Polypropylene composite film containing cyano-containing aromatic functional interface layer and preparation method thereof

Abstract

The application belongs to the technical field of capacitor film, and particularly relates to a polypropylene composite film containing a cyano-containing aromatic functional interface layer and a preparation method thereof. The polypropylene film for capacitors is used as a base body, a glycidyl methacrylate active layer is photo-grafted on the surface of the base body, and a cyano-containing aromatic functional molecule is further grafted to construct a cyano-containing aromatic functional interface layer composite film. On the basis of keeping low dielectric loss, high charge and discharge efficiency and good processing performance of the polypropylene film, the injection and migration behavior of the interface carrier is regulated. Compared with the unmodified polypropylene film, the composite film obtained by the application has obvious improvement in direct current conductivity, breakdown strength and energy storage performance, and can provide a medium material with optimized performance for a high energy storage density metalized film capacitor.

Description

Technical Field

[0001] This invention belongs to the field of capacitor film technology, specifically relating to a polypropylene composite film containing a cyano aromatic functional interface layer and its preparation method. Background Technology

[0002] The rapid development of power electronic systems and high-power energy storage devices has presented new challenges to capacitor performance. Under high-frequency, high-power, and rapid cycling conditions, capacitors not only need to achieve efficient energy release but also must maintain long-term stable operation. Especially in scenarios such as electric vehicles, aerospace, and renewable energy interfaces, where device size and weight are limited, higher demands are placed on the energy density and insulation reliability of the energy storage medium. The performance of the dielectric material directly affects the efficiency, safety, and lifespan of the entire system. Therefore, finding thin-film materials that can maintain high energy storage capacity and stable insulation performance under complex operating conditions has become an urgent technical problem to be solved.

[0003] Polypropylene (PP) film, as a mainstream dielectric material, possesses excellent thermal stability, low dielectric loss, and high breakdown strength, making it widely used in metallized film capacitors. However, due to the non-polar molecular structure of PP, its dielectric constant is relatively low, limiting its energy storage capacity per unit volume. Under high temperature, high electric field, and long-term charge-discharge conditions, PP films are prone to charge injection enhancement, space charge accumulation, and local electric field distortion, thereby reducing breakdown strength and energy storage efficiency. Furthermore, the thermal and charge loads on the film under extreme operating conditions accelerate material aging, further limiting its application in high-performance energy storage systems.

[0004] Currently, researchers have proposed various modification strategies to improve the dielectric properties and energy storage capacity of polypropylene films, such as inorganic nanofiller blending, polymer blending, molecular grafting, and surface coating. While introducing high-dielectric fillers into the polypropylene bulk can improve dielectric polarization, it easily leads to the formation of numerous interfacial regions between the filler and the polymer, potentially causing localized electric field concentration due to particle agglomeration and uneven interfacial bonding. Bulk chemical grafting can alter the molecular structure of polypropylene, typically requiring processes such as melt processing, irradiation, or re-film formation, which impacts the original biaxially oriented film structure and industrial manufacturing processes.

[0005] Compared to bulk modification, constructing a nanoscale functional interface layer on the surface of a polypropylene film allows for the modulation of charge carrier behavior at the electrode / polymer interface while preserving the bulk structure and mechanical properties of the polypropylene as much as possible. Therefore, there is an urgent need to develop a method that enables the stable and uniform immobilization of functional molecules on low surface energy polypropylene films to form an interface structure with clear carrier trapping and electron injection blocking effects, thereby achieving high-performance surface modification. Summary of the Invention

[0006] The purpose of this invention is to provide a polypropylene composite film containing a cyano aromatic functional interface layer, its preparation method, and its application. The polypropylene composite film provided by this invention can effectively suppress electrode carrier injection and migration, improve film breakdown strength and high-temperature insulation performance, thereby providing a new interface modification technology path for high-performance metallized film capacitors.

