A BOPP film modification method

Abstract

This invention relates to a BOPP film modification method, which belongs to the field of capacitor films. A polymer aerosol is used as a modifier to construct a functional layer on the BOPP film surface. This method improves the breakdown field strength and discharge energy density while maintaining the self-healing ability of the metallized modified BOPP film. This method addresses the issue of reduced mechanical properties of modified BOPP film; it also addresses the self-healing failure of organically modified BOPP film after metallization; and it achieves a sufficiently fast processing speed, potentially compatible with production lines, addressing processing efficiency issues.

Description

Technical Field

[0001] The invention belongs to the field of capacitor films, and in particular relates to a BOPP film modification method. Background Art

[0002] Biaxially oriented polypropylene (BOPP) film is widely used as an energy storage medium in metallized film capacitors (MFCs) due to its excellent insulation properties, low dielectric loss, and self-healing ability after metallization. Despite its many significant advantages, BOPP film's insulation performance degrades significantly with increasing operating temperature, particularly above 100°C. Inadequate insulation can cause the film to break down in strong electric fields, leading to operational failures and safety hazards. Furthermore, energy storage performance decreases dramatically with increasing temperature.

[0003] Commonly used BOPP film thicknesses range from several microns to over ten microns, with the thinnest industrially produced BOPP film only approximately 2μm thick. This thinness makes defects easily introduced during the film-forming process. In recent decades of engineering practice, surface defects introduced during the BOPP film production process have been a key factor limiting its insulation performance. According to energy storage principles, energy storage density is influenced by two parameters: breakdown field strength and relative dielectric constant. Breakdown field strength is the primary insulation performance parameter. Therefore, a decrease in insulation performance significantly reduces energy storage performance.

[0004] To improve the high-temperature energy storage density of BOPP film, numerous ingenious modification schemes have been developed. Generally, the goal of increasing energy storage density is very clear. On the one hand, the electric field distribution is optimized to reduce high-temperature leakage current, thereby achieving a high breakdown field strength for BOPP film at high temperatures. On the other hand, the dielectric constant can be increased by introducing high-dielectric fillers. Based on the different principles of these modification methods, these approaches can be categorized into three types: matrix manipulation, structural design, and surface modification. Matrix manipulation primarily involves doping with inorganic and organic fillers and grafting onto a polypropylene base. Structural design typically involves constructing high-dielectric constant layers and charge barrier layers. Surface modification involves grafting polar groups onto the BOPP film surface through various physical and chemical methods, such as atomic layer deposition, to finely control the surface. However, the introduction of inorganic components can lead to a degradation of the film's mechanical properties after modification. Maintaining the self-healing properties of metallized BOPP film after the introduction of organic components is also a pressing issue. Because BOPP film is produced industrially using high-speed roll-to-roll processes, surface modification strategies require high-speed processing. Refined modification schemes such as atomic layer deposition (ALD) are still unable to meet the efficiency requirements. Therefore, from the perspective of industrial production, efforts should be made to develop an efficient, low-cost, and environmentally friendly method to improve the energy storage performance of BOPP film.

[0005] Chinese patent application CN202110095371.2 discloses a method for preparing a thin-layer composite membrane using an electrospray method, which can achieve uniform growth of the thin-layer composite membrane in space or chemical composition. The premise for the electrospray method to be able to well atomize the modified liquid is that a suitable electric field is stimulated by a high-voltage power supply. For some low-voltage thin films, there is a risk of insulation damage. After the electrospray treatment driven by a high-voltage power supply, the dielectric film will accumulate charges on its surface due to its capacitance effect. If the electric field is too high, there is a risk of the film being broken down. If the electric field is too low, the modified liquid will not be well atomized or even fail to atomize, which is not conducive to uniform coverage of the film surface. Chinese patent application CN201010232379.0 provides a method for preparing a polymer organic membrane. This method only involves the preparation of a single-layer membrane and does not consider the interface bonding problem between the modified layer and the original film. The solution mentioned in the article cannot be used as a modifying liquid. Chinese patent application CN202111350408.8 introduces a maintenance device for maintaining cables. As part of the maintenance, it is necessary to spray insulating spray on the damaged parts of the cable and to dry the cable surface with dry particles at all times. Due to the low volatility of water, part of the moisture absorbed by the dry particles will adhere to the surface of the cable along with the particles, and part of it will remain on the surface of the cable. This is acceptable for cables with thicker insulation thickness (centimeter level). For BOPP films in the micron level, these moisture and dry particles will bring about surface defects that cannot be ignored, seriously affecting the insulation performance of the film. In addition, since the film to the capacitor needs to go through multiple processing processes, the residual moisture will trigger many side reactions, resulting in a decrease in the breakdown field strength.

