Low-thermal-shrinkage-rate PP film for capacitor and preparation method of low-thermal-shrinkage-rate PP film

Abstract

The invention provides a low-thermal-shrinkage-rate PP film for a capacitor and a preparation method thereof, and relates to the technical field of capacitor manufacturing. The preparation method comprises the following steps: respectively preparing an upper-layer component, a middle-layer component and a lower-layer component, respectively melting and mixing, co-extruding, casting and molding to obtain a membrane; performing two-way stretching and heat setting on the membrane to obtain a PP film; wherein the upper-layer component, the middle-layer component and the lower-layer component are prepared from the following raw materials: isotactic polypropylene, polytetrafluoroethylene and silicon dioxide in-situ polymerized polystyrene. The silicon dioxide is fixed on the surface of the polystyrene through in-situ polymerization, uniform distribution of the silicon dioxide is ensured, agglomeration of the silicon dioxide is avoided, and therefore the PP film with low roughness and low heat shrinkage rate is obtained.

Description

Technical Field

[0001] The present invention relates to the technical field of capacitor manufacturing, and in particular to a PP film for capacitors with low heat shrinkage and a preparation method thereof. Background Art

[0002] PP film (polypropylene film) for capacitors is an electrical-grade film material made from polypropylene through a biaxial stretching process. It has excellent electrical properties and stability, and is therefore widely used in high-frequency, high-voltage, and high-reliability power electronic equipment.

[0003] The primary raw material for capacitor PP film is polypropylene. Other polymers, such as polyolefins and polystyrene, can also be added, as well as additives such as inorganic fillers, compatibilizers, dispersants, and antioxidants. The production process for capacitor PP film primarily includes batching and mixing, melt extrusion, sheet casting, longitudinal and transverse stretching, and winding. Post-treatments such as corona treatment and irradiation are also possible. Film capacitors can be produced by forming a metal layer on the surface using processes such as magnetron sputtering and vacuum evaporation.

[0004] For PP film used in capacitors, low thermal shrinkage is key to improving capacitor performance and reliability. Capacitors generate Joule heat during charging and discharging. If the thermal shrinkage of PP film is too high, the film will shrink longitudinally or transversely after heating, causing delamination between the metallized coating and the film interface or the formation of gaps between the film and the electrodes. This delamination can cause localized electric field concentration, reducing the breakdown voltage and even causing the capacitor to short-circuit.

[0005] The existing technology for controlling the thermal shrinkage of capacitor PP film by adjusting the longitudinal and transverse stretch ratios to control the film's molecular chain orientation is well established. Furthermore, inorganic fillers or nucleating agents can be introduced into the film components to reduce anisotropy in thermal shrinkage. In addition to these shrinkage control methods, the roughness of capacitor PP film also significantly affects its thermal shrinkage.

[0006] The surface roughness of capacitor PP film primarily affects its electrical performance. Excessive surface roughness (above 0.5 microns) can lead to localized electric field concentration and reduced breakdown field strength. Excessive surface roughness (below 0.05 microns) can lead to insufficient oil-immersibility and potential for partial discharge. Therefore, the surface roughness of capacitor PP film commonly found on the market is typically between 0.05 and 0.5 microns, achieving a balance between oil-immersibility, voltage resistance, and processing costs.

[0007] The processing of polypropylene film involves a phase transition from β-crystals (hexagonal) to α-crystals (monoclinic). The higher the β-crystal content in the surface layer of capacitor PP film, the more pronounced the surface roughness formed after stretching. Within the aforementioned roughness range of 0.05 to 0.5 microns, the greater the film's roughness, the greater its thermal shrinkage. Therefore, while ensuring satisfactory electrical performance, reducing the surface roughness of capacitor PP film can help reduce its thermal shrinkage.

[0008] As new energy vehicles and smart grids place increasing demands on capacitor reliability, further reducing the thermal shrinkage of PP film is essential. Therefore, how to reduce the surface roughness and thermal shrinkage of PP film while maintaining both mechanical and electrical properties through component and process adjustments has become a key research and development focus in this field. Summary of the Invention

[0009] One of the problems solved by the present invention is how to provide a PP film for capacitors with low surface roughness and thermal shrinkage and good mechanical and electrical properties.

[0010] To solve at least one of the above problems, the present invention provides a method for preparing a PP film for capacitors with low heat shrinkage, the preparation method comprising: S100, preparing upper layer components, middle layer components and lower layer components respectively, melting and mixing them, co-extruding them, and casting them into sheets to obtain a membrane; S200, biaxially stretching and heat-setting the film to obtain a PP film; The raw materials used for the upper layer component, the middle layer component and the lower layer component all include isotactic polypropylene, polytetrafluoroethylene and silicon dioxide in-situ polymerized polystyrene.