[0007] To achieve the above objectives, the present invention provides the following technical solution: This invention provides a composite polypropylene film containing a cyano aromatic interface layer, wherein the cyano aromatic interface layer is a functional interface layer formed by grafting glycidyl methacrylate onto the surface of a polypropylene film and introducing cyano aromatic functional molecules.

[0008] This invention also provides a method for preparing the above-mentioned composite polypropylene film containing a cyano aromatic interface layer, comprising the following steps: S1. After cleaning and drying, the polypropylene film is immersed in a photoinitiator solution for 10 min. S2. The product processed in step S1 is immersed in a grafting solution containing glycidyl methacrylate to carry out a photo-induced grafting reaction. After washing, a poly(glycidyl methacrylate) modified film is obtained. S3. The poly(glycidyl methacrylate) modified film was placed in a 20 mmol / L 4-aminobenzonitrile solution for grafting reaction. After washing and drying, it was heat-treated at 70℃ for 30 min to obtain a composite polypropylene film containing a cyano aromatic interface layer.

[0009] Preferably, the drying process in step S1 is carried out at 60°C for 1 hour.

[0010] Preferably, the photoinitiator solution in step S1 is prepared by dissolving benzophenone in acetone; the mass concentration of the photoinitiator solution is 0.5%.

[0011] Preferably, the grafting solution containing glycidyl methacrylate in step S2 is prepared by adding glycidyl methacrylate to ethanol or an ethanol / water mixed solvent; the volume fraction of glycidyl methacrylate in the grafting solution is 10%.

[0012] Preferably, the reaction conditions for the photoinduced grafting reaction in step S2 are: irradiation with 365 nm ultraviolet light for 10 min.

[0013] Preferably, the 4-aminobenzonitrile solution in step S3 is prepared by dissolving 4-aminobenzonitrile in N,N-dimethylformamide.

[0014] Preferably, the reaction conditions for the grafting reaction in step S3 are: reaction at 70°C for 6 h.

[0015] The beneficial effects of this invention are: First, this invention introduces a cyano-containing aromatic functional interface layer onto the surface of a polypropylene film, forming an interface structure with carrier trapping capabilities. The aromatic structure and cyano groups in the cyano-containing aromatic functional molecule possess strong polarity and electronic modulation capabilities, which can alter the local electronic state of the interface region and form carrier traps. These traps capture carriers injected by the electrode, inhibiting carrier migration into the polypropylene film, thereby reducing interfacial charge accumulation and leakage current. This helps to mitigate the enhanced charge injection and space charge accumulation phenomena that occur in polypropylene films under high temperature and high electric field conditions.

[0016] Secondly, this invention uses glycidyl methacrylate as an active intermediate branching layer, utilizing the ring-opening reaction between its epoxy groups and amino-containing aromatic nitrile compounds to achieve stable covalent fixation of functional molecules on the surface of the polypropylene film. This method is beneficial for improving the bonding stability and coverage uniformity of the functional interface layer, reducing interface structural inhomogeneity and local defects, and lowering the local electric field concentration and breakdown probability caused by interface weaknesses, thereby improving the DC breakdown strength of the composite film.

[0017] Third, by adjusting the grafting degree of the glycidyl methacrylate active layer, the grafting concentration of cyano-containing aromatic functional molecules, and the reaction time, this invention can control the structure of the functional interface layer and the ability to regulate the interface charge. A suitable functional interface layer can form carrier blocking and trapping structures on the film surface, reducing the effective migration ability of carriers, thereby reducing the DC conductivity of the composite film under high temperature conditions and improving the insulation stability and reliability of the polypropylene film under high temperature and strong electric field environments.

