High-performance pp capacitor base film and preparation method thereof

Abstract

The application relates to the field of high-temperature-resistant thin film capacitor dielectric materials, in particular to a high-performance PP capacitor base film and a preparation method thereof, which comprises the following steps: melt granulation of capacitor-grade isotactic polypropylene resin, a multifunctional macromolecular modifier, an antioxidant and a crosslinking catalyst to obtain special material; the special material is extruded and cast into a thick sheet, preheated, stretched in the longitudinal and transverse directions, hot set by water vapor, cooled, wound and aged to obtain the high-performance PP capacitor base film. The modifier is prepared by the following steps: cooling (4-hydroxyphenyl)(4-trifluoromethylphenyl) ketone, an acid binding agent and a solvent, dropwise adding methyl acryloyl chloride to react and purify to obtain (4-methacryloyloxyphenyl)(4-trifluoromethylphenyl) ketone; melt grafting, cutting and drying of the (4-methacryloyloxyphenyl)(4-trifluoromethylphenyl) ketone, polyolefin backbone resin, vinyltrimethoxysilane and an initiator to obtain the modifier. The application solves the problems of easy migration and easy precipitation of traditional small-molecule additives, thereby significantly improving the overall uniformity and service life of the capacitor base film.

Description

Technical Field

[0001] This invention relates to the field of high-temperature resistant thin-film capacitor dielectric materials, specifically a high-performance PP capacitor base film and its preparation method. Background Technology

[0002] Film capacitors are widely used in various fields of electronics, power and modern industry. Existing capacitor base films are developing towards high temperature resistance, high breakdown field strength and low loss while ensuring basic electrical performance. Among them, the insulation stability of the base film under extreme conditions is receiving more and more attention, especially in the new generation of high power density power systems, where the high temperature withstand voltage requirements of film capacitors are extremely strict.

[0003] Isotactic polypropylene film is the core dielectric material of film capacitors. Its microstructure and molecular chain morphology directly affect the insulation and dielectric properties of the film. Existing pure isotactic polypropylene film is widely used at room temperature, but at high temperature, the slippage and relaxation of molecular chains in its amorphous region are significantly enhanced, resulting in an increase in the free volume inside the material, which in turn causes a sharp drop in breakdown field strength.

[0004] However, due to technological limitations, in the past, the physical blending strategy of small molecule voltage stabilizers was mostly used to improve the high temperature resistance of the base film. Because this method does not form covalent bonds with the matrix, it is prone to thermodynamic migration and precipitation under high temperature and strong electric field, resulting in severe distortion of the local electric field. Its modification control methods are limited, and the high temperature insulation performance of existing polypropylene films can no longer meet the needs of the development of high temperature resistant capacitors. Summary of the Invention

[0005] The purpose of this invention is to provide a high-performance PP capacitor base film and its preparation method to solve the problems mentioned in the background art.

[0006] Existing pure isotactic polypropylene films and small molecule blending modification techniques have significant shortcomings when facing extreme conditions of high temperature and strong electric field: the free volume of pure films increases significantly at high temperatures, leading to a sharp decrease in breakdown field strength; while small molecule voltage stabilizers are prone to thermodynamic migration and precipitation, resulting in severe distortion of the local electric field. Therefore, it is necessary to propose a high-performance PP capacitor base film and its preparation method that can covalently anchor deep trap groups through chemical bonds to prevent migration and precipitation, and construct a microscopic three-dimensional cross-linked network of silicon and oxygen in situ to effectively limit molecular chain slippage.

[0007] The technical solution of the present invention includes: mixing capacitor-grade isotactic polypropylene resin, multifunctional macromolecular modifier, antioxidant and crosslinking catalyst evenly, melt blending and granulating to obtain special material;

[0008] The special material is melted and extruded, then cast onto a rapid cooling roller to form a thick sheet;

[0009] After the thick sheet is preheated, it is stretched longitudinally and then stretched laterally. After stretching, it is heat-set. Water vapor is introduced into the heat-set area. The relative humidity of the water vapor is 60%-80%. After cooling and winding, it is cured to obtain a high-performance PP capacitor base film.

[0010] The preparation of multifunctional macromolecular modifiers includes the following steps:

[0011] Step (1) Mix (4-hydroxyphenyl)(4-trifluoromethylphenyl) ketone, acid-binding agent and solvent, cool, add methacryloyl chloride dropwise, react, wash, dry and purify to obtain (4-methacryloyloxyphenyl)(4-trifluoromethylphenyl) ketone;

[0012] Step (2) Mix the polyolefin skeleton resin, (4-methacryloyloxyphenyl)(4-trifluoromethylphenyl) methyl ketone, vinyltrimethoxysilane and initiator evenly, perform melt reactive grafting, remove volatiles, granulate and dry to obtain a multifunctional macromolecular modifier.

