Modified polypropylene film, method for producing the same, and capacitor

Abstract

The application discloses a modified polypropylene film, a preparation method thereof and a capacitor. The preparation method of the modified polypropylene film comprises the following steps: applying an electric field to pretreat the surface of a polypropylene film; and soaking the pretreated surface with natural ester vegetable oil to obtain the modified polypropylene film. The pretreatment is beneficial to inducing the polypropylene film to form a surface with physical combination, chemical combination and electrostatic attraction potential, and beneficial to improving the wettability of the polypropylene film in the natural ester vegetable oil. On this basis, when the surface of the polypropylene film is treated with the natural ester vegetable oil, the surface of the polypropylene film can be more fully infiltrated with the natural ester vegetable oil under the synergistic effect of the physical combination, the chemical combination and the electrostatic attraction, the natural ester vegetable oil is promoted to optimize the state of aggregation of the polypropylene film, and the pores near the combined interface of the polypropylene film and the natural ester vegetable oil are reduced. Therefore, the modified polypropylene film prepared by the application has good mechanical properties and electrical properties.

Description

Technical Field

[0001] This application relates to the field of polymer material modification technology, specifically to a modified polypropylene film and its preparation method, and a capacitor. Background Technology

[0002] Polypropylene (PP), a semi-crystalline thermoplastic polymer, is widely considered an ideal candidate material for power capacitor films due to its excellent dielectric properties, high heat resistance, low cost, and good processability. Although the breakdown field strength of pure polypropylene is already relatively high, with the trend of equipment miniaturization and high voltage, the breakdown field strength of existing polypropylene materials is still insufficient to meet the extreme requirements of equipment. Furthermore, in practical applications, polypropylene not only needs to withstand electric field stress but also cope with complex mechanical stresses, requiring a synergistic improvement in both mechanical and electrical properties. Summary of the Invention

[0003] In view of this, this application provides a modified polypropylene film, a method for preparing the same, and a capacitor to solve the above-mentioned technical problems.

[0004] To achieve the above objectives, in a first aspect, this application provides a method for preparing a modified polypropylene film, comprising: applying a direct current electric field to pretreat the surface of the polypropylene film; and immersing the pretreated surface in a natural ester vegetable oil to prepare the modified polypropylene film.

[0005] Based on the first aspect, in some embodiments, the electric field strength of the pretreatment is 10 kV / mm to 30 kV / mm.

[0006] Based on the first aspect, in some implementations, the preprocessing time is 10 min to 60 min.

[0007] Based on the first aspect, in some embodiments, the natural ester vegetable oil includes FR3.

[0008] Based on the first aspect, in some embodiments, the soaking treatment time is 12 h to 36 h.

[0009] Based on the first aspect, in some embodiments, the immersion treatment temperature is between 15 °C and 40 °C.

[0010] Based on the first aspect, in some embodiments, the immersion treatment is carried out under light-protected conditions.

[0011] Based on the first aspect, in some embodiments, after the immersion treatment, the preparation method further includes: drying the polypropylene film after immersion treatment, wherein the drying temperature is 40 ℃ to 60 ℃ and the drying time is 8 h to 14 h.

[0012] Secondly, this application provides a modified polypropylene film, which is prepared by the above-described preparation method.

[0013] Thirdly, this application provides a capacitor comprising insulating oil and an insulating medium impregnated in the insulating oil, the insulating medium comprising the aforementioned modified polypropylene film.