[0011] In the above technical solution, based on 100 parts by mass of the total amount of raw materials, the raw materials used for the upper layer component, the middle layer component and the lower layer component respectively include: isotactic polypropylene, 75 to 85 parts by mass; polytetrafluoroethylene, 5 to 8 parts by mass; silica in situ polymerized polystyrene, 8 to 12 parts by mass; dispersant, 0.5 to 2 parts by mass; antioxidant, 0.5 to 1.5 parts by mass.

[0012] In the above technical solution, the ratio of the thickness of each layer in the PP film is upper layer thickness: middle layer thickness: lower layer thickness = (0.5-0.6): 1: (0.5-0.6).

[0013] In the above technical solution, the temperature condition for melt mixing in S100 is 225°C to 245°C; the temperature condition for sheet casting in S100 is 85°C to 95°C.

[0014] In the above technical solution, before each longitudinal stretching or each transverse stretching of the biaxial stretching in S200, the multilayer film is preheated to 90°C to 100°C; the stretching temperature of the longitudinal stretching in S200 is 145°C to 155°C, and the stretching ratio is 3 times to 4 times; the stretching temperature of the transverse stretching in S200 is 165°C to 170°C, and the stretching ratio is 4 times to 5 times; the temperature condition for heat setting is 125°C to 130°C.

[0015] In the above technical solution, the method for preparing polystyrene by in-situ polymerization of silica includes: S310, preparing a silica sol using raw materials including an organosilicon source, an initiator, an acidic catalyst, an emulsifier, ethanol, and water; S320, using raw materials including styrene monomer, amino (2)-four-arm-polyethylene glycol-dopamine (2), and ethyl acetate to prepare a monomer dispersion emulsion; S330, under a protective atmosphere, gradually adding the silica sol dropwise into the monomer dispersion emulsion and mixing and stirring, and heating and continuously stirring the reaction after the addition is completed; S340: After the reaction is completed, the temperature is lowered, the emulsion is broken, the solids are separated, washed, and dried to obtain silica in-situ polymerized polystyrene.

[0016] In the above technical solution, the organosilicon source is ethyl silicate, the initiator is azobisisobutyronitrile, the acidic catalyst is a 6 vol% hydrochloric acid aqueous solution, and the emulsifier is Span 60; in S310, the mass ratio of organosilicon source: initiator: acidic catalyst: emulsifier: ethanol: water is (20-30): (0.02-0.06): (8-12): (2-6): 100: (30-40); in S320, the mass ratio of styrene monomer: amino (2)-four-arm-polyethylene glycol-dopamine (2): ethyl acetate is (6-12): (0.5-1): 100; in S330, the mass ratio of silica sol: monomer dispersed emulsion is 150:100.

[0017] In the above technical solution, the raw materials used for the middle layer component include silica in-situ polymerized polystyrene A, and the raw materials used for the upper layer component and the lower layer component both include silica in-situ polymerized polystyrene B; wherein, the raw material ratios used in the preparation process of silica in-situ polymerized polystyrene A and silica in-situ polymerized polystyrene B are different.

[0018] In the above technical solution, when preparing silica in situ polymerized polystyrene A, in S310, the mass ratio of organic silicon source: initiator: acidic catalyst: emulsifier: ethanol: water is 20:0.04:11.5:4:100:30; when preparing silica in situ polymerized polystyrene B, in S310, the mass ratio of organic silicon source: initiator: acidic catalyst: emulsifier: ethanol: water is 30:0.04:8.5:4:100:40; when preparing silica in situ polymerized polystyrene A, in S320, the mass ratio of styrene monomer: amino (2)-four-arm-polyethylene glycol-dopamine (2): ethyl acetate is 12:0.75: 100; When preparing silica in-situ polymerized polystyrene B, in S320, the mass ratio of styrene monomer: amino (2)-four-arm-polyethylene glycol-dopamine (2): ethyl acetate is 6:0.75:100.

[0019] The present invention also provides a PP film for capacitors with a low heat shrinkage rate, which is obtained by using the preparation method of any of the above technical solutions.

[0020] Beneficial effects The preparation method of the present invention first separately prepares the upper layer component, the middle layer component, and the lower layer component. These components are melt-mixed, co-extruded, and cast into a film. The film is then biaxially stretched and heat-set to obtain a PP film. In the present invention, the raw materials used for the upper, middle, and lower layer components all include isotactic polypropylene, polytetrafluoroethylene, and silicon dioxide in-situ polymerized polystyrene. Polystyrene has high insulation resistance and low dielectric loss, which can improve the dielectric properties and charge storage capacity of the PP film. Furthermore, polystyrene can optimize the interfacial structure of the PP film and enhance its mechanical strength. Silicon dioxide accelerates PP crystallization, enhancing the film's tensile strength and elongation at break. Furthermore, silicon dioxide imparts a certain degree of roughness to the PP film, improving its adhesion to metals. Finally, the present invention in situ polymerizes silica with styrene monomers, so that polystyrene coats and fixes the silica, and uses polystyrene molecular chains to form a physical barrier. By forming steric hindrance, the mutual contact and aggregation of silica particles are prevented. As a result, during the processes of melting, casting and stretching the PP film, silica is not easily agglomerated and forms secondary particles, thereby ensuring that the PP film has low roughness and thermal shrinkage. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is the FT-IR infrared absorption spectrum of silica in situ polymerized polystyrene sample 2. DETAILED DESCRIPTION

[0022] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, a detailed description is given below in conjunction with specific embodiments of the present invention.

