A method for making a multilayer composite film for a capacitor

Abstract

The application belongs to the technical field of capacitor film preparation, and particularly relates to a preparation method of a multilayer composite structure capacitor film. The film has a three-layer composite structure of a surface layer / a core layer / a surface layer, the surface layer material comprises high isotactic polypropylene, the core layer material comprises an electrical grade polypropylene, a bimodal polypropylene and an amino-terminated hyperbranched polyamide-amine with a specific structure. By adopting a multilayer co-extrusion casting, longitudinal stretching and segmented transverse stretching process, and performing high-temperature stress relaxation and segmented cooling solidification after transverse stretching, the film has a low high-temperature thermal shrinkage rate and a high high-temperature dielectric breakdown strength.

Description

Technical Field

[0001] This invention belongs to the field of thin film preparation technology for capacitors, specifically a method for preparing thin films for multilayer composite structure capacitors. Background Technology

[0002] With the rapid development of industries such as new energy vehicles, high-voltage power transmission and transformation, and photovoltaic power generation, film capacitors, as core energy storage components, are increasingly being applied in high-temperature, high-voltage, and high-energy-density environments. Biaxially oriented polypropylene (POP) film, due to its excellent dielectric strength, low dielectric loss, and good self-healing properties, has become the most widely used capacitor dielectric material. However, in practical industrial applications, existing POP films for capacitors still have the following problems: 1. Severe degradation of electrical insulation performance at high temperatures. In environments exceeding 105°C, the mobility of free electrons in conventional POP films increases dramatically, leading to a rapid decrease in breakdown strength, making it difficult to meet the long-term stable operation requirements at 120°C or even 150°C. Although existing technologies modify films by adding inorganic fillers, this often introduces impurities and can easily lead to micropores or tearing during the stretching process. 2. Controlling the surface morphology and roughness of the film is difficult. To ensure venting performance during capacitor winding and improve the impregnation effect of oil-immersed capacitors, a certain degree of roughness is required on the film surface. Existing casting and quenching processes typically use lower quenching temperatures, which, while beneficial for processing, result in overly smooth film surfaces and make it difficult to achieve precise control over the surface morphology while maintaining mechanical properties. 3. Poor synergy between material components and multilayer structures. Existing multilayer composite films often simply stack materials with different properties, but the compatibility between these materials is poor, making it difficult to improve overall performance. Chinese Patent CN119348120B discloses a method for preparing polypropylene film for film capacitors. The method involves melting, extruding, and cooling polypropylene film raw materials to obtain a cast sheet, followed by post-treatments such as simultaneous stretching, transverse stretching, secondary transverse stretching, secondary simultaneous stretching, heat setting, and corona treatment. The resulting film has uniform thickness, high longitudinal tensile strength, high transverse elongation, and low thermal shrinkage. However, the research on how to achieve a multilayer composite capacitor film that can balance high-temperature voltage resistance, ideal surface roughness, low thermal shrinkage, and good processing stability remains a key focus for researchers. Summary of the Invention

[0003] To address at least one of the above problems, the present invention provides a method for preparing a thin film for a multilayer composite capacitor, comprising the following steps: S1. The surface material uses raw materials including high isotactic polypropylene, and the core material uses raw materials including electrical grade polypropylene, bimodal polypropylene, and amino-terminated hyperbranched polyamide-amine. The surface material and the core material are added to a multi-layer co-extrusion casting equipment, extruded through an A / B / A co-extrusion distribution valve and a die, and then attached to a quenching roller for cooling and curing to obtain an initial casting sheet with a three-layer composite structure. S2. The initial casting is fed into a longitudinal stretching machine, preheated at 105-115℃, and then longitudinally stretched at 115-125℃, with a total longitudinal stretching ratio of 4.5-5.0 times. S3. The film is then fed into a transverse stretching machine. The transverse stretching is carried out in two stages: in the first stretching stage, the film is stretched laterally at a speed of 300% / s-400% / s at 150-155℃ to 60% of the total width; in the second stretching stage, the heating device is activated to raise the surface temperature of the film to 160-165℃, and the film continues to be stretched laterally at a speed of 80% / s-120% / s to a total transverse stretching ratio of 9.8-10.2 times. S4. After the transverse stretching is completed, control the film surface temperature to be maintained at 155-162℃. At the same time, control the two ends of the transverse stretching machine to retract and shrink inward, so that the film undergoes 9.0%-10.0% stress relaxation and shrinkage in the transverse direction. Maintain the temperature for 10-15 seconds, and then carry out segmented cooling curing and corona treatment. Finally, rewind to obtain the final product.

[0004] Furthermore, the thickness proportions of each layer of surface layer A, core layer B, and another surface layer A are 14%-16%, 68%-72%, and 14%-16%, respectively.

