A biaxially stretched polypropylene composite film for capacitors and a method for preparing the same

Abstract

The application relates to a bidirectional-stretching polypropylene composite film for capacitors and a preparation method thereof, which is an ABA three-layer structure; wherein the A-layer raw material comprises a composite filler of PP, a nucleating agent, MAH-g-PP and nanoparticles; and the B-layer raw material comprises a PET master batch, modified silicon dioxide and a brush-shaped polymer. The application prepares a three-layer structure composite film, the layers are firmly combined through covalent bonds, hydrogen bond forces and the like, and the interlayer peeling strength is significantly improved; the high-dielectric-constant nanoparticle filler is introduced into the A-layer structure, the dielectric strength of the film is improved, and the film has relatively high anti-breakdown strength; the B-layer effectively improves the mechanical stability of the film material, and compared with a traditional PP film, the film has higher mechanical properties and material rigidity.

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:

[0007] 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;

[0008] 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.

[0009] The brush-like polymer is obtained by free radical copolymerization of 2-isopropenyl-2-oxazoline and 2-bromomethyl acrylate to obtain a polymer skeleton, and then introducing brush-like polymer chains under the polymer skeleton by reaction of bromine and ethylenediamine.

[0010] Further, the components of the A layer raw material are 100 parts of PP, 0.5-2 parts of a nucleating agent, 12-25 parts of a MAH-g-PP and nanoparticle composite filler; the components of the B layer raw material are 100 parts of PET, 5-10 parts of modified silicon dioxide, and 10-20 parts of a brush-like polymer.

[0011] Further, the preparation process of the MAH-g-PP and nanoparticle composite filler is as follows: dispersing the barium titanate nanopowder in an ethanol solution, ultrasonic treatment to form a suspension, slowly adding KH-560 to the suspension, raising the temperature to 60-80℃, refluxing for 4-6h, adding carboxylated graphene to the suspension, stirring and refluxing overnight, centrifuging, washing and drying after the reaction is completed to obtain nanoparticles;

[0012] The MAH-g-PP and nanoparticles are put into an extruder, and a stearic acid lubricant and polyvinyl alcohol are added at the same time, and then extrusion granulation is performed to obtain the product.

[0013] Further, the mass ratio of the barium titanate nanopowder, KH-560 and carboxylated graphene in the nanoparticles is 20-60:2-6:100.

[0014] Further, in the MAH-g-PP and nanoparticle composite filler, 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.

[0015] Further, the nucleating agent is an aryl diamide β nucleating agent.

[0016] Further, the preparation process of the brush-like polymer is as follows:

[0017] Under the protection of inert gas, 2-isopropenyl-2-oxazoline and 2-bromomethyl acrylate are added to a reaction kettle, azobisisobutyronitrile and an organic solvent are added, and the reaction is carried out at 60-80℃ for 12-24h; after the reaction is completed, pour into cold ether, precipitate, filter and dry to obtain the polymer skeleton precursor;

[0018] The polymer skeleton precursor is dissolved in a polar solvent, an excess of ethylenediamine is added, and the reaction is carried out at 50-70℃ for 6-12h, precipitated in cold ether, filtered, washed and dried to obtain the product.

[0019] Further, the molar ratio of 2-isopropenyl-2-oxazoline and 2-bromomethyl acrylate is 1.5-3:1.

[0020] Further, the modified silica is a hydroxyl-functionalized silica.

[0021] The application further provides a preparation method of the bidirectional-stretching polypropylene composite film for capacitors as described above, comprising the following steps:

[0022] S1, mixing and granulating the raw materials of the A layer and the B layer respectively;

[0023] S2, taking the A layer as a surface layer and the B layer as a core layer, melt-extruding at 230-270 DEG C to obtain an A-B-A three-layer structure, the extrusion mass ratio being 2-5:1-3:2-5, and cooling the extruded sheet after extrusion;

[0024] S3, preheating the obtained sheet at 50-70 DEG C, then longitudinally stretching at 110-130 DEG C, the stretching ratio being 3-3.6, then preheating at 80-120 DEG C, transversely stretching at 110-130 DEG C, the stretching ratio being 2.6-3.2;

[0025] S4, heat-setting the film after bidirectional stretching at 200-230 DEG C, then cooling at 50 DEG C, drawing, and winding to obtain the film.