[0013] As a preferred embodiment, the weight ratio of capacitor-grade isotactic polypropylene resin, multifunctional macromolecular modifier, antioxidant and crosslinking catalyst is (85-95):(5-15):(0.1-0.5):(0.05-0.2).

[0014] Preferably, in step (1), the molar ratio of (4-hydroxyphenyl)(4-trifluoromethylphenyl) methyl ketone, the acid-binding agent, and methacryloyl chloride is 1:(1-1.5):(1-1.5);

[0015] The solvent is anhydrous dichloromethane;

[0016] The acid-binding agent is triethylamine.

[0017] Preferably, in step (2), the weight ratio of the polyolefin skeleton resin, (4-methacryloyloxyphenyl)(4-trifluoromethylphenyl) methyl ketone, vinyltrimethoxysilane and initiator is 100:(3-8):(2-5):(0.1-0.3).

[0018] Preferably, in step (2), the polyolefin skeleton resin is high melt strength isotactic polypropylene or ethylene-octene copolymer;

[0019] The initiator is 2,5-dimethyl-2,5-bis(tert-butylperoxy)hexane.

[0020] Preferably, in step (1), the cooling temperature is 0°C, the reaction temperature is 25°C, and the reaction time is 10-14 hours;

[0021] In step (2), the melt reactive grafting is carried out in a twin-screw extruder, and the temperature of each zone of the extruder is controlled at 170-200℃; the volatiles are removed by vacuuming.

[0022] Preferably, melt blending and granulation are carried out in a twin-screw extruder at a temperature of 190-220°C.

[0023] Melt extrusion is carried out in a single-screw extruder at a temperature of 220-240℃; the temperature of the quench roll is 80-90℃.

[0024] Preferably, the preheating temperature for longitudinal stretching is 140-150℃, and the stretching ratio for longitudinal stretching is 4-6 times.

[0025] The temperature for transverse stretching is 155-165℃, and the stretching ratio for transverse stretching is 6-9 times.

[0026] The heat setting temperature is 160℃, and the relative humidity of the water vapor is 60%-80%.

[0027] The maturation was carried out at a temperature of 60℃ and a relative humidity of 80% for 48 hours.

[0028] Preferably, the isotactic polypropylene resin for capacitor grade has an isotacticity ≥98%, ash content ≤30ppm, and melt flow index of 1.5-3.0g / 10min.

[0029] The crosslinking catalyst is dibutyltin dilaurate or dibutyltin diacetate;

[0030] A method for preparing a high-performance PP capacitor base film.

[0031] This invention provides a high-performance PP capacitor base film and its preparation method through improvements, which have the following improvements and advantages compared with the prior art:

[0032] 1. This invention prepares a multifunctional macromolecular modifier by grafting a specific benzophenone derivative and vinyltrimethoxysilane onto a polyolefin backbone resin. Since the modifier uses a polyolefin with a structure similar to that of capacitor-grade isotactic polypropylene resin as a backbone, the two have excellent compatibility during melt blending. This not only ensures the uniform dispersion of the modifier in the polypropylene matrix, but also solves the problems of easy migration and precipitation of traditional small molecule additives, thereby significantly improving the overall uniformity and service life of the capacitor base film.

[0033] 2. The preparation process of this invention includes the addition of a crosslinking catalyst and the innovative introduction of water vapor into the heat setting zone, followed by curing treatment at specific temperatures and humidity. Under the combined action of the crosslinking catalyst and water, the siloxane groups on the multifunctional macromolecular modifier can undergo hydrolysis and crosslinking reactions, forming a stable crosslinked three-dimensional network structure inside the polypropylene film. This process significantly enhances the intermolecular forces of the film and significantly improves the dimensional stability and heat resistance of the capacitor base film under high-temperature conditions.

[0034] 3. This invention employs a precisely temperature-controlled extrusion, rapid cooling, and sequential biaxial stretching process, first longitudinal and then transverse. Combined with a modifier skeleton material with high melt strength, it effectively improves the anti-breakage ability and processing stability of the special material during melt extrusion and stretching. Specific preheating and stretching ratio control enable the polypropylene molecular chains to achieve a high degree of uniform orientation. Combined with subsequent shaping and curing to release internal stress, this endows the final high-performance capacitor base film with excellent mechanical strength and thickness uniformity. Detailed Implementation

[0035] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0036] Example 1: A method for preparing a high-performance PP capacitor base film, comprising the following steps:

[0037] Isotactic polypropylene resin of capacitor grade, multifunctional macromolecular modifier, antioxidant and crosslinking catalyst are mixed evenly, melt-blended and granulated to obtain special material; the special material is melt-extruded and cast onto a quenching roller to form a thick sheet; the thick sheet is preheated and then stretched longitudinally, followed by stretching laterally, and then heat-set. Water vapor is introduced into the heat-setting zone, and after cooling and winding, it is cured to obtain a high-performance PP capacitor base film.

