Method for improving energy storage performance of polypropylene film for capacitor based on crystal form regulation and control

Abstract

The invention discloses a method for improving the energy storage performance of a polypropylene film for a capacitor based on crystal form regulation and control, which comprises the following steps of: regulating and controlling the adding proportion of a nucleating agent in polypropylene, dispensing, drying, annealing, cooling step by step and the like to induce the generation of beta crystals and regulate and control the aggregation state structure of the polypropylene, so that a deep trap is introduced into the polypropylene film, and the energy storage performance of the polypropylene film for the capacitor is improved. Further, the energy storage characteristic of the polypropylene film is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of capacitor films, and in particular relates to a method for improving the energy storage performance of flexible polypropylene films based on crystal form regulation. Background Art

[0002] As a key component of flexible DC transmission systems, metallized polypropylene film capacitors perform multiple electrical functions in converter stations, including energy storage, harmonic suppression, dynamic voltage compensation, potential equalization, and oscillation damping. The reliability of this equipment directly impacts the operational efficiency of new power systems integrated with renewable energy. With increasing voltage levels and more complex power environments, capacitors are becoming more compact and miniaturized, and the demand for their energy storage density is also increasing. Therefore, improving the energy storage performance of polypropylene film used in capacitors is a key issue in flexible DC transmission systems.

[0003] Crystal form manipulation is a key approach to improving the mechanical, optical, and thermal stability of polypropylene films, and is particularly important for the production of high-performance flexible films. Due to their unique performance advantages, flexible polypropylene films with controlled crystal forms have broad application prospects in a variety of fields, including packaging, healthcare, and electronics. However, the extrusion process for producing polypropylene films often operates at temperatures between 210°C and 230°C, which can limit the formation and optimization of certain specific crystal forms.

[0004] Existing patent CN 118956053A discloses a method for processing and modifying polypropylene, comprising the following steps: (1) mixing a polypropylene raw material, an additive, and a nucleating agent, and processing the mixture into a polypropylene profile of a desired shape; (2) subjecting the polypropylene profile to stretching and orientation processing to obtain a modified polypropylene product; wherein the polypropylene raw material is selected from polypropylene or a polypropylene composition, the amount of the nucleating agent added is 0-1.0% by weight of the polypropylene raw material; and the amount of the additive added is 0-1.0% by weight of the polypropylene raw material. This invention optimizes and modifies the polypropylene raw material by synergistically controlling the crystal form and stretching and orientation. The mechanical properties of the obtained polypropylene product are significantly improved, and the processing method is simple. However, it only focuses on the mechanical properties of the material and does not establish a correlation between the mechanical properties and the electrical properties of polypropylene, and therefore may not be applicable to the electrical field. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a method for improving the energy storage performance of polypropylene film by regulating the β crystal content in polypropylene.

[0006] To achieve the above object, the present invention adopts the following technical solutions: A method for improving the energy storage performance of polypropylene film for capacitors based on crystal form regulation includes adding a certain amount of nucleating agent to polypropylene, and performing steps such as drop coating, drying, annealing, and step-by-step cooling to regulate the polypropylene aggregate structure, introduce deep traps, and thereby improve the energy storage characteristics of the polypropylene film. The method specifically includes the following steps: (1) At room temperature, polypropylene is added to xylene and mixed evenly, and then the nucleating agent is added and stirred thoroughly at a certain temperature to ensure that the polypropylene particles are completely dissolved in the xylene and fully mixed with the nucleating agent particles; (2) Using a dropper, evenly apply the mixed solution obtained in step (1) onto a pre-cleaned and dried glass plate, and then place the coated glass plate in an oven to dry; (3) taking out the film sample dried in step (2) and placing it in a vacuum oven, then heating the vacuum oven under vacuum and then cooling it down in steps; (4) Soak the film sample treated in step (3) in deionized water. After the film is naturally peeled off from the glass substrate, dry it in a vacuum oven to remove residual moisture, thereby obtaining a modified polypropylene film.

[0007] Furthermore, the nucleating agent in step (1) is WBG-II.

[0008] Furthermore, the content of polypropylene in the mixed solution obtained in step (1) is 10 mg / mL, and the content of the nucleating agent is less than 1.00 wt‰ of the mass of polypropylene.

[0009] Furthermore, the stirring temperature in step (1) is 115° C. to 125° C., and the stirring time is 5.5 h to 6.5 h.