[0014] The modified polypropylene film preparation method of this application involves DC electric field pretreatment. On one hand, this roughens the surface of the polypropylene film, such as reducing sharp defects, which optimizes the surface and makes it easier to physically bond with natural ester vegetable oils, thus enhancing the degree of physical bonding. On the other hand, it injects charge carriers into the surface of the polypropylene film and optimizes the orientation of the polypropylene molecular chains through space charge polarization, thereby inducing the formation of active sites on the surface of the polypropylene film that are more conducive to chemical bonding with natural ester vegetable oils. Furthermore, the DC electric field pretreatment also allows the polypropylene film surface to carry a persistent electrostatic charge. When natural ester vegetable oil approaches, the surface charge can induce molecular polarization in the oil droplets through electrostatic interaction, thereby generating an additional attractive force. Therefore, the above pretreatment is beneficial for inducing the formation of a surface in the polypropylene film with potential for physical bonding, chemical bonding, and electrostatic attraction, which is beneficial for improving the wettability of the polypropylene film in natural ester vegetable oils. Building upon this, when the surface of a polypropylene film is treated with natural ester vegetable oil, the surface of the polypropylene film can achieve more thorough wetting with the natural ester vegetable oil through the synergistic effects of physical bonding, chemical bonding, and electrostatic attraction. This promotes the optimization of the aggregation state of the polypropylene film by the natural ester vegetable oil, reducing porosity near the interface. On the one hand, this reduces deep-level charge trapping and homogenizes the electric field distribution, thereby improving the breakdown field strength of the polypropylene film. On the other hand, the formation of a denser cross-linked network simultaneously enhances tensile strength. Therefore, the modified polypropylene film prepared in this application possesses both excellent mechanical and electrical properties. Attached Figure Description

[0015] Figure 1 Fourier transform infrared spectra of the polypropylene film provided in Comparative Example 1 of this application and the modified polypropylene films provided in Examples 1-3 and Comparative Example 2.

[0016] Figure 2 The oil contact angle test results are for the polypropylene film provided in Comparative Example 1 of this application and the modified polypropylene films provided in Examples 1-3 and Comparative Example 2.

[0017] Figure 3 The tensile strength and elongation at break of the polypropylene film provided in Comparative Example 1 of this application, as well as the modified polypropylene films provided in Examples 1-3 and Comparative Example 2, are shown in the test results.

[0018] Figure 4 The breakdown field strength spectra at 30 °C were fitted using Weibull distribution analysis for the polypropylene film provided in Comparative Example 1 of this application and the modified polypropylene films provided in Examples 1-3 and Comparative Example 2.

[0019] Figure 5 The breakdown field strength spectra at 50 °C were fitted using Weibull distribution analysis for the polypropylene film provided in Comparative Example 1 of this application and the modified polypropylene films provided in Examples 1-3 and Comparative Example 2.

[0020] Figure 6 The breakdown field strength spectra at 70 °C were fitted using Weibull distribution analysis for the polypropylene film provided in Comparative Example 1 of this application and the modified polypropylene films provided in Examples 1-3 and Comparative Example 2. Detailed Implementation

[0021] To facilitate understanding of the technical solutions of this application, a more comprehensive description of the technical solutions will be provided below. The technical solutions of this application can be implemented in many different forms and are not limited to the embodiments described herein. Rather, these embodiments are provided to make the disclosure of the technical solutions of this application more thorough and comprehensive.

[0022] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0023] In related technologies, conventional nanofiller composite modification methods can improve mechanical strength, but the inorganic-organic interface is prone to causing electric field distortion, leading to a decrease in breakdown performance. While chemical modification methods such as grafting and crosslinking can regulate electrical properties, they often come at the cost of sacrificing processability or mechanical toughness. This constraint between electromechanical properties has become a key scientific problem restricting the application of polypropylene in high-end insulation fields. Based on this, this application develops a polypropylene with synergistically improved mechanical and electrical properties.

[0024] Based on this, one embodiment of this application provides a method for preparing a modified polypropylene film, comprising:

[0025] Step 1: Apply a DC electric field to pretreat the surface of the polypropylene film.

