A method for preparing a dielectric film for a dry DC capacitor

Abstract

The present invention relates to the technical field of capacitor films, and more specifically, to a method for preparing a dielectric film for a dry DC capacitor. The method comprises the following steps: pre-treating polypropylene particles to form a melt, and then forming a sheet film blank; preheating the film blank, and then performing synchronous biaxial stretching on the film blank after the temperature is raised, and then performing rapid cooling to obtain a stretched film material; first heating the stretched film material for heat relaxation treatment, then performing heat setting treatment on the stretched film material, and then cooling to produce a dielectric film; performing surface corona treatment on the dielectric film, and then attaching a conductive material to the surface of the dielectric film and cutting the film to complete the preparation. In the present invention, by performing synchronous biaxial stretching on the film blank, the arrangement of the molecular chains in the longitudinal and transverse directions can be increased, and the stress generated by the film during biaxial stretching can be first released, and the film molecules can be rearranged, the orientation process of the film can be improved, and the internal stress of the film can be further eliminated, thereby producing a dielectric film with better performance.

Description

Technical Field

[0001] The invention relates to the technical field of capacitor films, and in particular to a method for preparing a dielectric film for a dry DC capacitor. Background Art

[0002] Dry-type DC capacitors refer to capacitors that do not contain liquid dielectrics. They use solid dielectrics to store charge and have the advantages of high stability and long service life. Dry-type DC capacitors are widely used in high-voltage and ultra-high-voltage power electronics fields, such as transformers, DC high-voltage transmission lines, etc., to improve the reliability of power transmission.

[0003] At present, in some preparation methods of polypropylene-based composite metallized films, a boron nitride solution is prepared, and a boron nitride solid powder and N,N-dimethylformamide are mixed according to a predetermined mass volume ratio to form a boron nitride solution; the metallized polypropylene film is placed on a glass plate of a flat-bed coating machine with the metal-deposited side facing down and the non-metallic side facing up, the boron nitride solution is added to a coating scraper, and multiple layers are coated at room temperature to generate a polypropylene-based composite metallized film. This invention utilizes the wide bandgap and high thermal conductivity characteristics of boron nitride, and proposes coating boron nitride on the metallized film, utilizing the high bandgap width to enhance the breakdown field strength, and utilizing the high thermal conductivity to promote self-healing plasma arc extinction, but the key properties of the film itself, such as tensile strength and elongation at break, have not been improved. When applied to dry DC capacitors, the film is prone to rupture, which in turn leads to failure of the capacitor.

[0004] In order to improve the performance of the film and ensure the normal use of the dry DC capacitor, a method for preparing a dielectric film for the dry DC capacitor is proposed. Summary of the invention

[0005] In view of the above analysis, an embodiment of the present invention aims to provide a method for preparing a dielectric film for a dry DC capacitor to solve the problems raised in the above background technology.

[0006] To achieve the above object, the present invention provides a method for preparing a dielectric film for a dry DC capacitor, comprising the following steps:

[0007] S1, pre-treating polypropylene particles to form a melt, and then extruding it through a narrow and long head of an extruder to form a sheet film blank;

[0008] S2, preheating the film blank, performing synchronous biaxial stretching on the film blank after the temperature is raised, and then performing rapid cooling treatment to obtain a stretched film material;

[0009] S3, first heating the stretched film material to perform heat relaxation treatment, then performing heat setting treatment on the stretched film material and then cooling it to produce a dielectric film;

[0010] S4. After performing corona treatment on the surface of the dielectric film, a conductive material is attached to the surface of the dielectric film and cut to complete the preparation.

[0011] As a further improvement of the technical solution, in S1, the feeding speed of the extruder during extrusion is in the range of 2.4-2.7 m / min, the extrusion temperature is in the range of 220-260°C, and the screw shear rate is in the range of 80-220s -1 .

[0012] As a further improvement of the technical solution, in S2, the film blank is preheated and treated at a constant temperature, and then the film blank is simultaneously stretched longitudinally and transversely, and the longitudinal stretching multiple is the same as the transverse stretching multiple.

[0013] As a further improvement of the present technical solution, in S2, the stretching rate during biaxial stretching is in the range of 30 to 140 mm / min.

[0014] As a further improvement of the technical solution, the stretching rate during the biaxial stretching is 50 mm / min, 65 mm / min, 72 mm / min, and 80 mm / min.