[0038] The preparation of the multifunctional macromolecular modifier includes the following steps: Step 1: (4-hydroxyphenyl)(4-trifluoromethylphenyl) methyl ketone, acid-binding agent and solvent are mixed, cooled, methacryloyl chloride is added dropwise, reacted, washed, dried and purified to obtain (4-methacryloyloxyphenyl)(4-trifluoromethylphenyl) methyl ketone; Step 2: Polyolefin skeleton resin, (4-methacryloyloxyphenyl)(4-trifluoromethylphenyl) methyl ketone, vinyltrimethoxysilane and initiator are mixed evenly, melt reactive grafting is performed, volatiles are removed, pelletized and dried to obtain the multifunctional macromolecular modifier; The weight ratio of capacitor-grade isotactic polypropylene resin, multifunctional macromolecular modifier, antioxidant and crosslinking catalyst is 90:10:0.2:0.1;

[0039] In step 1, the molar ratio of (4-hydroxyphenyl)(4-trifluoromethylphenyl) methyl ketone, acid-binding agent, and methacryloyl chloride is 1:1.2:1.2; the solvent is anhydrous dichloromethane; the acid-binding agent is triethylamine. In step 2, the weight ratio of polyolefin skeleton resin, (4-methacryloyloxyphenyl)(4-trifluoromethylphenyl) methyl ketone, vinyltrimethoxysilane, and initiator is 100:5:3:0.15; in step 2, the polyolefin skeleton resin is high melt strength isotactic polypropylene; the initiator is 2,5-dimethyl-2,5-ditert-butylperoxyhexane.

[0040] In step 1, the cooling temperature is 0℃, the reaction temperature is 25℃, and the reaction time is 12 hours. In step 2, the melt reactive grafting is carried out in a twin-screw extruder, with the temperature of each zone of the extruder controlled between 175℃ and 195℃. Volatile substances are removed by vacuum extraction. Melt blending and granulation are carried out in a twin-screw extruder at a temperature of 210℃. Melt extrusion is carried out in a single-screw extruder at a temperature of 230℃. The temperature of the quench roll is 85℃. The preheating temperature for longitudinal stretching is 145℃, and the stretching ratio for longitudinal stretching is 5 times.

[0041] The transverse stretching temperature is 160℃, and the stretching ratio is 8 times; the heat setting temperature is 160℃, and the relative humidity of water vapor is 70%; the curing temperature is 60℃, the relative humidity of curing is 80%, and the curing time is 48 hours; the isotacticity of the capacitor-grade isotactic polypropylene resin is 98.5%, the ash content is 15ppm, and the melt index is 2.0g / 10min; the crosslinking catalyst is dibutyltin dilaurate; a high-performance PP capacitor base film prepared by the above preparation method.

[0042] This embodiment provides a high-performance PP capacitor base film and its preparation method, aiming to solve the problem of a sharp decrease in breakdown field strength caused by a significant increase in the free volume of pure isotactic polypropylene film at high temperatures through molecular structure design. The specific process for preparing the multifunctional macromolecular modifier covers the synthesis and reactive extrusion grafting of (4-methacryloyloxyphenyl)(4-trifluoromethylphenyl) ketone. In the synthesis of (4-methacryloyloxyphenyl)(4-trifluoromethylphenyl) ketone, 0.1 moles of (4-hydroxyphenyl)(4-trifluoromethylphenyl) ketone and 0.12 moles of triethylamine acid binder are dissolved in 150 mL of anhydrous dichloromethane solvent, and the initial reaction environment is established under 0°C ice bath cooling conditions to suppress the occurrence of side reactions.

[0043] 0.12 mol of methacryloyl chloride was slowly added dropwise. After the addition was complete, the reaction was continued at 25°C for 12 hours to ensure the complete conversion of the esterification reaction and to avoid the increase of by-products. This esterification reaction combines the benzophenone structure with the strongly electron-withdrawing trifluoromethyl group to construct an electron-deficient deep trap monomer with extremely strong electron affinity in situ. The reaction solution was washed successively with 50 mL of 1 mol / L dilute hydrochloric acid, 50 mL of saturated sodium bicarbonate aqueous solution, and 50 mL of deionized water. The organic phase was collected and dried with 10 g of anhydrous magnesium sulfate. The solvent was removed by rotary evaporation and purified by silica gel column chromatography to obtain high-purity (4-methacryloyloxyphenyl)(4-trifluoromethylphenyl) methyl ketone.

[0044] The yield of this step was 86%, and the purity was 99.2% as determined by HPLC. Its structure was confirmed by 1H NMR spectroscopy (400MHz, CDCl3) δ 7.85-7.75 (m, 4H), 7.30 (d, J=8.0Hz, 2H), 7.15 (d, J=8.0Hz, 2H), 6.36 (s, 1H), 5.79 (s, 1H), 2.06 (s, 3H), and mass spectrometry (ESI-MS) m / z: 335.1 [M+H]+, confirming the successful synthesis of this key intermediate and meeting the requirements for subsequent reactions.

