Polypropylene film for capacitor and preparation method thereof

Abstract

The invention belongs to the technical field of capacitor films, and particularly relates to a polypropylene film for capacitors and a preparation method thereof. The raw materials include, by weight, 65-90 parts of polypropylene resin; 5-12 parts of maleic anhydride grafted polypropylene; 0.5-4 parts of modified flaky inorganic filler; 0.3-5 parts of silane coupling agent; 0.2-0.5 parts of antioxidant; and 0.01-0.05 parts of nucleating agent. The polypropylene film for capacitors prepared by the invention has large roughness, low maximum roughness, high consistency and orderliness of surface microstructure, good dielectric properties, breakdown strength and tensile strength, and excellent comprehensive performance.

Description

Technical Field

[0001] The invention belongs to the technical field of capacitor films, and in particular relates to a polypropylene film for capacitors and a preparation method thereof. Background Art

[0002] Film capacitors use metal foil as electrodes, overlap it with plastic films such as polyethylene, polypropylene, polystyrene or polycarbonate from both ends, and then wind it into a cylindrical structure. Film capacitors have the advantages of non-polarity, high insulation impedance, wide frequency response, and low dielectric loss. They are currently widely used in electronics, home appliances, communications, electricity, electrified railways, hybrid vehicles, wind power generation, solar power generation and other fields.

[0003] Typically, film capacitors use polymer films as dielectric materials. The most commonly used polymer films include polypropylene film, polyester film, and polystyrene film. Among them, polypropylene film is a non-polar material with a low dielectric constant and low dielectric loss. It has a very high breakdown voltage and is currently the most widely used polymer film, especially in the field of high-power power electronic capacitors.

[0004] However, ordinary polymer films cannot be directly used in film capacitors. As polymer films for capacitors, they are usually required to have appropriate surface roughness, appropriate friction coefficient, high breakdown voltage, low relative dielectric constant and low dielectric loss. In addition, polymer films are also required to have high tensile strength, moderate hardness, good winding properties, heat resistance and cold resistance. It can be seen that there are very strict requirements for the performance of polymer films used in capacitors in all aspects.

[0005] In the field of film capacitor manufacturing, polymer film is a key material, and the improvement and optimization of its properties has always been a difficult task. This is not only because the selection of raw material components has a direct and far-reaching impact on the performance of the film, but also because every subtle link in the preparation process may have a decisive effect on the final performance of the film. At the same time, the various properties of polymer films are not isolated. They are interrelated and influence each other, forming a complex and delicate balance. This means that when trying to improve the performance of a certain aspect, it is necessary to consider the impact that this change may have on other properties. This "one move affects the whole body" characteristic undoubtedly increases the difficulty of improving the performance optimization of polymer films. For example, a thin film capacitor is made by stacking several roughened films as dielectrics, placing them between electrode foils, rolling them into a capacitor element, and then impregnating them with insulating oil. In order to ensure the fluidity of the insulating oil between the layers of roughened films, the roughness of the two roughened surfaces is generally required to be as large as possible. However, if the roughness is too large, the thickness of the middle part of the film will be reduced, reducing the electrical withstand voltage carrying capacity of the capacitor. At the same time, too high a roughness will significantly reduce the electrical properties, mechanical properties and processing properties of the film. On the contrary, if the thickness of the middle part is guaranteed, the roughness of the two roughened surfaces will be reduced, the oil immersion of the insulating oil will be deteriorated, and the withstand voltage characteristics will be reduced.

[0006] In the production process of polymer films, surface roughness is a key physical property that directly affects the application performance and stability of the film. In order to adjust the surface roughness of the film, specific particles are usually added to the raw materials. Factors such as the type, shape, addition amount, and particle size distribution of these particles will directly affect the roughness of the surface of the final film produced. In the actual production process, there is great difficulty in controlling the roughness. This is mainly due to the uncertainty of the impact of fluctuations in factors such as raw materials and preparation processes on roughness; more importantly, differences in particle shape and particle size distribution will also make the control of roughness more complicated.

[0007] Especially in recent years, with the rapid development of the information industry, especially the integration and miniaturization of components, the capacitors used in these industries are developing in the direction of miniaturization and large capacity, resulting in a rapid increase in the demand for ultra-thin (1~6um) films as important dielectrics of capacitors. In the preparation process of polymer films, in order to obtain films with suitable surface roughness and overcome many difficulties in the film processing process, a certain amount of fine inorganic particles must be added as anti-adhesion agents during the processing process. The current main solution is to use fine, such as nano inorganic fillers as anti-adhesion agents to achieve the purpose of controlling the surface roughness of the film and reducing the thickness of the film. For ultra-thin films, the particle size of the inorganic particles added therein is smaller, usually around 50~150nm. The preparation of such uniformly sized, nano-scale inorganic particles is extremely difficult, especially the particle size control of nano-scale inorganic particles is difficult, and the preparation process is cumbersome and the raw materials are very rare. When the particle size distribution of nano-scale inorganic particles is too wide, the addition of inorganic particles in ultra-thin films will aggravate problems such as increased film breakage rate, increased electrical weak points, and decreased strength. At the same time, due to the very small particle size of inorganic particles in ultra-thin films, the surface roughness of ultra-thin films has always been low and uneven.

[0008] Therefore, providing a polypropylene film for capacitors and a preparation method thereof that can reduce the difficulty of controlling the roughness of polymer films, especially ultra-thin polymer films, while improving their comprehensive performance is one of the technical problems that technicians in this field need to solve urgently. Summary of the invention

[0009] The purpose of the present invention is to provide a polypropylene film for capacitors and a preparation method thereof in order to reduce the difficulty of controlling the roughness of the polymer film, improve the comprehensive performance of the polymer film for capacitors, and realize the preparation of ultra-thin films for high-performance film capacitors.

[0010] In view of this, the present invention provides a method for preparing a polypropylene film for capacitors, wherein the raw materials include, by weight:

[0011] 65-90 parts of polypropylene resin;

[0012] Maleic anhydride grafted polypropylene 5-12 parts;

[0013] 0.5~4 parts of modified flaky inorganic filler;

[0014] Silane coupling agent 0.3~5 parts;

[0015] Antioxidant 0.2~0.5 parts;

[0016] Nucleating agent 0.01~0.05 parts.

[0017] Furthermore, the polypropylene resin includes: linear polypropylene resin and long-chain branched polypropylene resin, wherein:

[0018] The linear polypropylene resin has an isotactic index of ≥95% and an ash content of ≤400ppm;

[0019] The long-chain branched polypropylene resin has an isotactic index of ≥97%, a melt flow rate of 2-6 g / 10 min, an ash content of ≤200 ppm, and a branching point density of 1 / 10000 carbon atom to 1 / 100 carbon atom;

[0020] The weight ratio of the linear polypropylene resin to the long-chain branched polypropylene resin is (0.3-0.6):1.

[0021] Furthermore, the linear polypropylene resin includes a low molecular weight linear polypropylene resin with a weight average molecular weight Mw of 200,000 to 300,000 and a melt flow rate of 5 to 8 g / 10 min, and a high molecular weight linear polypropylene resin with a weight average molecular weight Mw of 400,000 to 450,000 and a melt flow rate of 1 to 3 g / 10 min, wherein the weight ratio of the low molecular weight linear polypropylene resin to the high molecular weight linear polypropylene resin is (2 to 4):1.