[0015] As a further improvement of the technical solution, in S3, the heat relaxation treatment is to heat the stretched film material and then let it stand until the stretched film material stops deforming, and then perform a heat setting treatment, and the temperature during the heat setting treatment is the same as the temperature during the heat relaxation treatment.

[0016] As a further improvement of the technical solution, in S3, the time range of the heat setting treatment is 45 to 65 minutes.

[0017] As a further improvement of the technical solution, the heat setting treatment takes 48 minutes, 55 minutes, or 60 minutes.

[0018] As a further improvement of the present technical solution, in S4, the dielectric film is first cleaned to remove surface impurities, and then the film surface is corona treated by a corona treatment machine to form a fine concave-convex structure on the surface of the dielectric film. Then, the dielectric film is immersed in the conductive material by dip coating to complete the attachment of the conductive material.

[0019] As a further improvement of the technical solution, the corona treatment power during the surface corona treatment is 30kW, 45kW, or 50kW.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] In the preparation method of the dielectric film for the dry DC capacitor, the film blank made of polypropylene particles is subjected to synchronous biaxial stretching, so that the arrangement of the molecular chains in the longitudinal and transverse directions can be increased, and the stress generated by the biaxial stretching of the film is first released through heat relaxation treatment and heat setting treatment, and the film molecules are rearranged, the orientation process of the film is improved, and the internal stress of the film is further eliminated, thereby producing a dielectric film with better performance.

[0022] In the present invention, the above-mentioned technical solutions can also be combined with each other to achieve more preferred combination solutions. Other features and advantages of the present invention will be described in the subsequent description, and some advantages can become obvious from the description, or can be understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained through the contents particularly pointed out in the description and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The drawings are only for the purpose of illustrating particular embodiments and are not to be considered limiting of the present invention. Like reference symbols denote like components throughout the drawings.

[0024] Figure 1 It is the overall flow chart of the present invention;

[0025] Figure 2 It is a schematic diagram of the tensile strength of the present invention;

[0026] Figure 3 It is a schematic diagram of the elongation at break of the present invention;

[0027] Figure 4 Schematic diagram of volume resistivity of the present invention. DETAILED DESCRIPTION

[0028] The following will be combined with the accompanying drawings in the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0029] Example 1

[0030] See also Figure 1 As shown, the purpose of this embodiment is to provide a method for preparing a dielectric film for a dry DC capacitor, comprising the following steps:

[0031] S1. Pre-treat polypropylene particles to make a melt, and then extrude it through a narrow and long head of an extruder to form a sheet film blank, wherein the pre-treatment is to first screen, dry and pre-mix the polypropylene particles, add the pre-mixed polypropylene raw material into the melt cavity of the extruder, melt it by heating and mixing, and push the molten polypropylene into the die head through the screw of the extruder to form a continuous and uniform melt. The feed speed range of the extruder during extrusion is 2.4~2.7m / min, the extrusion temperature range is 220~260℃, and the screw shear rate range is 80~220s -1 ;

[0032] S2, preheating the film blank to an appropriate temperature and treating it at a constant temperature to improve its ductility, and then simultaneously performing longitudinal and transverse directional stretching on the film blank after the film blank is heated, and the longitudinal directional stretching multiple is the same as the transverse directional stretching multiple, which can increase the arrangement of the molecular chains along the longitudinal and transverse directions, and then rapidly cooling the film to obtain a stretched film material, wherein the stretching rate range during biaxial stretching is 30~140mm / min;

[0033] S3, first heat the stretched film material for heat relaxation treatment, then heat set the stretched film material and then cool it to produce a dielectric film, wherein the heat relaxation treatment is to heat the stretched film material and then let it stand until the stretched film material stops deforming, and the heat relaxation treatment is used to allow the stretched film material to undergo slow elastic deformation or stress release under stress to achieve stress balance, and then heat set treatment is performed, and the temperature during the heat setting treatment is the same as the temperature during the heat relaxation treatment. The purpose of the heat setting treatment is to utilize the melting characteristics of the polypropylene material to rearrange the film molecules during the heating process, improve the crystallization orientation process of the stretched film material, eliminate internal stress and produce a stable dielectric film, and in addition, the heat setting treatment takes 45 to 65 minutes;

[0034] S4. First, clean the dielectric film to remove surface impurities, and then use a corona treatment machine to perform corona treatment on the film surface, so as to improve the surface energy level and enable the conductive material to better adhere to the dielectric film surface. The corona treatment power range during the surface corona treatment is 25~55kW. After a fine concave-convex structure is formed on the surface of the dielectric film, the dielectric film is immersed in the conductive material by dip coating to complete the adhesion of the conductive material and the preparation is completed by cutting.