[0026] In the prior art, inorganic fillers such as silica and barium titanate are usually used to improve the dielectric constant of the PP film. However, the direct introduction of inorganic fillers may cause defects such as holes and cracks in the film due to poor compatibility, which in turn may result in a decrease in the anti-breakdown strength of the film, poor processing performance, and difficulty in practical application.

[0027] The application improves the performance by preparing a polypropylene composite film with A / B / A three-layer structure. The B layer is a middle support layer formed by co-extrusion of PET and modified silica and brush-shaped polymer, which provides mechanical support and dimensional stability for the composite film, and also brings high mechanical properties and high heat resistance. The brush-shaped polymer contains rich oxazoline structure, ester bond, cation and amino group. The modified silica is hydroxyl functionalized silica, which can be well dispersed in PET by forming interaction force with the brush-shaped polymer. The main raw material of the A layer is PP, which ensures good adhesion to the metal electrode and low loss, and also introduces nano-particle fillers with high dielectric constant. The nano-particle fillers are formed by covalent coupling of barium titanate and carboxylated graphene through silane coupling agent. The barium titanate is first reacted with the silane coupling agent and then inserted into the interlayer structure of the carboxylated graphene, thereby synergistically improving the dielectric strength of the composite film. However, the nano-particle fillers are not suitable for direct mixing and granulation with PP masterbatch, but need to be pretreated with maleic anhydride grafted polypropylene (MAH-g-PP). Since MAH-g-PP contains polypropylene segments, it has good compatibility with PP masterbatch. A small amount of lubricant and polyvinyl alcohol are added for coating, thereby greatly improving the dispersibility of the nano-particle fillers in the matrix and avoiding the deterioration of the electrical properties caused by agglomeration. In the ABA three-layer co-extrusion process, the hydroxyl functionalized silica contained in the B layer and the brush-shaped polymer containing multiple active sites can also act as an interfacial reinforcing agent. Through the chemical reaction of the hydroxyl, oxazoline and amino groups in the brush-shaped polymer with the epoxy, anhydride and carboxyl groups in the A layer structure, covalent bond, hydrogen bond and intermolecular force are formed, thereby solving the problem of weak interfacial adhesion and easy delamination of PP and PET due to thermodynamic incompatibility.

[0028] Compared with the prior art, the application has the following advantages: the A-B-A three-layer structure composite film is prepared, and the layers are firmly combined through covalent bond, hydrogen bond and other forces, thereby significantly improving the interlayer peeling strength; the nano-particle fillers with high dielectric constant are introduced into the A layer structure, thereby improving the dielectric strength of the film while having high anti-breakdown strength; and the B layer effectively improves the mechanical stability of the film material, and has higher mechanical properties and material rigidity compared with the traditional PP film. DETAILED DESCRIPTION

[0029] The technical solutions of the application will be described clearly and completely below with reference to the embodiments. Obviously, the described embodiments are only part of the embodiments of the application, rather than all the embodiments. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative work fall within the protection scope of the application.

[0030] In addition, the technical features involved in the different embodiments of the application described below can be combined with each other as long as they do not conflict with each other.

[0031] Embodiment: A bidirectional stretched polypropylene composite film for capacitor,

[0032] S1, mixing and granulating A layer and B layer raw materials respectively;

[0033] S2, A layer as a surface layer, B layer as a core layer, melt extrusion at 230-270 DEG C to obtain A-B-A three-layer structure, the extrusion mass ratio is 2-5:1-3:2-5, after extrusion, cooling the cast piece;

[0034] S3, the obtained cast piece is preheated at 50-70 DEG C, then stretched longitudinally at 110-130 DEG C, the stretching ratio is 3-3.6, then preheated at 80-120 DEG C, stretched transversely at 110-130 DEG C, the stretching ratio is 2.6-3.2;

[0035] S4, the film after bidirectional stretching is heat set at 200-230 DEG C, then cooled at 50 DEG C, drawn, and wound to obtain.

[0036] The A layer raw material includes 100 parts of PP, 0.5-2 parts of aryl diamide beta nucleating agent, 12-25 parts of MAH-g-PP and nano particle composite filler;

[0037] The B layer raw material includes 100 parts of PET, 5-10 parts of modified silicon dioxide, and 10-20 parts of brush-shaped polymer.