[0023] Unless otherwise specified, the reagents and raw materials used in the present invention can be purchased from commercial sources. Experimental methods in the following examples where specific conditions are not specified were performed according to conventional methods and conditions, or selected according to the product specifications.

[0024] The main purpose of the present invention is to provide a method for preparing a PP film for capacitors with low thermal shrinkage. The method for preparing a PP film for capacitors with low thermal shrinkage provided by the present invention comprises: S100, preparing upper layer components, middle layer components and lower layer components respectively, melting and mixing them, co-extruding them, and casting them into sheets to obtain a membrane; S200, biaxially stretching and heat-setting the film to obtain a PP film; The raw materials used for the upper layer component, the middle layer component and the lower layer component all include isotactic polypropylene, polytetrafluoroethylene and silicon dioxide in-situ polymerized polystyrene.

[0025] For PP films for capacitors, good electrical properties, high tensile strength, good elongation at break, reasonable roughness and low thermal shrinkage are all crucial.

[0026] The PP film of the present invention uses polypropylene as its primary raw material and is supplemented with polystyrene. Polystyrene has high insulation resistance and low dielectric loss, which can improve the dielectric properties and charge storage capacity of the PP film. Polystyrene particularly helps improve the stability of the PP film in high-temperature environments, reducing capacitor performance fluctuations caused by temperature changes. Furthermore, the introduction of polystyrene can optimize the interfacial structure of the PP film, reduce interlayer gaps and defects, and thus improve the mechanical strength, particularly toughness, of the PP film.

[0027] The PP film of the present invention further contains polytetrafluoroethylene, which has a relatively high melting point (approximately 327° C.), and thus helps to improve the thermal stability of the PP film. The inertness of polytetrafluoroethylene can also enhance the corrosion resistance and chemical stability of the PP film.

[0028] Finally, the PP film of the present invention also contains silicon dioxide. Specifically, silicon dioxide is porous nano-silicon dioxide particles, which act as nucleation centers to accelerate PP crystallization and enhance the film's tensile strength and elongation at break. Furthermore, silicon dioxide imparts a certain degree of roughness to the PP film, improving the adhesion between the PP film and the metal. Finally, because silicon dioxide can anchor the amorphous segments of polypropylene and inhibit the conversion of β crystals to α crystals under high temperature conditions, the addition of silicon dioxide not only increases the roughness of the PP film but also increases the thermal shrinkage of the PP film and reduces its dimensional stability at high temperatures. Therefore, the present invention also aims to control the preparation process of silicon dioxide to achieve the purpose of controlling the PP film to have a reasonable roughness and reducing its thermal shrinkage.

[0029] Regarding the preparation process of silica, one of the improvements of the present invention over the prior art is that silica and polystyrene are fixed by in-situ polymerization to achieve the purpose of controlling and reducing the roughness of the PP film and reducing its thermal shrinkage.

[0030] Specifically, the silica added to the PP film is nano-sized porous silica, which can be produced through processes such as sol-gel and hydrothermal methods. The reason silica increases the thermal shrinkage of PP film is that during the processing of PP film, the nano-sized silica, due to its high surface energy, easily agglomerates and produces secondary particles. This phenomenon increases the actual particle size of the silica. While this phenomenon can improve the roughness of the PP film and its bonding strength with metals, it also increases the thermal shrinkage of the PP film.

[0031] Taking porous silica with a particle size less than 100 nanometers as an example, theoretically, by adding porous silica with a particle size less than 100 nanometers, the roughness of PP films can be controlled to between 0.05 and 0.1 microns, while also imparting good dimensional stability. However, in practice, the use of silica with a particle size less than 100 nanometers in the prior art tends to cause a sudden increase in the roughness of the PP film and a significant increase in thermal shrinkage. This is because silica itself has a loose, porous structure with a large relative surface area, a small particle size, and high surface energy. Therefore, silica with a smaller particle size is more likely to aggregate in the polymer matrix. This aggregation of silica leads to the formation of secondary particles, which increase in size uncontrollably. Therefore, achieving a PP film with a roughness between 0.05 and 0.1 microns and low thermal shrinkage by adding silica is difficult to achieve with the prior art.

