A capacitor pp film with low heat shrinkage and a preparation method thereof

Abstract

The application provides a low-heat-shrinkage PP film for capacitors 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, an intermediate layer component and a lower layer component, co-extruding after melt mixing, casting sheet forming, and obtaining a film piece; bidirectional stretching and heat setting the film piece to obtain a PP film; wherein the raw materials of the upper layer component, the intermediate layer component and the lower layer component all comprise isotactic polypropylene, polytetrafluoroethylene and silica in-situ polymerized polystyrene. In the application, the silica is fixed on the surface of the polystyrene through in-situ polymerization, uniform distribution of the silica is ensured, and agglomeration of the silica is avoided, so that the PP film with low roughness and low heat shrinkage is obtained.

Description

TECHNICAL FIELD

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

[0002] The PP film for capacitors (polypropylene film) is a kind of electrical grade film material made of polypropylene as raw material through a two-way stretching process, which has excellent electrical properties and stability, and is therefore widely used in high-frequency, high-voltage and high-reliability power electronic devices.

[0003] The main raw material of the PP film for capacitors is polypropylene, and other polymer raw materials such as polyolefin, polystyrene, and inorganic fillers, compatibilizers, dispersants, antioxidants, and other additives can also be added. The preparation process of the PP film for capacitors mainly includes: batching and mixing, melt extrusion, sheet forming, longitudinal and transverse stretching, traction winding. On this basis, the PP film for capacitors can be subjected to post-processing such as corona and irradiation. For the PP film for capacitors, a metal layer can be formed on its surface by using processes such as magnetron sputtering and vacuum evaporation, and a film capacitor product can be obtained.

[0004] For the PP film for capacitors, a lower heat shrinkage is a key to improving the performance and reliability of the capacitor. During the charging and discharging process of the capacitor, Joule heat is generated. If the heat shrinkage of the PP film is too high, the film will shrink longitudinally or transversely after being heated, resulting in peeling of the metallized coating from the film interface or gaps between the film and the electrode. This delamination can cause local electric field concentration, reduce the breakdown voltage, and even cause short circuit of the capacitor.

[0005] In the prior art, the longitudinal and transverse stretching ratios are adjusted to control the molecular chain orientation degree of the film, thereby controlling the heat shrinkage of the PP film for capacitors. In addition, inorganic fillers or nucleating agents can be introduced into the film components to reduce the anisotropy of heat shrinkage. In addition to the above-mentioned heat shrinkage control means, the roughness of the PP film for capacitors also has a significant impact on its heat shrinkage.

[0006] The surface roughness of the PP film for capacitors mainly affects its electrical properties. If the surface roughness of the PP film for capacitors is too high (more than 0.5 microns), it will cause local concentration of electric field and decrease in breakdown field strength. If the surface roughness of the PP film for capacitors is too low (less than 0.05 microns), it will cause insufficient oil immersion and local discharge problems. Therefore, the surface roughness of the commonly used PP film for capacitors in the market is generally between 0.05 microns and 0.5 microns, so as to balance the oil immersion, voltage resistance and processing cost.

[0007] The processing of polypropylene film is accompanied by phase transition from beta crystal (hexagonal crystal) to alpha crystal (monoclinic crystal). The higher the content of surface beta crystal of the PP film for capacitor is, the more obvious the surface concave-convex structure formed after stretching is. The roughness of the film is between 0.05 microns and 0.5 microns, and the greater the roughness of the film is, the higher the thermal shrinkage rate is. Therefore, under the premise of ensuring that the electrical performance is satisfied, reducing the surface roughness of the PP film for capacitor helps to reduce the thermal shrinkage rate thereof.

[0008] With the improvement of the reliability requirement of capacitor for new energy vehicles and smart grid, it is necessary to further reduce the thermal shrinkage rate of the PP film. Therefore, how to reduce the surface roughness and thermal shrinkage rate of the PP film by adjusting the components and process, and taking into account the mechanical properties and electrical properties, has become the research focus in the field. SUMMARY

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

[0010] To solve at least one of the above problems, the present application provides a preparation method of a PP film for capacitor with low thermal shrinkage rate, the preparation method comprising:

[0011] S100, respectively preparing an upper layer component, an intermediate layer component and a lower layer component, melt mixing each component, co-extruding, casting sheet forming, and obtaining a film sheet;

[0012] S200, bidirectional stretching and heat setting the film sheet to obtain a PP film;

[0013] The raw materials used in the upper layer component, the intermediate layer component and the lower layer component each include isotactic polypropylene, polytetrafluoroethylene and silica in-situ polymerized polystyrene.