[0006] While existing modification schemes improve insulation and energy storage performance, they also present the following challenges: Traditional inorganic modification schemes can easily degrade the mechanical properties of modified BOPP films, leading to a decrease in breakdown field strength. Traditional organic modification schemes typically rely on doping with organic components with high carbon content, which can easily lead to self-healing failure of BOPP films after metallization. Newer modification schemes, such as atomic layer and molecular layer deposition, can precisely control the proportions of inorganic and organic components, thereby alleviating these issues. However, their processing speeds are extremely slow, making them unsuitable for production lines and unsuitable for mass production.

[0007] Therefore, there is an urgent need to solve the problems of decreased mechanical properties of modified BOPP films, failure of self-healing of organically modified BOPP films after metallization, and processing efficiency. Summary of the Invention

[0008] To solve the above technical problems, the present invention provides a BOPP film modification method, which is a method for modifying BOPP film by pressure spraying. By quickly and over a large area reconstructing the surface of the BOPP film, the shortcomings of the breakdown field strength caused by the insulation defects on the original film surface are repaired, thereby improving the energy storage performance.

[0009] In order to achieve the above object, the present invention adopts the following technical solutions:

[0010] A BOPP film modification method utilizes pressure spraying to improve the performance of the BOPP film. A pressure spray device is used to atomize a polymer modifier to form an aerosol without static electricity. The aerosol is then directed and directionally sputtered onto the surface of the BOPP film by the pressure spray device, thereby forming a functional layer on the surface of the BOPP film. The functional layer is used to repair micro-nano defects on the surface of the BOPP film, limit the injection of interfacial charges into the interior of the medium, and increase the polarization factor of the medium. When processing the BOPP film, an unwinding device and a rewinding device are linked to achieve continuous surface processing of the BOPP film through unwinding and rewinding.

[0011] Furthermore, the dielectric strength of the polymer modifier should not be lower than that of the BOPP film.

[0012] Furthermore, the polymer modifier is an organic medium.

[0013] Furthermore, an auxiliary agent is added to the polymer modifier.

[0014] Furthermore, the particle size of the aerosol formed by atomization is one order of magnitude or more smaller than the film thickness of the BOPP film.

[0015] Furthermore, the particle size of the aerosol is below the submicron level.

[0016] Furthermore, the straight-line distance between the pressure spray device and the surface to be treated of the BOPP film is on the order of tens of centimeters.

[0017] Furthermore, the processing time is less than 20 seconds.

[0018] Furthermore, the polymer modifier is a polytetrafluoroethylene dispersion or a polypropylene dispersion.

[0019] Furthermore, the auxiliary agent is tetrabutyl titanate, methyl formaldehyde acrylate, chlorinated polypropylene or polyurethane.

[0020] Beneficial effects:

[0021] The present invention provides a method for improving the insulation and energy storage properties of BOPP film while taking into account self-healing properties. A functional layer that can improve the insulation and energy storage properties of the BOPP film is constructed on the surface of the BOPP film using polymer aerosol, and the modified BOPP film still has self-healing properties after metallization. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 Schematic diagram of a BOPP film modification method of the present invention;

[0023] Figure 2 Schematic diagram of the average breakdown field strength of BOPP film and its modified film;

[0024] Figure 3 This is a schematic diagram of the distribution of self-healing points and their surrounding elements;

[0025] Figure 4 Schematic diagram of discharge energy density of BOPP film and its modified film;

[0026] Figure 5 Schematic diagram of the mechanical properties of BOPP film and its modified film; (a) is the tensile strength and (b) is the breaking strength.

[0027] Among them, 1-pressure spray device; 2-BOPP film; 3-unwinding device; 4-winding device. DETAILED DESCRIPTION

[0028] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely for the purpose of explaining the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

[0029] like Figure 1 As shown, the present invention discloses a method for modifying BOPP film using pressure spraying to enhance the film's performance. A pressure spray device 1 is used to atomize the modifier, forming a static-free aerosol. The aerosol is then directed and directionally sputtered onto the surface of a BOPP film 2 by the pressure spray device 1, thereby forming a functional layer on the surface of the BOPP film 2. During the treatment of the BOPP film 2, an unwinding device 3 and a rewinding device 4 are linked, achieving continuous surface treatment of the BOPP film 2 through a sequential unwinding and rewinding process. This method features a large treatment area and extremely high processing speed.

[0030] Preferably, in order to repair the defects on the surface of the BOPP film 2 , the dielectric strength of the modifier used should not be lower than the dielectric strength of the BOPP film 2 .

[0031] Preferably, in order to prevent the interface bonding problem between the inorganic layer and the organic layer, such as the BOPP film 2 becoming brittle, the modifier used should be an organic medium.

[0032] Preferably, in order to prevent the functional layer from falling off, an auxiliary agent should be added to the modifier.

[0033] Preferably, to avoid the formation of large particles attached to the surface of the BOPP film 2 after pressure spraying and the introduction of new defects, the aerosol particle size formed by atomization should be at least one order of magnitude smaller than the film thickness of the BOPP film 2. The aerosol particle size should be below the submicron level.