[0045] In the reactive extrusion grafting stage of the multifunctional macromolecular modifier, 100 parts of high melt strength isotactic polypropylene skeleton resin, 5 parts of the above-synthesized (4-methacryloyloxyphenyl)(4-trifluoromethylphenyl) methyl ketone, 3 parts of vinyltrimethoxysilane and 0.15 parts of 2,5-dimethyl-2,5-bis-tert-butylperoxyhexane initiator are fully homogenized in a high-speed mixer.

[0046] The mixture is fed into a co-rotating twin-screw extruder with a length-to-diameter ratio of 40 for melt reactive grafting. The temperatures of each zone of the extruder are set to 175℃, 185℃, 195℃, 195℃, and 190℃ to ensure appropriate decomposition of the initiator and stable grafting reaction. A vacuum of -0.09 MPa is applied at the end of the extruder to remove unreacted volatiles to ensure efficient removal of volatiles. The extrudate is then pelletized and dried underwater to obtain a multifunctional macromolecular modifier.

[0047] This multifunctional macromolecular modifier covalently anchors the deep-trap group chemical bonds to the long chain of polyolefin, thermodynamically preventing the migration and precipitation of small-molecule voltage stabilizers under strong electric fields. In the preparation stage of high-performance PP capacitor base film, 90 parts of capacitor-grade isotactic polypropylene resin with an isotacticity of 98.5%, an ash content of 15 ppm and a melt index of 2.0 g, 10 parts of multifunctional macromolecular modifier, 0.2 parts of antioxidant pentaerythritol tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] and 0.1 parts of dibutyltin dilaurate crosslinking catalyst are premixed.

[0048] The mixture is melt-blended and granulated in a twin-screw extruder at 210°C. The main chain of the multifunctional macromolecular modifier and the isotactic polypropylene matrix achieve molecular-level chain entanglement and co-crystallization in the molten state, avoiding phase interface defects caused by the introduction of inorganic fillers. The special material is melt-extruded in a single-screw extruder at 230°C and cast onto a quench roll at 85°C to form a thick sheet. The thick sheet is preheated at 145°C and then stretched 5 times longitudinally, followed by 8 times transversely at 160°C, giving the film excellent mechanical strength. The stretched film enters a heat-setting zone at 160°C, during which it is sprayed with high-temperature water vapor with a relative humidity of 70% for 5 seconds.

[0049] The siloxane groups in the film undergo hydrolysis and condensation reactions under the synergistic cascade mechanism of moisture and crosslinking catalyst, constructing a microscopic three-dimensional crosslinked network of siloxane in situ in the amorphous region and the edge of the crystalline region. The three-dimensional crosslinked structure formed by this network effectively restricts the slippage and relaxation of molecular chains in the amorphous region at high temperature, and controls the free volume expansion rate to an extremely low level. After the film is cooled and rolled up, it is placed in a constant temperature and humidity environment of 60°C and 80% relative humidity for 48 hours to promote the complete locking of molecular chain morphology by the micro-crosslinking reaction, and finally obtain a capacitor base film with both high heat resistance and high breakdown strength.

[0050] To further verify the rationality of the above-mentioned optimal range of process parameters, the inventors conducted a series of supplementary experiments based on this embodiment:

[0051] (1) The cooling temperature of the synthesis of (4-methacryloyloxyphenyl)(4-trifluoromethylphenyl) methyl ketone was adjusted to -5℃ and 5℃, respectively, and the reaction time was adjusted to 10 hours and 14 hours, respectively. The results showed that the yield was above 84% and the purity was above 99.0%, which proved that this range could ensure the full conversion of the esterification reaction and effectively suppress the side reaction.

[0052] (2) The devolatilization vacuum degree in the preparation stage of the multifunctional macromolecular modifier was adjusted to -0.08 MPa and -0.1 MPa, respectively. The results showed that both could efficiently remove unreacted volatiles and effectively ensure the purity of the modifier.

[0053] (3) The steam treatment time during the preparation of high-performance PP capacitor base film was adjusted to 3 seconds and 8 seconds, respectively; the curing temperature was adjusted to 50℃ and 70℃, respectively; and the curing time was adjusted to 36 hours and 60 hours, respectively. The test results showed that the films prepared under these end conditions had a DC breakdown field strength of 120℃ that was stable between 455-470kV / mm, a dielectric loss of less than 0.00020, and a gel rate that remained between 11% and 13%.