[0032] To address the above issues, the present invention utilizes silica in-situ polymerized polystyrene as one of its raw materials. In addition to leveraging the dielectric properties of polymerized polystyrene and the positive impact of silica on improving the metal adhesion of PP films, silica can also be solidified within the polystyrene. By adding this silica in-situ polymerized polystyrene material to polypropylene, the polystyrene molecular chains form a physical barrier that, through steric hindrance, prevents the aggregation of silica particles, preventing silica agglomeration and the formation of secondary particles, ensuring that the PP film has low roughness and thermal shrinkage.

[0033] In the present invention, the PP film is produced through a multi-layer coextrusion process. Multi-layer coextrusion is a molding process in which different molten resins are extruded simultaneously through multiple extruders, and then layered and fused through a composite die (such as a coextrusion die or a lamination die) to form a multi-layer composite film or sheet.

[0034] It is understood that in the prior art, a single-layer homogeneous PP film can also be directly produced through a process other than multi-layer co-extrusion. The present invention utilizes a multi-layer co-extrusion process to produce a heterogeneous PP film with different properties in each layer. Each layer is primarily made of isotactic polypropylene, polytetrafluoroethylene, and silica in-situ polymerized polystyrene, with the aforementioned raw materials added in the same or similar proportions in each layer.

[0035] For example, based on 100 parts by mass of the total amount of raw materials, the raw materials used for the upper layer component, the middle layer component, and the lower layer component each include: isotactic polypropylene, 75 to 85 parts by mass; polytetrafluoroethylene, 5 to 8 parts by mass; silica in situ polymerized polystyrene, 8 to 12 parts by mass; dispersant, 0.5 to 2 parts by mass; and antioxidant, 0.5 to 1.5 parts by mass. Preferably, in a preferred embodiment of the present invention, the preparation process used for the silica in situ polymerized polystyrene in each layer is different. Such differences can be reflected in different types of raw materials or solvents for the silica in situ polymerized polystyrene, different proportions of raw materials or solvents added, and different process parameters such as heating and stirring. The purpose of adopting such process differences is to prepare silica in situ polymerized polystyrene with different performance properties, so that the properties of each layer in the PP film are similar, but also different.

[0036] Preferably, the preparation process can be adjusted so that the silica in-situ polymerized polystyrene used in the middle layer has a lower silica content and a smaller silica particle size, thereby making the roughness of the middle layer smaller and the overall thermal shrinkage of the PP film lower.

[0037] Further preferably, the preparation process can be adjusted so that the silica in situ polymerized polystyrene used in the upper and lower surface layers has a higher silica content and a larger silica particle size, thereby making the upper and lower surface layers rougher. On the basis of ensuring that the overall thermal shrinkage rate of the PP film is low, it ensures that it has good bonding with the metal layer, and also makes the electrical properties of the PP film better.

[0038] Preferably, the thickness ratio of each layer in the PP film is (upper layer thickness: middle layer thickness: lower layer thickness) = (0.5-0.6):1:(0.5-0.6). It should be noted that the thickness of PP film typically ranges from 2 microns to 50 microns. While adhering to the aforementioned thickness ratios of each layer, those skilled in the art can adjust the total thickness of the PP film according to actual needs.

[0039] In general, the present invention reduces the amount of styrene monomer used when preparing the silicon dioxide in-situ polymerized polystyrene used in the middle layer and the upper and lower surface layers. Different from the technical solution that aims to achieve a uniform mixture of silicon dioxide and polystyrene with a content close to or little difference between the two, the present invention aims to form a polystyrene film layer on the surface of small-sized nano-silica sol particles to utilize steric hindrance to limit the mutual contact and van der Waals attraction of silicon dioxide. Therefore, the present invention uses less styrene monomer and a lower concentration, and uses a mixture containing silicon dioxide sol particles, an emulsifier, and an initiator as the aqueous phase, and uses styrene monomer to prepare the oil phase. By dripping the dispersed aqueous phase into the continuous oil phase, an oil-in-water emulsion is prepared. After obtaining the emulsion, a heat polymerization reaction is carried out so that the in-situ polymerized polystyrene is coated on the surface of the silicon dioxide particles, thereby adjusting the roughness and thermal shrinkage of the entire PP film.

[0040] It should also be noted that increasing the extrusion temperature, lowering the casting temperature, avoiding excessively high chilling temperatures, increasing the transverse and longitudinal stretching temperatures, and increasing the heat setting temperature can all help reduce the thermal shrinkage of PP films. Therefore, the present invention also adjusts the above process parameters.

[0041] Specifically, the present invention adopts a higher melt mixing temperature, stretching temperature, and heat setting temperature, and adopts a lower casting sheet forming temperature. Preferably, the temperature condition of melt mixing in S100 is 225°C to 245°C. The temperature condition of casting sheet forming in S100 is 85°C to 95°C. Before each longitudinal stretching or each transverse stretching of the biaxial stretching in S200, the multilayer film is preheated to 90°C to 100°C; the stretching temperature of the longitudinal stretching in S200 is 145°C to 155°C, and the stretching ratio is 3 times to 4 times; the stretching temperature of the transverse stretching in S200 is 165°C to 170°C, and the stretching ratio is 4 times to 5 times; the temperature condition of heat setting is 125°C to 130°C.