[0038] The raw materials used in the application can be purchased from the market without special instructions, wherein the preparation process of the MAH-g-PP and nano particle composite filler is as follows:

[0039] 30 g of nano barium titanate powder is dispersed in 400 ml of ethanol solution, ultrasonic treatment is carried out to form a suspension, 1.5 g of KH-560 is slowly added to the suspension, the temperature is raised to 60-80 DEG C, and the reflux reaction is carried out for 4-6 h, 50 g of carboxylated graphene is added to the suspension, stirred and refluxed overnight, after the reaction is completed, centrifugation, washing and drying are carried out to obtain the nano particle filler;

[0040] 10 g of MAH-g-PP and 15-20 g of nano particle filler are put into an extruder, 0.1-0.2 g of stearic acid lubricant and 0.2-1 g of polyvinyl alcohol are added at the same time, and extrusion granulation is carried out to obtain.

[0041] The preparation process of the brush-shaped polymer is as follows:

[0042] In inert gas protection, 2-isopropenyl-2-oxazoline and 2-bromomethyl acrylate with a molar ratio of 1.5-3:1 were added to a reaction kettle, and an appropriate amount of initiator azobisisobutyronitrile and organic solvent toluene were added, and the reaction was carried out at 60-80℃ for 12-24h; after the reaction was completed, it was poured into cold ether, precipitated, filtered and dried to obtain the polymer skeleton precursor;

[0043] The polymer skeleton precursor was dissolved in DMF, an excess of ethylenediamine (molar ratio of 1.2:1 with 2-bromomethyl acrylate) was added, and the reaction was carried out at 50-70℃ for 6-12h, and then precipitated in cold ether, filtered, washed and dried.

[0044] Example 1:

[0045] Layer A: 100 parts of PP, 1 part of aryl diamide β nucleating agent, 12 parts of MAH-g-PP and nano particle composite filler;

[0046] Layer B: 100 parts of PET, 5 parts of modified silica, 10 parts of brush-shaped polymer;

[0047] Layer A: Layer B: Layer A = 3:2:3.

[0048] The preparation process of the MAH-g-PP and nano particle composite filler is as follows:

[0049] 30 g of nano barium titanate powder was dispersed in 400 ml of ethanol solution, ultrasonic treatment was carried out to form a suspension, 1.5 g of KH-560 was slowly added to the suspension, the temperature was raised to 60-80℃, and the reflux reaction was carried out for 4-6h, 50 g of carboxylated graphene was added to the suspension, and the stirring reflux was carried out overnight, and then centrifuged, washed and dried to obtain the nano particle filler;

[0050] 10 g of MAH-g-PP and 15 g of nano particle filler were put into an extruder, 0.1 g of stearic acid lubricant and 0.5 g of polyvinyl alcohol were added, and then extrusion granulation was carried out.

[0051] The preparation process of the brush-shaped polymer is as follows:

[0052] In inert gas protection, 2-isopropenyl-2-oxazoline and 2-bromomethyl acrylate with a molar ratio of 1.5:1 were added to a reaction kettle, and an appropriate amount of initiator azobisisobutyronitrile and organic solvent toluene were added, and the reaction was carried out at 60-80℃ for 12-24h; after the reaction was completed, it was poured into cold ether, precipitated, filtered and dried to obtain the polymer skeleton precursor;

[0053] The polymer skeleton precursor is dissolved in DMF, an excess of ethylenediamine (molar ratio of 1.2:1 with 2-bromomethyl acrylate) is added, and the mixture is reacted at 50-70℃ for 6-12h, precipitated in cold ether, filtered, washed, and dried to obtain the product.

[0054] Example 2:

[0055] Layer A: 100 parts of PP, 1 part of aryl diamide β nucleating agent, 18 parts of MAH-g-PP and nanoparticle composite filler;

[0056] Layer B: 100 parts of PET, 8 parts of modified silica, 15 parts of brush polymer;

[0057] Layer A: Layer B: Layer A = 3:2:3;

[0058] The preparation process of the MAH-g-PP and nanoparticle composite filler is as follows:

[0059] 30 g of nanometer barium titanate powder is dispersed in 400 ml of ethanol solution, and a suspension is formed by ultrasonic treatment. 1.5 g of KH-560 is slowly added to the suspension, the temperature is raised to 60-80℃, and the mixture is refluxed for 4-6 h. 50 g of 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 the nanoparticle filler.

[0060] 10 g of MAH-g-PP and 18 g of nanoparticle filler are added to an extruder, 0.1 g of stearic acid lubricant and 0.7 g of polyvinyl alcohol are added at the same time, and the mixture is extruded and granulated to obtain the product.