[0014] In the above technical solution, the raw materials used in the upper layer component, the intermediate layer component and the lower layer component each include, based on 100 parts by mass of the total amount of raw materials: 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.

[0015] In the above technical solution, the thickness ratio of each layer in the PP film is upper layer thickness: intermediate layer thickness: lower layer thickness = (0.5-0.6):1:(0.5-0.6).

[0016] In the above technical solution, the temperature condition for melt mixing in S100 is 225 to 245 DEG C; and the temperature condition for casting sheet forming in S100 is 85 to 95 DEG C.

[0017] In the above technical solution, the multilayer film is preheated to 90-100 DEG C before each longitudinal stretching or each transverse stretching in S200; the stretching temperature of the longitudinal stretching in S200 is 145-155 DEG C, and the stretching ratio is 3-4 times; the stretching temperature of the transverse stretching in S200 is 165-170 DEG C, and the stretching ratio is 4-5 times; the temperature condition of heat setting is 125-130 DEG C.

[0018] In the above technical solution, the preparation method of the silica in-situ polymerized polystyrene comprises:

[0019] S310, using raw materials including an organic silicon source, an initiator, an acidic catalyst, an emulsifier, ethanol, and water to prepare a silica sol;

[0020] S320, using raw materials including a styrene monomer, amino (2) -four-arm-polyethylene glycol-dopamine (2), and ethyl acetate to prepare a monomer dispersion emulsion;

[0021] S330, under a protective atmosphere, gradually adding the silica sol into the monomer dispersion emulsion and mixing and stirring, heating and continuously stirring after the addition is completed;

[0022] S340, after the reaction is completed, cooling, demulsification, separation of solid, washing, drying, and obtaining the silica in-situ polymerized polystyrene.

[0023] In the above technical solution, the organic silicon source is ethyl silicate, the initiator is azobisisobutyronitrile, the acidic catalyst is a 6vol% hydrochloric acid aqueous solution, and the emulsifier is Span 60; in S310, the mass ratio of the organic silicon 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 the 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 the silica sol: monomer dispersion emulsion is 150: 100.

[0024] In the above technical solution, the raw materials used by the intermediate layer component include silica in-situ polymerized polystyrene A, and the raw materials used by the upper layer component and the lower layer component both include silica in-situ polymerized polystyrene B; wherein the raw material ratio used by the silica in-situ polymerized polystyrene A and the silica in-situ polymerized polystyrene B in the preparation process is different.

[0025] In the technical solution, in the preparation of the silica in-situ polystyrene A, in S310, the mass ratio of the organic silicon source, initiator, acidic catalyst, emulsifier, ethanol and water is 20:0.04:11.5:4:100:30; in the preparation of the silica in-situ polystyrene B, in S310, the mass ratio of the organic silicon source, initiator, acidic catalyst, emulsifier, ethanol and water is 30:0.04:8.5:4:100:40; in the preparation of the silica in-situ polystyrene A, in S320, the mass ratio of the styrene monomer, amino (2) -four-arm-polyethylene glycol-dopamine (2) and ethyl acetate is 12:0.75:100; in the preparation of the silica in-situ polystyrene B, in S320, the mass ratio of the styrene monomer, amino (2) -four-arm-polyethylene glycol-dopamine (2) and ethyl acetate is 6:0.75:100.

[0026] The application further provides a PP film with low thermal shrinkage for capacitors, which is obtained by the preparation method in any of the above technical solutions.

[0027] Advantages

[0028] The preparation method of the application first prepares the upper layer component, the middle layer component and the lower layer component respectively, melts and mixes each component, co-extrudes, and forms a cast sheet to obtain a film; and then bidirectionally stretches and heat sets the film to obtain the PP film. In the application, the raw materials of the upper layer component, the middle layer component and the lower layer component all include isotactic polypropylene, polytetrafluoroethylene and silica in-situ polystyrene. The polystyrene has high insulation resistance and low dielectric loss, which can improve the dielectric properties and charge storage capacity of the PP film. In addition, the polystyrene can optimize the interface structure of the PP film and improve the mechanical strength of the PP film. The silica can accelerate the crystallization of PP, enhance the tensile strength and elongation at break of the film. Moreover, the silica can give the PP film a certain roughness and improve the adhesion between the PP film and metal. Finally, the application in-situ polymerizes the silica and the styrene monomer, so that the polystyrene coats and fixes the silica to form a physical barrier by using the polystyrene molecular chain to prevent the mutual contact and aggregation of silica particles by forming steric hindrance, so that the silica is not easy to aggregate and form secondary particles in the processes of melting, casting and stretching of the PP film, thereby ensuring that the PP film has low roughness and thermal shrinkage. BRIEF DESCRIPTION OF DRAWINGS

[0029] Figure 1 FT-IR infrared absorption spectrum of the silica in-situ polystyrene sample 2. DETAILED DESCRIPTION

[0030] In order to make the above objectives, characteristics and advantages of the present application more apparent, more comprehensible, the following will make a detailed description in combination with specific embodiments of the present application.