[0026] The modified polypropylene film preparation method of this application involves DC electric field pretreatment. On one hand, this roughens the surface of the polypropylene film, such as reducing sharp defects, which optimizes the surface and makes it easier to physically bond with natural ester vegetable oils, thus enhancing the degree of physical bonding. On the other hand, it injects charge carriers into the surface of the polypropylene film and optimizes the orientation of the polypropylene molecular chains through space charge polarization, thereby inducing the formation of active sites on the surface of the polypropylene film that are more conducive to chemical bonding with natural ester vegetable oils. Furthermore, the DC electric field pretreatment also allows the polypropylene film surface to carry a persistent electrostatic charge. When natural ester vegetable oil approaches, the surface charge can induce molecular polarization in the oil droplets through electrostatic interaction, thereby generating an additional attractive force. Therefore, the above pretreatment is beneficial for inducing the formation of a surface in the polypropylene film with potential for physical bonding, chemical bonding, and electrostatic attraction, which is beneficial for improving the wettability of the polypropylene film in natural ester vegetable oils.

[0027] Understandably, the above-mentioned pretreatment of applying a DC electric field can be performed on one surface of the polypropylene film or on both surfaces of the polypropylene film. Understandably, when the above-mentioned electric field pretreatment is performed on both surfaces of the polypropylene film, the resulting polypropylene film as a whole has better wettability to natural ester vegetable oil.

[0028] In some embodiments, the electric field strength for pretreatment is between 10 kV / mm and 30 kV / mm. For example, the electric field strength for pretreatment can be 10 kV / mm, 12 kV / mm, 14 kV / mm, 16 kV / mm, 18 kV / mm, 20 kV / mm, 22 kV / mm, 24 kV / mm, 26 kV / mm, 28 kV / mm, 30 kV / mm, or any value within the range of any two of the above values. This application has found that controlling the electric field strength for pretreatment within the above range is beneficial for overcoming the resistance to eliminating sharp-point defects on the surface of the polypropylene film, more effectively improving the physical bonding between the polypropylene film and the natural ester vegetable oil, and simultaneously optimizing the orientation of the polypropylene molecular chains more specifically, thus more effectively improving the chemical bonding between the polypropylene film and the natural ester vegetable oil, thereby further enhancing the potential for physical and chemical bonding between the surface of the polypropylene film after electric field pretreatment and the natural ester vegetable oil. Controlling the above-mentioned electric field strength range also helps to reduce the risk of space charge accumulation when the field strength is large, which could lead to local breakdown, molecular chain breakage, stress concentration points, and weakening of the integrity of the crystal region, resulting in a decrease in tensile strength. It also helps to avoid the problem of insignificant pretreatment effect when the field strength is weak.

[0029] In some embodiments, the pretreatment time is from 10 min to 60 min. For example, the pretreatment time can be 10 min, 15 min, 20 min, 25 min, 30 min, 35 min, 40 min, 45 min, 50 min, 55 min, 60 min, or any value within the range of any two of the above values. Controlling the pretreatment time within the above range helps the applied electric field to adequately pretreat the polypropylene film and achieve the desired effect.

[0030] Step 2: Immerse the pretreated surface with natural ester vegetable oil to prepare a modified polypropylene film.

[0031] Based on the above pretreatment, when the surface of the polypropylene film is treated with natural ester vegetable oil, the surface of the polypropylene film can be more fully wetted by the natural ester vegetable oil under the synergistic effect of physical bonding, chemical bonding, and electrostatic attraction. This promotes the optimization of the aggregation state of the polypropylene film by the natural ester vegetable oil, reducing the porosity near the interface between the two. On the one hand, this reduces deep-level charge trapping and homogenizes the electric field distribution, thus improving the breakdown field strength of the polypropylene film. On the other hand, it also enhances the tensile strength by forming a denser cross-linked network. Therefore, the modified polypropylene film prepared in this application possesses both good mechanical and electrical properties.

[0032] Understandably, without the above-mentioned electric field pretreatment, when the surface of the polypropylene film is treated with natural ester vegetable oil, there are many bubbles or pores at the interface between the natural ester vegetable oil and the polypropylene film, and the wetting of the two is not sufficient, resulting in limited improvement on the breakdown field strength and tensile strength.