[0022] Furthermore, the modified flaky inorganic filler is a mixture of a first modified flaky inorganic filler and a second modified flaky inorganic filler, wherein the second modified flaky inorganic filler has a porous structure.

[0023] Furthermore, the preparation process of the modified flaky inorganic filler is as follows:

[0024] a1, placing large particle raw materials of flaky inorganic fillers in the inner chamber of a ball mill, adding steel balls into the inner chamber of the ball mill, then sealing the chamber of the ball mill, introducing inert gas of a certain pressure into the chamber, and then starting the ball mill for grinding. During the ball milling process, inorganic filler particles ground to a certain size can pass through the through holes on the wall of the inner chamber of the ball mill into the outer chamber of the ball mill, and then the inorganic filler in the outer chamber of the ball mill is sucked out by the circulating exhaust unit of the ball mill;

[0025] a2, dispersing 50-100 parts by weight of the inorganic filler after ball milling in step a1 in 200-500 parts by weight of water, fully dispersing by mechanical stirring or ultrasound, placing in a shear dispersion device, shearing and dispersing at a speed of 3000-8000 r / min for 1-3 hours, then filtering and drying;

[0026] a3, placing 40-80 parts by weight of the inorganic filler sheared and dispersed in step a2 into 100-300 parts by weight of the intercalation treatment agent solution, pressurizing and heating to a pressure of 0.5-2MPa and a temperature of 40-70°C in a closed container, then starting an ultrasonic device, and performing ultrasonic peeling for 0.5-1h at a power of 500-1000W, a frequency of 30-50KHz, and a temperature of 40-70°C. After the ultrasonic peeling is completed, the pressure is quickly released to normal pressure at a speed of 0.2-0.5MPa / min. After the pressure is released, the inorganic filler is centrifuged, washed with water, and dried to obtain a flaky inorganic filler with a thickness of less than 50nm and a plane particle size of less than 1um;

[0027] a4. Prepare the first flaky inorganic filler and the second flaky inorganic filler respectively according to the method shown in the above steps a1 to a3, then modify the first flaky inorganic filler and the second flaky inorganic filler respectively to obtain the first modified flaky inorganic filler and the second modified flaky inorganic filler, and then compound them according to the proportion to obtain the modified flaky inorganic filler.

[0028] Furthermore, in the step a4, the process of modifying the first flaky inorganic filler to obtain the first modified flaky inorganic filler is:

[0029] b1, placing the first flaky inorganic filler treated in step a3 in a fluidized bed, introducing an inert gas as an operating gas into the fluidized bed, heating the organic matter, introducing the gas generated by the organic matter into the fluidized bed, and maintaining the temperature of the fluidized bed at 600-800° C., so that the gas generated by the organic matter is carbonized and the formed tiny particles are attached to the surface of the inorganic filler, forming carbon particle protrusions on the surface of the inorganic filler;

[0030] b2, add 20-40 parts by weight of unsaturated fatty acid to 80-100 parts of deionized water, stir to mix evenly, then add acid or alkali solution dropwise to adjust the pH value of the solution to 3-5 under stirring, then add 30-50 parts by weight of the inorganic filler treated in step b1, react at 70-90° C. for 2-3 hours under stirring, centrifuge, wash with water, and dry to obtain the first modified flaky inorganic filler.

[0031] Furthermore, in the step a4, the process of modifying the second flaky inorganic filler to obtain the second modified flaky inorganic filler is:

[0032] c1, add 10-20 parts by weight of the second flaky inorganic filler treated in step a4 and 2-5 parts by weight of a surfactant to 250-500 parts by weight of deionized water, and after uniform dispersion, heat the mixed system to 40-70° C. under the protection of an inert gas, and slowly drop 5-10 parts by weight of styrene and an initiator while stirring. The styrene and the initiator are added dropwise within 0.5-1 hour, and then continue to stir and react for 2-3 hours. After the reaction is completed, filter, separate, wash with water, and dry to obtain the second modified flaky inorganic filler.

[0033] Furthermore, the method for preparing the polypropylene film for capacitors comprises the steps of:

[0034] S1, placing polypropylene resin pellets, maleic anhydride grafted polypropylene, modified flaky inorganic filler, silane coupling agent, antioxidant, and nucleating agent in a double-roll mill, and melt blending at 170-190° C. for 30-60 minutes to obtain a mixture;

[0035] S2, preheating the mixture at 230-240° C. for 3-5 minutes on a hot press, and then maintaining it at a pressure of 5-8 MPa for 3-5 minutes; then placing the mixture in a hot press at 160-180° C., maintaining it at a pressure of 3-6 MPa for 7-10 minutes, and naturally cooling it to room temperature to obtain a polypropylene casting sheet;

[0036] S3, preheating the polypropylene cast sheet obtained in step S2 at a temperature range of 140-150° C. for 2-4 minutes, and then performing a synchronous biaxial stretching process on the polypropylene cast sheet to obtain a single synchronous biaxial stretching polypropylene film; then placing the single synchronous biaxial stretching polypropylene film at a temperature range of 150-160° C., and performing a second synchronous biaxial stretching process on the polypropylene cast sheet to obtain a polypropylene film for capacitors.

[0037] Furthermore, the stretching ratio of the first simultaneous biaxial stretching is 2.0*2.0~4.0*4.0; the stretching ratio of the second simultaneous biaxial stretching is 5.0*5.0~8.0*8.0.

[0038] A polypropylene film for capacitors is prepared by the above-mentioned preparation method.

[0039] The polypropylene film for capacitors prepared by the present invention has a larger roughness than conventional ultra-thin films, a lower maximum roughness, a high consistency and orderliness of the surface microstructure, good dielectric properties, breakdown strength and tensile strength, and excellent comprehensive performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1Schematic diagram of the overall structure of the ball mill of the present invention (cut along the central axis of the ball mill chamber);

[0041] Figure 2 It is a structural schematic diagram of the ball milling chamber in the ball mill of the present invention;

[0042] The symbols in the figure are:

[0043] 1. Ball mill chamber; 101. Ball mill inner chamber; 1011. Through hole; 102. Ball mill outer chamber; 103. Steel ball; 2. Circulating ventilation unit; 201. Air inlet; 202. Air outlet; 203. Separation device; 204. Fan; 3. Driving system. DETAILED DESCRIPTION

[0044] The technical solutions in this application will be described clearly below in conjunction with the specific embodiments of this application. Obviously, the described embodiments are part of the embodiments of this application, rather than all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments in this application belong to the scope of protection of this application.

[0045] In the description of the present application, it should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present application. The techniques, methods and devices known to those of ordinary skill in the relevant art may not be discussed in detail, but in appropriate cases, the techniques, methods and devices should be considered as part of the authorization specification. In all examples shown and discussed here, any specific value should be interpreted as being merely exemplary and not as a limitation. Therefore, other examples of the exemplary embodiments may have different values.