[0031] Unless otherwise specified, the reagents and raw materials used in the present application can be purchased through commercial channels. The experimental methods in the following examples without specific conditions are selected according to conventional methods and conditions, or according to the product instructions.

[0032] The main purpose of the present application is to provide a preparation method of a PP film for capacitor with low heat shrinkage. The preparation method of the PP film for capacitor with low heat shrinkage provided by the present application comprises:

[0033] S100, respectively preparing an upper layer component, a middle layer component and a lower layer component, melt mixing each component, co-extruding, casting sheet forming, and obtaining a film piece;

[0034] S200, bidirectional stretching and heat setting the film piece to obtain a PP film;

[0035] The raw materials used in the upper layer component, the middle layer component and the lower layer component all include isotactic polypropylene, polytetrafluoroethylene and silica in-situ polymerized polystyrene.

[0036] For the PP film for capacitor, good electrical properties, high tensile strength, good elongation at break, reasonable roughness and low heat shrinkage are all crucial.

[0037] The PP film of the present application takes polypropylene as the main raw material and adds 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 is especially helpful to improve the stability of the PP film in high temperature environment, reducing the fluctuation of capacitor performance caused by temperature change. In addition, the introduction of polystyrene can optimize the interface structure of the PP film, reduce its interlayer voids and defects, thereby improving the mechanical strength of the PP film, especially the toughness.

[0038] The PP film of the present application also adds polytetrafluoroethylene, which has a high melting point (about 327℃), which helps to improve the thermal stability of the PP film, and the inertness of polytetrafluoroethylene can also enhance the corrosion resistance and chemical stability of the PP film.

[0039] Finally, the PP film of the present application also adds silica. The silica is specifically porous nanosilica particles, which as nucleation centers can accelerate PP crystallization, enhance the tensile strength and elongation at break of the film. In addition, silica can impart a certain roughness to the PP film, improving the adhesion between the PP film and the metal. Finally, since silica can anchor the amorphous segments of polypropylene and inhibit the transformation of beta crystals to alpha crystals under high temperature conditions. Therefore, the addition of silica not only leads to an increase in the roughness of the PP film, but also increases the thermal shrinkage of the PP film, reducing its dimensional stability at high temperatures. Therefore, the present application also aims to regulate the preparation process of silica to achieve the purpose of controlling the PP film to have reasonable roughness and reducing its thermal shrinkage.

[0040] Regarding the preparation process of silica, one of the improvements of the present application over the prior art is to use in-situ polymerization to fix silica and polystyrene, in order to control and reduce the roughness of the PP film and reduce its thermal shrinkage.

[0041] Specifically, the silica added to the PP film is porous nanosilica, which can be prepared by sol-gel method, hydrothermal method, etc. The reason why silica causes the thermal shrinkage of the PP film to increase is that during the processing of the PP film, nanoscale silica is prone to agglomeration due to its high surface energy, resulting in the formation of secondary particles. This phenomenon leads to an increase in the actual particle size of silica. Although this phenomenon can improve the roughness of the PP film and its adhesion to metal, it will cause the thermal shrinkage of the PP film to increase.

[0042] Taking porous silica with a particle size of less than 100 nanometers as an example, in theory, by adding porous silica with a particle size of less than 100 nanometers, the roughness of the PP film can be controlled between 0.05 microns and 0.1 microns, and at the same time, the PP film is given good dimensional stability. However, in fact, in the prior art, the use of silica with a particle size of less than 100 nanometers tends to cause a sudden increase in the roughness of the PP film and a significant increase in the thermal shrinkage. This is because silica itself has a loose and porous structure, with a large specific area, small particle size and high surface energy, so smaller silica particles are also more prone to agglomeration in the polymer matrix. Agglomeration of silica leads to the formation of secondary particles and uncontrolled size increase. Therefore, it is difficult to achieve a PP film with a roughness of 0.05 microns to 0.1 microns and a low thermal shrinkage by adding silica in the prior art.