[0033] In some embodiments, the natural ester vegetable oil includes FR3. The method for preparing the modified polypropylene film of this application, through electric field pretreatment, facilitates the formation of a surface in the polypropylene film with physical bonding, chemical bonding, and electrostatic attraction potential, thereby improving the wettability of the polypropylene film in the natural ester vegetable oil. Therefore, selecting the above-mentioned materials as natural ester vegetable oils allows for sufficient wetting of the polypropylene film after electric field pretreatment, thereby improving the electrical and mechanical properties of the film. Furthermore, these natural ester vegetable oils are commonly used as insulating oils for capacitors, and the modified polypropylene film of this application can be used in such capacitors, being compatible with insulating oil components.

[0034] In some embodiments, the soaking time is from 12 h to 36 h. For example, the soaking time can be 12 h, 14 h, 16 h, 18 h, 20 h, 22 h, 24 h, 26 h, 28 h, 30 h, 32 h, 34 h, 36 h, or any value within the range of any two of the above values. Controlling the soaking time of the natural ester vegetable oil within the above range is beneficial to promoting sufficient bonding and wetting of the pretreated polypropylene film with the natural ester vegetable oil, thereby more effectively reducing the porosity near the bonding interface between the two.

[0035] In some embodiments, the immersion treatment temperature is between 15°C and 40°C. For example, the immersion treatment temperature can be 15°C, 18°C, 21°C, 24°C, 27°C, 30°C, 33°C, 36°C, 39°C, 40°C, or any value within the range of any two of the above values. Controlling the temperature of the natural ester vegetable oil immersion treatment within the above range is beneficial for promoting the molecular movement of the natural ester vegetable oil while reducing excessive disturbance to the polypropylene molecular chains at higher temperatures, which could affect the full wetting of the polypropylene film and the natural ester vegetable oil.

[0036] In some embodiments, the immersion treatment is carried out under light-protected conditions. Controlling the immersion treatment under light-protected conditions helps reduce photo-oxidation reactions of natural ester vegetable oils rich in unsaturated double bonds during the immersion process, thereby reducing the generation of polar small molecules such as acids and aldehydes, as well as free radicals, thus ensuring that the insulating properties of the natural ester vegetable oil medium itself are not degraded. It should be noted that the light-protected conditions in this application include, but are not limited to, light-protected conditions generated by reducing light absorption; these can be complete light protection or partial light protection.

[0037] In some embodiments, after the immersion treatment, the preparation method further includes: drying the immersion-treated polypropylene film at a temperature of 40°C to 60°C for a time of 8 h to 14 h. For example, the drying temperature can be 40°C, 42°C, 44°C, 46°C, 48°C, 50°C, 52°C, 54°C, 56°C, 58°C, 60°C, or any value within the range of any two of the above values. The drying time can be 8 h, 8.5 h, 9 h, 9.5 h, 10 h, 10.5 h, 11 h, 11.5 h, 12 h, 12.5 h, 13 h, 13.5 h, 14 h, or any value within the range of any two of the above values. Drying helps remove free or residual natural ester vegetable oils from the surface of the soaked polypropylene film. In this application, reducing free or residual natural ester vegetable oils on the surface of the soaked polypropylene film helps to further maintain adequate wetting between the polypropylene film and the natural ester vegetable oils.

[0038] An embodiment of this application also provides a modified polypropylene film, which is prepared by the above-described preparation method.

[0039] The modified polypropylene film of this application was tested by Fourier transform infrared spectroscopy and showed performance at a wavenumber of 1750±10 cm⁻¹. -1 An absorption peak is formed at a certain point. The inventors reasonably speculate that the formation of this absorption peak is attributed to the stretching vibration of the carbonyl groups in the bonded and impregnated natural ester vegetable oil. Compared to polypropylene films that are directly impregnated with natural ester vegetable oil without an applied DC electric field (i.e., without pretreatment), the modified polypropylene film of this application exhibits an absorption peak at a wavenumber of 1750±10 cm⁻¹. -1 The peak area of ​​the stretching vibration peak is larger, and the ratio of the relative peak areas is about 1.004-1.032, indicating that the modified polypropylene film prepared in this application forms a more sufficient bond and wetting with the natural ester vegetable oil. The preparation method of this application helps to improve the wettability between the polypropylene film and the natural ester vegetable oil.