[0046] It should be noted that, in the present application, the terms "comprise", "include" or any other variant thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, an element defined by the sentence "comprises one..." does not exclude the presence of other identical elements in the process, method, article or device including the element. In addition, it should be noted that the scope of the method and device in the embodiment of the present application is not limited to performing functions in the order shown or discussed, and may also include performing functions in a substantially simultaneous manner or in reverse order according to the functions involved, for example, the described method may be performed in an order different from that described, and various steps may also be added, omitted, or combined. In addition, the features described with reference to certain examples may be combined in other examples.

[0047] A polypropylene film for capacitors, the raw materials of which include, by weight:

[0048] 65-90 parts of polypropylene resin;

[0049] Maleic anhydride grafted polypropylene 5-12 parts;

[0050] 0.5~4 parts of modified flaky inorganic filler;

[0051] Silane coupling agent 0.3~5 parts;

[0052] Antioxidant 0.2~0.5 parts;

[0053] Nucleating agent 0.01~0.05 parts.

[0054] Preferably, the polypropylene resin comprises: a linear polypropylene resin and a long-chain branched polypropylene resin, wherein the weight ratio of the linear polypropylene resin to the long-chain branched polypropylene resin is (0.3-0.6):1.

[0055] Preferably, the isotactic index of the linear polypropylene resin is ≥95%, and the ash content is ≤400ppm; the isotactic index of the long-chain branched polypropylene resin is ≥97%, the melt flow rate is 2-6g / 10min, the ash content is ≤200ppm, and the branching point density is 1 / 10000 carbon atom to 1 / 100 carbon atom.

[0056] Straight-chain polypropylene resins generally have better fluidity and processability, while long-chain branched polypropylene resins may provide higher melt strength and better tensile strength. In the present invention, the combination of straight-chain polypropylene resins and long-chain branched polypropylene resins can combine the advantages of both, improve the impact resistance and toughness of the material, improve the thickness uniformity of the prepared polypropylene film, and reduce the film breakage rate. In particular, the application of a large number of long-chain branched polypropylene resins can utilize the interlacing and entanglement between the flexible long-chain functional groups in its structure to form a three-dimensional network structure, thereby improving the compatibility of the polypropylene resin and the modified flaky inorganic filler.

[0057] As some examples of the present invention, the long-chain branched polypropylene resin is a polypropylene heterophasic copolymer prepared by polymerization of propylene, ethylene and α-olefin.

[0058] More preferably, the linear polypropylene resin comprises a low molecular weight linear polypropylene resin having a weight average molecular weight Mw of 200,000 to 300,000 and a melt flow rate of 5 to 8 g / 10 min, and a high molecular weight linear polypropylene resin having a weight average molecular weight Mw of 400,000 to 450,000 and a melt flow rate of 1 to 3 g / 10 min, wherein the weight ratio of the low molecular weight linear polypropylene resin to the high molecular weight linear polypropylene resin is (2 to 4):1.

[0059] In the present invention, by using a low molecular weight linear polypropylene resin and a high molecular weight linear polypropylene resin in combination, a more complex molecular chain network structure can be formed, which can improve the tensile strength, tear strength and impact strength of the film while maintaining the flexibility of the film, and at the same time, it is helpful to improve the heat resistance, voltage resistance and dielectric properties of the film, and can improve the processing performance and stability of the polypropylene film. In addition, low molecular weight resins are usually relatively low in price. By reasonably mixing linear polypropylene resins of different molecular weights, the cost can be effectively reduced while ensuring the required performance.

[0060] As some examples of the present invention, the silane coupling agent is selected from one or more of KH550 and KH-560.

[0061] As some examples of the present invention, the antioxidant is selected from one or more of antioxidant 164, antioxidant 168, antioxidant 1010, antioxidant 1076, and antioxidant 1098.

[0062] As some examples of the present invention, the nucleating agent is a β-nucleating agent, such as an amide compound or a rare earth nucleating agent.

[0063] Furthermore, the modified flaky inorganic filler is a mixture of a first modified flaky inorganic filler and a second modified flaky inorganic filler, wherein the second modified flaky inorganic filler has a porous structure.

[0064] In the prior art, spherical or quasi-spherical inorganic filler particles are mostly added to polypropylene films in order to improve the performance of the films. However, in the process of preparing inorganic filler particles, the particle size and morphology of spherical or quasi-spherical inorganic filler particles are difficult to control. The particle size of the prepared inorganic filler particles varies greatly and the particle size distribution is wide. For example, the particle size of most nano inorganic filler particles can vary by hundreds or even thousands of times. At the same time, the morphology of inorganic filler particles is also ever-changing. In the process of film preparation, factors such as the type, shape, addition amount and particle size distribution of inorganic filler particles have a direct impact on the roughness of the film. When the particle size distribution of inorganic filler particles is wide and the morphology differs greatly, it is easy to reduce the consistency and order of the microstructure of the film surface, so that the surface roughness, thickness and performance of the obtained film are uneven, which easily leads to the reduction of film performance. At the same time, the addition of large-sized inorganic particles with different phases in the thickness direction will also have an adverse effect on the mechanical properties of the film.

[0065] In the present invention, when the flaky inorganic filler is used to modify the polypropylene film, due to the special shape of the flaky inorganic filler and the specific preparation process, the flaky inorganic filler will be subjected to the pressure of the hot press and the stretching of the stretching machine during the film preparation process, especially the high pressure of the hot press. In this process, since the flaky inorganic filler has a larger specific surface area and a lower energy state in the direction parallel to the film plane, it is more stable in this direction, so the filler particles are more inclined to be oriented and arranged in the direction parallel to the film plane. This distribution state of the flaky inorganic filler is parallel to the deformation direction of the film, weakening the influence of the two-dimensional plane size of the flaky inorganic filler on the film. At the same time, since the thickness of the flaky inorganic filler is relatively easy to control, its thickness deviation can be relatively easily controlled within hundreds of times or even tens of times. Therefore, when it is used for polypropylene film, the consistency and order performance of the microstructure of the film surface can be greatly improved, which can promote the improvement of film performance.

[0066] Although the flaky inorganic filler can improve the consistency and orderliness of the microstructure of the surface of the polypropylene film, it is more likely to agglomerate in the matrix material than spherical or quasi-spherical inorganic filler particles. It is known that good dispersion of the inorganic filler in the matrix material is an important basis for its function. In order to promote good dispersion of the flaky inorganic filler in the matrix material, the present invention compounds a certain proportion of a second modified flaky inorganic filler having a porous structure in the modified flaky inorganic filler. The advantages of doing so are:

[0067] First, the porous structure can significantly increase the specific surface area of ​​the inorganic filler. The increase in specific surface area means that the contact area between the inorganic filler and the polypropylene matrix is ​​also increased accordingly, thereby enhancing the interaction force between the inorganic filler and the matrix, such as van der Waals force or chemical bonding, which is conducive to the uniform dispersion of the filler.