[0054] The above supplementary verification fully demonstrates that, within the specified range of cooling temperature, reaction time, devolatilization vacuum, processing time, and curing temperature and time, deep electron traps and microscopic three-dimensional cross-linked networks of silicon and oxygen can be effectively constructed, stably achieving the technical effect of both high heat resistance and high breakdown strength, thus strongly supporting the rationality of the protected area.

[0055] Example 2: The weight ratio of capacitor-grade isotactic polypropylene resin, multifunctional macromolecular modifier, antioxidant, and crosslinking catalyst is 85:15:0.5:0.2; melt blending and granulation are carried out in a twin-screw extruder at a temperature of 190°C; melt extrusion is carried out in a single-screw extruder at a temperature of 220°C; the temperature of the quench roll is 80°C.

[0056] This embodiment verifies the lower and upper limits of the polymer composition ratio and melt extrusion process parameters. In the batching stage, the amount of capacitor-grade isotactic polypropylene resin is adjusted to 85 parts, while the amount of multifunctional macromolecular modifier is increased to the upper limit of 15 parts, along with 0.5 parts of antioxidant and 0.2 parts of crosslinking catalyst. Increasing the proportion of macromolecular modifier aims to further improve the density of deep electron traps and the degree of crosslinking of the micro-crosslinking network inside the film, in order to obtain stronger carrier trapping ability under extreme high temperature environment.

[0057] With a higher concentration of crosslinking catalyst, the siloxane condensation kinetics under subsequent hydration conditions can be accelerated. In the melt blending and granulation stage, the temperature of the twin-screw extruder is set to a lower 190°C, and the melt extrusion temperature of the single-screw extruder is simultaneously lowered to 220°C, while the quench roll temperature is controlled at 80°C. Appropriately reducing the processing temperature can effectively suppress the risk of thermal degradation of high-concentration modifiers under strong shear force and maintain the structural integrity of the grafted side chains. The lower quench roll temperature accelerates the crystallization nucleation rate of the polymer melt, forming a finer and more uniform spherulite structure.

[0058] This embodiment demonstrates the process adaptability under high modifier dosage and verifies the robustness of this technical solution when adjusting the phase region structure ratio. Although the processing window is narrowed, a base film with a continuous phase morphology is still successfully prepared.

[0059] Example 3: The weight ratio of polyolefin skeleton resin, (4-methacryloyloxyphenyl)(4-trifluoromethylphenyl) methyl ketone, vinyltrimethoxysilane, and initiator was 100:3:2:0.1; the preheating temperature for longitudinal stretching was 150°C, and the stretching ratio for longitudinal stretching was 6 times; the temperature for transverse stretching was 165°C, and the stretching ratio for transverse stretching was 9 times; the heat setting temperature was 160°C, and the relative humidity of water vapor was 80%; the curing temperature was 60°C, the relative humidity for curing was 80%, and the curing time was 48 hours.

[0060] This embodiment focuses on verifying the synergistic effect of low concentration ratio of grafted monomer and high ratio biaxial stretching process; in the preparation of multifunctional macromolecular modifier, 100 parts of polyolefin skeleton resin are combined with only 3 parts of electron-deficient aromatic ring deep trap monomer and 2 parts of vinyltrimethoxysilane, and the amount of initiator is reduced to the lower limit of 0.1 parts.

[0061] Reducing the grafting rate of polar monomers aims to preserve the non-polar characteristics of the polyolefin backbone to the greatest extent possible, in order to verify the effectiveness of the trap energy level under extremely low dielectric loss requirements. In the film forming stage, the preheating temperature for longitudinal stretching is increased to 150°C, and the stretching ratio reaches the upper limit of 6 times. The subsequent transverse stretching temperature is set at 165°C, and the stretching ratio is increased to 9 times. High stretching ratio promotes the high orientation of polymer molecular chains, which increases the density and crystallinity of the crystalline regions.

[0062] During the heat setting process at 160℃, water vapor with a relative humidity of up to 80% was introduced. The high humidity environment compensated for the insufficient density of crosslinking groups caused by the low siloxane grafting rate. By strengthening the water penetration kinetics, it was ensured that the limited crosslinking groups could fully undergo hydrolysis and condensation.

[0063] This embodiment demonstrates that by improving the physical stretching orientation and strengthening the hydration crosslinking conditions, even with low chemical modifier dosage, the free volume of the amorphous region can still be locked, reflecting the complementarity between physical morphology control and chemical micro-crosslinking.

[0064] Example 4: The weight ratio of capacitor-grade isotactic polypropylene resin, multifunctional macromolecular modifier, antioxidant, and crosslinking catalyst was 88:12:0.3:0.15; the preheating temperature for longitudinal stretching was 142°C, and the stretching ratio for longitudinal stretching was 4.5 times; the temperature for transverse stretching was 158°C, and the stretching ratio for transverse stretching was 7 times; the heat setting temperature was 160°C, and the relative humidity of water vapor was 60%; the curing temperature was 60°C, the relative humidity for curing was 80%, and the curing time was 48 hours.