[0010] Furthermore, the temperature of the drop coating in step (2) is 115°C to 125°C, and the drop coating amount is 0.6 mL / cm 2 .

[0011] Furthermore, the drying temperature in step (2) is 115° C. to 125° C., and the drying time is 11 h to 13 h.

[0012] Furthermore, in step (3), the vacuum oven is heated to 195°C to 205°C and maintained for 115min to 125min.

[0013] Furthermore, the stepwise cooling in step (3) is to first lower the temperature to 125°C~135°C, maintain it for 30 minutes, and then lower it to room temperature at a rate of 10°C every 30 minutes.

[0014] Furthermore, the drying temperature in step (4) is 75°C to 85°C, and the drying time is 55 min to 65 min.

[0015] The significant advantages of the present invention are: The present invention controls the addition ratio of a nucleating agent and step-by-step temperature reduction to induce the formation of β crystals, and regulates the content and morphology of β crystals in a polypropylene film, thereby introducing deep traps into the polypropylene film, hindering charge transport, and further improving the energy storage characteristics of polypropylene for film capacitors, thereby achieving the purpose of improving the electrical performance of the film. The method is low-cost and easy to operate. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 This is a crystal structure diagram of the unmodified polypropylene film prepared in the comparative example.

[0017] Figure 2 The crystal structure diagram of the modified polypropylene film prepared in Examples 1 to 3 (ac).

[0018] Figure 3 2 is a comparison chart of the discharge energy density and charge-discharge efficiency of the polypropylene films prepared in the comparative example and Examples 1 to 3 at room temperature (a) and high temperature (b).

[0019] Figure 4 This is a comparison chart of the maximum discharge energy density of the polypropylene films prepared in the comparative example and Examples 1 to 3 when the energy storage efficiency is greater than 90% under normal temperature and high temperature conditions. DETAILED DESCRIPTION

[0020] A method for improving the energy storage performance of polypropylene film for capacitors based on crystal form regulation, comprising the following steps: (1) At room temperature, polypropylene was added to xylene and mixed evenly. Then, WBG-II nucleating agent was added and placed on a magnetic stirring table. The mixture was stirred at 115°C to 125°C for 5.5 h to 6.5 h to ensure that the polypropylene particles were completely dissolved in the xylene and fully mixed with the nucleating agent particles. The content of polypropylene in the resulting mixture was 10 mg / mL, and the content of nucleating agent was less than 1.00 wt‰ of the mass of polypropylene. (2) At a temperature of 115°C to 125°C, the mixture obtained in step (1) was stirred at 0.6 mL / cm 2 Use a dropper to evenly apply the amount of the coating on the pre-cleaned and dried glass plate, then put the coated glass plate into an oven and dry it at 115℃~125℃ for 11h~13h; (3) Take out the film sample dried in step (2) and place it in a vacuum oven. Then, heat the vacuum oven to 195°C~205°C under vacuum and keep it for 115min~125min. Then, reduce the temperature to 125°C~135°C and keep it for 30min. Then, reduce the temperature to room temperature at a rate of 10°C every 30min. (4) Soak the film sample treated in step (3) in deionized water. After the film is naturally peeled off from the glass substrate, dry it in a vacuum oven at 75°C to 85°C for 55 minutes to 65 minutes to remove residual moisture, thereby obtaining a modified polypropylene film.

[0021] In order to make the contents of the present invention easier to understand, the technical solutions of the present invention are further described below in conjunction with specific implementation methods, but the present invention is not limited thereto.

[0022] The polypropylene (PP) used in the examples was obtained from Beijing Mairuida Technology Co., Ltd., with a model number of M057077-500g. The xylene used was obtained from Shanghai Aladdin Biochemical Technology Co., Ltd., with a model number of X112051-500ml.