[0018] Fourth, this invention employs an interface modification method combining surface photografting and functional molecule covalent grafting, rather than traditional inorganic nanoparticle filling or polypropylene bulk modification methods. This avoids localized electric field distortion caused by particle agglomeration, poor interfacial compatibility, or changes in the bulk structure. The functional interface layer mainly acts on the surface of the polypropylene film, without significantly altering the main structure and flexibility of the polypropylene film. This helps maintain the film's original advantages of low dielectric loss, good processing performance, and suitability for metallized film capacitors. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 Schematic diagram of a polypropylene composite film containing a cyano aromatic functional interface layer; Figure 2 This is a statistical graph showing the conductivity of BOPP and composite films. Detailed Implementation

[0021] This invention provides a polypropylene composite film containing a cyano aromatic functional interface layer and its preparation method. Using a capacitor-grade polypropylene film as a substrate, this invention constructs a cyano aromatic functional interface layer composite film by photografting a glycidyl methacrylate active layer onto its surface and further grafting cyano aromatic functional molecules. While maintaining the low dielectric loss, high charge-discharge efficiency, and good processability of the polypropylene film, this invention achieves the control of interfacial carrier injection and migration behavior. Compared with unmodified polypropylene films, the composite film obtained by this invention exhibits significant improvements in DC conductivity, breakdown strength, and energy storage performance, providing an optimized dielectric material for high-energy-density metallized film capacitors.

[0022] This invention introduces a cyano-containing aromatic functional interface layer onto the surface of a polypropylene film, forming an interface structure with carrier trapping capabilities. The aromatic structure and cyano groups in the cyano-containing aromatic functional molecule possess strong polarity and electronic modulation capabilities, which can alter the local electronic state of the interface region and form carrier traps. These traps capture carriers injected by the electrode, inhibiting carrier migration into the polypropylene film, thereby reducing interfacial charge accumulation and leakage current. This helps mitigate the enhanced charge injection and space charge accumulation phenomena that occur in polypropylene films under high temperature and high electric field conditions.

[0023] This invention employs glycidyl methacrylate as an active intermediate branching layer, utilizing the ring-opening reaction between its epoxy groups and amino-containing aromatic nitrile compounds to achieve stable covalent fixation of functional molecules on the surface of a polypropylene film. This method is beneficial for improving the bonding stability and coverage uniformity of the functional interface layer, reducing interface structural inhomogeneity and local defects, and lowering the probability of local electric field concentration and breakdown caused by interface weaknesses, thereby improving the DC breakdown strength of the composite film.

[0024] This invention achieves control over the structure of the functional interface layer and the ability to regulate the interface charge by adjusting the grafting degree of the glycidyl methacrylate active layer, the grafting concentration of cyano-containing aromatic functional molecules, and the reaction time. A suitable functional interface layer can form carrier blocking and trapping structures on the film surface, reducing the effective migration ability of carriers, thereby reducing the DC conductivity of the composite film under high-temperature conditions and improving the insulation stability and reliability of the polypropylene film under high-temperature and strong electric field environments.

[0025] This invention employs an interface modification method combining surface photografting and covalent grafting of functional molecules, rather than traditional inorganic nanoparticle filling or polypropylene bulk modification methods. This avoids localized electric field distortions caused by particle agglomeration, poor interfacial compatibility, or alterations to the bulk structure. The functional interface layer primarily acts on the surface of the polypropylene film, without significantly altering the film's main structure and flexibility. This helps maintain the film's original advantages of low dielectric loss, good processability, and suitability for metallized film capacitors.

[0026] In summary, this invention achieves a synergistic effect of interfacial carrier trapping, carrier migration suppression, interfacial binding stabilization, and local electric field modulation by constructing a cyano-containing aromatic functional interface layer on the surface of a polypropylene film. This results in a composite film with low DC conductivity, high breakdown strength, and good energy storage performance. This method can effectively improve the overall performance of metallized film capacitor dielectric materials under high-temperature and high-electric-field conditions, providing technical support for the development of capacitors towards miniaturization, high energy density, and high reliability.

[0027] To further illustrate the present invention, the technical solutions provided by the present invention will be described in detail below with reference to the accompanying drawings and embodiments, but these should not be construed as limiting the scope of protection of the present invention.

[0028] Unless otherwise specified, the production processes, experimental methods, or testing methods involved in the embodiments of this invention are all conventional methods in the prior art, and their names and / or abbreviations are all conventional names in the field, which are very clear and distinct in the relevant application areas. Those skilled in the art can understand the conventional process steps based on the names and apply the corresponding equipment, and implement them according to conventional conditions or the conditions recommended by the manufacturer.