[0065] This embodiment explores the adaptation mechanism of a moderately high modification ratio with mild stretching and low humidity setting conditions. The formulation system uses 88 parts isotactic polypropylene resin and 12 parts multifunctional macromolecular modifier, supplemented with 0.3 parts antioxidant and 0.15 parts crosslinking catalyst. This relatively high modifier ratio provides a relatively rich number of deep electron traps and crosslinking anchors. In the film stretching process, the longitudinal preheating temperature is set at 142°C with a stretching ratio of 4.5 times, and the transverse stretching temperature is set at 158°C with a stretching ratio of 7 times.

[0066] The relatively mild tensile field strength reduces the internal stress accumulation of the molecular chains, retains a moderate proportion of amorphous regions, and provides sufficient steric hindrance margin for the subsequent construction of cross-linked networks. In the heat setting stage, water vapor with a relative humidity of 60% is introduced. The lower humidity environment slows down the transient reaction rate of hydrolysis and condensation, making the formation of cross-linked networks more uniform and avoiding uneven internal stress or embrittlement of the film caused by local over-cross-linking.

[0067] This combination of parameters, while ensuring the improvement of high-temperature breakdown field strength, also gives the film better flexibility and winding processing compliance, demonstrating the design flexibility of this process system in pursuing mechanical balance.

[0068] Example 5: The weight ratio of polyolefin skeleton resin, (4-methacryloyloxyphenyl)(4-trifluoromethylphenyl) methyl ketone, vinyltrimethoxysilane and initiator is 100:6:4:0.2;

[0069] The preheating temperature for longitudinal stretching is 148℃, and the stretching ratio for longitudinal stretching is 5.5 times.

[0070] The transverse stretching temperature is 162℃, and the transverse stretching ratio is 8.5 times.

[0071] The heat setting temperature was 160℃, and the relative humidity of the water vapor was 75%.

[0072] The curing temperature was 60℃, the relative humidity was 80%, and the curing time was 48 hours.

[0073] This embodiment verifies the cascade matching effect of a relatively high grafted monomer concentration with a high-intensity orientation field and a high-humidity environment; in the preparation of the modifier, 100 parts of polyolefin backbone resin are combined with 6 parts of deep trap monomer and 4 parts of vinyltrimethoxysilane, and the amount of initiator is set to 0.2 parts; increasing the concentration of reaction substrate improves the multifunctionality of polymer side chains and constructs a denser electron trapping center and crosslinking precursor;

[0074] During the film stretching stage, longitudinal preheating at 148℃ and stretching ratio of 5.5 times are used, combined with transverse stretching process at 162℃ and 8.5 times. The high-intensity bidirectional strain field forces the polymer chain segments with large side chains grafted to stretch and rearrange highly along the direction of force, effectively overcoming the steric hindrance effect caused by high grafting ratio.

[0075] In the heat-setting zone, high-temperature water vapor with a relative humidity of 75% is introduced; the ample water supply and the highly extended state of the polymer chains work synergistically, enabling the siloxane groups hidden inside the amorphous region to quickly contact water molecules and complete in-situ construction; this example demonstrates that while improving the functional density of the modifier, by strengthening the driving force of physical orientation and chemical crosslinking, the kinetic mismatch problem of multi-component systems can be successfully overcome, achieving a deep integration of high field strength tolerance and structural stability.

[0076] Comparative Example 1: This comparative example provides a method for preparing a pure isotactic polypropylene film without the addition of a modifier, serving as a benchmark to verify the technical contribution of the macromolecular modifier. The preparation process uses the same capacitor-grade isotactic polypropylene resin as in Example 1, and adds the same amount of antioxidant as in Example 1, but without adding any multifunctional macromolecular modifier or crosslinking catalyst. The process strictly follows the extrusion, casting, and biaxial stretching parameters of Example 1 to prepare a pure biaxially oriented polypropylene film with a thickness of 5 micrometers.

[0077] It should be noted that no water vapor was sprayed into the heat setting zone, and no constant temperature and humidity curing treatment was performed after winding; this comparative example reflects the intrinsic electrical properties of existing conventional polypropylene films when they do not possess deep electron traps and amorphous micro-crosslinked networks.

[0078] Comparative Example 2: This comparative example explores the limitations of the traditional physical blending strategy for small molecule voltage stabilizers; during formulation, 99 parts of capacitor-grade isotactic polypropylene resin and 1 part of small molecule voltage stabilizer (4-methacryloyloxyphenyl)(4-trifluoromethylphenyl) methyl ketone were directly physically blended.