[0023] Comparative Example (1) At room temperature, add 150 mg of polypropylene to 15 mL of xylene and mix thoroughly. Then place on a magnetic stirring table and stir thoroughly at 120 °C for 6 h. (2) At a temperature of 120°C, the mixture obtained in step (1) was heated to 0.6 mL / cm 2 Use a dropper to evenly apply the amount of the coating on the pre-cleaned and dried glass plate, then put the coated glass plate into an oven and dry it at 120℃ for 12h; (3) Take out the film sample dried in step (2) and place it in a vacuum oven. Then, heat the vacuum oven to 200°C under vacuum and keep it for 120 minutes. Then, reduce the temperature to 130°C and keep it for 30 minutes. Then, reduce the temperature to room temperature at a rate of 10°C every 30 minutes. (4) The film sample treated in step (3) was immersed in deionized water. After the film was naturally peeled off from the glass substrate, it was dried in a vacuum oven at 80°C for 60 minutes to remove residual moisture, thereby obtaining a polypropylene film, which was labeled as PP-0.

[0024] Example 1 (1) At room temperature, 150 mg of polypropylene was added to 15 mL of xylene and mixed evenly. Then, 0.25 wt‰ of WBG-II nucleating agent was added and the mixture was placed on a magnetic stirring table and stirred at 120 °C for 6 h. (2) At a temperature of 120°C, the mixture obtained in step (1) was heated to 0.6 mL / cm 2 Use a dropper to evenly apply the amount of the coating on the pre-cleaned and dried glass plate, then put the coated glass plate into an oven and dry it at 120℃ for 12h; (3) Take out the film sample dried in step (2) and place it in a vacuum oven. Then, heat the vacuum oven to 200°C under vacuum and keep it for 120 minutes. Then, reduce the temperature to 130°C and keep it for 30 minutes. Then, reduce the temperature to room temperature at a rate of 10°C every 30 minutes. (4) The film sample treated in step (3) was immersed in deionized water. After the film was naturally peeled off from the glass substrate, it was dried in a vacuum oven at 80°C for 60 minutes to remove residual moisture, thereby obtaining a modified polypropylene film, which was labeled as PP-β0.25‰.

[0025] Example 2 (1) At room temperature, 150 mg of polypropylene was added to 15 mL of xylene and mixed evenly. Then, 0.50 wt‰ of WBG-II nucleating agent was added and the mixture was placed on a magnetic stirring table and stirred at 120 °C for 6 h. (2) At a temperature of 120°C, the mixture obtained in step (1) was heated to 0.6 mL / cm 2 Use a dropper to evenly apply the amount of the coating on the pre-cleaned and dried glass plate, then put the coated glass plate into an oven and dry it at 120℃ for 12h; (3) Take out the film sample dried in step (2) and place it in a vacuum oven. Then, heat the vacuum oven to 200°C under vacuum and keep it for 120 minutes. Then, reduce the temperature to 130°C and keep it for 30 minutes. Then, reduce the temperature to room temperature at a rate of 10°C every 30 minutes. (4) The film sample treated in step (3) was immersed in deionized water. After the film was naturally peeled off from the glass substrate, it was dried in a vacuum oven at 80°C for 60 minutes to remove residual moisture, thereby obtaining a modified polypropylene film, which was labeled as PP-β0.5‰.

[0026] Example 3 (1) At room temperature, 150 mg of polypropylene was added to 15 mL of xylene and mixed evenly. Then, 1.00 wt‰ of WBG-II nucleating agent was added to the mixture. The mixture was placed on a magnetic stirring table and stirred at 120 °C for 6 h. (2) At a temperature of 120°C, the mixture obtained in step (1) was heated to 0.6 mL / cm 2 Use a dropper to evenly apply the amount of the coating on the pre-cleaned and dried glass plate, then put the coated glass plate into an oven and dry it at 120℃ for 12h; (3) Take out the film sample dried in step (2) and place it in a vacuum oven. Then, heat the vacuum oven to 200°C under vacuum and keep it for 120 minutes. Then, reduce the temperature to 130°C and keep it for 30 minutes. Then, reduce the temperature to room temperature at a rate of 10°C every 30 minutes. (4) The film sample treated in step (3) was immersed in deionized water. After the film was naturally peeled off from the glass substrate, it was dried in a vacuum oven at 80°C for 60 minutes to remove residual moisture, thereby obtaining a modified polypropylene film, which was labeled as PP-β1‰.

[0027] Figure 1 、 2 The following diagrams illustrate the crystal structures of the polypropylene films prepared in the comparative example and Examples 1-3, respectively. A comparison of the images reveals that the appropriate addition of a nucleating agent not only effectively promotes the growth and distribution of β crystals but also further refines the size of α crystals, resulting in a denser structure. This increases the density of the crystalline regions, introducing deeper traps than previously possible, thereby improving the energy storage properties of the polypropylene film at the crystalline level. The polypropylene film prepared with a 0.5‰ addition of a nucleating agent exhibited the smallest crystal size.