[0005] Furthermore, the high isotactic polypropylene has an isotacticity index greater than 98%, ash content < 10 ppm, and melt flow rate of 2.0-3.0 g / 10 min; the electrical grade polypropylene has an isotacticity index of 94%-96%, ash content < 20 ppm, and melt flow rate of 2.5-3.5 g / 10 min.

[0006] Furthermore, in the bimodal polypropylene, the high molecular weight component Mw > 650,000 and the low molecular weight component Mw < 80,000.

[0007] Furthermore, the surface material also includes an electrical-grade antioxidant, and the core material also includes an electrical-grade antioxidant and a compatibilizer.

[0008] Furthermore, in step S1, the surface temperature of the quenching roller is 85-90℃, and during casting, hot air at a temperature of 70-75℃ is continuously sprayed onto the outer surface of the film away from the quenching roller using an air knife, with an air pressure of 0.2-0.4MPa.

[0009] Furthermore, in step S4, the segmented cooling and curing process is as follows: first, the temperature is lowered to 130-135℃, and a relaxation of 2%-3% is maintained; then, the temperature is lowered to 100-105℃, and finally, the temperature is allowed to cool naturally to room temperature.

[0010] Furthermore, the terminal amino hyperbranched polyamide-amine is prepared by polycondensation reaction using raw materials including pentaerythritol, maleic anhydride and diethylenetriamine.

[0011] Further, the specific preparation process of the terminal amino hyperbranched polyamide-amine is as follows: pentaerythritol and maleic anhydride are added to a reaction vessel, the temperature is raised to 125-135℃ under an inert atmosphere, and the reaction is carried out at a constant temperature for 3-4 hours. The temperature is then lowered to 75-85℃, and diethylenetriamine is slowly added to it. After the addition is completed, the temperature is raised to 165-175℃ and a vacuum is drawn until the pressure is below 0.05MPa. The reaction is carried out for 4-6 hours. The material is poured out while hot, cooled, and then ground to obtain the terminal amino hyperbranched polyamide-amine.

[0012] A thin film for a multilayer composite capacitor is prepared by the preparation method of a thin film for a multilayer composite capacitor as described in any of the above technical solutions, wherein the total thickness of the thin film is 3.2-6 μm.

[0013] The present invention has the following beneficial effects: By adding terminal amino-terminated hyperbranched polyamide-amine to the core layer material, the numerous polar groups it contains form a high-density charge trap in the polypropylene matrix, effectively capturing free electrons migrating under high-temperature conditions, thereby suppressing leakage current. Furthermore, compared to the low-temperature quenching process in existing technologies, this invention employs a high-temperature quenching roller and hot air knife system. By controlling the heating of the melt on both sides, it effectively induces the homogenization of the polypropylene crystal morphology, resulting in a film with suitable surface roughness, which significantly improves the venting performance of the film during capacitor winding. A two-stage stretching process is used in the transverse stretching process, combined with multi-stage stress relaxation and segmented cooling. By activating the heating device in the middle of the oven to increase the film surface temperature and combining it with a slowed expansion rate, the residual stress inside the film is released, allowing the film to maintain a low thermal shrinkage rate even under high-temperature conditions. Detailed Implementation

[0014] The technical solutions in the embodiments of this application will be clearly and completely described below with reference to the embodiments of this application. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0015] With the rapid development of modern electronic power equipment, the long-term continuous operating temperature requirement for film capacitors, as basic energy storage and filtering components, has increased significantly from the traditional 85℃ to 105-125℃. Currently, BOPP capacitor films capable of stable operation under such extreme high temperature and high pressure environments rely entirely on expensive imported high-isotactic polypropylene granules. To reduce raw material costs, this invention provides a method for preparing a multilayer composite structure capacitor film, comprising the following steps: S1. The surface material includes high-isotactic polypropylene raw material, and the core material includes electrical grade polypropylene, bimodal polypropylene, and amino-terminated hyperbranched polyamide-amine raw material. The surface material and the core material are added to a multi-layer co-extrusion casting equipment, extruded through an A / B / A co-extrusion distribution valve and a die, and attached to a quenching roller for cooling and curing to obtain an initial casting sheet with a three-layer composite structure. S2. The initial casting is introduced into the longitudinal stretching machine, preheated at 105-115℃ and then longitudinally stretched at 115-125℃, with a total longitudinal stretching ratio of 4.5-5.0 times. S3. The film is then fed into a transverse stretching machine. The transverse stretching is carried out in two stages: in the first stretching stage, the film is stretched laterally at a speed of 300% / s-400% / s at 150-155℃ to 60% of the total width; in the second stretching stage, the heating device is activated to raise the surface temperature of the film to 160-165℃, and the film continues to be stretched laterally at a speed of 80% / s-120% / s to a total transverse stretching ratio of 9.8-10.2 times. S4. After the transverse stretching is completed, control the film surface temperature to be maintained at 155-162℃. At the same time, control the two ends of the transverse stretching machine to retract and shrink inward, so that the film undergoes 9.0%-10.0% stress relaxation and shrinkage in the transverse direction. Maintain the temperature for 10-15 seconds, and then carry out segmented cooling curing and corona treatment. Finally, rewind to obtain the final product.