[0029] The various instruments, equipment, raw materials or reagents used in the embodiments of this invention are not subject to any special restrictions on their source. They are all conventional products that can be purchased through regular commercial channels and can be prepared according to conventional methods known to those skilled in the art.

[0030] Example 1: Preparation of Polypropylene Composite Film 1.1 Pretreatment of Polypropylene Film The polypropylene film was sequentially immersed in anhydrous ethanol and deionized water for 10 minutes each to remove impurities and residual contaminants from the film surface. After cleaning, the film was placed in a vacuum drying oven at 60°C for approximately 1 hour.

[0031] 1.2 Photoinitiator loading A 0.5% (w / w) photoinitiator solution was prepared by dissolving benzophenone in acetone. The dried polypropylene film was then immersed in this solution for 10 min to ensure uniform benzophenone loading onto the film surface. After treatment, the film was removed and allowed to evaporate naturally under light-protected conditions to remove excess solvent, providing active sites for subsequent photoinduced grafting reactions.

[0032] 1.3 Preparation of glycidyl methacrylate graft solution Prepare a 10% (v / v) grafting solution by adding glycidyl methacrylate to ethanol or an ethanol / water mixture, and stir thoroughly to ensure homogeneity before use. If necessary, nitrogen gas can be purged for 10 minutes before the reaction to reduce the influence of oxygen on the free radical grafting reaction.

[0033] 1.4 Photografting of glycidyl methacrylate active layer A polypropylene film loaded with a photoinitiator was immersed in a glycidyl methacrylate grafting solution, and a photoinduced grafting reaction was carried out under 365 nm ultraviolet light irradiation for 10 min. This allowed glycidyl methacrylate to graft and polymerize on the surface of the polypropylene film, forming a glycidyl methacrylate layer containing epoxy active groups. After the reaction was completed, the film was repeatedly washed with anhydrous ethanol and deionized water for 10 min each time to remove unreacted monomers and free polymers.

[0034] 1.5 Grafting of cyano-containing aromatic functional molecules 4-Aminobenzonitrile was dissolved in N,N-dimethylformamide (DMF) to prepare a 20 mmol / L 4-aminobenzonitrile solution. A glycidyl methacrylate-modified film was placed in this solution and a grafting reaction was carried out at 70 °C with gentle stirring. Grafting times of 2 h, 4 h, 6 h, and 8 h were set to prepare composite films with different grafting degrees. The ring-opening reaction between the amino group in 4-aminobenzonitrile and the epoxy group in the active layer allows cyano-containing aromatic functional molecules to be stably grafted onto the surface of the polypropylene film, forming a cyano-containing aromatic functional interface layer.

[0035] 1.6 Cleaning and Drying After grafting, the film was sequentially washed multiple times with N,N-dimethylformamide (DMF), anhydrous ethanol, and deionized water for 10 min each time to remove unreacted and physically adsorbed functional molecules from the surface. The film was then placed in a clean environment or a vacuum drying oven and dried at 60°C for 1 h to remove residual solvents and improve the stability of the functional interface layer.

[0036] 1.7 Post-processing As needed, the dried composite film can be heat-treated at 70℃ for 30 min to promote further reaction between residual epoxy groups and amino groups, improve the grafting stability of functional molecules and the integrity of the interfacial layer structure, thereby obtaining a uniform, stable, and tunably grafted cyano-aromatic functional interfacial layer composite polypropylene film, the structure of which is as follows: Figure 1 As shown.

[0037] 1.8 Performance Analysis Unmodified polypropylene film was used as PP sample; different grafting times were set to 2 h, 4 h, 6 h and 8 h in 1.5 grafting of cyano aromatic functional molecules, respectively, to prepare composite film samples with different grafting degrees, namely Example 1, Example 2, Example 3 and Example 4.

