Polypropylene composite film for capacitor based on interface modification and preparation method thereof

Abstract

The invention relates to a capacitor polypropylene film preparation method based on interface modification and volume modification, which comprises the following steps: mixing linear polypropylene granules with an ultraviolet light initiator and a cross-linking agent, heating on a double-roller device at 180-190 DEG C for 10-20 minutes, setting the pressure to be 18-22 MPa, and cooling to room temperature to obtain the capacitor polypropylene film. Naturally cooling for 20-30 minutes to obtain a mixed film containing the cross-linking agent, 1.5 wt% of the cross-linking agent and 1 wt% of the ultraviolet light initiator; irradiating the mixed film for 45-60 seconds by using ultraviolet light to obtain a cross-linked and modified polypropylene film; the preparation method comprises the following steps: dissolving N, N-dimethylformamide (DMF) and xylene according to a volume ratio of 3: 1 to obtain a mixed organic solvent, dissolving maleic anhydride and benzophenone into the mixed organic solvent to obtain a mixed solution, and dripping the mixed solution onto the upper surface of a cross-linked polypropylene film to obtain a cross-linked grafted modified polypropylene film; according to the invention, the actual operation requirement of the metallized film capacitor can be better met.

Description

Technical Field

[0001] The invention belongs to the technical field of high-voltage equipment, and in particular relates to a polypropylene composite film for capacitors based on interface modification and a preparation method thereof. Background Art

[0002] Metallized film capacitors are core equipment of modern power systems and renewable energy systems. They rely on power electronics technology and have many advantages, such as small size, light weight, low inductance and self-healing function. Among film capacitors, polypropylene (PP) is a commonly used dielectric material, which is widely used due to its reliable insulation performance and excellent charge and discharge efficiency. However, with the continuous increase in voltage levels, its insulation performance is seriously deteriorated, which may eventually lead to a decline in the overall performance of metallized film capacitors, thereby reducing the performance and efficiency of the entire DC transmission system.

[0003] The present invention proposes a method for improving the dielectric properties of polypropylene film based on interface modification and body modification. Maleic anhydride is grafted on the surface of the polypropylene film to increase the charge concentration on the electrode / film interface, thereby weakening the interface electric field and inhibiting charge injection. In addition, the polypropylene film is cross-linked and modified by ultraviolet radiation. The cross-linking reaction can enhance the entanglement of macromolecular chains inside the polypropylene film, thereby changing the crystal morphology of polypropylene (PP), reducing the migration rate of carriers, and improving the dielectric properties of the capacitor film from the perspective of charge transport. The method has good effect, simple operation and low cost. Summary of the invention

[0004] The present invention takes polypropylene film for capacitors as the research object, aims to make up for the shortcomings of existing polypropylene materials in dielectric properties, proposes a capacitor polypropylene film with high dielectric properties based on interface modification and body modification and a preparation method thereof, the modification method adopts surface grafting and radiation cross-linking, the surface grafting monomer adopts maleic anhydride, and the radiation cross-linking adopts ultraviolet radiation cross-linking, on the basis of regulating the charge injection and carrier migration of the polypropylene film, the breakdown field strength and energy storage characteristics of the polypropylene are improved, the dielectric properties of the capacitor medium can be improved, and then the overall performance of the metallized film capacitor is improved.

[0005] The present invention solves the technical problem by adopting the following technical solutions:

[0006] The method for preparing capacitor polypropylene film based on interface modification and volume modification comprises the following steps:

[0007] The linear polypropylene pellets are mixed with an ultraviolet light initiator and a crosslinking agent, heated on a double roller device at 180° C. to 190° C. for 10 to 20 minutes, the pressure is set to 18 to 22 MPa, and naturally cooled for 20 to 30 minutes to obtain a mixed film containing linear polypropylene, 1.5 wt% trimethylolpropane triacrylate (TMPTA) and 1 wt% benzophenone;

[0008] The mixed film is irradiated with ultraviolet light for 45-60 seconds to obtain a cross-linked modified polypropylene film;

[0009] N, N-dimethylformamide (DMF) and xylene are dissolved in a volume ratio of 3:1 to obtain a mixed organic solvent, maleic anhydride and benzophenone are dissolved in the mixed organic solvent to obtain a mixed solution, and then the mixed solution is dropped onto the upper surface of a cross-linked polypropylene film to obtain a cross-linked graft-modified polypropylene film.

[0010] Furthermore, the ultraviolet light initiator is benzophenone.

[0011] Furthermore, the crosslinking agent is trimethylolpropane triacrylate TMPTA.

[0012] Furthermore, the length and width of the modified polypropylene film are equal, both of which are 9 cm×9 cm.

[0013] Furthermore, the thickness of the modified polypropylene film is 20±5 μm.

