoparticles; the components of the B layer raw material by weight are: 100 parts PET, 5-10 parts modified silica, and 10-20 parts brush polymer.

[0008] Furthermore, the preparation process of the composite filler of MAH-g-PP and nanoparticles is as follows: the nano barium titanate powder is dispersed in an ethanol solution, ultrasonically treated to form a suspension, KH-560 is slowly added dropwise to the suspension, the temperature is raised to 60-80℃, and the reaction is refluxed for 4-6 hours. Carboxylated graphene is added to the suspension, and the mixture is stirred and refluxed overnight. After the reaction is completed, the mixture is centrifuged, washed and dried to obtain nanoparticles. MAH-g-PP and nanoparticles are fed into an extruder, along with stearic acid lubricant and polyvinyl alcohol, and then extruded and granulated to obtain the final product.

[0009] Furthermore, the mass ratio of barium titanate nanoparticles, KH-560, and carboxylated graphene in the nanoparticles is 20~60:2~6:100.

[0010] Furthermore, in the composite filler of MAH-g-PP and nanoparticles, the mass ratio of MAH-g-PP, nanoparticles, stearic acid lubricant and polyvinyl alcohol is 10:15~20:0.1~0.2:0.2~1.

[0011] Furthermore, the nucleating agent is an aryl diamide β-nucleating agent.

[0012] Furthermore, the preparation process of the brush-like polymer is as follows: Under inert gas protection, 2-isopropenyl-2-oxazoline and ethyl 2-bromomethacrylate were added to a reaction vessel, along with azobisisobutyronitrile and an organic solvent. The reaction was carried out at 60-80°C for 12-24 h. After the reaction was completed, the mixture was poured into cold diethyl ether, precipitated, filtered, and dried to obtain the polymer backbone precursor. The polymer backbone precursor is dissolved in a polar solvent, excess ethylenediamine is added, and the mixture is reacted at 50-70°C for 6-12 hours. The precipitate is then collected in cold diethyl ether, filtered, washed, and dried to obtain the final product.

[0013] Furthermore, the molar ratio of 2-isopropenyl-2-oxazoline to ethyl 2-bromomethacrylate is 1.5 to 3:1.

[0014] Furthermore, the modified silica is hydroxyl-functionalized silica.

[0015] The present invention further provides a method for preparing the biaxially oriented polypropylene composite film for capacitors as described above, comprising the following steps: S1. Mix and granulate the raw materials of layer A and layer B separately; S2 and A layers are used as the surface layer and B layer is used as the core layer. The ABA three-layer structure is obtained by melt extrusion at 230~270℃ with an extrusion mass ratio of 2~5:1~3:2~5. After extrusion, the sheet is cooled and cast. S3. The resulting casting is preheated at 50~70℃, and then longitudinally stretched at 110~130℃ with a stretching ratio of 3-3.6; then preheated at 80~120℃, and transversely stretched at 110~130℃ with a stretching ratio of 2.6~3.2. S4. The biaxially stretched film is heat-set at 200~230℃, then cooled at 50℃, drawn, and wound to obtain the final product.

[0016] In existing technologies, inorganic fillers such as silica and barium titanate are commonly used to improve the dielectric constant of PP films. However, the direct introduction of inorganic fillers may lead to defects such as pores and cracks in the film due to poor compatibility. This, in turn, can reduce the film's breakdown strength, result in poor processing performance, and hinder practical applications.