[0043] To solve the above problems, the present application uses silica in-situ polymerized polystyrene as one of the raw materials. In addition to taking advantage of the dielectric properties of polymerized polystyrene and the positive effect of silica on the adhesion of PP film metal, it can also solidify silica in polystyrene. By adding this silica in-situ polymerized polystyrene material to polypropylene, the effect achieved is that the polystyrene molecular chain can form a physical barrier to prevent the aggregation of silica particles through steric hindrance to avoid silica agglomeration and prevent the formation of secondary particles, ensuring that the PP film has lower roughness and thermal shrinkage.

[0044] In the present application, the PP film is prepared by a multi-layer co-extrusion process. Multi-layer co-extrusion is a molding process that simultaneously extrudes different molten resins through multiple extruders, merges them in layers through a composite die (such as a co-extrusion die, a layering die), and finally forms a multi-layer composite film or sheet.

[0045] It can be understood that in the prior art, a single-layer homogeneous PP film can also be directly prepared by a non-multi-layer co-extrusion process. The purpose of using a multi-layer co-extrusion process in the present application is to obtain a heterogeneous PP film with different properties in each layer. Each layer uses isotactic polypropylene, polytetrafluoroethylene, and silica in-situ polymerized polystyrene as the main raw materials, and the addition ratio of the above-mentioned raw materials in each layer is the same or close.

[0046] Exemplarily, taking the total amount of raw materials as 100 parts by mass, the raw materials used in 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; antioxidant, 0.5 to 1.5 parts by mass. Preferably, in the preferred embodiments of the present application, the preparation process of the silica in-situ polymerized polystyrene in each layer is different. The above-mentioned differences can be reflected in the different types of raw materials or solvents, the addition ratio of raw materials or solvents, and process parameters such as heating and stirring. The purpose of using the above-mentioned process differences is to prepare silica in-situ polymerized polystyrene with different properties, so that the properties of each layer in the PP film are similar but also different.

[0047] 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.

[0048] Further preferably, the preparation process can be adjusted so that the silica 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 have a larger roughness, ensuring that the PP film has a better bonding force with the metal layer and better electrical performance while ensuring that the overall heat shrinkage of the PP film is low.

[0049] Preferably, the thickness ratio of the layers in the PP film is upper layer thickness: intermediate layer thickness: lower layer thickness = (0.5-0.6):1:(0.5-0.6). It should be noted that the thickness of the PP film is usually in the range of 2-50 microns, and under the premise of following the above thickness ratio of the layers, the total thickness of the PP film can be adjusted according to actual needs by those skilled in the art.

[0050] Overall, the use amount of styrene monomer is reduced when the silica used in the preparation of the intermediate layer and the upper and lower surface layers is in-situ polymerized polystyrene. Unlike the technical solution that aims to achieve uniform mixing of silica and polystyrene with similar or small differences in content, the present application aims to form a polystyrene film layer on the surface of small-sized nanosilica sol particles to limit the mutual contact and van der Waals attraction of silica by steric hindrance. Therefore, the present application uses less styrene monomer and has a lower concentration, and uses a mixture containing silica sol particles and emulsifiers and initiators as the water phase, uses styrene monomer to prepare the oil phase, and obtains a water-in-oil emulsion by dropping the dispersed water phase into the continuous oil phase. After obtaining the emulsion, a heating polymerization reaction is performed to coat the in-situ polymerized polystyrene on the surface of the silica particles, thereby adjusting the overall roughness and heat shrinkage of the PP film.

[0051] It should also be noted that increasing the extrusion temperature, reducing the cast sheet forming temperature, avoiding excessively high quenching temperature, increasing the temperature of transverse and longitudinal stretching, and increasing the heat setting temperature also help to reduce the heat shrinkage of the PP film. Therefore, the above process parameters are also adjusted in the present application.

[0052] Specifically, the present application uses higher melt mixing temperature, stretching temperature, and heat setting temperature, and uses a lower cast sheet forming temperature. Preferably, the temperature condition for melt mixing in S100 is 225-245℃. The temperature condition for cast sheet forming in S100 is 85-95℃. In S200, the multilayer film is preheated to 90-100℃ before each longitudinal stretching or each transverse stretching; the stretching temperature for longitudinal stretching in S200 is 145-155℃, and the stretching ratio is 3-4 times; the stretching temperature for transverse stretching in S200 is 165-170℃, and the stretching ratio is 4-5 times; the temperature condition for heat setting is 125-130℃.

[0053] In the present application, the silica is prepared by sol-gel method, and the polystyrene is formed on the surface of the silica nanoparticles by in-situ polymerization. In order to ensure the in-situ polymerization of polystyrene on the silica nanoparticles to obtain the core-shell structure silica with polystyrene as the shell, the present application uses silica sol as the water phase and styrene monomer as the oil phase, drops the dispersed water phase into the continuous oil phase, continuously stirs, obtains the water-in-oil emulsion, and heats the emulsion to make the styrene polymerize on the surface of the silica nanoparticles.