[0016] Specifically, in step S1, the mass ratio of high-isotactic polypropylene, electrical-grade polypropylene, bimodal polypropylene, and amino-terminated hyperbranched polyamide-amine is 100:60-75:20-35:1-2; the thickness ratios of surface layer A, core layer B, and another surface layer A are 14%-16%:68%-72%:14%-16%, respectively; the high-isotactic polypropylene has an isotacticity index greater than 98%, ash content <10ppm, and melt flow rate of 2.0-3.0g / 10min (230℃, 2.16kg); the electrical-grade polypropylene has an isotacticity index of 94%-96%, ash content <20ppm, and melt flow rate of 2.0-3.0g / 10min (230℃, 2.16kg). The melt flow rate is 2.5-3.5 g / 10 min; the bimodal polypropylene, wherein the high molecular weight component Mw > 650,000 provides high melt tension and high tensile orientation potential; the low molecular weight component Mw < 80,000 is responsible for providing excellent shear thinning properties and processing lubrication; the surface material also includes an electrical grade antioxidant, wherein the electrical grade antioxidant is electrical grade antioxidant 1010; the core material also includes an electrical grade antioxidant and a compatibilizer, wherein the electrical grade antioxidant is electrical grade antioxidant 1010; the compatibilizer is maleic anhydride-grafted polypropylene, with a grafting rate of 1%-1.2%, a melt flow rate of 20-100 g / 10 min, and an ash content of < 200 ppm.

[0017] Multilayer co-extrusion cast film: The surface layer material and the core layer material are added separately to different extruders of the multilayer co-extrusion casting equipment, extruded through A / B / A co-extrusion distribution valves and dies, and then attached to a quenching roller with a surface temperature of 85-90℃ for cooling and solidification. During casting, hot air at a temperature of 70-75℃ and a pressure of 0.2-0.4MPa is continuously sprayed onto the outer surface of the film away from the quenching roller using an air knife, resulting in an initial cast film with a three-layer composite structure.

[0018] When the high-temperature melt is extruded from the die, if it is directly quenched, the polypropylene will mainly form an amorphous morphology, resulting in a lack of crystalline domain support and film breakage during stretching. If the cooling is too slow, coarse α-spherulites will form, causing severe grain boundary tearing during biaxial stretching. Therefore, in this step, the quenching roller temperature is set to a relatively high 85-90°C, while the outer surface is swept with a hot air knife at 70-75°C. This relatively gentle cooling rate induces the polypropylene melt to form a large number of small, uniformly distributed pseudo-hexagonal crystals or small, immature α-crystals. This nascent crystalline structure can significantly reduce stress concentration at grain boundaries during subsequent longitudinal and transverse stretching processes, accompanied by the orderly arrangement and slippage of molecular chains. This change in microstructure enables the cast sheet to have excellent ductility, ensuring a dense film.

[0019] Specifically, in step S2, longitudinal stretching: the initial casting is fed into a longitudinal stretching machine, first preheated by multiple sets of preheating rollers at 105-115℃, and then longitudinally stretched in one or more stages between stretching rollers at 115-125℃, with a total longitudinal stretching ratio of 4.5-5.0 times.

[0020] Specifically, in step S3, the film is stretched in two stages: it is then fed into the oven of the transverse stretching machine, and the transverse stretching is carried out in two stages: the first stretching stage is carried out at 150-155℃ with a high shear rate of 300% / s-400% / s to expand the film to 60% of the total width; the second stretching stage is carried out by starting the heating device to precisely raise the surface temperature of the film to 160-165℃, and continuing to expand the film laterally at a lower speed of 80% / s-120% / s until the total transverse stretching ratio reaches 9.8-10.2 times.

[0021] In step S2, longitudinal stretching aligns the molecular chains along the longitudinal direction. In step S3, transverse stretching, the first segment rapidly expands at 300%-400% / s at 150-155℃. This is because the longitudinally oriented crystal domains are very strong, requiring rapid dissociation of the folded chains under strong shear force to rearrange them laterally. When the expansion reaches 60%, the internal stress of the film is extremely accumulated; continued high-speed stretching would inevitably break the film. Therefore, the second segment increases the temperature to 160-165℃ and slows the speed to 80%-120% / s. At this point, the ultra-long entangled chains of the bimodal polypropylene high molecular weight component maintain melt strength, allowing the film to continue sliding and expanding to 9.8-10.2 times under localized micro-melting and low speed, achieving ultra-high transverse orientation and ensuring continuous production stability.