[0040] Understandably, without electric field pretreatment, the resulting polypropylene film, when analyzed by Fourier transform infrared spectroscopy, exhibits poor performance at a wavenumber of 1750±10 cm⁻¹. -1 The peak area of ​​the absorption peak formed at this point will be smaller than the lower limit of the preset range, indicating that the polypropylene film is not sufficiently impregnated with the natural ester vegetable oil at this time, and the improvement on the breakdown field strength and tensile strength is also limited.

[0041] One embodiment of this application also provides a capacitor, including insulating oil and an insulating medium impregnated in the insulating oil, the insulating medium including the modified polypropylene film described above.

[0042] The capacitors using the modified polypropylene film of this application exhibit improved dielectric strength of the insulating dielectric layer, enabling them to withstand higher operating voltages within the same volume. The improved mechanical strength of the film provides better dimensional stability and resistance to deformation during winding and long-term operation, thereby improving manufacturing yield and operational reliability. Therefore, the capacitors of this application possess improved service life and performance stability, and exhibit good adaptability to high-voltage conditions.

[0043] The present application will be described below through specific embodiments and comparative examples. Those skilled in the art should understand that the preparation methods described in this application are merely examples, and any other suitable preparation methods are within the scope of this application.

[0044] Example 1:

[0045] A method for preparing a modified polypropylene film, comprising:

[0046] Step 1: Place the polypropylene film, prepared by extrusion using a twin-screw extruder, between parallel plate electrodes. Connect the electrodes to a high-voltage DC power supply and a grounding terminal. Perform pretreatment by applying an electric field of 10 kV / mm to both surfaces of the polypropylene film for 30 min.

[0047] Step 2: Immerse the pretreated polypropylene film in natural ester vegetable oil FR3 and place it under light-proof conditions for 24 hours. Remove the film after immersion and dry it in a vacuum oven at 50 ℃ for 12 hours to obtain the modified polypropylene film.

[0048] Example 2:

[0049] The difference from Example 1 is that in the first step, the electric field strength is adjusted to 20 kV / mm.

[0050] Example 3:

[0051] The difference from Example 1 is that in the first step, the electric field strength is adjusted to 30 kV / mm.

[0052] Comparative Example 1:

[0053] The polypropylene film from Example 1 was used without any treatment.

[0054] Comparative Example 2:

[0055] The difference from Comparative Example 1 is that the untreated polypropylene film was subjected to the following treatment:

[0056] Step 1: Immerse the polypropylene film in natural ester vegetable oil FR3 and place it under light-proof conditions for 24 hours.

[0057] Step 2: Remove the soaked film and dry it in a vacuum oven at 50 ℃ for 12 h to obtain a modified polypropylene film.

[0058] The modified polypropylene films obtained in Examples 1-3 and Comparative Examples 1-2 were subjected to the following performance tests:

[0059] 1. Fourier transform infrared spectroscopy analysis and testing: The Nicolet 6700 Fourier transform infrared spectrometer is manufactured by Shenzhen Ruisheng Technology Co., Ltd.

[0060] 2. Oil contact angle test of polypropylene film: Contact angle measuring instrument model JC2000C1 manufactured by Shanghai Zhongchen Digital Technology Equipment Co., Ltd.

[0061] 3. Tensile strength and elongation at break of polypropylene film: tensile testing machine model DR-507A manufactured by Dongwan Dongri Instrument Co., Ltd.