[0017] This invention comprehensively improves the performance of a polypropylene composite film with an A / B / A three-layer structure. Layer B serves as the intermediate support layer, co-extruded from PET, modified silica, and a brush polymer, providing mechanical support and dimensional stability while also offering high mechanical properties and high heat resistance. The brush polymer contains abundant oxazoline structures, ester bonds, cations, and amino groups. The modified silica is hydroxyl-functionalized silica, which interacts with the brush polymer and disperses well in PET. Layer A is primarily made of PP, ensuring good adhesion and low loss to the metal electrode. It also incorporates high-dielectric-constant nanoparticle fillers, formed by covalent coupling of barium titanate and carboxylated graphene using a silane coupling agent. The barium titanate first reacts with the silane coupling agent and then inserts into the interlayer structure of the carboxylated graphene, synergistically enhancing the dielectric strength of the composite film. However, nanoparticle fillers are not suitable for direct mixing and granulation with PP masterbatch. Instead, they must be pretreated with maleic anhydride-grafted polypropylene (MAH-g-PP). Since MAH-g-PP contains polypropylene segments, it has good compatibility with PP masterbatch. Furthermore, by adding a small amount of lubricant and polyvinyl alcohol for coating, the dispersibility of the nanoparticle fillers in the matrix is ​​greatly improved, avoiding the degradation of electrical properties caused by agglomeration. In the ABA three-layer co-extrusion process, the hydroxyl-functionalized silica and the brush-like polymer containing multiple active sites in the B layer can also act as interface reinforcing agents. Through chemical reactions between the hydroxyl, oxazoline, and amino groups in the B layer and the epoxy, anhydride, and carboxyl groups in the A layer structure, covalent bonds, hydrogen bonds, and intermolecular forces are formed, solving the problem of weak interfacial adhesion and easy delamination caused by thermodynamic incompatibility between PP and PET.

[0018] Compared with the prior art, the beneficial effects of the present invention are as follows: The present application prepares an ABA three-layer composite film, in which the layers are firmly bonded by covalent bonds, hydrogen bonds and other forces, which significantly improves the interlayer peel strength; high dielectric constant nanoparticle filler is introduced into the A layer structure, which improves the dielectric strength of the film and has high breakdown strength; the B layer effectively improves the mechanical stability of the film material, and has higher mechanical properties and material rigidity compared with traditional PP film. Detailed Implementation

[0019] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] Furthermore, the technical features involved in the different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0021] Example: A biaxially oriented polypropylene composite film for capacitors. S1. Mix and granulate the raw materials of layer A and layer B separately; S2 and A layers are used as the surface layer and B layer is used as the core layer. The ABA three-layer structure is obtained by melt extrusion at 230~270℃ with an extrusion mass ratio of 2~5:1~3:2~5. After extrusion, the sheet is cooled and cast. S3. The resulting casting is preheated at 50~70℃, and then longitudinally stretched at 110~130℃ with a stretching ratio of 3-3.6; then preheated at 80~120℃, and transversely stretched at 110~130℃ with a stretching ratio of 2.6~3.2. S4. The biaxially stretched film is heat-set at 200~230℃, then cooled at 50℃, drawn, and wound to obtain the final product.

[0022] Among them, the raw materials of layer A include 100 parts of PP, 0.5~2 parts of aryl diamide β nucleating agent, and 12~25 parts of MAH-g-PP and nanoparticle composite filler; The B-layer material consists of 100 parts PET, 5-10 parts modified silica, and 10-20 parts brush polymer.

[0023] Unless otherwise specified, all raw materials used in this invention can be purchased from the market. The preparation process of the MAH-g-PP and nanoparticle composite filler is as follows: 30 g of barium titanate nanoparticles were dispersed in 400 ml of ethanol solution and sonicated to form a suspension. 1.5 g of KH-560 was slowly added dropwise to the suspension and the temperature was raised to 60-80 °C. The mixture was refluxed for 4-6 h. 50 g of carboxylated graphene was added to the suspension and stirred and refluxed overnight. After the reaction was completed, the mixture was centrifuged, washed and dried to obtain the nanoparticle filler. 10 g of MAH-g-PP and 15-20 g of nanoparticle filler are fed into an extruder, along with 0.1-0.2 g of stearic acid lubricant and 0.2-1 g of polyvinyl alcohol. The mixture is then extruded and granulated to obtain the final product.

[0024] The preparation process of the brush-like polymer is as follows: Under inert gas protection, 2-isopropenyl-2-oxazoline and ethyl 2-bromomethacrylate in a molar ratio of 1.5 to 3:1 were added to a reactor, along with an appropriate amount of azobisisobutyronitrile (AIBN) initiator and toluene as an organic solvent. The reaction was carried out at 60 to 80 °C for 12 to 24 hours. After the reaction was completed, the mixture was poured into cold diethyl ether, precipitated, filtered, and dried to obtain the polymer backbone precursor. The polymer backbone precursor is dissolved in DMF, and excess ethylenediamine (molar ratio of ethylenediamine to ethyl 2-bromomethacrylate is 1.2:1) is added. The mixture is reacted at 50-70℃ for 6-12 hours, precipitated in cold diethyl ether, and then filtered, washed, and dried to obtain the final product.