[0079] The mixture was prepared into a 5-micron film using the conventional thin film processing technology of Comparative Example 1. Although this scheme introduced an electron-withdrawing benzophenone structure into the system, the small molecules were not covalently anchored to the polymer matrix by chemical bonds. Under the continuous action of high temperature and strong electric field, they easily overcame the binding of the matrix molecular chains and underwent thermodynamic migration. These free-state small molecules not only could not stably play the electron-capturing function, but were also prone to agglomeration and precipitation on the film surface or at defects in amorphous regions, resulting in severe distortion of the local electric field.

[0080] Comparative Example 3: This comparative example aims to independently verify the respective contribution weights of deep electron traps and micro-crosslinked networks in improving the high-temperature breakdown field strength; in the preparation steps of the multifunctional macromolecular modifier, the addition of vinyltrimethoxysilane and crosslinking catalyst was removed, and only the deep trap monomer was grafted onto the polyolefin skeleton to prepare a monofunctional modifier.

[0081] Subsequently, a 5-micron film was prepared according to the formulation and extrusion stretching process of Example 1, and the hydration micro-crosslinking treatment was cancelled in the heat setting stage. Although the system has deep trap energy levels that can capture high-energy free electrons, due to the lack of physical locking of the silicon-oxygen three-dimensional crosslinking network in the amorphous region, the slippage and relaxation of polymer molecular chain segments are still severe at a high temperature of 120°C, and the free volume expansion cannot be effectively suppressed, thus limiting further breakthroughs in high-temperature insulation performance.

[0082] Verification experiments: In order to comprehensively evaluate the insulation stability and structural reliability of the high-performance PP capacitor base film provided by the present invention under extreme working conditions, a comprehensive microstructure characterization and high-temperature electrical performance test were carried out on the film samples prepared in Examples 1 to 5 and Comparative Examples 1 to 3.

[0083] The testing dimensions cover the depth analysis of trap energy levels, the density assessment of cross-linked networks in amorphous regions, and the determination of dielectric loss and DC breakdown field strength under high temperature conditions. This series of experiments aims to reveal the synergistic mechanism of macromolecular anchored deep traps and in-situ micro-cross-linked networks in suppressing carrier migration and locking free volume through multi-dimensional data cross-validation.

[0084] Test standards: The trap level depth test was performed in accordance with the relevant academic specifications of the thermally stimulated current method, with the polarization temperature set at 100℃, the polarization electric field at 50 MV / m, and the heating rate controlled at 3℃ / min; the cross-linked network gelation rate test was conducted according to GB / T25498-2010, using boiling xylene as the extraction solvent; the high-temperature DC breakdown field strength test followed the specification for breakdown voltage testing of electrical insulation materials, with the test environment temperature kept constant at 120℃, using a ball-plate electrode system, and a voltage rise rate of 500V / s; the dielectric loss test was conducted at 120℃ and 1 kHz using a broadband dielectric spectrometer.

[0085] The specific testing process is as follows: In the trap energy level test, the film sample is placed in a vacuum test chamber, a polarization electric field is applied and the temperature is lowered to -50°C to freeze the polarization charge. Then, the temperature is raised at a constant rate and the release curve of the depolarization current with temperature is recorded. The depth of the trap energy level is calculated by the initial rise method. In the crosslinking network characterization step, a certain mass of film sample is accurately weighed and placed in a stainless steel mesh bag. It is then immersed in boiling xylene for continuous extraction for 24 hours. After drying to constant weight, the mass percentage of insoluble gel is calculated to characterize the formation of the micro-crosslinking network.

[0086] In the high-temperature dielectric and breakdown performance test, the thin film sample was placed in a constant temperature oil bath at 120℃ to eliminate the effect of surface flashover. A DC voltage was applied until the sample broke down. The voltage value at the moment of breakdown was recorded and divided by the actual thickness of the film to obtain the breakdown field strength. For the sample of Comparative Example 2, an additional pretreatment of aging at 120℃ for 100 hours was added to observe the destructive effect of small molecule precipitation on long-term insulation performance.

[0087] Table 1 Performance test data of Examples 1-5 and Comparative Examples 1-3

[0088]

[0089] The experimental data clearly reveal the superior performance of the technical solution of the present invention; the trap energy level depths of Examples 1 to 5 are all significantly higher than the shallow trap level of Comparative Example 1, proving that the introduction of the electron-deficient aromatic ring structure successfully constructs a deep electron trap, effectively improving the ability to capture high-energy free electrons; the gel rate data show that the example groups all successfully constructed a moderate silicon-oxygen micro-crosslinked network in the amorphous region, while the comparative example groups had zero gel rate due to the lack of crosslinking groups.

[0090] At an extreme high temperature of 120°C, the DC breakdown field strength of the example group remained above 445 kV / mm, which was a significant improvement over the pure polypropylene film, i.e., Comparative Example 1. In the aging test, the breakdown field strength of the small molecule blend system of Comparative Example 2 dropped sharply to 260 kV / mm due to physical migration, while the example group, thanks to the macromolecular chemical covalent anchoring mechanism, showed minimal attenuation of the breakdown field strength before and after aging, demonstrating excellent long-term insulation stability.