[0061] The preparation process of the brush polymer is as follows:

[0062] Under the protection of inert gas, 2-isopropenyl-2-oxazoline and 2-bromomethyl acrylate with a molar ratio of 2:1 are added to a reaction kettle, an appropriate amount of initiator azobisisobutyronitrile and organic solvent toluene are added, and the mixture is reacted at 60-80℃ for 12-24 h. After the reaction is completed, the mixture is poured into cold ether, precipitated, filtered, and dried to obtain the polymer skeleton precursor.

[0063] The polymer skeleton precursor is dissolved in DMF, an excess of ethylenediamine (molar ratio of 1.2:1 with 2-bromomethyl acrylate) is added, and the mixture is reacted at 50-70℃ for 6-12h, precipitated in cold ether, filtered, washed, and dried to obtain the product.

[0064] Example 3:

[0065] Layer A: 100 parts of PP, 1 part of aryl diamide β nucleating agent, 25 parts of MAH-g-PP and nanoparticle composite filler;

[0066] Layer B: 100 parts of PET, 10 parts of modified silica, 20 parts of brush polymer;

[0067] Layer A: Layer B: Layer A = 3:2:3.

[0068] The preparation process of the composite filler of MAH-g-PP and nanoparticles is as follows:

[0069] 30 g of nano-barium titanate powder was dispersed in 400 ml of ethanol solution, and a suspension was formed by ultrasonic treatment. 1.5 g of KH-560 was slowly added to the suspension, and the temperature was raised to 60-80 ℃. The reaction was refluxed for 4-6 h. 50 g of carboxylated graphene was added to the suspension, and the mixture was stirred and refluxed overnight. After the reaction was completed, the mixture was centrifuged, washed and dried to obtain the nanoparticle filler.

[0070] 10 g of MAH-g-PP and 20 g of nanoparticle filler were added to an extruder, and 0.1 g of stearic acid lubricant and 0.5 g of polyvinyl alcohol were added at the same time. The mixture was extruded and granulated to obtain the product.

[0071] The preparation process of the brush polymer is as follows:

[0072] Under the protection of inert gas, 2-isopropenyl-2-oxazoline and 2-bromomethyl acrylate with a molar ratio of 2:1 were added to a reaction kettle, and an appropriate amount of initiator azobisisobutyronitrile and organic solvent toluene were added. The mixture was reacted at 60-80 ℃ for 12-24 h. After the reaction was completed, the mixture was poured into cold ether, and the product was obtained by precipitation, filtration and drying.

[0073] The polymer skeleton precursor was dissolved in DMF, and an excess of ethylenediamine (with a molar ratio of 1.2:1 to 2-bromomethyl acrylate) was added. The mixture was reacted at 50-70 ℃ for 6-12 h, and then precipitated in cold ether. The product was obtained by filtration, washing and drying.

[0074] Comparative Example 1:

[0075] Layer A: 100 parts of PP, 1 part of aryl diamide β nucleating agent, 10 parts of MAH-g-PP, and 20 parts of nanoparticle filler;

[0076] Layer B: 100 parts of PET, 8 parts of modified silica, and 15 parts of brush polymer;

[0077] Layer A: Layer B: Layer A = 3:2:3.

[0078] The preparation process of the nanoparticle filler is as follows:

[0079] 30 g of nano-barium titanate powder was dispersed in 400 ml of ethanol solution, ultrasonic treatment to form a suspension, 1.5 g of KH-560 was slowly added to the suspension, the temperature was raised to 60-80 ℃, and refluxed for 4-6 h, 50 g of carboxylated graphene was added to the suspension, stirred and refluxed overnight, and then centrifuged, washed and dried to obtain the nanoparticle filler;

[0080] 10 g of MAH-g-PP and 20 g of the nanoparticle filler were put into an extruder, and then extruded and granulated to obtain the product.

[0081] The preparation process of the brush-shaped polymer is as follows:

[0082] Under the protection of inert gas, 2-isopropenyl-2-oxazoline and 2-bromomethyl acrylate with a molar ratio of 2:1 were added to a reaction kettle, and an appropriate amount of initiator azobisisobutyronitrile and organic solvent toluene were added, and the reaction was carried out at 60-80 ℃ for 12-24 h; after the reaction was completed, it was poured into cold ether, precipitated, filtered and dried to obtain the polymer skeleton precursor;

[0083] The polymer skeleton precursor was dissolved in DMF, an excess of ethylenediamine (molar ratio of 1.2:1 with 2-bromomethyl acrylate) was added, and the reaction was carried out at 50-70 ℃ for 6-12 h, and then precipitated in cold ether, filtered, washed and dried.