[0028] Figure 3 The following chart compares the discharge energy density and charge-discharge efficiency of polypropylene films prepared in the comparative example and Examples 1-3 at room temperature (25°C) and high temperature (85°C). The figure shows that the addition of a nucleating agent improves the material's energy storage performance. Both at room temperature (25°C) and high temperature (85°C), the maximum discharge energy density and energy storage efficiency of the polypropylene film at the same applied electric field strength are significantly increased, with the addition of 0.5‰ of a nucleating agent achieving the best results.

[0029] Figure 4 The following chart compares the maximum discharge energy density of polypropylene films prepared in the comparative example and Examples 1-3 at room temperature (25°C) or high temperature (85°C) when the energy storage efficiency exceeds 90%. As can be seen, the addition of a nucleating agent significantly increases the maximum discharge energy density of the modified polypropylene film at an efficiency exceeding 90%. This is due, in part, to the effective increase in the trap depth within the polypropylene film's crystalline regions, which macroscopically manifests as an increase in the film's withstandable breakdown electric field strength. Furthermore, it effectively improves energy storage efficiency. These two factors mutually reinforce each other, resulting in a significant increase in the maximum discharge energy density of the polypropylene film at an efficiency exceeding 90%. The most significant improvement is achieved with the addition of 0.5‰ of the nucleating agent.

[0030] The above description is only a preferred embodiment of the present invention. All equivalent changes and modifications made according to the scope of the patent application of the present invention should fall within the scope of the present invention.

Claims

1. A method for improving the energy storage performance of polypropylene film for capacitors based on crystal form regulation, characterized in that: The steps include: (1) At room temperature, add polypropylene to xylene and mix well, then add nucleating agent and stir thoroughly at a certain temperature; (2) evenly applying the mixture obtained in step (1) on a pre-cleaned and dried glass plate, and then placing the coated glass plate in an oven to dry; (3) taking out the film sample dried in step (2) and placing it in a vacuum oven, then heating the vacuum oven under vacuum and then cooling it down in steps; (4) Soaking the film sample treated in step (3) in deionized water, and after the film is naturally peeled off from the glass substrate, vacuum drying is performed to remove residual moisture, thereby obtaining a modified polypropylene film.

2. The method for improving the energy storage performance of polypropylene film for capacitors based on crystal form regulation according to claim 1, characterized in that: The nucleating agent in step (1) is WBG-II.

3. The method for improving the energy storage performance of polypropylene film for capacitors based on crystal form regulation according to claim 1, characterized in that: The content of polypropylene in the mixed solution obtained in step (1) is 10 mg / mL, and the content of the nucleating agent is less than 1.00 wt‰ of the mass of polypropylene.

4. The method for improving the energy storage performance of polypropylene film for capacitors based on crystal form regulation according to claim 1, characterized in that: The stirring temperature in step (1) is 115°C to 125°C, and the stirring time is 5.5h to 6.5h.

5. The method for improving the energy storage performance of polypropylene film for capacitors based on crystal form regulation according to claim 1, characterized in that: The temperature of the drop coating in step (2) is 115°C to 125°C, and the drop coating volume is 0.6 mL / cm 2 .

6. The method for improving the energy storage performance of polypropylene film for capacitors based on crystal form regulation according to claim 1, characterized in that: The drying temperature in step (2) is 115°C to 125°C, and the drying time is 11h to 13h.

7. The method for improving the energy storage performance of polypropylene film for capacitors based on crystal form regulation according to claim 1, characterized in that: In step (3), the vacuum oven is heated to 195°C to 205°C and maintained for 115 min to 125 min.

8. The method for improving the energy storage performance of polypropylene film for capacitors based on crystal form regulation according to claim 1, characterized in that: The stepwise cooling in step (3) specifically involves first lowering the temperature to 125°C~135°C, maintaining it for 30 minutes, and then gradually lowering it to room temperature at a rate of 10°C every 30 minutes.

9. The method for improving the energy storage performance of polypropylene film for capacitors based on crystal form regulation according to claim 1, characterized in that: The vacuum drying in step (4) is carried out at a temperature of 75°C to 85°C and for a time of 55 min to 65 min.

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