[0038] The breakdown test employed a ball-and-plate electrode structure, with an upper electrode diameter of 1 cm and a lower circular plate electrode diameter of 10 cm. The sample was immersed in a silicone oil bath during testing to avoid the effects of surface discharge and air breakdown. Temperatures were set at 25℃ and 125℃, and the voltage increased linearly at a rate of approximately 0.2 kV / s. Each sample was tested 15 times at each temperature. The breakdown strength data were statistically analyzed using a two-parameter Weibull distribution.

[0039] The results are shown in Table 1. The DC breakdown field strength of the composite film prepared by this invention is significantly better than that of ordinary unmodified polypropylene film.

[0040] Table 1. DC breakdown field strength of polypropylene film and composite film at different temperatures

[0041] DC conductivity was measured using the standard three-electrode method. Under conditions of 25℃ and 125℃, a voltage was continuously applied for 40 min under an electric field of 40 kV / mm. The conductivity was calculated by taking the average current in the last 30 s of the test to reduce the influence of polarization effect on the results.

[0042] The results are as follows Figure 2 As shown, the electrical conductivity of the composite film prepared by this invention is significantly lower than that of ordinary unmodified polypropylene film.

[0043] In summary, this invention achieves a synergistic effect of interfacial carrier trapping, carrier migration suppression, interfacial binding stabilization, and local electric field modulation by constructing a cyano-containing aromatic functional interface layer on the surface of a polypropylene film. This results in a composite film with low DC conductivity, high breakdown strength, and good energy storage performance. This method can effectively improve the overall performance of metallized film capacitor dielectric materials under high-temperature and high-electric-field conditions, providing technical support for the development of capacitors towards miniaturization, high energy density, and high reliability.

[0044] Although the above embodiments have provided a detailed description of the present invention, they are only some embodiments of the present invention, and not all embodiments. People can obtain other embodiments based on these embodiments without creative effort, and these embodiments all fall within the protection scope of the present invention.

Claims

1. A composite polypropylene film containing a cyano aromatic interface layer, characterized in that, The cyano-containing aromatic interface layer is a functional interface layer formed by grafting glycidyl methacrylate onto the surface of a polypropylene film and introducing cyano-containing aromatic functional molecules.

2. The method for preparing a composite polypropylene film containing a cyano aromatic interface layer as described in claim 1, characterized in that, Includes the following steps: S1. After cleaning and drying, the polypropylene film is immersed in a photoinitiator solution for 10 min. S2. Under nitrogen atmosphere, the product treated in step S1 is immersed in a grafting solution containing glycidyl methacrylate to carry out a photo-induced grafting reaction. After washing, a poly(glycidyl methacrylate) modified film is obtained. S3. The poly(glycidyl methacrylate) modified film was placed in a 20 mmol / L 4-aminobenzonitrile solution for grafting reaction. After washing and drying, it was heat-treated at 70℃ for 30 min to obtain a composite polypropylene film containing a cyano aromatic interface layer.

3. The preparation method according to claim 2, characterized in that, The drying process described in step S1 is at 60°C for 1 hour.

4. The preparation method according to claim 2, characterized in that, The photoinitiator solution in step S1 is prepared by dissolving benzophenone in acetone; the mass concentration of the photoinitiator solution is 0.5%.

5. The preparation method according to claim 2, characterized in that, The grafting solution containing glycidyl methacrylate in step S2 is prepared by adding glycidyl methacrylate to ethanol or an ethanol / water mixed solvent; the volume fraction of glycidyl methacrylate in the grafting solution is 10%.

6. The preparation method according to claim 2, characterized in that, The reaction conditions for the photoinduced grafting reaction in step S2 are: irradiation with 365 nm ultraviolet light for 10 min.

7. The preparation method according to claim 2, characterized in that, The 4-aminobenzonitrile solution mentioned in step S3 is prepared by dissolving 4-aminobenzonitrile in N,N-dimethylformamide.

8. The preparation method according to claim 2, characterized in that, The reaction conditions for the grafting reaction in step S3 are: 70°C for 6 h.

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