[0084] Comparative Example 2:

[0085] 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.

[0086] Comparative Example 3:

[0087] Layer A: 100 parts of PP, 1 part of aryl diamide β nucleating agent, 25 parts of MAH-g-PP and nanoparticle composite filler;

[0088] Layer B: 100 parts of PET, 10 parts of modified silicon dioxide, and 20 parts of brush-shaped polymer;

[0089] A layer: B layer: A layer = 3:2:3.

[0090] The preparation process of the MAH-g-PP and nanoparticle composite filler is as follows:

[0091] 30 g of nano-barium titanate powder was dispersed in 400 ml of ethanol solution, ultrasonic treatment to form a suspension, 1.5 g of KH-560 was slowly added to the suspension, the temperature was raised to 60-80 ℃, and refluxed for 4-6 h, 50 g of carboxylated graphene was added to the suspension, stirred and refluxed overnight, and then centrifuged, washed and dried to obtain the nanoparticle filler;

[0092] 10 g of MAH-g-PP and 20 g of nanoparticle filler were put into an extruder, 0.1 g of stearic acid lubricant and 0.5 g of polyvinyl alcohol were added, and extrusion granulation was performed to obtain the product.

[0093] The brush-shaped polymer was prepared as follows:

[0094] Under the protection of inert gas, 2-isopropenyl-2-oxazoline and 2-bromomethyl acrylate with a molar ratio of 2:1 were added into a reaction kettle, an appropriate amount of initiator azobisisobutyronitrile and organic solvent toluene were added, and the reaction was carried out at 60-80℃ for 12-24h; after the reaction was completed, it was poured into cold ether, precipitated, filtered and dried to obtain the product.

[0095] Comparative Example 4:

[0096] The difference from Comparative Example 3 is that the brush-shaped polymer is not contained in the structure of layer B.

[0097] The capacitive film prepared in the above examples and comparative examples was tested for related performance, and the results are recorded in Table 1.

[0098] Table 1

[0099]

[0100] As can be seen from Table 1, the nanoparticles in the A layer structure and the brush-like polymer in the B layer structure synergistically improve the interlayer peeling strength. As can be seen from the lowest peeling strength of Comparative Example 4, the brush-like polymer in the B layer structure has an important influence on the interfacial bonding. The introduction of the nanoparticles can effectively improve the dielectric constant of the composite film. The nanoparticles are pretreated with MAH-g-PP, a stearate lubricant and polyvinyl alcohol, so that they can be well dispersed in the PP. As can be seen from Comparative Example 1, when the nanoparticles are directly extruded with MAH-g-PP to form composite particles, the performance of the prepared film is reduced, especially the breakdown strength. In addition, the nanoparticles are obtained by first coupling modification of barium titanate and then intercalation into the interlayer structure of carboxylated graphene. This process greatly improves the dispersibility of the nanoparticles in the PP material. As can be seen from Comparative Example 2, without this key step, the performance is significantly reduced. In Comparative Example 3, the brush-like polymer is obtained by free radical copolymerization of 2-isopropyl-2-oxazoline and 2-bromomethyl acrylate. The oxazoline structure can be covalently connected to the carboxyl and other functional groups contained in the A layer structure, but the effect is limited. It is not as good as introducing ethylenediamine into the polymer backbone through the bromine element to form a brush-like polymer chain and a firm connection with the A layer structure. Therefore, the peeling strength of Comparative Example 3 is reduced, but the breakdown strength is higher than that of Comparative Examples 1 and 2. In summary, the A-B-A three-layer structure composite film is prepared. The layers are firmly connected through covalent bonding, hydrogen bonding and other forces, which significantly improves the interlayer peeling strength. The high dielectric constant nanoparticles are introduced into the A layer structure, which improves the dielectric strength of the film while having high breakdown strength. The B layer effectively improves the mechanical stability of the film material, and the film has higher mechanical properties and material rigidity compared with the traditional PP film.

[0101] Although the embodiments of the present application have been disclosed as above, they are not limited to the application listed in the specification and the embodiments, and can be fully applied to various fields suitable for the present application. Those skilled in the art can easily make other modifications, and therefore the present application is not limited to specific details, without departing from the general concept defined by the claims and the equivalent scope.