[0022] Specifically, in step S4, high-temperature stress relaxation and segmented cooling heat setting: After transverse stretching, the film enters the heat setting section. The film surface temperature is controlled to be maintained at 155-162℃, while the tracks at both ends of the transverse stretching machine are controlled to retract inward, causing the film to undergo 9.0%-10.0% stress relaxation and retraction in the transverse direction, and this temperature is maintained for 10-15 seconds. Subsequently, the film enters the segmented cooling curing section: first, it is cooled to the 130-135℃ region, while the tracks are controlled to continue to retract inward, so that the film maintains 2%-3% of the second stage of micro-relaxation; then, it is cooled to the 100-105℃ region to stabilize the molecular chains, and finally, it is drawn out by the lead-out roller and naturally cooled to room temperature. After that, the film is subjected to 12-14 W·min / m 2 The product is obtained by corona treatment with a corona roller followed by winding.

[0023] Due to the high tensile strength, a large amount of unbalanced elastic strain energy accumulates inside the film. Therefore, in step S4, the surface temperature is controlled at 160-165℃ and maintained for 10-15 seconds. During this process, the amorphous chains in the amorphous regions of the polymer and the binding molecules at the grain boundaries exhibit high mobility. At this time, causing the orbitals to actively retract inward by a large proportion (9.0%-10.0%) promotes the instantaneous untangling and local relaxation of these tensile amorphous chains, thereby greatly eliminating lateral internal stress without damaging the existing crystal orientation structure. Finally, through segmented cooling and natural curing, the crystallization is fully stabilized.

[0024] Specifically, the preparation process of the terminal amino hyperbranched polyamide-amine is as follows: Pentaerythritol, maleic anhydride, and diethylenetriamine are weighed in a molar ratio of 1:4:4.2-4.5; Pentaerythritol and maleic anhydride are added to a reaction vessel, and the temperature is raised to 125-135℃ under an inert atmosphere and reacted at a constant temperature for 3-4 hours. Then the temperature is lowered to 75-85℃, and diethylenetriamine is slowly added dropwise, controlling the dropwise addition process to be completed within 1-1.5 hours; After the addition is completed, the temperature is raised to 165-175℃ and a vacuum is drawn until the pressure is below 0.05MPa. The reaction is carried out for 4-6 hours to remove the polycondensation byproduct water. Finally, the material is poured out while hot, and after natural cooling, it is ground to a particle size of <45μm to obtain the terminal amino hyperbranched polyamide-amine.

[0025] In this process, pentaerythritol undergoes an esterification ring-opening reaction with maleic anhydride at 125-135℃ to form a star-shaped prepolymer with four-terminated carboxyl groups. When the temperature is lowered to 75-85℃, diethylenetriamine is slowly added dropwise. The amino groups at both ends of the diethylenetriamine molecule undergo an amidation reaction with the four-terminated carboxyl prepolymer, generating a branched macromolecule containing amide bonds and terminal amino groups. Cooling and slow dropwise addition effectively control the self-polymerization of carbon-carbon double bonds caused by localized and severe overheating. Furthermore, dehydration condensation occurs under high-temperature vacuum conditions. By continuously removing the byproduct water, the reaction continues in the forward direction, ultimately yielding the terminal-amino hyperbranched polyamide-amine.

[0026] In the core layer, the terminal amino hyperbranched polyamide-amine, as a highly polar macromolecule, has poor compatibility with non-polar polypropylene. However, under the shear-thinning fluxing effect of the low molecular weight component in bimodal polypropylene, combined with high-shear twin-screw compounding, the terminal amino hyperbranched polyamide-amine can be uniformly dispersed in the polypropylene matrix. The terminal amino and amide groups on it form high-density deep charge traps in space. When the capacitor film is under high temperature and high field strength, the random migration of electrons injected from the electrode and metal impurity ions is spontaneously captured by these deep traps, converting their kinetic energy into heat energy dissipation. This significantly reduces the high-temperature leakage current of the film, suppresses conductivity loss, and significantly improves the high-temperature breakdown strength.

[0027] If terminal amino hyperbranched polyamide-amine is directly placed in a single-layer film, its polar groups will accumulate on the film surface, leading to severe moisture absorption and a sharp increase in dielectric loss. This invention employs a three-layer A / B / A structure, confining it in the core layer B, while the surface layer A is made of high isotactic polypropylene without polar additives. The high crystallinity of surface layer A not only effectively prevents moisture penetration but also ensures a smooth, precipitation-free surface, providing an excellent adhesion substrate for subsequent metallization vacuum evaporation of zinc-aluminum electrodes.