[0014] The capacitor polypropylene film based on interface modification and volume modification is prepared based on the above preparation method.

[0015] The advantages and positive effects of the present invention are:

[0016] The method of the invention modifies polypropylene by surface grafting maleic anhydride and ultraviolet radiation cross-linking. The method improves the polarity of the polypropylene surface, strengthens the entanglement of macromolecular chains inside the polypropylene film, improves the charge injection on the polypropylene surface and the internal carrier migration, improves the dielectric properties of the polypropylene, can better meet the needs of the actual operation of the metallized film capacitor, and has important value and significance for the overall efficiency and reliability of the direct current transmission system; the method is reasonably designed and easy to operate. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The technical solution of the present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments, but it should be understood that these drawings are designed only for explanation purposes and are not intended to limit the scope of the present invention. In addition, unless otherwise specified, these drawings are intended only to conceptually illustrate the structural configurations described herein and are not necessarily drawn to scale.

[0018] Figure 1is a hot pressing process diagram of a mixed film sample of the present invention;

[0019] Figure 2 are the breakdown field strength and Weibull distribution shape parameters of the films obtained by different modification methods provided in the embodiments of the present invention;

[0020] Figure 3 It is the energy storage density and charge-discharge efficiency of the films with different modification methods provided in the embodiments of the present invention. DETAILED DESCRIPTION

[0021] First of all, it should be noted that the specific structure, features and advantages of the present invention will be specifically described below by way of example, but all descriptions are only for illustration and should not be understood as limiting the present invention in any way. In addition, any single technical feature described or implied in the embodiments mentioned herein, or any single technical feature displayed or implied in the drawings, can still be combined or deleted between these technical features (or their equivalents) to obtain more other embodiments of the present invention that may not be directly mentioned in this document. In addition, in order to simplify the drawings, the same or similar technical features may be marked only in one place in the same drawing.

[0022] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application may be combined with each other.

[0023] like Figure 1-3 As shown, this embodiment provides a capacitor polypropylene film with high dielectric properties based on interface modification and body modification and a preparation method thereof, and the hot pressing process for preparing the mixed film sample is as follows Figure 1 As shown, the preparation process is mainly divided into three steps.

[0024] The first step is to mix linear polypropylene pellets with benzophenone and trimethylolpropane triacrylate in a double-roll mill, mix them evenly at 180-190° C. and then cool them, and take an appropriate amount of the cooled polypropylene, benzophenone, trimethylolpropane triacrylate mixture and place it in a hot press, hot press it at 180-190° C. and 18-22Mpa for 5-10 minutes, and then cool it naturally to obtain a mixed film containing linear PP, 1.5wt% TMPTA and 1wt% benzophenone;

[0025] The second step is to place the mixed film in an irradiation device and irradiate it with ultraviolet light using a high-pressure mercury lamp (2000W). After 45-60s of ultraviolet irradiation, a cross-linked modified polypropylene film is obtained, and the film is washed three times in DMF to remove residual reactants.

[0026] The third step is to dissolve an appropriate amount of maleic anhydride (MAH) and ultraviolet light initiator (benzophenone) in an organic solvent, which is composed of DMF and xylene in a volume ratio of 3:1. Then, about 100 μL of the mixed solution is dropped onto the upper surface of the cross-linked polypropylene film, and after standing for 1-5 minutes, it is placed in a vacuum drying oven at 60°C and dried for 24 hours to obtain a cross-linked grafted modified polypropylene film (PP-CG) with a length and width of 9±0.5 cm and a thickness of about 20 μm;

[0027] According to the above method, a cross-linked grafted polypropylene film (PP-CG film) was obtained, and the obtained cross-linked grafted polypropylene film (PP-CG film) was tested for breakdown strength and energy storage density. The breakdown test used a built breakdown test platform. Before the test, the thickness of the sample was measured with a spiral micrometer, and an area with uniform thickness was selected and cut into a 3cm×3cm square sample, which was then placed between the test electrodes and filled with silicone oil to start the breakdown strength test. The film was broken down by gradually applying pressure, and the voltage at the time of breakdown was recorded. The breakdown test was performed at different positions on the film, and the recorded data was processed. After Weibull distribution statistics were performed, the breakdown strength was obtained. The energy storage density test used a Radiant Premier II ferroelectric analyzer produced in Germany. The sample to be tested, which was plated with electrodes, was placed between the test electrodes of the ferroelectric analysis test system, and filled with silicone oil to start the hysteresis loop test. Different hysteresis loops can be obtained by gradually increasing the polarization electric field. This study started from 50kV / mm and tested every 50kV / mm until the sample broke down. Finally, a set of hysteresis loop data was obtained when the polarization electric field increased from 50kV / mm to before the sample broke down. Through hysteresis loop analysis, the energy storage characteristics of the film sample can be calculated and inferred. Among them, the calculation formulas for discharge energy density (Ue) and charge and discharge efficiency (η) are as follows:

[0028]

[0029] In the formula, E is the electric field intensity; D is the electric displacement field, the unit is coulomb per square meter (C / m 2 );D max is the maximum electric displacement field, which indicates the maximum electric displacement reached by the material under the action of an external electric field; D r is the residual electric displacement field, which usually represents the residual electric displacement in the material after the electric field is completely removed; U loss It represents the loss energy density, that is, the energy lost by the material due to various loss mechanisms under the action of the electric field.