[0042] In the present invention, silica is prepared via a sol-gel process, and polystyrene is coated onto the surface of silica nanoparticles through in-situ polymerization. To ensure the production of a core-shell structure of silica in-situ polymerized polystyrene with silica as the core and polystyrene as the shell, the present invention uses silica sol as the aqueous phase and styrene monomer as the oil phase. The dispersed aqueous phase is dropwise added to the continuous oil phase with continuous stirring to form a water-in-oil emulsion. The emulsion is then heated to polymerize the styrene on the surface of the silica nanoparticles.

[0043] Specifically, the method for preparing polystyrene by in-situ polymerization of silica includes: S310, preparing a silica sol using raw materials including an organosilicon source, an initiator, an acidic catalyst, an emulsifier, ethanol, and water; S320, using raw materials including styrene monomer, amino (2)-four-arm-polyethylene glycol-dopamine (2), and ethyl acetate to prepare a monomer dispersion emulsion; S330, under a protective atmosphere, gradually adding the silica sol dropwise into the monomer dispersion emulsion and mixing and stirring, and heating and continuously stirring the reaction after the addition is completed; S340: After the reaction is completed, the temperature is lowered, the emulsion is broken, the solids are separated, washed, and dried to obtain silica in-situ polymerized polystyrene.

[0044] In S310, the mass ratio of organosilicon source: initiator: acidic catalyst: emulsifier: ethanol: water is (20-30): (0.02-0.06): (8-12): (2-6): 100: (30-40); In S320, based on the mass ratio, styrene monomer: amino (2)-four-arm-polyethylene glycol-dopamine (2): ethyl acetate = (6-12): (0.5-1): 100; In S330 , the mass ratio of silica sol to monomer dispersed emulsion is 150:100.

[0045] In the above steps, the organosilicon source is ethyl silicate, the initiator is azobisisobutyronitrile, the acidic catalyst is a 6 vol% hydrochloric acid aqueous solution, and the emulsifier is Span 60. The reason for using azobisisobutyronitrile as the initiator is that it is an organic initiator and is not sensitive to pH. The reason for using Span 60 as the emulsifier is that it is a typical oil-in-water emulsifier. The stearate chain gives Span 60 a strong lipophilicity, which can effectively reduce the oil-water interfacial tension and form a stable oil-in-water interfacial film. It is understood that there are many types of organosilicon sources, such as methyl silicate. The present invention uses ethyl silicate as an example to illustrate the proportion relationship of each raw material. It should be noted that when using the sol-gel method to prepare silica, both acidic catalysts and alkaline catalysts can promote the formation of silica sol. However, acidic catalysts help to form silica sols with smaller particle size and more uniform particle size, so the present invention uses acidic catalysts.

[0046] In the present invention, silica sol serves as the aqueous phase, and a monomer-dispersed emulsion oil phase is used. Before the dispersed aqueous phase is added dropwise to the continuous oil phase, the pH of the silica sol is preferably adjusted to neutral using a buffer such as sodium bicarbonate, prepared as a 10 to 12 wt% aqueous solution.

[0047] It is understood that the styrene monomer used in the present invention is preferably washed with a sodium hydroxide aqueous solution of, for example, about 4 wt % to 6 wt % to remove polymerization inhibitors such as hydroquinone, and then used after washing to neutrality.

[0048] Ethyl acetate is used as the solvent of the oil phase, and amino (2)-four-arm-polyethylene glycol-dopamine (2) is used as a modifier. Pentaerythritol is used as the core, and each arm has a hydroxyl group at the end. After modification, two amino groups and two dopamine groups are connected as functional groups. The above structure makes amino (2)-four-arm-polyethylene glycol-dopamine (2) have good hydrophilicity and reactivity, and can help to achieve the combination of styrene and silica particles. Specifically, the dopamine group can improve the hydrophilicity of the oil phase containing styrene monomer, promote the uniform dispersion of the water phase in the oil phase, and improve the uniformity of emulsification. In the process of preparing silica by the sol-gel method, the dissociation of silanol groups causes the release of hydrogen ions on the silica surface. The four alcoholic hydroxyl groups of amino (2)-tetraarm-polyethylene glycol-dopamine (2) in the oil phase can attract hydrogen ions due to hydrogen bonding, further promoting the uniform dispersion of the water phase in the oil phase, thereby improving the dispersibility of silica and the uniformity of polystyrene coating on silica.