[0034] Preferably, to prevent the functional layer formed by the spray from being too thick and causing it to fall off, the straight-line distance between the pressure spray device 1 and the surface to be treated of the BOPP film 2 should be reasonably controlled to be on the order of tens of centimeters. At the same time, to improve the treatment efficiency, the treatment time should be less than 20 seconds, preferably less than a dozen seconds.

[0035] Preferably, the modifier can be a polymer dispersion such as polytetrafluoroethylene dispersion, polypropylene dispersion, etc., which has potential self-healing ability and insulation performance not lower than that of polypropylene.

[0036] Preferably, the auxiliary agent may be tetrabutyl titanate, methyl methacrylate, chlorinated polypropylene, polyurethane, etc.

[0037] Example:

[0038] A pressure spray device was used to atomize a polytetrafluoroethylene dispersion containing tetrabutyl titanate into a submicron-sized aerosol, which was then sputtered onto the surface of a 15 μm thick BOPP film to form a modified BOPP film. During sputtering, the linear distance between the pressure spray device 1 and the surface of the BOPP film 2 to be treated was 30 to 35 cm. Figure 2 As shown in the figure, F-0, F-3, F-6, F-9, and F-12 represent the number of treatment cycles 0, 3, 6, 9, and 12, respectively. A complete coverage of a specific area on the surface of the BOPP film is counted as one cycle.

[0039] The increase in electrode-dielectric contact area means an increase in the number and probability of defects involved in the film breakdown process. Therefore, the use of large-diameter electrodes makes it easier to verify the repair effect of the modification on defects. Figure 2 In a high-temperature (100°C) DC breakdown test using an electrode with a diameter of 25 mm, it was found that the average breakdown field strength increased with the increase in the number of cycles, from 360.5 V / μm of the original BOPP film 2 to a maximum of 462.5 V / μm, an increase of about 28.3%.

[0040] To maintain stable operation of film capacitors, improving the high-temperature performance of BOPP film 2 should not come at the expense of self-healing properties. BOPP film 2 with 30nm gold electrodes sputtered on both sides still exhibits good self-healing properties during the boost process. Figure 3The diameter of the self-healing point is about 50μm, and the diameter of the electrode removal area is about 200μm. The self-healing process usually occurs at the electrical weakness (defect) of the BOPP film. The local high temperature at the electrical weakness causes the surrounding gold electrodes to be removed, exposing the dielectric, thereby restoring a certain degree of insulation performance.

[0041] Figure 4 The maximum discharge energy density of the original BOPP film 2 at medium and high temperatures (120°C) is only 2.8 J / cm 3 (applied electric field strength is 550V / μm); the discharge energy density of modified BOPP film 2 is significantly improved, reaching 4.9J / cm 3 , which is 1.75 times the discharge energy density of unmodified BOPP film under the same conditions.

[0042] The tensile deformation process of polymers needs to go through three stages: elastic deformation zone, yield zone, and molecular slip after yield, and finally form cracks and break. When the film strain reaches the yield point, the initial cracks tend to be generated from the amorphous region between adjacent lamellae. The modification causes the crystallinity and grain size of BOPP film to change, so the mechanical properties of the composite film change accordingly. Figure 5 (a), Figure 5 As shown in (b), the strain at break of BOPP Film 2 and its modified film is no less than 200% (much greater than 50%), indicating that they are semi-rigid plastics with good flexibility. With increasing cycles, the tensile properties of the modified films first increase and then decrease, but remain no less than those of the unmodified BOPP Film 2.

[0043] It will be easily understood by those skilled in the art that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A BOPP film modification method, characterized in that: Utilizing pressure spraying to enhance the performance of BOPP film, a pressure spray device is used to atomize a polymer dispersion with self-healing capabilities and insulation properties comparable to those of polypropylene as a polymer modifier, forming a static-free aerosol with submicron particle size. The aerosol is then directed and deposited onto the surface of the BOPP film through the pressure spray device, thereby forming a functional layer on the surface of the BOPP film. The functional layer is used to repair micro-nano defects on the BOPP film surface, limit the injection of interfacial charges into the dielectric, and increase the dielectric polarization factor. During the processing of the BOPP film, an unwinding device and a rewinding device are linked together to achieve continuous surface treatment of the BOPP film. The dielectric strength of the polymer modifier should not be lower than that of the BOPP film and should be an organic medium; The straight-line distance between the pressure spray device and the surface to be treated of the BOPP film is 30-35 cm; The time for the pressure spray device to spray the film surface is less than 20 seconds.

2. A BOPP film modification method according to claim 1, characterized in that, An auxiliary agent is added to the polymer modifier.

3. A BOPP film modification method according to claim 1, characterized in that, The polymer modifier is a polytetrafluoroethylene dispersion or a polypropylene dispersion.

4. The BOPP film modification method according to claim 2, characterized in that: The auxiliary agent is tetrabutyl titanate, methyl formaldehyde acrylate, chlorinated polypropylene or polyurethane.

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