[0068] Second, the pores in the porous structure can adsorb polypropylene molecular chains to form physical or chemical cross-linking points, which help to fix the inorganic filler particles and prevent them from agglomerating in the matrix, thereby improving their dispersibility;

[0069] Third, the pores inside the porous filler particles can act as a "buffer zone" to reduce direct contact between particles through the pore effect and reduce the possibility of agglomeration. At the same time, the gas or liquid in the pores may play a lubricating role during the processing, further promoting the dispersion of the filler;

[0070] Fourth, the porous structure makes it easier to modify the surface of the inorganic filler particles, which then improves the compatibility of the filler with the matrix and reduces the interfacial tension, thereby facilitating the uniform dispersion of the filler.

[0071] Preferably, the first modified flaky inorganic filler is made of flaky talc powder, mica, kaolinite, silicon dioxide, titanium dioxide, etc. with a thickness of less than 3 mm and a particle size of less than 10 mm as raw materials, and is prepared by mechanical crushing and chemical treatment.

[0072] Preferably, the second modified flaky inorganic filler is prepared from porous flaky boehmite having a thickness of less than 5 mm and a particle size of less than 10 mm as raw material through mechanical crushing and chemical treatment.

[0073] Furthermore, in the modified flaky inorganic filler, the weight proportion of the first modified flaky inorganic filler is 60-80wt%, and the weight proportion of the second modified flaky inorganic filler is 20-40wt%.

[0074] Furthermore, the preparation process of the modified flaky inorganic filler is as follows:

[0075] a1, placing large particle raw materials of flaky inorganic fillers in the inner chamber of a ball mill, adding steel balls into the inner chamber of the ball mill, then sealing the chamber of the ball mill, introducing inert gas of a certain pressure into the chamber, and then starting the ball mill for grinding. During the ball milling process, inorganic filler particles ground to a certain size can pass through the through holes on the wall of the inner chamber of the ball mill into the outer chamber of the ball mill, and then the inorganic filler in the outer chamber of the ball mill is sucked out by the circulating exhaust unit of the ball mill;

[0076] a2, dispersing 50-100 parts by weight of the inorganic filler after ball milling in step a1 in 200-500 parts by weight of water, fully dispersing by mechanical stirring or ultrasound, placing in a shear dispersion device, shearing and dispersing at a speed of 3000-8000 r / min for 1-3 hours, then filtering and drying;

[0077] a3, placing 40-80 parts by weight of the inorganic filler sheared and dispersed in step a2 into 100-300 parts by weight of the intercalation treatment agent solution, pressurizing and heating to a pressure of 0.5-2MPa and a temperature of 40-70°C in a closed container, then starting an ultrasonic device, and performing ultrasonic peeling for 0.5-1h at a power of 500-1000W, a frequency of 30-50KHz, and a temperature of 40-70°C. After the ultrasonic peeling is completed, the pressure is quickly released to normal pressure at a speed of 0.2-0.5MPa / min. After the pressure is released, the inorganic filler is centrifuged, washed with water, and dried to obtain a flaky inorganic filler with a thickness of less than 50nm and a plane particle size of less than 1um;

[0078] a4. Prepare the first flaky inorganic filler and the second flaky inorganic filler respectively according to the method shown in the above steps a1 to a3, then modify the first flaky inorganic filler and the second flaky inorganic filler respectively to obtain the first modified flaky inorganic filler and the second modified flaky inorganic filler, and then compound them according to the proportion to obtain the modified flaky inorganic filler.

[0079] Specifically, the ball mill used in step a1 has the following Figure 1-Figure 2 The structure shown has a ball mill chamber 1, wherein the ball mill chamber 1 includes a ball mill inner chamber 101 and a ball mill outer chamber 102 sleeved on the outer periphery of the ball mill inner chamber 101, wherein a ball mill steel ball 103 is located in the ball mill inner chamber 101, and a through hole 1011 is provided on the cavity wall of the ball mill inner chamber 101, and the diameter of the through hole 1011 is 0.5-1 mm. The ball mill also includes a circulating exhaust unit 2, wherein the air inlet 201 is connected to the ball mill outer chamber 102, and the air outlet 202 is connected to the ball mill inner chamber 101; a separation device 203 is provided in the circulating exhaust unit 2, and the separation device 203 can separate the inorganic filler sucked out by gas through filtering or sedimentation.

[0080] In addition, the ball mill further comprises a driving system 3, and the driving system 3 can drive the ball mill inner cavity 101 to rotate so as to grind the inorganic filler therein.

[0081] Preferably, the separation device 203 is a filter screen or a cyclone separation device, which separates the inorganic filler sucked out of the gas through filtering action or centrifugal force.

[0082] The ball milling process in step a1 is as follows: during ball milling, the inorganic filler is stratified and its particle size gradually decreases under the grinding and impact of the steel ball 103. When its particle size is reduced to be smaller than the through hole 1011, it can be sucked into the ball mill outer cavity 102 by the suction port 201 of the circulating exhaust unit 2, and then further sucked into the circulating exhaust unit 2. After separation by the separation device 203 in the circulating exhaust unit 2, the inorganic filler is collected to complete the ball milling process.

[0083] According to the above ball milling process, it can be known that the inorganic filler of the present invention can separate the small-size filler in time by sucking gas during the ball milling process, so that the filler particles after ball milling are uniform in size, which can lay a good foundation for subsequent processing. In addition, during ball milling, the high-speed airflow generated by the circulating exhaust unit 2 can blow up the inorganic filler in the ball mill cavity 101, promote its full mixing, collision and grinding; at the same time, it provides favorable conditions for the timely separation of small-size fillers and improves the uniformity of the filler particles after grinding.

[0084] Furthermore, in the step a1, the ball milling speed is set to 300-600 r / min.

[0085] Furthermore, in step a1, an inert gas is introduced into the ball mill chamber so that the pressure inside the ball mill chamber is 0.3-0.6 MPa.

[0086] Furthermore, in step a3, the intercalation treatment agent solution is a mixed solution of hydrochloric acid and nitrate, and the nitrate is sodium nitrate or potassium nitrate.

[0087] Preferably, in the intercalation treatment agent solution, the content of hydrochloric acid is 20-30wt%, and the content of nitrate is 25-35wt%.

[0088] In the intercalation treatment process described in the present invention, the gas generated by the reaction of hydrochloric acid and nitrate can be used to quickly release the pressure, so that the Cl2 and NOCl gases generated by the reaction can quickly overflow, thereby promoting further peeling of the inorganic filler layer.

[0089] Furthermore, in the step a4, the process of modifying the first flaky inorganic filler to obtain the first modified flaky inorganic filler is:

[0090] b1, placing the first flaky inorganic filler treated in step a3 in a fluidized bed, introducing an inert gas as an operating gas into the fluidized bed, heating the organic matter, introducing the gas generated by the organic matter into the fluidized bed, and maintaining the temperature of the fluidized bed at 600-800° C., so that the gas generated by the organic matter is carbonized and the formed tiny particles are attached to the surface of the inorganic filler, forming carbon particle protrusions on the surface of the inorganic filler;

[0091] b2, add 20-40 parts by weight of unsaturated fatty acid to 80-100 parts of deionized water, stir to mix evenly, then add acid or alkali solution dropwise to adjust the pH value of the solution to 3-5 under stirring, then add 30-50 parts by weight of the inorganic filler treated in step b1, react at 70-90° C. for 2-3 hours under stirring, centrifuge, wash with water, and dry to obtain the first modified flaky inorganic filler.