[0062] 4. Breakdown field strength test of polypropylene film based on Weibull distribution analysis: a breakdown device composed of asymmetric columnar electrodes (high voltage electrode diameter 25 mm, ground electrode diameter 75 mm).

[0063] The test results for this application are as follows:

[0064] Please see Figure 1 The polypropylene film of Comparative Example 1 and the modified polypropylene films obtained in Examples 1-3 and Comparative Example 2 were compared at 2938 cm⁻¹. -1 The absorption peak at 1455 cm⁻¹ is attributed to the asymmetric stretching vibration of the methylene group in polypropylene; the absorption peak at 1455 cm⁻¹ is attributed to the asymmetric stretching vibration of the methylene group in polypropylene. -1 The absorption peak at 1375 cm⁻¹ is due to the asymmetric bending vibrations of the methylene and methyl groups; -1 The absorption peak at 997 cm⁻¹ is attributed to the symmetric bending vibration of the methyl group. -1 The absorption peak at 1750 cm⁻¹ is a characteristic peak closely related to the stereoregularity of polypropylene and is attributed to skeletal vibrations. Among them, the polypropylene films impregnated with natural ester vegetable oil in Examples 1-3 and Comparative Example 2 show an absorption peak at 1750 cm⁻¹. -1 A new absorption peak appeared at 1750 cm⁻¹, mainly attributed to the stretching vibration of the carbonyl group in the natural ester vegetable oil. The modified polypropylene films of Examples 1-3 underwent electric field pretreatment, which improved their wettability to the natural ester vegetable oil. The modified polypropylene films of Examples 1-3 and Comparative Example 2 showed similar absorption peaks at 1750 cm⁻¹. -1 ±10cm -1 The ratio of the relative peak areas of the stretching vibration peaks is 1.004-1.032. Therefore, the peak areas of the natural ester vegetable oil characteristic peaks of the modified polypropylene films of Examples 1-3 are greater than the peak areas of the corresponding peaks in Comparative Example 2.

[0065] Please see Figure 2 According to the oil contact angle test results, compared with Comparative Example 1 (23.79°) and Comparative Example 2 (23.01°), the oil contact angles of Examples 1-3 (18.32°, 16.64°, 13.91°) decreased significantly. This indicates that the electric field pretreatment roughens the surface of the polypropylene film, such as reducing sharp defects, which is beneficial to optimizing the surface of the polypropylene film, making it easier to physically combine with natural ester vegetable oil and improving the degree of physical combination. On the other hand, it injects charge carriers into the surface of the polypropylene film and optimizes the orientation of the polypropylene molecular chains through space charge polarization, thereby inducing the formation of active sites on the surface of the polypropylene film that are more likely to chemically combine with natural ester vegetable oil. In addition, the electric field pretreatment can also make the surface of the polypropylene film carry a persistent electrostatic charge. When natural ester vegetable oil approaches, the surface charge can induce molecular polarization in the oil droplet through electrostatic interaction, thereby generating an additional attractive force. Therefore, the above pretreatment is beneficial for inducing the formation of a surface in the polypropylene film with physical bonding, chemical bonding, and electrostatic attraction potential, which is beneficial for improving the wettability of the polypropylene film in natural ester vegetable oils. Thus, the embodiments of this application improve the wettability of the polypropylene film.