[0030] The cross-linked grafted polypropylene film (PP-CG) obtained in this example was subjected to a breakdown strength test with a polypropylene film (PP film), a cross-linked polypropylene film (PP-C), and a grafted polypropylene film (PP-G). Figure 2As shown, the breakdown strength of the cross-linked grafted polypropylene film is increased by about 14.8% compared with the polypropylene film (PP-P), which is the largest increase.

[0031] The cross-linked grafted polypropylene film (PP-CG) obtained in this example was tested for energy storage density with a polypropylene film (PP film), a cross-linked polypropylene film (PP-C), and a grafted polypropylene film (PP-G). Figure 3 As shown in the figure, the energy storage density of the cross-linked grafted polypropylene film (PP-CG) is higher than that of the polypropylene film (PP film), cross-linked polypropylene film (PP-C), and grafted polypropylene film (PP-G) as the field strength increases, and the efficiency is also higher. The charge and discharge efficiency decays more slowly as the field strength increases.

[0032] like Figure 2 , Figure 3 As shown, the modified polypropylene film designed according to the above method has greatly improved breakdown field strength and energy storage density compared with pure polypropylene film (PP-P), cross-linked polypropylene film (PP-C), and grafted polypropylene film (PP-G), which meets the working requirements of high-voltage DC metallized film capacitors.

[0033] The method of the present invention performs ultraviolet radiation cross-linking modification on polypropylene film, and grafts maleic anhydride on the surface of polypropylene film. The surface grafting of maleic anhydride will increase the charge concentration on the electrode / film interface, thereby weakening the interface electric field and inhibiting charge injection; in addition, the cross-linking reaction can enhance the entanglement of macromolecular chains, thereby changing the crystal morphology of PP and reducing the migration rate of carriers. The method improves the dielectric properties of capacitor film from the perspective of charge transport; the grafted maleic anhydride monomer double bond and carbonyl structure can enhance the polarity of the polymer, improve the compatibility in the polymer blend, make polymers of different polarities dispersed evenly, and at the same time improve the relative dielectric constant of the polypropylene film.

[0034] The above embodiments describe the present invention in detail, but the contents are only preferred embodiments of the present invention and cannot be considered to limit the scope of the present invention. All equivalent changes and improvements made within the scope of the present invention should still fall within the scope of the present invention.

Claims

1. A method for preparing a capacitor polypropylene film based on interface modification and volume modification, characterized in that: The steps include: The linear polypropylene pellets are mixed with an ultraviolet initiator and a crosslinking agent, heated on a double-roll device at 180° C. to 190° C. for 10 to 20 minutes, the pressure is set to 18 to 22 MPa, and naturally cooled for 20 to 30 minutes to obtain a mixed film containing linear polypropylene, 1.5 wt% of the crosslinking agent and 1 wt% of the ultraviolet initiator; The mixed film is irradiated with ultraviolet light for 45-60 seconds to obtain a cross-linked modified polypropylene film; N, N-dimethylformamide and xylene are dissolved in a volume ratio of 3:1 to obtain a mixed organic solvent, maleic anhydride and benzophenone are dissolved in the mixed organic solvent to obtain a mixed solution, and then the mixed solution is dropped onto the upper surface of the cross-linked polypropylene film to obtain a cross-linked grafted modified polypropylene film.

2. The method for preparing a polypropylene composite film for capacitors based on interface modification according to claim 1, characterized in that: The ultraviolet light initiator is benzophenone.

3. The method for preparing a polypropylene composite film for capacitors based on interface modification according to claim 1, characterized in that: The crosslinking agent is trimethylolpropane triacrylate TMPTA.

4. The method for preparing a polypropylene composite film for capacitors based on interface modification according to claim 1, characterized in that: The length and width of the modified polypropylene film are equal, both of which are 9cm×9cm.

5. The method for preparing a polypropylene composite film for capacitors based on interface modification according to claim 1, characterized in that: The thickness of the modified polypropylene film is 20±5 μm.

6. A capacitor polypropylene film based on interface modification and bulk modification prepared by the preparation method according to any one of claims 1 to 5.

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