Claims

1. A bidirectionally stretched polypropylene composite film for capacitors, characterized by, The composite film is an ABA three-layer co-extrusion structure; wherein the A layer raw material comprises PP, nucleating agent, MAH-g-PP and nano-particle composite filler; the B layer raw material comprises PET master batch, modified silica, brush-shaped polymer; The MAH-g-PP and nano-particle composite filler is obtained by putting MAH-g-PP and nano-particles into an extruder, adding stearic acid lubricant and polyvinyl alcohol at the same time, and then extruding and granulating; The brush-shaped polymer is obtained by free radical copolymerization of 2-isopropenyl-2-oxazoline and 2-bromomethyl acrylate to obtain a polymer skeleton, and then introducing brush-shaped polymer chains under the polymer skeleton by reaction of bromine and ethylenediamine; The preparation process of the nano-particles is as follows: nano-barium titanate powder is dispersed in an ethanol solution, ultrasonic treatment is performed to form a suspension, KH-560 is slowly added to the suspension, the temperature is raised to 60-80℃, reflux reaction is performed for 4-6h, carboxylated graphene is added to the suspension, stirring and reflux are performed overnight, centrifugation, washing and drying are performed after the reaction is completed to obtain the nano-particles.

2. The bidirectionally stretched polypropylene composite film for a capacitor according to claim 1, wherein The components of the A layer raw material are as follows in terms of weight parts: 100 parts of PP, 0.5-2 parts of nucleating agent, 12-25 parts of MAH-g-PP and nano-particle composite filler; the components of the B layer raw material are as follows in terms of weight parts: 100 parts of PET, 5-10 parts of modified silica, and 10-20 parts of brush-shaped polymer.

3. The bidirectionally stretched polypropylene composite film for a capacitor according to claim 1, wherein The mass ratio of nano-barium titanate powder, KH-560 and carboxylated graphene in the nano-particles is 20-60:2-6:

100.

4. The bidirectionally stretched polypropylene composite film for a capacitor according to claim 1, wherein The mass ratio of MAH-g-PP, nano-particles, stearic acid lubricant and polyvinyl alcohol in the MAH-g-PP and nano-particle composite filler is 10:15-20:0.1-0.2:0.2-1.

5. The bidirectionally stretched polypropylene composite film for a capacitor according to claim 1, wherein The nucleating agent is aryl diamide β nucleating agent.

6. The bidirectionally stretched polypropylene composite film for a capacitor according to claim 1, wherein The preparation process of the brush-shaped polymer is as follows: Under the protection of inert gas, 2-isopropenyl-2-oxazoline and 2-bromomethyl acrylate are added to a reaction kettle, azobisisobutyronitrile and organic solvent are added, and reaction is performed at 60-80℃ for 12-24h; after the reaction is completed, the product is poured into cold ether, precipitated, filtered and dried to obtain the polymer skeleton precursor; The polymer skeleton precursor is dissolved in a polar solvent, excess ethylenediamine is added, and reaction is performed at 50-70℃ for 6-12h; the product is precipitated in cold ether, filtered, washed and dried.

7. The bidirectionally stretched polypropylene composite film for a capacitor according to claim 1, wherein The molar ratio of 2-isopropenyl-2-oxazoline and 2-bromomethyl acrylate is 1.5-3:

1.

8. The bidirectionally stretched polypropylene composite film for a capacitor according to claim 1, wherein The modified silica is hydroxyl functionalized silica.

9. A method for producing the bidirectionally stretched polypropylene composite film for a capacitor as claimed in any one of claims 1 to 8, characterized by, The method comprises the following steps: S1, the A layer and the B layer raw material are mixed and granulated respectively; S2, the A layer is used as the surface layer, and the B layer is used as the core layer, and the A-B-A three-layer structure is obtained by melt extrusion at 230-270℃, the extrusion mass ratio is 2-5:1-3:2-5, and the extruded sheet is cooled; S3, the obtained sheet is preheated at 50-70℃, then longitudinally stretched at 110-130℃, the stretching ratio is 3-3.6; then preheated at 80-120℃, and transversely stretched at 110-130℃, the stretching ratio is 2.6-3.2; S4, the biaxially stretched film is heat set at 200-230°C, and then cooled at 50°C, drawn, and wound to obtain the film.

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