[0028] Preparation Example The specific preparation process of amine-terminated hyperbranched polyamide-amine is as follows: 136g pentaerythritol and 392g maleic anhydride are added to a reaction vessel, the temperature is raised to 130℃ under a nitrogen atmosphere, and the reaction is kept at a constant temperature for 3.5h. Then the temperature is lowered to 80℃, and 444g diethylenetriamine is slowly added dropwise, controlling the dropwise addition process to be completed within 1h. During the dropwise addition, the reaction system is strictly maintained between 80-85℃. After the addition is completed, the temperature is raised to 170℃ and the vacuum is drawn to a pressure of 0.01MPa. The reaction is carried out for 5h, and the polycondensation byproduct water is removed in time. After the reaction is completed, the material is poured out while hot, and after natural cooling to room temperature, it is ground to a particle size of <45μm to obtain amine-terminated hyperbranched polyamide-amine.

[0029] Example 1 A method for preparing a thin film for a multilayer composite capacitor includes the following steps: S1. Pretreatment of core layer materials: By weight, weigh 1.5 parts of terminal amino hyperbranched polyamide-amine, 67 parts of electrical grade polypropylene (isotacticity 95%, ash content 15ppm, MFR=2.8g / 10min), 30 parts of bimodal polypropylene (high molecular weight component Mw=700,000, low molecular weight component Mw=60,000), 0.3 parts of electrical grade antioxidant 1010, and 0.25 parts of maleic anhydride-grafted polypropylene; put all the above raw materials into a high-speed mixer and mix at room temperature. After mixing at 800 rpm for 10 minutes, the mixture was added in three batches to a high-shear co-rotating twin-screw extruder (L / D ratio L / D=48) for blending and granulation. The temperature control of each zone of the extruder was as follows: Zone 1 180℃, Zone 2 210℃, Zone 3 230℃, Zone 4 230℃, Zone 5 225℃, and Die head 220℃. The screw speed was 350 rpm. After the melt was extruded through the die, it was cooled in a high-purity water cooling tank, air-dried, pelletized, and then dried in an 80℃ vacuum drying oven for 12 hours to obtain the core layer masterbatch. Surface material pretreatment: Weigh 100 parts by weight of high isotactic polypropylene (isotacticity 98.5%, ash content 8ppm, MFR=2.2g / 10min) and 0.3 parts by weight of electrical grade antioxidant 1010, and dry them in a drying oven at 90℃ for 6 hours to obtain the dried surface material. S2. The dried surface material is fed into the surface main extruder (single screw, 65mm diameter) at an extrusion temperature of 235-245℃; the core masterbatch is fed into the core main extruder (single screw, 90mm diameter) at an extrusion temperature of 230-240℃; the two high-pressure melts are combined in the A / B / A co-extrusion distribution valve and extruded together through a multi-layer casting flat die (die gap 1.2mm). The rheological distribution valve is adjusted to make the thickness ratio of the three extruded melts 15%:75%:15%. The melt is then attached to a quenching roller with a surface temperature of 88℃. At the same time, the air knife system is started to continuously spray hot air at a temperature of 72℃ onto the outer surface of the film away from the quenching roller. The air knife pressure is controlled at 0.3MPa to ensure that the melt is heated evenly and crystallized slowly on both sides. After solidification, an initial casting with a total thickness of about 92μm is obtained. S3, Longitudinal Stretching: The initial cast film is introduced into the longitudinal stretching unit. It first passes through 5 slow preheating rollers with roller surface temperatures of 105℃, 108℃, 112℃, 115℃ and 115℃ respectively. Then the film enters the fast stretching roller section with the stretching roller surface temperature set at 122℃. The total longitudinal stretching ratio is 4.7 times.

[0030] S4. Lateral stretching: The longitudinally stretched film is fed into the lateral stretching machine through the transition roller. Lateral stretching is carried out in two stages: First stretching stage: The temperature of the front section of the oven is controlled at 152℃, and the chain clamp track rapidly expands to both sides, with the stretching speed controlled at 350% / s, until the film width reaches 60% of the final target total width; Second stretching stage: The film then immediately slides into the middle section of the oven, and the heating device is activated to raise the film surface temperature to 163℃. The track expansion speed is slowed down and controlled at 100% / s, and lateral expansion continues until the total lateral stretching ratio of the film reaches 10 times. S5. After transverse stretching, the film enters the heat setting oven section: the film surface temperature is maintained at 158℃, and the tracks at both ends of the transverse stretching machine actively retract inward, causing the film to undergo a 9.5% first-stage stress relaxation and shrinkage in the transverse direction. This state is maintained at a constant temperature for 12 seconds. Then, the film slides into the first cooling and setting zone, where the temperature is controlled at 132℃. Simultaneously, the tracks continue to retract inward, maintaining a 2.5% second-stage micro-relaxation in the film. Next, the film enters the second cooling and setting zone, where the temperature is controlled at 102℃ to stabilize the molecular chains. Finally, the film is drawn out by the lead-out roller and naturally cooled to room temperature, then subjected to a 12W·min / m... 2 After corona treatment by the corona roller, the film for multilayer composite capacitors with a total thickness of 3.2 μm is obtained by winding.