[0035] In this embodiment, by performing simultaneous biaxial stretching on the film blank made of polypropylene particles, the arrangement of the molecular chains in the longitudinal and transverse directions can be increased, and through heat relaxation treatment and heat setting treatment, the stress generated by the film during biaxial stretching is first released, and the film molecules are rearranged, thereby improving the orientation process of the film and further eliminating the internal stress of the film, thereby producing a dielectric film with better performance.

[0036] Example 2

[0037] In the process of biaxially stretching the film blank to form a stretched film material, the polypropylene molecular chains of the film blank will be arranged along the stretching direction, i.e., longitudinally and transversely. Therefore, the physical properties of the film can be effectively improved by biaxial stretching. In the stretching process, the stretching rate of the film blank will affect the arrangement effect of the polypropylene molecular chains. A faster stretching rate can promote the rapid orientation of the molecular chains, but may cause an increase in the internal stress of the film, thereby affecting its stability and durability. On the contrary, a slower stretching rate can reduce the internal stress, but may affect the production efficiency.

[0038] In order to determine the effect of stretching rate on the tensile strength of the film, in this embodiment, only the stretching rate is changed while other parameters remain unchanged to prepare the dielectric film. The tensile strength of the dielectric film is tested according to "GB / T13542.2-2021 Electrical Insulation Film Part 2: Test Method". The stretching rate parameters are shown in Table 1, and the tensile strength test results are shown in Table 1. Figure 2 shown.

[0039] Table 1 - Stretching rate parameters

[0040]

[0041] It can be seen from the chart that when the stretching rate is 72 mm / min, the transverse tensile strength and longitudinal tensile strength of the prepared dielectric film are relatively high.

[0042] Example 3

[0043] After the stretched film material is subjected to heat relaxation treatment to release the stress generated in the stretching step, the prepared dielectric film is subjected to heat setting treatment to achieve stress balance. During the heat setting treatment, the interaction between the polypropylene molecules in the dielectric film can be promoted, making it easier to rearrange, eliminating the internal stress generated in the dielectric film during the stretching process, and causing the polypropylene macromolecules to relax to a certain extent, so as to fix the shape of the dielectric film and improve the physical properties of the dielectric film. It is worth noting that if the heat setting treatment time is too long or too short, it will affect the treatment effect on the dielectric film;

[0044] In order to ensure the treatment effect of the dielectric film and determine the optimal heat setting treatment time, in this embodiment, only the heat setting treatment time is changed while other parameters remain unchanged, and the dielectric film is prepared. According to "GB / T13542.2-2021 Electrical Insulation Film Part 2: Test Method", the elongation at break of the dielectric film is measured. The heat setting treatment time parameters are shown in Table 2. The elongation at break of the dielectric film is measured as shown in Table 2. Figure 3 shown.

[0045] Table 2 - Heat setting treatment time parameters

[0046]

[0047] According to the chart, when the heat setting treatment time is 55 minutes, the transverse and longitudinal elongation at break of the prepared dielectric film is relatively high.

[0048] Example 4

[0049] Corona treatment generates corona discharge on the surface of plastic film through the action of high-frequency and high-voltage electric field, thus forming a layer of active material covering on the surface of the material. This treatment method can break the chemical bonds of polypropylene molecules on the surface of dielectric film and degrade them, while generating a large amount of ozone. Ozone, as a strong oxidant, can oxidize polypropylene molecules and generate carbonyl and peroxide groups with strong polarity, thereby increasing its surface energy, facilitating the adhesion of conductive materials, and thus improving the electrical properties of dielectric film.

[0050] When the dielectric film is corona treated with a corona treatment machine, the size of the fine concave-convex structure on the surface of the dielectric film can be adjusted by adjusting the output power of the corona treatment machine, that is, the roughness of the surface of the dielectric film is changed, and the adhesion effect of the conductive material on the surface of the dielectric film is improved by adjusting the roughness. In order to determine the optimal corona treatment power during corona treatment, this embodiment only changes the corona treatment power while other parameters remain unchanged to prepare the dielectric film. According to "GB / T13542.2-2021 Electrical Insulation Film Part 2: Test Method", a Φ25mm / Φ40mm two-electrode system is used, and the sample is stacked to (0.151~0.160mm) to measure the volume resistivity of the dielectric film. The corona treatment power parameters are shown in Table 3, and the volume resistivity measurement results of the dielectric film are shown in Table 3. Figure 4 shown.