[0054] Specifically, the preparation method of the silica in-situ polymerized polystyrene comprises:

[0055] S310, preparing silica sol by using raw materials including an organic silicon source, an initiator, an acidic catalyst, an emulsifier, ethanol, and water;

[0056] S320, preparing monomer dispersion emulsion by using raw materials including styrene monomer, amino (2)-four-arm-polyethylene glycol-dopamine (2), and ethyl acetate;

[0057] S330, gradually dropping the silica sol into the monomer dispersion emulsion and mixing and stirring under a protective atmosphere, heating and continuously stirring after the dropping is completed, and reacting;

[0058] S340, after the reaction is completed, cooling, demulsification, separating the solid, washing, drying, and obtaining the silica in-situ polymerized polystyrene.

[0059] In S310, the mass ratio of the organic silicon source, the initiator, the acidic catalyst, the emulsifier, the ethanol, and the water is (20-30):(0.02-0.06):(8-12):(2-6):100:(30-40);

[0060] In S320, the mass ratio of the styrene monomer, the amino (2)-four-arm-polyethylene glycol-dopamine (2), and the ethyl acetate is (6-12):(0.5-1):100;

[0061] In S330, the mass ratio of the silica sol and the monomer dispersion emulsion is 150:100.

[0062] In the above steps, the organic silicon source is ethyl silicate, the initiator is azobisisobutyronitrile, the acidic catalyst is 6vol% hydrochloric acid aqueous solution, and the emulsifier is Span 60. The reason for using azobisisobutyronitrile as the initiator is that it belongs to an organic initiator and is not sensitive to pH. The reason for using Span 60 as the emulsifier is that it belongs to a typical water-in-oil emulsifier, and the stearate chain gives Span 60 strong lipophilicity, which can effectively reduce the oil-water interfacial tension and form a stable water-in-oil interfacial film. It can be understood that there are many types of organic silicon sources, such as methyl silicate. The present application takes ethyl silicate as an example to explain the proportion relationship of each raw material. It should be noted that when preparing silica by sol-gel method, both acidic catalyst and basic catalyst can promote the formation of silica sol. However, the acidic catalyst helps to form silica sol with smaller particle size and more uniform particle size, so the acidic catalyst is used in the present application.

[0063] In the present application, the silica sol is used as the water phase, and the monomer dispersion emulsion is used as the oil phase. Before the water phase as the dispersed phase is added into the oil phase as the continuous phase, a buffer such as sodium bicarbonate is preferably prepared into an aqueous solution of about 10wt% to 12wt% to adjust the pH value of the silica sol to neutral.

[0064] It can be understood that the styrene monomer used in the present application is preferably washed with, for example, an aqueous solution of sodium hydroxide of about 4wt% to 6wt% to remove, for example, the polymerization inhibitor hydroquinone, and is ready for use after washing to neutral.

[0065] Ethyl acetate is used as the solvent of the oil phase, and amino (2)-tetra-armed-polyethylene glycol-dopamine (2) is used as the modifier. With pentaerythritol as the core, the terminal of each arm is hydroxyl, and after modification, two amino groups and two dopamine groups are connected as functional groups. The above structure makes amino (2)-tetra-armed-polyethylene glycol-dopamine (2) have good hydrophilic performance, reactivity, and can help to realize the combination of styrene and silica particles. Specifically, the dopamine group can improve the hydrophilic performance of the oil phase containing styrene monomer, promote the uniform dispersion of the water phase in the oil phase, and improve the emulsification uniformity. In the process of preparing silica by sol-gel method, the dissociation of silicon hydroxyl group causes the hydrogen ion to be released from the surface of silica. The four alcohol hydroxyl groups of amino (2)-tetra-armed-polyethylene glycol-dopamine (2) in the oil phase can attract each other by 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.

[0066] As mentioned above, the multi-layer co-extrusion process can obtain a non-homogeneous PP film with different properties of each layer. In the present application, the raw materials used in the middle layer component include silica in-situ polystyrene A, and the raw materials used in the upper layer component and the lower layer component include silica in-situ polystyrene B; wherein the raw material ratio used in the preparation of silica in-situ polystyrene A and silica in-situ polystyrene B is different.