[0025] Example 1: Layer A: 100 parts PP, 1 part aryl diamide β nucleating agent, 12 parts MAH-g-PP and nanoparticle composite filler; Layer B: 100 parts PET, 5 parts modified silica, 10 parts brush polymer; Layer A: Layer B: Layer A = 3:2:3.

[0026] The preparation process of the composite filler of MAH-g-PP and nanoparticles is as follows: 30 g of barium titanate nanoparticles were dispersed in 400 ml of ethanol solution and sonicated to form a suspension. 1.5 g of KH-560 was slowly added dropwise to the suspension and the temperature was raised to 60-80 °C. The mixture was refluxed for 4-6 h. 50 g of carboxylated graphene was added to the suspension and stirred and refluxed overnight. After the reaction was completed, the mixture was centrifuged, washed and dried to obtain the nanoparticle filler. 10 g of MAH-g-PP and 15 g of nanoparticle filler were fed into an extruder, along with 0.1 g of stearic acid lubricant and 0.5 g of polyvinyl alcohol. The mixture was then extruded and granulated to obtain the final product.

[0027] The preparation process of the brush-like polymer is as follows: Under inert gas protection, 2-isopropenyl-2-oxazoline and ethyl 2-bromomethacrylate in a molar ratio of 1.5:1 were added to a reactor, along with an appropriate amount of azobisisobutyronitrile (AIBN) initiator and toluene as an organic solvent. The reaction was carried out at 60-80°C for 12-24 hours. After the reaction was completed, the mixture was poured into cold diethyl ether, precipitated, filtered, and dried to obtain the polymer backbone precursor. The polymer backbone precursor is dissolved in DMF, and excess ethylenediamine (molar ratio of ethylenediamine to ethyl 2-bromomethacrylate is 1.2:1) is added. The mixture is reacted at 50-70℃ for 6-12 hours, precipitated in cold diethyl ether, and then filtered, washed, and dried to obtain the final product.

[0028] Example 2: Layer A: 100 parts PP, 1 part aryl diamide β nucleating agent, 18 parts MAH-g-PP and nanoparticle composite filler; Layer B: 100 parts PET, 8 parts modified silica, 15 parts brushed polymer; Layer A: Layer B: Layer A = 3:2:3; The preparation process of the composite filler of MAH-g-PP and nanoparticles is as follows: 30 g of barium titanate nanoparticles were dispersed in 400 ml of ethanol solution and sonicated to form a suspension. 1.5 g of KH-560 was slowly added dropwise to the suspension and the temperature was raised to 60-80 °C. The mixture was refluxed for 4-6 h. 50 g of carboxylated graphene was added to the suspension and stirred and refluxed overnight. After the reaction was completed, the mixture was centrifuged, washed and dried to obtain the nanoparticle filler. 10 g of MAH-g-PP and 18 g of nanoparticle filler were fed into an extruder, along with 0.1 g of stearic acid lubricant and 0.7 g of polyvinyl alcohol. The mixture was then extruded and granulated to obtain the final product.

[0029] The preparation process of the brush-like polymer is as follows: Under inert gas protection, 2-isopropenyl-2-oxazoline and ethyl 2-bromomethacrylate in a molar ratio of 2:1 were added to a reactor, along with an appropriate amount of azobisisobutyronitrile (AIBN) initiator and toluene as an organic solvent. The reaction was carried out at 60-80°C for 12-24 hours. After the reaction was completed, the mixture was poured into cold diethyl ether, precipitated, filtered, and dried to obtain the polymer backbone precursor. The polymer backbone precursor is dissolved in DMF, and excess ethylenediamine (molar ratio of ethylenediamine to ethyl 2-bromomethacrylate is 1.2:1) is added. The mixture is reacted at 50-70℃ for 6-12 hours, precipitated in cold diethyl ether, and then filtered, washed, and dried to obtain the final product.