[0031] Example 2 This embodiment differs from Embodiment 1 in the following ways: In step S1, the core layer material pretreatment is as follows: by weight, 2 parts of terminal amino hyperbranched polyamide-amine, 60 parts of electrical grade polypropylene, 35 parts of bimodal polypropylene, 0.5 parts of electrical grade antioxidant 1010 and 0.28 parts of maleic anhydride grafted polypropylene are weighed, and mixed, granulated and dried according to the method of Example 1 to obtain the core layer masterbatch; the surface layer material pretreatment is as follows: by weight, 100 parts of high isotactic polypropylene and 0.2 parts of electrical grade antioxidant 1010 are weighed, and dried according to the method of Example 1 to obtain the dried surface layer material.

[0032] In step S2, the thickness ratio of the co-extrusion distribution valve is adjusted to 14%:72%:14%, the surface temperature of the quench roll is controlled at 85°C, the hot air temperature of the air knife is 70°C, and the air pressure is 0.4MPa, resulting in an initial casting with a total thickness of approximately 137μm.

[0033] In step S3, longitudinal stretching: the material is fed into a longitudinal stretching machine, preheated to 105°C, and longitudinally stretched at 115°C, with a total longitudinal stretching ratio of 4.5 times.

[0034] In step S4, the transverse stretching is as follows: First stretching section: at 150°C, the film is laterally expanded to 60% of its total width at a speed of 300% / s; Second stretching section: the heating device is activated to raise the film surface temperature to 160°C, and the film is further laterally expanded at a speed of 80% / s until the total transverse stretching ratio is 10.2 times.

[0035] In step S5, the film surface temperature is maintained at 155°C, and the tracks at both ends retract inwards, causing a 10% stress relaxation and retraction in the transverse direction of the film. This temperature is held constant for 15 seconds. Subsequently, segmented cooling and curing are performed: first, the temperature is lowered to 130°C, maintaining a 2.0% relaxation; then, it is lowered to 100°C, and finally, it is allowed to cool naturally to room temperature. (The temperature is measured at 14 W·min / m².) 2 After corona treatment by the corona roller, the film for multilayer composite capacitors with a total thickness of 4μm is obtained by winding.

[0036] Example 3 This embodiment differs from Embodiment 1 in the following ways: In step S1, the core material pretreatment is as follows: by weight, 1 part of terminal amino hyperbranched polyamide-amine, 75 parts of electrical grade polypropylene, 23 parts of bimodal polypropylene, 0.2 parts of electrical grade antioxidant 1010 and 0.22 parts of maleic anhydride grafted polypropylene are weighed, and mixed, granulated and dried according to the method of Example 1 to obtain the core masterbatch; the surface material pretreatment is as follows: by weight, 100 parts of high isotactic polypropylene and 0.5 parts of electrical grade antioxidant 1010 are weighed, and dried according to the method of Example 1 to obtain the dried surface material.

[0037] In step S2, the thickness ratio of the co-extrusion distribution valve is adjusted to 16%:68%:16%, the surface temperature of the quench roll is controlled at 90°C, the hot air temperature of the air knife is 75°C, and the air pressure is 0.2MPa, resulting in an initial casting with a total thickness of approximately 192μm.

[0038] In step S3, longitudinal stretching: the material is fed into a longitudinal stretching machine, preheated to 115°C, and longitudinally stretched at 125°C, with a total longitudinal stretching ratio of 4.8 times.

[0039] In step S4, the transverse stretching is as follows: First stretching section: at 155°C, the film is laterally expanded to 60% of its total width at a speed of 400% / s; Second stretching section: the heating device is activated to raise the film surface temperature to 165°C, and the film is further laterally expanded at a speed of 120% / s until the total transverse stretching ratio is 10 times.

[0040] In step S5, the film surface temperature is maintained at 162°C, and the tracks at both ends retract inwards, causing the film to undergo 9% stress relaxation and shrinkage in the transverse direction. This temperature is held constant for 10 seconds. Subsequently, segmented cooling and curing are performed: first, the temperature is lowered to 135°C, maintaining a 3% relaxation; then, it is lowered to 105°C, and finally, it is allowed to cool naturally to room temperature. (The temperature is measured at 14 W·min / m².) 2 After corona treatment by the corona roller, the film for multilayer composite capacitors with a total thickness of 5μm is obtained by winding.