[0067] Preferably, in the preparation of silica in-situ polystyrene A, in S310, the mass ratio of organic silicon source: initiator: acidic catalyst: emulsifier: ethanol: water = 20:0.04:11.5:4:100:30; in the preparation of silica in-situ polystyrene B, in S310, the mass ratio of organic silicon source: initiator: acidic catalyst: emulsifier: ethanol: water = 30:0.04:8.5:4:100:40; in the preparation of silica in-situ polystyrene A, in S320, the mass ratio of styrene monomer: amino (2) -four-arm-polyethylene glycol-dopamine (2): ethyl acetate = 12:0.75:100; in the preparation of silica in-situ polystyrene B, in S320, the mass ratio of styrene monomer: amino (2) -four-arm-polyethylene glycol-dopamine (2): ethyl acetate = 6:0.75:100. The test results of the present application show that the roughness and thermal shrinkage of the PP film are affected by the raw material ratio of the silica in-situ polystyrene. Among them, by adjusting the ratio of organic silicon source and acidic catalyst in the preparation of aqueous phase, and adjusting the addition amount of styrene monomer in the preparation of oil phase, the roughness and thermal shrinkage of the PP film can be directly controlled. Therefore, by adjusting the raw material ratio of the silica in-situ polystyrene during preparation, the relatively thick middle layer can maintain a relatively low roughness and thermal shrinkage, and the relatively thin upper and lower layers have a relatively high roughness and metal bonding capacity. Therefore, the present application can reduce the thermal shrinkage of the PP film while considering its metal adhesion, so as to ensure that the PP film has good electrical properties.

[0068] The preparation method of the silica in-situ polystyrene and the PP film of the present application will be described in the form of specific examples. In the raw materials used in the present application, amino (2) -four-arm-polyethylene glycol-dopamine (2) is purchased from Xi'an Kaixin Biological Technology, styrene monomer is purchased from Shandong Yaojia Chemical Industry, and the rest of the raw materials are purchased from Shanghai National Medicine.

[0069] Example One

[0070] A series of silica in-situ polystyrene samples are prepared in this example. The raw material ratio and process parameters of this example are shown in Table 1, and the preparation method is as follows.

[0071] S1, according to the proportion of Table 1, the silicon ethylate is weighed, mixed with ethanol and water, and then azobisisobutyronitrile and Span 60 are added and mixed uniformly, and then the water bath is kept at 30°C, then 6vol% hydrochloric acid aqueous solution is added dropwise at a rate of 2mL / min, and stirring is carried out at a stirring speed of 300 rpm, after the dropwise addition is completed, stirring is continued for 40min, and then the sample is left to stand for 2h, then sodium bicarbonate is added to adjust the pH value to neutral, and a silica sol is obtained;

[0072] S2, according to the proportion of Table 1, the styrene monomer (with removed inhibitor) is weighed, mixed with ethyl acetate, and then amino (2)-tetra-arm-polyethylene glycol-dopamine (2) is added and mixed uniformly, and then the mixture is homogenized in a homogenizer at a speed of 2000 rpm for 15min, to obtain a monomer dispersion emulsion;

[0073] S3, the monomer dispersion emulsion in the homogenizer is kept in a 30°C constant temperature water bath, and the silica sol is gradually added into the monomer dispersion emulsion at a rate of 1.5mL / min, and simultaneously homogenized at a speed of 100 rpm, after the dropwise addition is completed, the temperature of the constant temperature water bath is increased to 75°C under a protective atmosphere, the homogenization speed is increased to 2000 rpm, and the constant temperature reaction is carried out for 4.5h;

[0074] S4, after the reaction is completed, the sample is naturally cooled to room temperature, 5wt% calcium chloride aqueous solution is added and stirred to break the emulsion, after the emulsion breaking is completed, the solid is centrifuged, washed with ethanol, washed with water, and dried in a vacuum oven at 55°C for 12h, to obtain a silica in-situ polymerized polystyrene sample.

[0075] Table 1

[0076]

[0077] Comparative Example One

[0078] A series of silica in-situ polymerized polystyrene samples are prepared in this comparative example, the raw material ratio and process parameters of this comparative example are shown in Table 2, the raw materials are selected and prepared in the same way as in Example 1, the only difference is that no amino (2)-tetra-arm-polyethylene glycol-dopamine (2) is used in the samples of this comparative example, or other raw materials are used instead of amino (2)-tetra-arm-polyethylene glycol-dopamine (2).

[0079] Table 2

[0080]

[0081] Example Two

[0082] A series of PP film samples are prepared in this example, and the preparation method is as follows.