[0049] As mentioned above, a multi-layer co-extrusion process can produce heterogeneous PP films with varying properties across each layer. In the present invention, the raw materials used for the middle layer include silica in-situ polymerized polystyrene A, while the raw materials used for both the upper and lower layers include silica in-situ polymerized polystyrene B. Specifically, silica in-situ polymerized polystyrene A and silica in-situ polymerized polystyrene B are prepared using different raw material ratios.

[0050] Preferably, when preparing silica in situ polymerized polystyrene A, in S310, the mass ratio of organic silicon source: initiator: acidic catalyst: emulsifier: ethanol: water is 20:0.04:11.5:4:100:30; when preparing silica in situ polymerized polystyrene B, in S310, the mass ratio of organic silicon source: initiator: acidic catalyst: emulsifier: ethanol: water is 30:0.04:8.5:4:100:40; when preparing silica in situ polymerized polystyrene A, in S320, the mass ratio of styrene monomer: amino (2)-four-arm-polyethylene glycol-dopamine (2): ethyl acetate is 12:0.75: 100; when preparing silica in situ polymerized polystyrene B, in S320, the mass ratio of styrene monomer: amino (2)-four-arm-polyethylene glycol-dopamine (2): ethyl acetate is 6:0.75:100. The test results of the present invention show that the roughness and thermal shrinkage of the PP film are affected by the raw material ratio of silica in situ polymerized polystyrene. Among them, by adjusting the ratio of the organic silicon source and the acid catalyst during the aqueous phase preparation, and adjusting the amount of styrene monomer added during the oil phase preparation, the roughness and thermal shrinkage of the PP film can be directly controlled. Therefore, by adjusting the raw material ratio when preparing silica in situ polymerized polystyrene, the relatively thick middle layer can maintain a lower roughness and thermal shrinkage, and the relatively thin upper and lower layers can have a higher roughness and metal bonding ability. Therefore, the present invention can achieve the purpose of reducing the thermal shrinkage of the PP film and taking into account its metal adhesion, thereby ensuring that the PP film has good electrical properties.

[0051] The following is a specific example of the preparation method of the silica in-situ polymerization of polystyrene and PP film of the present invention. Among the raw materials used in the present invention, amino (2)-four-arm-polyethylene glycol-dopamine (2) was purchased from Xi'an Kaixin Biotechnology, styrene monomer was purchased from Shandong Yaojia Chemical, and the remaining raw materials were purchased from Shanghai Sinopharm.

[0052] Example 1 In this example, a series of silicon dioxide in-situ polymerized polystyrene samples were prepared. The raw material ratios and process parameters of this example are detailed in Table 1. The preparation method is as follows.

[0053] S1. Ethyl silicate was weighed according to the proportions in Table 1, and mixed uniformly with ethanol and water. Azobisisobutyronitrile and Span 60 were then added and mixed uniformly. The mixture was kept constant at 30°C in a water bath, and a 6 vol% aqueous hydrochloric acid solution was added dropwise at a rate of 2 mL / min. The mixture was stirred at 300 rpm. After the addition was complete, stirring was continued for 40 min. The mixture was allowed to stand for 2 h, and sodium bicarbonate was added to adjust the pH to neutral to obtain a silica sol. S2. According to the ratio in Table 1, weigh styrene monomer (after removing the polymerization inhibitor), mix it with ethyl acetate, then add amino (2)-four-arm-polyethylene glycol-dopamine (2) and continue to mix evenly. Homogenize the mixture in a homogenizer at a speed of 2000 rpm for 15 minutes to prepare a monomer dispersion emulsion; S3. Place the monomer dispersion emulsion in the homogenizer in a constant temperature water bath at 30°C. Gradually add the silica sol at a drop rate of 1.5 mL / min into the monomer dispersion emulsion at a mass ratio of silica sol to monomer dispersion emulsion of 150:100, and homogenize at a speed of 100 rpm. After the addition is completed, raise the temperature of the constant temperature water bath to 75°C under a protective atmosphere, increase the homogenization speed to 2000 rpm, and react at this constant temperature for 4.5 hours. S4. After the reaction is completed, the mixture is naturally cooled to room temperature, and a 5 wt% calcium chloride aqueous solution is added and stirred to break the emulsion. After the emulsion is broken, the solid is separated by centrifugation, washed with ethanol and water, and dried in a vacuum oven at 55°C for 12 hours to obtain a silica in situ polymerized polystyrene sample.

[0054] Table 1 Comparative Example 1 In this comparative example, a series of silica in situ polymerized polystyrene samples were prepared. The raw material ratios and process parameters of this comparative example are shown in Table 2. The raw material selection and preparation method are the same as those in Example 1. The only difference is that the samples of this comparative example do not use amino (2)-four-arm-polyethylene glycol-dopamine (2), or other raw materials are used instead of amino (2)-four-arm-polyethylene glycol-dopamine (2).