[0041] Example 4 This embodiment differs from Embodiment 1 in the following ways: In step S1, the core layer material pretreatment is as follows: by weight, 1.65 parts of terminal amino hyperbranched polyamide-amine, 70 parts of electrical grade polypropylene, 26 parts of bimodal polypropylene, 0.4 parts of electrical grade antioxidant 1010 and 0.26 parts of maleic anhydride grafted polypropylene are weighed, mixed, granulated and dried according to the method of Example 1 to obtain the core layer masterbatch; the surface layer material pretreatment is the same as in Example 1.

[0042] In step S2, the thickness ratio of the co-extrusion distribution valve is adjusted to 15%:70%:15%, the surface temperature of the quench roll is controlled at 86°C, the hot air temperature of the air knife is 73°C, and the air pressure is 0.35MPa, resulting in an initial casting with a total thickness of approximately 267μm.

[0043] In step S3, longitudinal stretching: the material is fed into a longitudinal stretching machine, preheated to 112°C, and longitudinally stretched at 120°C, with a total longitudinal stretching ratio of 5 times.

[0044] In step S4, the transverse stretching is as follows: First stretching section: at 153°C, the film is laterally expanded to 60% of its total width at a speed of 360% / s; Second stretching section: the heating device is activated to raise the film surface temperature to 162°C, and the film is further laterally expanded at a speed of 90% / s until the total transverse stretching ratio is 9.8 times.

[0045] In step S5, the film surface temperature is maintained at 156°C, and the tracks at both ends retract inwards, causing the film to undergo 9.2% stress relaxation and shrinkage in the lateral direction. This temperature is held constant for 14 seconds. Subsequently, segmented cooling and curing are performed: first, the temperature is lowered to 133°C, maintaining a 2.2% relaxation; then, it is lowered to 101°C, and finally, it is allowed to cool naturally to room temperature. (The temperature is measured at 13 W·min / m².) 2 After corona treatment by the corona roller, the film for multilayer composite capacitors with a total thickness of 6μm is obtained by winding.

[0046] Comparative Example 1 Compared with Example 1, this comparative example did not add terminal amino hyperbranched polyamide-amine during the preparation of the core layer masterbatch, and the electrical grade polypropylene was changed to 70 parts, finally obtaining a multilayer composite capacitor film with a total thickness of 3.2 μm. All other aspects were the same as in Example 1.

[0047] Comparative Example 2 Compared with Example 1, this comparative example changed the surface material from high-isotactic polypropylene to electrical-grade polypropylene, while the rest remained the same as in Example 1. The final result was a multilayer composite capacitor film with a thickness of 3.2 μm.

[0048] Comparative Example 3 Compared with Example 1, this comparative example did not add bimodal polypropylene during the preparation of the core layer masterbatch, changed the electrical grade polypropylene to 97 parts, and followed the same procedure as in Example 1. Finally, a small number of samples with a thickness of 3.2 μm were obtained for performance testing.

[0049] Comparative Example 4 Compared with Example 1, in step S2, the surface temperature of the quenching roller was reduced to 30°C, which is commonly used in conventional polypropylene film casting, and the air knife system was turned off. All other steps were the same as in Example 1. Finally, a small number of samples with a thickness of 3.2 μm were obtained for performance testing.

[0050] Comparative Example 5 Compared with Example 1, in this comparative example, during the transverse stretching process in step S4, the transverse stretching was not segmented. The temperature of the entire transverse stretching oven was set to 152°C, and the stretching was expanded to a total transverse stretching ratio of 10 times at a constant stretching speed of 250% / s. The film surface was not heated and decelerated separately through an infrared device. The rest was the same as in Example 1. Finally, a small number of samples with a thickness of 3.2μm were obtained for performance testing.

[0051] Related tests The polarity-related properties of the thin film samples prepared in Examples 1-4 and Comparative Examples 1-5 were tested as follows: the thermal shrinkage rate was calculated after heating at 120℃ for 15 min; the film roughness Sa and Sz were tested using a white light interferometer; the breakdown strength was tested according to standard GB / T13542.2-2021; the dielectric constant was tested using a WK65120B high-precision impedance analyzer; the dielectric loss tangent (tanδ) was measured using a high-precision broadband dielectric spectrometer at 120℃ and a test frequency of 1kHz, according to standard GB / T13542.2-2021. The test results are shown in Table 1.

[0052] Table 1 Relevant test performance results

[0053] The test results of Example 1 and Comparative Example 1 show that the breakdown strength is significantly improved after adding terminal amino hyperbranched polyamide-amine. This is because terminal amino hyperbranched polyamide-amine can form a high-density trap energy level at the interface, inhibiting the migration of free electrons at high temperatures, thereby significantly enhancing the voltage withstand capability. The test results of Example 1 and Comparative Example 5 show that segmented stretching and sufficient heat setting result in lower thermal shrinkage rates in the examples, while Comparative Example 5, without segmented stretching and without local heating using an infrared device, leads to higher film stress and increased thermal shrinkage.