[0083] S1, mass ratio of isotactic polypropylene: polytetrafluoroethylene: silica in-situ polymerized polystyrene: polypropylene wax: antioxidant 1010 = 82:6:11:0.5:0.5, the raw materials of each layer are mixed and prepared into upper layer component, middle layer component and lower layer component respectively, wherein the silica in-situ polymerized polystyrene used in each layer component is from Table 3;

[0084] S2, the raw materials of each layer are added into a high-speed mixer at 110°C and mixed for 15 minutes, and then extruded and melted through a twin-screw extruder (screw rotation speed 120 rpm), the temperature conditions of the melted extrusion are 180°C (feeding section)-225°C (homogenization section)-245°C (die). The melted materials of each layer are extruded into a sheet-shaped fluid through the die of the twin-screw extruder (die lip gap 1.1 mm), and the thickness ratio of each layer is controlled to be upper layer thickness: middle layer thickness: lower layer thickness = 0.5:1:0.5;

[0085] S3, the sheet-shaped fluid is cast into a sheet through a quenching roller at 85°C and a high-pressure air knife (pulling speed 10 m / min), and a film sheet is obtained;

[0086] S4, the film sheet is preheated to 90°C (preheating time 35 s), and is stretched longitudinally at a stretching ratio of 3.5 times at 150°C by using a two-way stretching machine (stretching rate 250% per second), and then the film sheet is preheated to 90°C again after being cooled to room temperature, and is stretched transversely at a stretching ratio of 4.5 times at 170°C (stretching rate 250% per second). The film sheet is heat set at 130°C for 25 s, and is naturally cooled and wound to obtain a PP film sample, and the thickness of the PP film sample is 4.4 microns (upper and lower error 3%).

[0087] Table 3

[0088]

[0089] Performance test

[0090] The infrared absorption spectra of the silica in-situ polymerized polystyrene samples 1 to 12 are determined by using a Fourier transform infrared spectrometer (FTIR), and potassium bromide is used for tabletting, and the scanning wavelength range is 400 to 4000 cm −1 . Figure 1 The infrared absorption spectrum of the silica in-situ polymerized polystyrene sample 2 is shown in the figure, and the infrared absorption spectra of the other samples have similar absorption peaks to the infrared absorption spectrum of the silica in-situ polymerized polystyrene sample 2, so only the infrared absorption spectrum of the silica in-situ polymerized polystyrene sample 2 is shown. In the infrared absorption spectrum of the silica in-situ polymerized polystyrene sample, the anti-symmetrical stretching vibration peak of the silicon-oxygen-silicon bond at 1100 cm −1 , and the symmetrical stretching vibration peak of the silicon-oxygen-silicon bond at 460 cm −1The symmetric stretching vibration peak of the siloxane bond can also be observed at 760 cm −1 , 1500 cm −1 , 760 cm −1 near the polystyrene characteristic peak. This proves that there is silica and polystyrene in the sample. In addition, a wide peak of primary amino group stretching vibration from 3300 cm-1 to 3500 cm-1 can also be observed, which indicates that the sample 2 of silica in-situ polymerized polystyrene contains amino groups (2) - four-arm-polyethylene glycol-dopamine (2).

[0091] The surface roughness, thermal shrinkage (120℃ / 10min), tensile strength, and elongation at break of the PP film samples 1 to 12 were tested, and the test results are shown in Table 4. The test results show that the roughness of the test PP film samples 1 to 12 obtained by the present application is low. Among them, the roughness of the PP film samples 9 to 12 is relatively high, which is due to the uneven coating of polystyrene, resulting in a small amount of aggregation of part of the silica. The thermal shrinkage MD of the PP film samples 1 to 8 can be controlled to be less than 5.5%, and the thermal shrinkage TD can be controlled to be less than 0.5%. The thermal shrinkage MD of the PP film sample 2, the PP film sample 3, the PP film sample 4, and the PP film sample 8 can be controlled to be less than 4.7%, and the thermal shrinkage TD can be controlled to be less than 0.4%. Among them, the thermal shrinkage of the PP film sample 3 is the lowest. The thermal shrinkage of the PP film samples 8 to 12 is relatively high, which is also due to the uneven coating of polystyrene, resulting in a small amount of aggregation of part of the silica. There is little difference in tensile strength and elongation at break among the samples. The tensile strength MD can exceed 200MPa, the tensile strength TD can exceed 250MPa, the elongation at break MD can exceed 60%, and the elongation at break TD can exceed 40%.