[0092] Preferably, in step b1, the organic gas is selected from one or more of benzene and ethylbenzene, and the amount of the organic gas introduced into the fluidized bed is 5-15% of the weight of the inorganic filler in the fluidized bed.

[0093] In the modification process of the first flaky inorganic filler, tiny carbon particle protrusions are formed on the surface of the inorganic filler by carbonization of organic gas, the surface morphology and surface properties of the inorganic filler are improved, and physical barriers are formed between the fillers by the protruding structure, which reduces the chance of direct contact between the particles and reduces agglomeration. At the same time, the surface area of ​​the inorganic filler is increased, and the dispersion of the inorganic filler in the matrix is ​​promoted. On this basis, through the modification of unsaturated fatty acids, double bonds can be formed on the inorganic filler to obtain inorganic filler particles containing double bonds on the surface. The chemical reaction or physical entanglement between the inorganic filler and the matrix material is promoted by the action of the double bonds, forming a more complex network structure, enhancing the interfacial bonding force, and improving the tensile strength, tear strength, toughness and heat resistance of the film. At the same time, the double bonds can promote the uniform dispersion of the inorganic filler in the polyester matrix through chemical reactions or physical actions, reduce agglomeration, and thus improve the fluidity and formability of the raw materials during processing.

[0094] Furthermore, in the step a4, the process of modifying the second flaky inorganic filler to obtain the second modified flaky inorganic filler is:

[0095] c1, add 10-20 parts by weight of the second flaky inorganic filler treated in step a4 and 2-5 parts by weight of a surfactant to 250-500 parts by weight of deionized water, and after uniform dispersion, heat the mixed system to 40-70° C. under the protection of an inert gas, and slowly drop 5-10 parts by weight of styrene and an initiator while stirring. The styrene and the initiator are added dropwise within 0.5-1 hour, and then continue to stir and react for 2-3 hours. After the reaction is completed, filter, separate, wash with water, and dry to obtain the second modified flaky inorganic filler.

[0096] Preferably, the initiator is sodium persulfate, and the amount of the initiator used is 0.1-0.5% of the weight of styrene.

[0097] As some examples of the present invention, the surfactant is sodium dodecyl sulfate.

[0098] By modifying the second flaky inorganic filler with styrene, styrene can form an organic layer on the surface of the inorganic filler through copolymerization, grafting or other chemical reactions. This organic layer has better chemical compatibility with the matrix resin. This improved compatibility helps to reduce the interfacial tension between the inorganic filler and the matrix, making it easier for the filler particles to be infiltrated and wrapped by the resin melt to form a stable dispersion system. At the same time, after the second flaky inorganic filler is modified with styrene, the organic chain segments on the surface of the inorganic filler may be physically entangled with the molecular chains of the matrix resin to form a stronger interfacial bonding force. This physical entanglement helps to firmly anchor the inorganic filler in the matrix to prevent it from agglomerating during processing.

[0099] Furthermore, the present invention also provides a method for preparing a polypropylene film for capacitors, which comprises the steps of:

[0100] S1, placing polypropylene resin pellets, maleic anhydride grafted polypropylene, modified flaky inorganic filler, silane coupling agent, antioxidant, and nucleating agent in a double-roll mill, and melt blending at 170-190° C. for 30-60 minutes to obtain a mixture;

[0101] S2, preheating the mixture at 230-240° C. for 3-5 minutes on a hot press, and then maintaining it at a pressure of 5-8 MPa for 3-5 minutes; then placing the mixture in a hot press at 160-180° C., maintaining it at a pressure of 3-6 MPa for 7-10 minutes, and naturally cooling it to room temperature to obtain a polypropylene casting sheet;

[0102] S3, preheating the polypropylene cast sheet obtained in step S2 at a temperature range of 140-150° C. for 2-4 minutes, and then performing a synchronous biaxial stretching process on the polypropylene cast sheet to obtain a single synchronous biaxial stretching polypropylene film; then placing the single synchronous biaxial stretching polypropylene film at a temperature range of 150-160° C., and performing a second synchronous biaxial stretching process on the polypropylene cast sheet to obtain a polypropylene film for capacitors.

[0103] Preferably, the stretching ratio of the first simultaneous biaxial stretching is 2.0*2.0~4.0*4.0; the stretching ratio of the second simultaneous biaxial stretching is 5.0*5.0~8.0*8.0.

[0104] In the prior art, the BOPP roughened film used in oil-immersed capacitors is mostly prepared by an asynchronous stretching process. However, the asynchronous double-stretching process will produce a certain bow effect, which is mainly manifested in insufficient stretching uniformity during production, poor balance of film products, and difficulty in maintaining uniform thermal shrinkage of the diagonal line. It is easy to cause warping at both ends. The film prepared in this way has large differences in tensile deformation at different positions, which ultimately affects the surface morphology and mechanical, electrical and thermodynamic properties of the film, thereby reducing the performance of the BOPP roughened film for oil-immersed capacitors. In the present invention, a polypropylene film with more uniform performance is prepared by hot pressing and two-stage synchronous biaxial stretching. The polypropylene film prepared by the present invention has a thickness of about 1~6um, and has a suitable surface roughness, a suitable friction coefficient, a high breakdown voltage, a low relative dielectric constant, a low dielectric loss and excellent mechanical properties, and can be used as an ultra-thin polypropylene film for capacitors.

[0105] The polypropylene film for capacitors and the preparation method thereof according to the present invention are described below by way of specific examples:

[0106] Example 1

[0107] Preparation of the first modified flaky inorganic filler:

[0108] (1) placing large particles of flaky silicon dioxide in the inner chamber of a ball mill, setting a through hole with a diameter of 1 mm on the wall of the inner chamber of the ball mill, adding a steel ball into the inner chamber of the ball mill, and then sealing the chamber of the ball mill, introducing an inert gas into the chamber to keep the pressure inside the chamber at about 0.5 MPa, and then starting the ball mill for grinding. During the ball milling process, inorganic filler particles ground to a certain size can pass through the through hole on the wall of the inner chamber of the ball mill into the outer chamber of the ball mill, and then the inorganic filler in the outer chamber of the ball mill is sucked out by the circulating exhaust unit of the ball mill;

[0109] (2) Dispersing 80 parts by weight of the inorganic filler after ball milling in step (1) in 300 parts by weight of water, placing in a shear dispersion device after being fully dispersed, and performing shear dispersion at a speed of 5000 r / min for 2 hours, then filtering and drying;

[0110] (3) placing 60 parts by weight of the inorganic filler sheared and dispersed in step (2) into 200 parts by weight of an intercalation treatment agent solution, wherein the content of hydrochloric acid in the intercalation treatment agent solution is 20 wt %, and the content of sodium nitrate is 25 wt %; in a closed container, pressurizing and heating to a pressure of 1 MPa and a temperature of 50° C., then starting an ultrasonic device, and ultrasonically peeling for 1 hour at a power of 600 W, a frequency of 40 kHz, and a temperature of 50° C. After the ultrasonic peeling is completed, the pressure is rapidly released to normal pressure at a speed of 0.2 MPa / min. After the pressure is released, the inorganic filler is centrifuged, washed with water, and dried to obtain a first sheet-like inorganic filler with a thickness of less than 50 nm and a plane particle size of less than 1 um;