[0030] Example 3: Layer A: 100 parts PP, 1 part aryl diamide β nucleating agent, 25 parts MAH-g-PP and nanoparticle composite filler; Layer B: 100 parts PET, 10 parts modified silica, 20 parts brush polymer; Layer A: Layer B: Layer A = 3:2:3.

[0031] The preparation process of the composite filler of MAH-g-PP and nanoparticles is as follows: 30 g of barium titanate nanoparticles were dispersed in 400 ml of ethanol solution and sonicated to form a suspension. 1.5 g of KH-560 was slowly added dropwise to the suspension and the temperature was raised to 60-80 °C. The mixture was refluxed for 4-6 h. 50 g of carboxylated graphene was added to the suspension and stirred and refluxed overnight. After the reaction was completed, the mixture was centrifuged, washed and dried to obtain the nanoparticle filler. 10 g of MAH-g-PP and 20 g of nanoparticle filler were fed into an extruder, along with 0.1 g of stearic acid lubricant and 0.5 g of polyvinyl alcohol. The mixture was then extruded and granulated to obtain the final product.

[0032] The preparation process of the brush-like polymer is as follows: Under inert gas protection, 2-isopropenyl-2-oxazoline and ethyl 2-bromomethacrylate in a molar ratio of 2:1 were added to a reactor, along with an appropriate amount of azobisisobutyronitrile (AIBN) initiator and toluene as an organic solvent. The reaction was carried out at 60-80°C for 12-24 hours. After the reaction was completed, the mixture was poured into cold diethyl ether, precipitated, filtered, and dried to obtain the polymer backbone precursor. The polymer backbone precursor is dissolved in DMF, and excess ethylenediamine (molar ratio of ethylenediamine to ethyl 2-bromomethacrylate is 1.2:1) is added. The mixture is reacted at 50-70℃ for 6-12 hours, precipitated in cold diethyl ether, and then filtered, washed, and dried to obtain the final product.

[0033] Comparative Example 1: Layer A: 100 parts PP, 1 part aryl diamide β nucleating agent, 10 parts MAH-g-PP, 20 parts nanoparticle filler; Layer B: 100 parts PET, 8 parts modified silica, 15 parts brushed polymer; Layer A: Layer B: Layer A = 3:2:3; The preparation process of the nanoparticle filler is as follows: 30 g of barium titanate nanoparticles were dispersed in 400 ml of ethanol solution and sonicated to form a suspension. 1.5 g of KH-560 was slowly added dropwise to the suspension and the temperature was raised to 60-80 °C. The mixture was refluxed for 4-6 h. 50 g of carboxylated graphene was added to the suspension and stirred and refluxed overnight. After the reaction was completed, the mixture was centrifuged, washed and dried to obtain the nanoparticle filler. 10 g of MAH-g-PP and 20 g of nanoparticle filler are added to an extruder and extruded to granulate.

[0034] The preparation process of the brush-like polymer is as follows: Under inert gas protection, 2-isopropenyl-2-oxazoline and ethyl 2-bromomethacrylate in a molar ratio of 2:1 were added to a reactor, along with an appropriate amount of azobisisobutyronitrile (AIBN) initiator and toluene as an organic solvent. The reaction was carried out at 60-80°C for 12-24 hours. After the reaction was completed, the mixture was poured into cold diethyl ether, precipitated, filtered, and dried to obtain the polymer backbone precursor. The polymer backbone precursor is dissolved in DMF, and excess ethylenediamine (molar ratio of ethylenediamine to ethyl 2-bromomethacrylate is 1.2:1) is added. The mixture is reacted at 50-70℃ for 6-12 hours, precipitated in cold diethyl ether, and then filtered, washed, and dried to obtain the final product.

[0035] Comparative Example 2: The difference from Comparative Example 1 is that the nanoparticle filler is obtained by mixing nano-barium titanate and graphene in a mass ratio of 3:5.

[0036] Comparative Example 3: Layer A: 100 parts PP, 1 part aryl diamide β nucleating agent, 25 parts MAH-g-PP and nanoparticle composite filler; Layer B: 100 parts PET, 10 parts modified silica, 20 parts brush polymer; Layer A: Layer B: Layer A = 3:2:3.