[0092] Table 4

[0093]

[0094] The surface of the PP film samples 1 to 12 was evaporated with a metal aluminum layer to obtain metal film samples 1 to 12. The vacuum evaporation conditions include: the vacuum degree is not more than 2×10 -5 Pa, the evaporation rate is 0.08A / s, the evaporation time is 220s, the substrate temperature is 140℃, the wire feeding speed is 800mm / min, and the evaporation boat temperature is 650℃. The breakdown strength of the metal film samples 1 to 12 was tested, and the test results are shown in Table 5. The test results show that the breakdown strength of the metal film samples 1 to 12 obtained by the present application can exceed 625 V / μm.

[0095] Table 5

[0096]

[0097] Although the present application has been disclosed with reference to various implementations, it is understood that equivalents can be employed and substitutions made herein without departing from the spirit and scope of the application as defined in the following claims.

Claims

1. A method for preparing a PP film for capacitors with low thermal shrinkage, characterized in that, The preparation method includes: S100: Prepare the upper layer component, middle layer component and lower layer component separately, melt and mix them separately, co-extrude them, and cast them into sheets to obtain a film. S200: The film is biaxially stretched and heat-set to obtain a PP film; The raw materials used in the upper layer component, the middle layer component, and the lower layer component all include isotactic polypropylene, polytetrafluoroethylene, and in-situ polymerized polystyrene with silica. Based on a total raw material quantity of 100 parts by mass, the raw materials used for the upper layer component, the middle layer component, and the lower layer component each respectively include: The isotactic polypropylene, 75 to 85 parts by weight; The polytetrafluoroethylene, 5 to 8 parts by weight; The in-situ polymerized polystyrene from silica, 8 to 12 parts by weight; Dispersant, 0.5 to 2 parts by weight; Antioxidant, 0.5 to 1.5 parts by weight; The method for preparing the in-situ polymerized polystyrene from silica includes: S310. Silica sol is prepared using raw materials including organosilicon source, initiator, acid catalyst, emulsifier, ethanol, and water. S320. A monomer dispersion emulsion is prepared using raw materials including styrene monomer, amino(2)-tetraarm-polyethylene glycol-dopamine(2), and ethyl acetate. S330. Under a protective atmosphere, the silica sol is gradually added dropwise into the monomer dispersion emulsion and mixed and stirred. After the addition is complete, the mixture is heated and stirred continuously to react. S340. After the reaction is complete, the temperature is lowered, the emulsion is broken, the solids are separated, washed, and dried to obtain the in-situ polymerized polystyrene from silica. In S310, by mass ratio, the ratio of organosilicon source: initiator: acid catalyst: emulsifier: ethanol: water = (20-30): (0.02-0.06): (8-12): (2-6): 100: (30-40); In S320, the mass ratio of styrene monomer: amino(2)-tetraarm-polyethylene glycol-dopamine(2):ethyl acetate is (6-12):(0.5-1):100; In S330, the mass ratio of silica sol to monomer dispersion emulsion is 150:

100.

2. 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).

3. The preparation method according to claim 1, characterized in that, The temperature conditions for melt mixing described in S100 are 225°C to 245°C; The temperature conditions for casting in S100 are 85°C to 95°C.

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

5. The preparation method according to claim 1, characterized in that, The organosilicon source is ethyl silicate, the initiator is azobisisobutyronitrile, the acidic catalyst is a 6 vol% aqueous hydrochloric acid solution, and the emulsifier is Span 60.

6. The preparation method according to claim 5, characterized in that, The intermediate layer component uses in-situ polymerized polystyrene A made from silica, and the upper and lower layer components both use in-situ polymerized polystyrene B made from silica. The proportions of the in-situ polymerized polystyrene A and the in-situ polymerized polystyrene B are different during the preparation process.

7. The preparation method according to claim 6, characterized in that, In the preparation of the silica in-situ polymerized polystyrene A, in S310, the mass ratio of organosilicon source: initiator: acid catalyst: emulsifier: ethanol: water is 20:0.04:11.5:4:100:30; In the preparation of the silica in-situ polymerized polystyrene B, in S310, the mass ratio of organosilicon source: initiator: acid catalyst: emulsifier: ethanol: water is 30:0.04:8.5:4:100:

40. In the preparation of the silica in-situ polymerized polystyrene A, in S320, the mass ratio of styrene monomer: amino(2)-tetraarm-polyethylene glycol-dopamine(2):ethyl acetate is 12:0.75:100; In the preparation of the silica in-situ polymerized polystyrene B, in S320, the mass ratio of styrene monomer: amino(2)-tetraarm-polyethylene glycol-dopamine(2):ethyl acetate is 6:0.75:

100.

8. A PP film for capacitors with low heat shrinkage, characterized in that, The PP film is obtained by the preparation method as described in any one of claims 1 to 7.

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