[0111] (4) placing the first flaky inorganic filler treated in step (3) in a fluidized bed, introducing an inert gas as an operating gas into the fluidized bed, heating ethylbenzene, introducing the gas generated by ethylbenzene into the fluidized bed, and maintaining the temperature of the fluidized bed at 650° C. so that the gas generated by ethylbenzene is carbonized and the formed tiny particles adhere to the surface of the inorganic filler, thereby forming carbon particle protrusions on the surface of the inorganic filler, wherein the amount of ethylbenzene used is 6% of the weight of the inorganic filler in the fluidized bed;

[0112] (5) 30 parts by weight of unsaturated fatty acid are added to 100 parts of deionized water, and stirred to mix evenly. Then, acid or alkali solution is added dropwise under stirring to adjust the pH value of the solution to 4. Then, 50 parts by weight of the inorganic filler treated in step (4) is added, and the mixture is reacted at 70° C. for 2 h under stirring. The mixture is centrifuged, washed with water, and dried to obtain the first modified flaky inorganic filler. The first modified flaky inorganic filler has an average thickness of 38 nm and a two-dimensional plane particle size of 0.62 um.

[0113] Example 2

[0114] Preparation of the second modified flaky inorganic filler:

[0115] (1) placing a flake boehmite large particle raw material in a ball mill inner chamber of a ball mill, setting a through hole with a diameter of 1 mm on the wall of the ball mill inner chamber, adding a steel ball into the ball mill inner chamber, then sealing the ball mill chamber, introducing an inert gas into the ball mill chamber, keeping the pressure inside the ball mill chamber at about 0.4 MPa, and then starting the ball mill for grinding. During the ball milling process, inorganic filler particles ground to a certain size can pass through the through hole on the wall of the ball mill inner chamber into the ball mill outer chamber, and then the inorganic filler in the ball mill outer chamber is sucked out by the circulating exhaust unit of the ball mill;

[0116] (2) dispersing 100 parts by weight of the inorganic filler after ball milling in step (1) in 200 parts by weight of water, placing in a shear dispersion device after being fully dispersed, and performing shear dispersion at a speed of 8000 r / min for 1 hour, followed by filtering and drying;

[0117] (3) placing 40 parts by weight of the inorganic filler sheared and dispersed in step (2) into 100 parts by weight of an intercalation treatment agent solution, wherein the content of hydrochloric acid in the intercalation treatment agent solution is 25 wt %, and the content of potassium nitrate is 30 wt %; in a closed container, pressurizing and heating to a pressure of 2 MPa and a temperature of 40° C., then starting an ultrasonic device, and performing ultrasonic peeling for 0.8 h at a power of 1000 W, a frequency of 50 kHz, and a temperature of 40° C. After the ultrasonic peeling is completed, the pressure is rapidly released to normal pressure at a rate of 0.5 MPa / min. After the pressure is released, the inorganic filler is centrifuged, washed with water, and dried to obtain a second flaky inorganic filler with a thickness of less than 50 nm and a plane particle size of less than 1 um;

[0118] (4) 20 parts by weight of the second flaky inorganic filler treated in step (3) and 3 parts by weight of a surfactant are added to 300 parts by weight of deionized water. After being uniformly dispersed, the mixed system is heated to 70° C. under the protection of an inert gas, and 8 parts by weight of styrene and an initiator are slowly added dropwise while stirring. The styrene and the initiator are added dropwise within 1 hour, and then the stirring and reaction are continued for 2 hours. After the reaction is completed, the second modified flaky inorganic filler is filtered, separated, washed with water, and dried to obtain a second modified flaky inorganic filler. The average thickness of the second modified flaky inorganic filler is 35 nm, and the two-dimensional plane particle size is 0.34 μm.

[0119] Example 3

[0120] Preparation of polypropylene film for capacitors:

[0121] S1, placing 65 parts by weight of polypropylene resin pellets, 5 parts by weight of maleic anhydride grafted polypropylene, 0.5 parts by weight of modified flaky inorganic filler, 0.3 parts by weight of silane coupling agent, 0.2 parts by weight of antioxidant, and 0.01 parts by weight of nucleating agent in a double-roll mill, and melt blending at 190° C. for 30 minutes to obtain a mixture; wherein the polypropylene resin comprises 15 parts by weight of linear polypropylene resin and 50 parts of long-chain branched polypropylene resin, the linear polypropylene resin comprises 10 parts by weight of low molecular weight linear polypropylene resin and 5 parts by weight of high molecular weight linear polypropylene resin; the modified flaky inorganic filler comprises 60 wt% of the first modified flaky inorganic filler prepared in Example 1 and 40 wt% of the second modified flaky inorganic filler prepared in Example 2;

[0122] S2, preheating the mixture at 230° C. for 5 min on a hot press, and then maintaining it at a pressure of 5 MPa for 5 min; then placing the mixture in a hot press at 160° C., maintaining it at a pressure of 6 MPa for 7 min, and naturally cooling it to room temperature to obtain a polypropylene casting sheet;

[0123] S3, preheating the polypropylene cast sheet obtained in step S2 at a temperature of 140° C. for 2 minutes, and then performing a synchronous biaxial stretching process on the polypropylene cast sheet to obtain a single synchronous biaxial stretching polypropylene film; then placing the single synchronous biaxial stretching polypropylene film in a temperature of 150° C., and performing a second synchronous biaxial stretching process on the polypropylene cast sheet, and air-cooling the stretched film at room temperature to obtain a polypropylene film for capacitors; wherein the stretching ratio of the single synchronous biaxial stretching is 2.0*2.0; and the stretching ratio of the second synchronous biaxial stretching is 5.0*5.0.

[0124] Example 4

[0125] Preparation of polypropylene film for capacitors:

[0126] S1, placing 75 parts by weight of polypropylene resin pellets, 10 parts by weight of maleic anhydride grafted polypropylene, 2 parts by weight of modified flaky inorganic filler, 1 part by weight of silane coupling agent, 0.3 parts by weight of antioxidant, and 0.03 parts by weight of nucleating agent in a double-roll mill, and melt blending at 170° C. for 50 minutes to obtain a mixture; wherein the polypropylene resin comprises 25 parts by weight of linear polypropylene resin and 50 parts of long-chain branched polypropylene resin, the linear polypropylene resin comprises 20 parts by weight of low molecular weight linear polypropylene resin and 5 parts by weight of high molecular weight linear polypropylene resin; the modified flaky inorganic filler comprises 70 wt% of the first modified flaky inorganic filler prepared in Example 1 and 30 wt% of the second modified flaky inorganic filler prepared in Example 2;

[0127] S2, preheating the mixture at 235° C. for 4 minutes on a hot press, and then maintaining it at a pressure of 6 MPa for 3 minutes; then placing the mixture in a hot press at 180° C., maintaining it at a pressure of 5 MPa for 10 minutes, and then naturally cooling it to room temperature to obtain a polypropylene casting sheet;

[0128] S3, preheating the polypropylene cast sheet obtained in step S2 at a temperature of 150° C. for 4 minutes, and then performing a synchronous biaxial stretching process on the polypropylene cast sheet to obtain a single synchronous biaxial stretching polypropylene film; then placing the single synchronous biaxial stretching polypropylene film in a temperature of 160° C., and performing a second synchronous biaxial stretching process on the polypropylene cast sheet, and air-cooling the stretched film at room temperature to obtain a polypropylene film for capacitors; wherein the stretching ratio of the single synchronous biaxial stretching is 3.0*3.0; and the stretching ratio of the second synchronous biaxial stretching is 7.0*7.0.