[0037] The preparation process of the composite filler of MAH-g-PP and nanoparticles is as follows: 30 g of barium titanate nanoparticles were dispersed in 400 ml of ethanol solution and sonicated to form a suspension. 1.5 g of KH-560 was slowly added dropwise to the suspension and the temperature was raised to 60-80 °C. The mixture was refluxed for 4-6 h. 50 g of carboxylated graphene was added to the suspension and stirred and refluxed overnight. After the reaction was completed, the mixture was centrifuged, washed and dried to obtain the nanoparticle filler. 10 g of MAH-g-PP and 20 g of nanoparticle filler were fed into an extruder, along with 0.1 g of stearic acid lubricant and 0.5 g of polyvinyl alcohol. The mixture was then extruded and granulated to obtain the final product.

[0038] The preparation process of the brush-like polymer is as follows: Under inert gas protection, 2-isopropenyl-2-oxazoline and ethyl 2-bromomethacrylate in a molar ratio of 2:1 are added to a reaction vessel, along with an appropriate amount of azobisisobutyronitrile (AIBN) initiator and toluene as an organic solvent. The reaction is carried out at 60-80°C for 12-24 hours. After the reaction is completed, the mixture is poured into cold diethyl ether, precipitated, filtered, and dried to obtain the final product.

[0039] Comparative Example 4: The difference from Comparative Example 3 is that the B layer structure does not contain brush polymers.

[0040] The capacitor films prepared in the above embodiments and comparative examples were subjected to relevant performance tests, and the results are recorded in Table 1.

[0041] Table 1 Table 1 shows that the synergistic effect of the nanoparticle filler in layer A and the brush-like polymer in layer B effectively improves the peel strength between layers. The lowest peel strength observed in Comparative Example 4 indicates that the brush-like polymer in layer B has a significant impact on interfacial bonding. The introduction of nanoparticles can effectively improve the dielectric constant of the composite film. The nanoparticles are pretreated with MAH-g-PP, stearic acid lubricant, and polyvinyl alcohol to ensure good dispersion in PP. Comparative Example 1 shows that when nanoparticles are directly extruded with MAH-g-PP to form composite particles, the properties of the resulting film decrease, especially the breakdown strength. Furthermore, the nanoparticles are obtained by first coupling-modifying barium titanate and then intercalating it into the interlayer structure of carboxylated graphene. This process also greatly improves the dispersibility of nanoparticles in PP materials. Comparative Example 2 shows that without this crucial step, all properties show a significant decline. In Comparative Example 3, the brush-like polymer was obtained by free radical copolymerization of 2-isopropenyl-2-oxazoline and ethyl 2-bromomethacrylate. The oxazoline structure can covalently connect with functional groups such as carboxyl groups in the A-layer structure, but the effect is limited. It is not as effective as the connection between the brush-like polymer chain formed by introducing ethylenediamine below the polymer backbone with bromine, which results in a weaker bond between the brush-like polymer chain and the A-layer structure. Therefore, the peel strength of Comparative Example 3 is lower, but its breakdown strength is significantly higher than that of Comparative Examples 1 and 2. In summary, this application prepared an ABA three-layer composite film. The layers are firmly bonded through covalent bonds and hydrogen bonds, significantly improving the interlayer peel strength. The A-layer structure incorporates nanoparticle fillers with high dielectric constants, enhancing the film's dielectric strength while maintaining high breakdown strength. The B-layer effectively improves the mechanical stability of the film material, exhibiting higher mechanical properties and material rigidity compared to traditional PP films.

[0042] Although the embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and embodiments. They can be applied to various fields suitable for the present invention. For those skilled in the art, other modifications can be easily made. Therefore, without departing from the general concept defined by the claims and their equivalents, the present invention is not limited to the specific details.

Claims

1. A biaxially oriented polypropylene composite film for capacitors, characterized in that, The composite membrane has an ABA three-layer structure; wherein, the A layer raw materials include PP, nucleating agent, MAH-g-PP and nanoparticle composite filler; the B layer raw materials include PET masterbatch, modified silica and brush polymer; The composite filler of MAH-g-PP and nanoparticles is obtained by feeding MAH-g-PP and nanoparticles into an extruder, while adding stearic acid lubricant and polyvinyl alcohol, and then extruding and granulating. The brush polymer is obtained by free radical copolymerization of 2-isopropenyl-2-oxazoline and ethyl 2-bromomethacrylate to obtain a polymer backbone, and then by reacting bromine and ethylenediamine to introduce brush polymer chains under the polymer backbone.