[0051] Table 3-Corona treatment power parameters

[0052]

[0053] According to the chart, when the corona treatment power is 45 kW, the volume resistivity of the prepared dielectric film is higher.

[0054] Example 5

[0055] In this embodiment, the dielectric film is prepared according to the method provided in the above embodiment and the determined preferred parameters, and the relative permittivity and dielectric loss factor of the dielectric film are tested by using a Φ25mm / Φ40mm two-electrode system and stacking the sample to (0.151~0.160mm) according to GB / T13542.2-2021 Electrical Insulation Film Part 2: Test Method. The relative permittivity of the dielectric film is 2.2, and the dielectric loss factor is 2.7×10 -4Under the working conditions of this embodiment, the performance of the dielectric film is good and can ensure the normal use of the dry DC capacitor.

[0056] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited by the above embodiments. The above embodiments and descriptions are only preferred examples of the present invention and are not intended to limit the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, which fall within the scope of the present invention. The scope of protection of the present invention is defined by the attached claims and their equivalents.

Claims

1. A method for preparing a dielectric film for a dry DC capacitor, characterized in that: It consists of the following steps: S1. Pre-treat polypropylene particles to make a melt, and then extrude it through a narrow and long head of an extruder to form a sheet film blank; the pre-treatment is to first screen, dry and pre-mix the polypropylene particles, add the pre-mixed polypropylene raw material into the melt cavity of the extruder, melt it by heating and mixing, and push the molten polypropylene into the die head through the screw of the extruder to form a continuous and uniform melt. The feed speed range of the extruder during extrusion is 2.4~2.7m / min, the extrusion temperature range is 220~260℃, and the screw shear rate range is 80~220s -1 ; S2, preheating the film blank, and after the film blank is heated, performing synchronous biaxial stretching and then rapidly cooling the film blank to obtain a stretched film material; the longitudinal directional stretching multiple is the same as the transverse directional stretching multiple, so as to increase the arrangement of the molecular chains in the longitudinal and transverse directions; S3, first heating the stretched film material for heat relaxation treatment, then performing heat setting treatment on the stretched film material and then cooling it to produce a dielectric film; the heat relaxation treatment is to allow the stretched film material to stand still after being heated until the stretched film material stops deforming, and the heat relaxation treatment is used to allow the stretched film material to undergo slow elastic deformation or stress release under stress to achieve stress balance, and then perform heat setting treatment; the temperature during the heat setting treatment is the same as the temperature during the heat relaxation treatment; S4, after performing surface corona treatment on the dielectric film, attaching a conductive material to the surface of the dielectric film and cutting it to complete the preparation; after the surface corona treatment forms a fine concave-convex structure on the surface of the dielectric film, the dielectric film is immersed in the conductive material by dip coating to complete the attachment of the conductive material; In S2, the stretching rate during biaxial stretching is in the range of 30 to 140 mm / min; In S3, the heat setting treatment takes 45 to 65 minutes; In S4, the corona treatment power range of the surface corona treatment is 25-55 kW.

2. The method for preparing a dielectric film for a dry DC capacitor according to claim 1, characterized in that: In S2, the film blank is preheated and treated at a constant temperature, and then the film blank is longitudinally and transversely oriented and stretched simultaneously.

3. The method for preparing a dielectric film for a dry DC capacitor according to claim 1, characterized in that: In S2, the stretching rate during biaxial stretching is 50 mm / min, 65 mm / min, 72 mm / min, and 80 mm / min.

4. The method for preparing a dielectric film for a dry DC capacitor according to claim 1, characterized in that: In S3, the heat setting treatment takes 48 min, 55 min, and 60 min.

5. The method for preparing a dielectric film for a dry DC capacitor according to claim 1, characterized in that: In S4, the dielectric film is first cleaned to remove surface impurities, and then the film surface is subjected to corona treatment by a corona treatment machine.

6. The method for preparing a dielectric film for a dry DC capacitor according to claim 1, characterized in that: In the above S4, the corona treatment power during the surface corona treatment is 30 kW, 45 kW, or 50 kW.

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