[0129] Example 5

[0130] Preparation of polypropylene film for capacitors:

[0131] S1, 90 parts by weight of polypropylene resin pellets, 12 parts by weight of maleic anhydride grafted polypropylene, 4 parts by weight of modified flaky inorganic filler, 5 parts by weight of silane coupling agent, 0.5 parts by weight of antioxidant, and 0.05 parts by weight of nucleating agent are placed in a double-roll mill, and melt-blended at 180° C. for 60 minutes to obtain a mixture; wherein the polypropylene resin comprises 33 parts by weight of linear polypropylene resin and 57 parts of long-chain branched polypropylene resin, the linear polypropylene resin comprises 25 parts by weight of low molecular weight linear polypropylene resin and 8 parts by weight of high molecular weight linear polypropylene resin; the modified flaky inorganic filler comprises 80 wt% of the first modified flaky inorganic filler prepared in the above embodiment 1 and 20 wt% of the second modified flaky inorganic filler prepared in the above embodiment 2;

[0132] S2, preheating the mixture at 240° C. for 3 min on a hot press, and then maintaining it at a pressure of 8 MPa for 4 min; then placing the mixture in a hot press at 170° C., maintaining it at a pressure of 3 MPa for 8 min, and naturally cooling it to room temperature to obtain a polypropylene casting sheet;

[0133] S3, preheating the polypropylene cast sheet obtained in step S2 at a temperature of 145° C. for 3 minutes, and then performing a synchronous biaxial stretching process on the polypropylene cast sheet to obtain a single synchronous biaxial stretching polypropylene film; then placing the single synchronous biaxial stretching polypropylene film in a temperature of 155° C., and performing a second synchronous biaxial stretching process on the polypropylene cast sheet, and air-cooling the stretched film at room temperature to obtain a polypropylene film for capacitors; wherein the stretching ratio of the single synchronous biaxial stretching is 4.0*4.0; and the stretching ratio of the second synchronous biaxial stretching is 8.0*8.0.

[0134] Comparative Example 1

[0135] Preparation of polypropylene film for capacitors:

[0136] The only difference between it and the above-mentioned embodiment 4 is that:

[0137] The polypropylene resins used are all straight-chain polypropylene resins.

[0138] Comparative Example 2

[0139] Preparation of polypropylene film for capacitors:

[0140] The only difference between it and the above-mentioned embodiment 4 is that:

[0141] The polypropylene resins used are all long-chain branched polypropylene resins.

[0142] Comparative Example 3

[0143] Preparation of polypropylene film for capacitors:

[0144] The only difference between it and the above-mentioned embodiment 4 is that:

[0145] The modified flaky inorganic fillers used are all the first modified flaky inorganic fillers prepared in Example 1 above.

[0146] Comparative Example 4

[0147] Preparation of polypropylene film for capacitors:

[0148] The only difference between it and the above-mentioned embodiment 4 is that:

[0149] The modified flaky inorganic fillers used are all the second modified flaky inorganic fillers prepared in Example 2 above.

[0150] Comparative Example 5

[0151] Preparation of polypropylene film for capacitors:

[0152] The only difference between it and the above-mentioned embodiment 4 is that:

[0153] The modified flaky inorganic filler used includes 70 wt % of the first flaky inorganic filler prepared according to steps (1) to (3) of the above embodiment 1 and 30 wt % of the second modified flaky inorganic filler prepared according to the above embodiment 2.

[0154] Comparative Example 6

[0155] Preparation of polypropylene film for capacitors:

[0156] The only difference between it and the above-mentioned embodiment 4 is that:

[0157] The modified flaky inorganic filler used includes 70 wt % of the first modified flaky inorganic filler prepared in the above Example 1 and 30 wt % of the second flaky inorganic filler prepared according to steps (1) to (3) of the above Example 2.

[0158] Comparative Example 7

[0159] Preparation of polypropylene film for capacitors:

[0160] The only difference between it and the above-mentioned embodiment 4 is that:

[0161] The modified flaky inorganic filler used includes 70wt% of silica flaky inorganic filler and 30wt% of the second modified flaky inorganic filler prepared in the above Example 2. The preparation process of the silica flaky inorganic filler is different from that of the above Example 1 in that: in step (3), after the ultrasonic stripping is completed, the pressure in the reactor is slowly reduced to normal pressure at a rate of 0.01MPa / min.

[0162] Comparative Example 8

[0163] Preparation of polypropylene film for capacitors:

[0164] The only difference between it and the above-mentioned embodiment 4 is that:

[0165] The modified flaky inorganic filler used includes 70wt% of silica flaky inorganic filler and 30wt% of the second modified flaky inorganic filler prepared in the above Example 2. The preparation process of the silica flaky inorganic filler is different from that of the above Example 1 in that: in step (3), 300r / min magnetic stirring is used instead of ultrasound for intercalation and exfoliation.

[0166] Comparative Example 9

[0167] Preparation of polypropylene film for capacitors:

[0168] The only difference between it and the above-mentioned embodiment 4 is that:

[0169] The modified flaky inorganic filler used includes 70 wt % of the silica flaky inorganic filler prepared according to steps (1) to (4) of the above embodiment 1 and 30 wt % of the second modified flaky inorganic filler prepared in the above embodiment 2.

[0170] Comparative Example 10

[0171] Preparation of polypropylene film for capacitors:

[0172] The only difference between it and the above-mentioned embodiment 4 is that:

[0173] The modified flaky inorganic filler used comprises 70 wt % of a silica flaky inorganic filler and 30 wt % of a second modified flaky inorganic filler prepared in the above-mentioned Example 2. The preparation process of the silica flaky inorganic filler is different from that of the above-mentioned Example 1 in that the first flaky inorganic filler is not treated in step (4), and the first flaky inorganic filler obtained in step (3) is directly modified in step (5).

[0174] Performance test: The performance of the polypropylene films prepared in the above Examples 3-5 and Comparative Examples 1-10 was tested, and the results are shown in Table 1 below:

[0175] Table 1 Test results of polypropylene film properties

[0176]

[0177] The embodiments of the present application are described above in conjunction with the accompanying drawings. In the absence of conflict, the embodiments in the present application and the features in the embodiments can be combined with each other. The present application is not limited to the above-mentioned specific implementation methods. The above-mentioned specific implementation methods are merely illustrative and not restrictive. Under the guidance of the present application, ordinary technicians in this field can also make many forms without departing from the purpose of the present application and the scope of protection of the claims, all of which are within the protection of the present application.