2. The biaxially oriented polypropylene composite film for capacitors as described in claim 1, characterized in that, The components of the A-layer raw material, by weight, are: 100 parts PP, 0.5-2 parts nucleating agent, and 12-25 parts composite filler of MAH-g-PP and nanoparticles; the components of the B-layer raw material, by weight, are: 100 parts PET, 5-10 parts modified silica, and 10-20 parts brush polymer.

3. The biaxially oriented polypropylene composite film for capacitors as described in claim 1, characterized in that, The preparation process of the composite filler of MAH-g-PP and nanoparticles is as follows: the nano barium titanate powder is dispersed in an ethanol solution, ultrasonically treated to form a suspension, KH-560 is slowly added dropwise to the suspension, the temperature is raised to 60-80℃, and the reaction is refluxed for 4-6 hours. Carboxylated graphene is added to the suspension, and the mixture is stirred and refluxed overnight. After the reaction is completed, the nanoparticles are obtained by centrifugation, washing and drying. MAH-g-PP and nanoparticles are fed into an extruder, along with stearic acid lubricant and polyvinyl alcohol, and then extruded and granulated to obtain the final product.

4. The biaxially oriented polypropylene composite film for capacitors as described in claim 3, characterized in that, The mass ratio of barium titanate powder, KH-560, and carboxylated graphene in the nanoparticles is 20~60:2~6:

100.

5. The biaxially oriented polypropylene composite film for capacitors as described in claim 1, characterized in that, In the composite filler of MAH-g-PP and nanoparticles, the mass ratio of MAH-g-PP, nanoparticles, stearic acid lubricant and polyvinyl alcohol is 10:15~20:0.1~0.2:0.2~1.

6. The biaxially oriented polypropylene composite film for capacitors as described in claim 1, characterized in that, The nucleating agent is an aryl diamide β-nucleating agent.

7. The biaxially oriented polypropylene composite film for capacitors as described in claim 1, characterized in that, The preparation process of the brush-like polymer is as follows: Under inert gas protection, 2-isopropenyl-2-oxazoline and ethyl 2-bromomethacrylate were added to a reaction vessel, along with azobisisobutyronitrile and an organic solvent. The reaction was carried out at 60-80°C for 12-24 hours. After the reaction was completed, the mixture was poured into cold diethyl ether, precipitated, filtered, and dried to obtain the polymer backbone precursor. The polymer backbone precursor is dissolved in a polar solvent, excess ethylenediamine is added, and the mixture is reacted at 50-70°C for 6-12 hours. The precipitate is then collected in cold diethyl ether, filtered, washed, and dried to obtain the final product.

8. The biaxially oriented polypropylene composite film for capacitors as described in claim 1, characterized in that, The molar ratio of 2-isopropenyl-2-oxazoline to ethyl 2-bromomethacrylate is 1.5 to 3:

1.

9. The biaxially oriented polypropylene composite film for capacitors as described in claim 1, characterized in that, The modified silica is hydroxyl-functionalized silica.

10. A method for preparing a biaxially oriented polypropylene composite film for capacitors as described in any one of claims 1-9, characterized in that, Includes the following steps: S1. Mix and granulate the raw materials of layer A and layer B separately; S2 and A layers are used as the surface layer and B layer is used as the core layer. The ABA three-layer structure is obtained by melt extrusion at 230~270℃ with an extrusion mass ratio of 2~5:1~3:2~5. After extrusion, the sheet is cooled and cast. S3. The resulting casting is preheated at 50~70℃, and then longitudinally stretched at 110~130℃ with a stretching ratio of 3-3.6; then preheated at 80~120℃, and transversely stretched at 110~130℃ with a stretching ratio of 2.6~3.

2. S4. The biaxially stretched film is heat-set at 200~230℃, then cooled at 50℃, drawn, and wound to obtain the final product.

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