[0091] Comparing Example 1 and Comparative Example 3, it can be seen that when there is only a deep electron trap without a micro-crosslinked network, the breakdown field strength is only 385 kV / mm. This further confirms the indispensable synergistic effect of the micro-crosslinked network in locking the high-temperature free volume and suppressing carrier migration. The composition system of the present invention significantly improves the withstand voltage characteristics while maintaining an extremely low dielectric loss of less than 0.02%, taking into account both energy storage density and energy conversion efficiency, and providing a practical technical solution for the development of dielectric materials for the next generation of high-temperature resistant thin-film capacitors.

[0092] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for preparing a high-performance PP capacitor base film, characterized in that, Includes the following steps: The capacitor-grade isotactic polypropylene resin, multifunctional macromolecular modifier, antioxidant and crosslinking catalyst are mixed evenly, melt-blended and granulated to obtain the special material. The special material is melted and extruded, then cast onto a rapid cooling roller to form a thick sheet; After the thick sheet is preheated, it is stretched longitudinally and then stretched laterally. After stretching, it is heat-set. Water vapor is introduced into the heat-set area. The relative humidity of the water vapor is 60%-80%. After cooling and winding, it is cured to obtain a high-performance PP capacitor base film. The preparation of multifunctional macromolecular modifiers includes the following steps: Step (1) Mix (4-hydroxyphenyl)(4-trifluoromethylphenyl) ketone, acid-binding agent and solvent, cool, add methacryloyl chloride dropwise, react, wash, dry and purify to obtain (4-methacryloyloxyphenyl)(4-trifluoromethylphenyl) ketone; Step (2) Mix polyolefin skeleton resin, (4-methacryloyloxyphenyl)(4-trifluoromethylphenyl) methyl ketone, vinyltrimethoxysilane and initiator evenly, perform melt reactive grafting, remove volatiles, granulate, dry, and obtain multifunctional macromolecular modifier. In step (2), the weight ratio of polyolefin skeleton resin, (4-methacryloyloxyphenyl)(4-trifluoromethylphenyl) methyl ketone, vinyltrimethoxysilane and initiator is 100:(3-8):(2-5):(0.1-0.3); In step (2), the polyolefin backbone resin is high melt strength isotactic polypropylene or ethylene-octene copolymer; The preheating temperature for longitudinal stretching is 140-150℃, and the stretching ratio for longitudinal stretching is 4-6 times. The temperature for transverse stretching is 155-165℃, and the stretching ratio for transverse stretching is 6-9 times. The heat setting temperature is 160℃, and the relative humidity of the water vapor is 60%-80%. The curing temperature was 60℃, the relative humidity was 80%, and the curing time was 48 hours.

2. The method for preparing a high-performance PP capacitor base film according to claim 1, characterized in that, The weight ratio of capacitor-grade isotactic polypropylene resin, multifunctional macromolecular modifier, antioxidant and crosslinking catalyst is (85-95):(5-15):(0.1-0.5):(0.05-0.2).

3. The method for preparing a high-performance PP capacitor base film according to claim 1, characterized in that, In step (1), the molar ratio of (4-hydroxyphenyl)(4-trifluoromethylphenyl) ketone, the acid-binding agent, and methacryloyl chloride is 1:(1-1.5):(1-1.5); The solvent is anhydrous dichloromethane; The acid-binding agent is triethylamine.

4. The method for preparing a high-performance PP capacitor base film according to claim 1, characterized in that, The initiator is 2,5-dimethyl-2,5-bis(tert-butylperoxy)hexane.

5. The method for preparing a high-performance PP capacitor base film according to claim 1, characterized in that, In step (1), the cooling temperature is 0℃, the reaction temperature is 25℃, and the reaction time is 10-14 hours; In step (2), the melt reactive grafting is carried out in a twin-screw extruder, and the temperature of each zone of the extruder is controlled at 170-200℃; the volatiles are removed by vacuuming.

6. The method for preparing a high-performance PP capacitor base film according to claim 1, characterized in that, Melt blending and granulation are carried out in a twin-screw extruder at a temperature of 190-220℃. Melt extrusion is carried out in a single-screw extruder at a temperature of 220-240℃; the temperature of the quench roll is 80-90℃.

7. The method for preparing a high-performance PP capacitor base film according to claim 1, characterized in that, The isotacticity of the capacitor-grade isotactic polypropylene resin is ≥98%, the ash content is ≤30ppm, and the melt index is 1.5-3.0g / 10min. The crosslinking catalyst is dibutyltin dilaurate or dibutyltin diacetate.

8. A high-performance PP capacitor base film prepared by the preparation method according to any one of claims 1-7.

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