[0055] Table 2 Example 2 In this example, a series of PP film samples were prepared, and the preparation method was as follows.

[0056] S1. According to the mass ratio of isotactic polypropylene: polytetrafluoroethylene: silica in situ polymerized polystyrene: polypropylene wax: antioxidant 1010 = 82:6:11:0.5:0.5, the materials were weighed and mixed to prepare the upper layer component, the middle layer component, and the lower layer component respectively. The source of silica in situ polymerized polystyrene used in each layer component is shown in Table 3; S2. Add the raw materials for each layer to a high-speed mixer and mix at 110°C for 15 minutes. Then, melt them through a twin-screw extruder (screw speed 120 rpm). The melt extrusion temperature is 180°C (feeding section) - 225°C (homogenizing section) - 245°C (die head). The melted materials are extruded into sheet-like fluid through the die head of the twin-screw extruder (die lip gap 1.1 mm). The thickness ratio of each layer is controlled to be upper layer thickness: middle layer thickness: lower layer thickness = 0.5:1:0.5; S3, the sheet fluid is cast into a sheet by passing it through a chilled roller at 85°C and a high-pressure air knife (pulling speed 10 m / min) to obtain a membrane; S4. Preheat the film to 90°C (preheating time 35 seconds) and stretch it longitudinally at 150°C at a stretch ratio of 3.5 (stretching rate 250% per second) using a biaxial stretching machine. After cooling to room temperature, preheat the film again to 90°C and stretch it transversely at 170°C at a stretch ratio of 4.5 (stretching rate 250% per second). Heat set at 130°C for 25 seconds, cool naturally, and wind up to obtain a PP film sample with a thickness of 4.4 μm (±3% error).

[0057] Table 3 Performance Testing The infrared absorption spectra of the silica in situ polymerized polystyrene samples 1 to 12 were measured using a Fourier transform infrared spectrometer (FTIR) with a potassium bromide pellet scanning wavelength range of 400 to 4000 cm −1 . Figure 1 This is the infrared absorption spectrum of silica in situ polymerized polystyrene sample 2. The infrared absorption spectrum absorption characteristic peaks of the other samples are similar to that of silica in situ polymerized polystyrene sample 2, so only the infrared absorption spectrum of silica in situ polymerized polystyrene sample 2 is shown. In the infrared absorption spectrum of silica in situ polymerized polystyrene sample, it can be observed that the 1100cm −1 The antisymmetric stretching vibration peak of the silicon-oxygen-silicon bond at 460 cm −1 The symmetrical stretching vibration peak of the silicon-oxygen bond at 3050cm −1 , 1500 cm −1 , 760cm −1The characteristic peaks of polystyrene near the silica were observed. This proves the presence of silica and polystyrene in the sample. In addition, a broad peak of primary amino stretching vibration from 3300 cm⁻¹ to 3500 cm⁻¹ can be observed, which indicates that the silica in situ polymerized polystyrene sample 2 contains amino (2)-four-arm-polyethylene glycol-dopamine (2).

[0058] PP film samples 1 to 12 were tested for surface roughness, thermal shrinkage (120°C / 10 min), tensile strength, and elongation at break. The test results are listed in Table 4. The test results show that the roughness of the PP film samples 1 to 12 obtained by the present invention was relatively low. PP film samples 9 to 12 had relatively high roughness, which was due to uneven polystyrene coating, which resulted in a small amount of silica agglomeration. The thermal shrinkage (MD) of PP film samples 1 to 8 was controlled below 5.5%, and the thermal shrinkage (TD) was controlled below 0.5%. The thermal shrinkage (MD) of PP film samples 2, 3, 4, and 8 was controlled below 4.7%, and the thermal shrinkage (TD) was controlled below 0.4%. PP film sample 3 had the lowest thermal shrinkage. PP film samples 8 to 12 had relatively high thermal shrinkage, which was also due to uneven polystyrene coating, which resulted in a small amount of silica agglomeration. In terms of tensile strength and elongation at break, there is little difference among the samples. The tensile strength MD can exceed 200 MPa, the tensile strength TD can exceed 250 MPa, the elongation at break MD can exceed 60%, and the elongation at break TD can exceed 40%.

[0059] Table 4 A metal aluminum layer was deposited on the surface of PP film samples 1 to 12 to obtain metal film samples 1 to 12. The conditions for vacuum deposition include: the vacuum degree does not exceed 2×10 -5 Pa, the evaporation rate was 0.08 A / s, the evaporation time was 220 s, the substrate temperature was 140°C, the wire feed speed was 800 mm / min, and the evaporation boat temperature was 650°C. The breakdown strength of metal film samples 1 to 12 was tested, and the test results are listed in Table 5. The test results show that the breakdown strength of metal film samples 1 to 12 obtained by the present invention can exceed 625 V / micron.

[0060] Table 5 Although the present invention is disclosed as above, the present invention is not limited thereto. Any person skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention should be based on the scope defined by the claims.