[0054] It should be noted that, in this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Although embodiments of this application have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of this application, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a thin film for a multilayer composite capacitor, characterized in that, Includes the following steps: S1. The surface material uses raw materials including high isotactic polypropylene, and the core material uses raw materials including electrical grade polypropylene, bimodal polypropylene, and amino-terminated hyperbranched polyamide-amine. The surface material and the core material are added to a multi-layer co-extrusion casting equipment, extruded through an A / B / A co-extrusion distribution valve and a die, and then attached to a quenching roller for cooling and curing to obtain an initial casting sheet with a three-layer composite structure. S2. The initial casting is fed into a longitudinal stretching machine, preheated at 105-115℃, and then longitudinally stretched at 115-125℃, with a total longitudinal stretching ratio of 4.5-5.0 times. S3. The film is then fed into a transverse stretching machine. The transverse stretching is carried out in two stages: in the first stretching stage, the film is stretched laterally at a speed of 300% / s-400% / s at 150-155℃ to 60% of the total width; in the second stretching stage, the heating device is activated to raise the surface temperature of the film to 160-165℃, and the film continues to be stretched laterally at a speed of 80% / s-120% / s to a total transverse stretching ratio of 9.8-10.2 times. S4. After the transverse stretching is completed, control the film surface temperature to be maintained at 155-162℃. At the same time, control the two ends of the transverse stretching machine to retract and shrink inward, so that the film undergoes 9.0%-10.0% stress relaxation and shrinkage in the transverse direction. Maintain the temperature for 10-15 seconds, and then carry out segmented cooling curing and corona treatment. Finally, rewind to obtain the final product.

2. The method for preparing a thin film for a multilayer composite capacitor according to claim 1, characterized in that, The thickness proportions of each layer—surface layer A, core layer B, and another surface layer A—are 14%-16%, 68%-72%, and 14%-16%, respectively.

3. The method for preparing a thin film for a multilayer composite capacitor according to claim 1, characterized in that, The high-isotactic polypropylene has an isotacticity index greater than 98%, ash content <10ppm, and melt flow rate of 2.0-3.0g / 10min; the electrical grade polypropylene has an isotacticity index of 94%-96%, ash content <20ppm, and melt flow rate of 2.5-3.5g / 10min.

4. The method for preparing a thin film for a multilayer composite capacitor according to claim 1, characterized in that, The bimodal polypropylene has a high molecular weight component Mw > 650,000 and a low molecular weight component Mw < 80,000.

5. The method for preparing a thin film for a multilayer composite capacitor according to claim 1, characterized in that, The surface material also includes electrical-grade antioxidants, and the core material also includes electrical-grade antioxidants and compatibilizers.

6. The method for preparing a thin film for a multilayer composite capacitor according to claim 1, characterized in that, In step S1, the surface temperature of the quenching roller is 85-90℃. During casting, hot air at a temperature of 70-75℃ is continuously sprayed onto the outer surface of the film away from the quenching roller using an air knife, with an air pressure of 0.2-0.4MPa.

7. The method for preparing a thin film for a multilayer composite capacitor according to claim 1, characterized in that, In step S4, the segmented cooling and curing process is as follows: first, cool down to 130-135℃, maintain a relaxation of 2%-3%, then cool down to 100-105℃, and finally cool naturally to room temperature.

8. The method for preparing a thin film for a multilayer composite capacitor according to claim 1, characterized in that, The terminal amino hyperbranched polyamide-amine is prepared by polycondensation reaction using raw materials including pentaerythritol, maleic anhydride and diethylenetriamine.

9. The method for preparing a thin film for a multilayer composite capacitor according to claim 8, characterized in that, The specific preparation process of the terminal amino hyperbranched polyamide-amine is as follows: pentaerythritol and maleic anhydride are added to a reaction vessel, the temperature is raised to 125-135℃ under an inert atmosphere, and the reaction is carried out at a constant temperature for 3-4 hours. The temperature is then lowered to 75-85℃, and diethylenetriamine is slowly added to it. After the addition is completed, the temperature is raised to 165-175℃ and a vacuum is drawn until the pressure is below 0.05MPa. The reaction is carried out for 4-6 hours. The material is poured out while hot, cooled, and then ground to obtain the terminal amino hyperbranched polyamide-amine.

10. A thin film for a multilayer composite capacitor, characterized in that, The film is prepared by the method for preparing a multilayer composite structure capacitor as described in any one of claims 1-9, and the total thickness of the film is 3.2-6 μm.

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