Claims

1. A method for preparing a polypropylene film for capacitors, characterized in that: By weight, the raw materials include: Polypropylene resin 65-90 parts; Maleic anhydride grafted polypropylene 5-12 parts; 0.5~4 parts of modified flaky inorganic filler; Silane coupling agent 0.3~5 parts; Antioxidant 0.2~0.5 parts; Nucleating agent 0.01~0.05 parts; The polypropylene resin includes: linear polypropylene resin and long-chain branched polypropylene resin, wherein: The linear polypropylene resin has an isotactic index of ≥95% and an ash content of ≤400ppm; The long-chain branched polypropylene resin has an isotactic index of ≥97%, a melt flow rate of 2-6 g / 10 min, an ash content of ≤200 ppm, and a branching point density of 1 / 10000 carbon atom to 1 / 100 carbon atom; The weight ratio of the linear polypropylene resin to the long-chain branched polypropylene resin is (0.3-0.6):1; The modified flaky inorganic filler is a mixture of a first modified flaky inorganic filler and a second modified flaky inorganic filler, wherein the second modified flaky inorganic filler has a porous structure; The preparation process of the modified flaky inorganic filler is as follows: a1, placing large particle raw materials of flaky inorganic fillers in the inner chamber of a ball mill, adding steel balls into the inner chamber of the ball mill, then sealing the chamber of the ball mill, introducing inert gas of a certain pressure into the chamber, and then starting the ball mill for grinding. During the ball milling process, inorganic filler particles ground to a certain size can pass through the through holes on the wall of the inner chamber of the ball mill into the outer chamber of the ball mill, and then the inorganic filler in the outer chamber of the ball mill is sucked out by the circulating exhaust unit of the ball mill; a2, dispersing 50-100 parts by weight of the inorganic filler after ball milling in step a1 in 200-500 parts by weight of water, fully dispersing by mechanical stirring or ultrasound, placing in a shear dispersion device, shearing and dispersing at a speed of 3000-8000 r / min for 1-3 hours, then filtering and drying; a3, placing 40-80 parts by weight of the inorganic filler sheared and dispersed in step a2 into 100-300 parts by weight of the intercalation treatment agent solution, pressurizing and heating to a pressure of 0.5-2MPa and a temperature of 40-70°C in a closed container, then starting an ultrasonic device, and performing ultrasonic peeling for 0.5-1h at a power of 500-1000W, a frequency of 30-50KHz, and a temperature of 40-70°C. After the ultrasonic peeling is completed, the pressure is quickly released to normal pressure at a speed of 0.2-0.5MPa / min. After the pressure is released, the inorganic filler is centrifuged, washed with water, and dried to obtain a flaky inorganic filler with a thickness of less than 50nm and a plane particle size of less than 1um; a4, preparing a first flaky inorganic filler and a second flaky inorganic filler respectively according to the method shown in the above steps a1 to a3, and then modifying the first flaky inorganic filler and the second flaky inorganic filler respectively to obtain a first modified flaky inorganic filler and a second modified flaky inorganic filler, and compounding them according to a proportion to obtain the modified flaky inorganic filler; In the step a4, the process of modifying the first flaky inorganic filler to obtain the first modified flaky inorganic filler is: b1, placing the first flaky inorganic filler treated in step a3 in a fluidized bed, introducing an inert gas as an operating gas into the fluidized bed, heating the organic matter, introducing the gas generated by the organic matter into the fluidized bed, and maintaining the temperature of the fluidized bed at 600-800° C., so that the gas generated by the organic matter is carbonized and the formed tiny particles are attached to the surface of the inorganic filler, forming carbon particle protrusions on the surface of the inorganic filler; b2, adding 20-40 parts by weight of unsaturated fatty acid to 80-100 parts of deionized water, stirring to mix evenly, then adding acid or alkali solution dropwise to adjust the pH value of the solution to 3-5 under stirring, then adding 30-50 parts by weight of the inorganic filler treated in step b1, reacting at 70-90° C. for 2-3 hours under stirring, centrifuging, washing with water, and drying to obtain the first modified flaky inorganic filler; In the step a4, the process of modifying the second flaky inorganic filler to obtain the second modified flaky inorganic filler is: c1, add 10-20 parts by weight of the second flaky inorganic filler treated in step a4 and 2-5 parts by weight of a surfactant to 250-500 parts by weight of deionized water, and after uniform dispersion, heat the mixed system to 40-70° C. under the protection of an inert gas, and slowly drop 5-10 parts by weight of styrene and an initiator while stirring. The styrene and the initiator are added dropwise within 0.5-1 hour, and then continue to stir and react for 2-3 hours. After the reaction is completed, filter, separate, wash with water, and dry to obtain the second modified flaky inorganic filler.

2. The method for preparing a polypropylene film for capacitors according to claim 1, characterized in that: The linear polypropylene resin includes a low molecular weight linear polypropylene resin with a weight average molecular weight Mw of 200,000 to 300,000 and a melt flow rate of 5 to 8 g / 10 min, and a high molecular weight linear polypropylene resin with a weight average molecular weight Mw of 400,000 to 450,000 and a melt flow rate of 1 to 3 g / 10 min, wherein the weight ratio of the low molecular weight linear polypropylene resin to the high molecular weight linear polypropylene resin is (2 to 4):

1.

3. The method for preparing a polypropylene film for capacitors according to claim 1, characterized in that: The method for preparing the polypropylene film for capacitors comprises the following steps: S1, placing polypropylene resin pellets, maleic anhydride grafted polypropylene, modified flaky inorganic filler, silane coupling agent, antioxidant, and nucleating agent in a double-roll mill, and melt blending at 170-190° C. for 30-60 minutes to obtain a mixture; S2, preheating the mixture at 230-240° C. for 3-5 minutes on a hot press, and then maintaining it at a pressure of 5-8 MPa for 3-5 minutes; then placing the mixture in a hot press at 160-180° C., maintaining it at a pressure of 3-6 MPa for 7-10 minutes, and naturally cooling it to room temperature to obtain a polypropylene casting sheet; S3, preheating the polypropylene cast sheet obtained in step S2 at a temperature range of 140-150° C. for 2-4 minutes, and then performing a synchronous biaxial stretching process on the polypropylene cast sheet to obtain a single synchronous biaxial stretching polypropylene film; then placing the single synchronous biaxial stretching polypropylene film at a temperature range of 150-160° C., and performing a second synchronous biaxial stretching process on the polypropylene cast sheet to obtain a polypropylene film for capacitors.

4. The method for preparing a polypropylene film for capacitors according to claim 3, characterized in that: The stretching ratio of the first synchronous biaxial stretching is 2.0*2.0~4.0*4.0; the stretching ratio of the second synchronous biaxial stretching is 5.0*5.0~8.0*8.

0.

5. A polypropylene film for capacitors, characterized in that: The polypropylene film for capacitor is prepared by the preparation method described in any one of claims 1 to 4 above.

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