Claims

1. A method for preparing a PP film for capacitors with low heat shrinkage, characterized in that: The preparation method comprises: S100, preparing upper layer components, middle layer components and lower layer components respectively, melting and mixing them, co-extruding them, and casting them into sheets to obtain a membrane; S200, biaxially stretching and heat-setting the film to obtain a PP film; The raw materials used for the upper layer component, the middle layer component and the lower layer component all include isotactic polypropylene, polytetrafluoroethylene and silicon dioxide in-situ polymerized polystyrene.

2. The preparation method according to claim 1, characterized in that Based on 100 parts by mass of the total amount of raw materials, the raw materials used for the upper layer component, the middle layer component and the lower layer component respectively include: 75 to 85 parts by mass of the isotactic polypropylene; 5 to 8 parts by mass of the polytetrafluoroethylene; The silicon dioxide in situ polymerized polystyrene, 8 to 12 parts by mass; Dispersant, 0.5 to 2 parts by mass; Antioxidant, 0.5 to 1.5 parts by mass.

3. The preparation method according to claim 1, characterized in that The thickness ratio of each layer in the PP film is upper layer thickness: middle layer thickness: lower layer thickness = (0.5-0.6): 1: (0.5-0.6).

4. The preparation method according to claim 1, characterized in that The temperature condition of the melt mixing in S100 is 225° C. to 245° C.; The temperature condition for the casting in S100 is 85°C to 95°C.

5. The preparation method according to claim 1, characterized in that Before each longitudinal stretching or each transverse stretching of the biaxial stretching in S200, the multilayer film is preheated to 90° C. to 100° C.; The stretching temperature of the longitudinal stretching in S200 is 145° C. to 155° C., and the stretching ratio is 3 to 4 times; The stretching temperature of the transverse stretching in S200 is 165° C. to 170° C., and the stretching ratio is 4 to 5 times; The heat setting temperature condition is 125°C to 130°C.

6. The preparation method according to any one of claims 1 to 5, characterized in that The method for preparing polystyrene by in-situ polymerization of silicon dioxide comprises: S310, preparing a silica sol using raw materials including an organosilicon source, an initiator, an acidic catalyst, an emulsifier, ethanol, and water; S320, using raw materials including styrene monomer, amino (2)-four-arm-polyethylene glycol-dopamine (2), and ethyl acetate to prepare a monomer dispersion emulsion; S330, under a protective atmosphere, gradually adding the silica sol dropwise into the monomer dispersion emulsion and mixing and stirring, and heating and continuously stirring to react after the addition is completed; S340, after the reaction is completed, the temperature is lowered, the emulsion is broken, the solids are separated, washed, and dried to obtain the silica in-situ polymerized polystyrene.

7. The preparation method according to claim 6, characterized in that The organosilicon source is ethyl silicate, the initiator is azobisisobutyronitrile, the acidic catalyst is a 6 vol% hydrochloric acid aqueous solution, and the emulsifier is Span 60; In S310, the mass ratio of organosilicon source: initiator: acidic catalyst: emulsifier: ethanol: water is (20-30): (0.02-0.06): (8-12): (2-6): 100: (30-40); In S320, based on the mass ratio, styrene monomer: amino (2)-four-arm-polyethylene glycol-dopamine (2): ethyl acetate = (6-12): (0.5-1): 100; In S330 , the mass ratio of silica sol to monomer dispersed emulsion is 150:

100.

8. The preparation method according to claim 7, characterized in that The raw materials used for the middle layer component include silica in-situ polymerized polystyrene A, and the raw materials used for the upper layer component and the lower layer component both include silica in-situ polymerized polystyrene B; wherein the silica in-situ polymerized polystyrene A and the silica in-situ polymerized polystyrene B use different raw material ratios during the preparation process.

9. The preparation method according to claim 8, characterized in that When preparing the silica in-situ polymerized polystyrene A, in S310, the mass ratio of organic silicon source: initiator: acidic catalyst: emulsifier: ethanol: water is 20:0.04:11.5:4:100:30; When preparing the silica in-situ polymerized polystyrene B, in S310, the mass ratio of the organosilicon source: initiator: acidic catalyst: emulsifier: ethanol: water is 30:0.04:8.5:4:100:40; When preparing the silica in-situ polymerized polystyrene A, in S320, the mass ratio of styrene monomer: amino (2)-four-arm-polyethylene glycol-dopamine (2): ethyl acetate is 12:0.75:100; When preparing the silica in-situ polymerized polystyrene B, in S320, the mass ratio of styrene monomer: amino (2)-four-arm-polyethylene glycol-dopamine (2): ethyl acetate is 6:0.75:

100.

10. A PP film for capacitors with low heat shrinkage, characterized in that: The PP film is obtained by the preparation method according to any one of claims 1 to 9.

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