[0066] Please see Figure 3 The mechanical properties of the untreated polypropylene film in Comparative Example 1 were poor (tensile strength 8.93 MPa, elongation at break 69.3%), and the mechanical properties of the modified polypropylene film in Comparative Example 2 without electric field pretreatment were also only slightly improved (tensile strength 9.17 MPa, elongation at break 243%). The modified polypropylene films of Examples 1-3 in this application exhibit significantly improved mechanical properties: In terms of tensile strength, the tensile strengths of Examples 1-3 are 18 MPa, 30.8 MPa, and 26.1 MPa, respectively, with Example 2 showing the highest tensile strength; in terms of elongation at break, the elongation at break of Examples 1-3 is 589%, 541%, and 508%, respectively. This indicates that electric field pretreatment is beneficial in inducing the formation of a surface in the polypropylene film with physical bonding, chemical bonding, and electrostatic attraction potential, which improves the wettability of the polypropylene film in natural ester vegetable oil. Furthermore, when the surface of the polypropylene film is treated with natural ester vegetable oil, the surface of the polypropylene film can achieve more thorough wetting with the natural ester vegetable oil under the synergistic effect of physical bonding, chemical bonding, and electrostatic attraction, promoting the optimization of the aggregation state of the polypropylene film by the natural ester vegetable oil. This reduces the porosity near the interface between the two, and through the formation of a tighter cross-linked network, it is beneficial to enhance tensile strength. Therefore, the toughness of the modified polypropylene film obtained in the examples of this application is significantly improved.

[0067] Please refer to the following: Figure 4 , Figure 5 and Figure 6 At 30 °C, the breakdown field strength of Comparative Example 1 was 138.9 kV / mm, and that of Comparative Example 2 was 151.7 kV / mm. The breakdown field strengths of Examples 1-3 were 156.5 kV / mm, 166.2 kV / mm, and 160.8 kV / mm, respectively. This indicates that electric field pretreatment is beneficial for inducing the formation of a surface with physical bonding, chemical bonding, and electrostatic attraction potential in the polypropylene film, and is beneficial for improving the wettability of the polypropylene film in natural ester vegetable oil. Based on this, when the surface of the polypropylene film is treated with natural ester vegetable oil, under the synergistic effect of physical bonding, chemical bonding, and electrostatic attraction, the surface of the polypropylene film can form more sufficient wettability with the natural ester vegetable oil, promoting the optimization of the aggregation state of the polypropylene film by the natural ester vegetable oil, reducing the porosity near the interface between the two, reducing deep-level charge traps and homogenizing the electric field distribution, thereby improving the breakdown field strength of the polypropylene film. Therefore, the modified polypropylene film obtained in the examples of this application has a significantly improved breakdown field strength. At 50 ℃ and 70 ℃, the breakdown field strength decreases with increasing temperature. This is because increasing temperature increases the concentration and mobility of charge carriers, which increases the probability of collisional ionization and thus reduces the breakdown field, but the overall trend remains unchanged.

[0068] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0069] The embodiments described above are merely illustrative of several implementations of the technical solution of this application, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the technical solution of this application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the inventive concept of this application, and these modifications and improvements all fall within the protection scope of this application. Therefore, the protection scope of this application should be determined by the appended claims.

Claims

1. A method for preparing a modified polypropylene film, characterized in that, include: A DC electric field is applied to pretreat the surface of the polypropylene film; The modified polypropylene film is prepared by immersing the pretreated surface in natural ester vegetable oil.

2. The preparation method according to claim 1, characterized in that, The electric field strength of the pretreatment is 10 kV / mm to 30 kV / mm.

3. The preparation method according to claim 1, characterized in that, The pretreatment time is 10 min to 60 min.

4. The preparation method according to claim 1, characterized in that, The natural ester vegetable oil includes FR3.

5. The preparation method according to claim 1, characterized in that, The soaking treatment time is 12 h to 36 h.

6. The preparation method according to claim 1, characterized in that, The soaking temperature is between 15 ℃ and 40 ℃.

7. The preparation method according to claim 1, characterized in that, The soaking treatment was carried out under light-protected conditions.

8. The preparation method according to claim 1, characterized in that, After the soaking treatment, the preparation method further includes: The polypropylene film after the soaking treatment is dried at a temperature of 40°C to 60°C for 8 to 14 hours.

9. A modified polypropylene film, characterized in that, The modified polypropylene film is prepared by the preparation method according to any one of claims 1-8.

10. A capacitor comprising insulating oil and an insulating dielectric impregnated in the insulating oil, characterized in that, The insulating medium includes the modified polypropylene film as described in claim 9.

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