A flexible resistive film based on polyvinylidene fluoride (PVDF) and a preparation method and application thereof

Abstract

This invention belongs to the technical field of electromagnetic functional materials, specifically relating to a resistive film based on polyvinylidene fluoride (PVDF), its preparation method, and its application. The resistive film comprises PVDF and a conductive agent; wherein PVDF acts as a binder, existing in a uniformly mixed form with the conductive agent, and is obtained by film formation through dispersion, which can be achieved, for example, by blade coating, where the conductive dispersion is blade-coated onto a carrier substrate; and the sheet resistance of the resistive film is determined by the mass ratio of PVDF to the conductive agent and the film thickness. This invention utilizes the high-bond-energy carbon-fluorine bond structure in the PVDF polymer, combined with a conductive agent, to prepare a resistive film with high physicochemical stability and uniformly controllable sheet resistance. Even under extremely harsh service environments, this material can maintain stable dielectric parameters and absorption efficiency, significantly extending the engineering service life of electromagnetic protection and reducing maintenance costs.

Description

Technical Field

[0001] This invention belongs to the technical field of electromagnetic functional materials, and more specifically, relates to a flexible resistive film based on polyvinylidene fluoride (PVDF), its preparation method, and its application. Background Technology

[0002] Thin resistive films with controllable sheet resistance are commonly used in classic electromagnetic absorbing structures such as Salisbury screens and Jaumann absorbers. These resistive films often employ metal oxides (such as ITO), polymer matrices, and carbon-based nano-conductive fillers. The resistivity of ITO resistive films is difficult to control, leading to challenges in achieving uniformity and accuracy of sheet resistance (i.e., sheet resistance) during large-area fabrication. Furthermore, the high cost restricts their engineering applications due to limitations imposed by both fabrication processes and material properties. Polymer-based resistive films with carbon-based nano-conductive fillers are typically fabricated by screen printing conductive paste onto a base film (such as PI or PET). This process is simple and low-cost, making them suitable for structural absorbers. However, screen printing requires a long lead time and is expensive, has limited resolution, and the film thickness and uniformity are affected by printing parameters. The film thickness is constrained by the screen printing, resulting in insufficient sheet resistance control accuracy and impacting repeatability and consistency.

[0003] Therefore, it is necessary to develop a resistive film and its preparation method that makes the sheet resistance of the resistive film uniform, stable and simple to control, with simple process and low cost, while ensuring that the absorbing material has stable and predictable electromagnetic response characteristics in a wide frequency band, significantly improving product yield and batch consistency, and providing a reliable material basis for the application of high-performance electromagnetic shielding and protection. Summary of the Invention

[0004] In view of the above-mentioned defects or improvement needs of the existing technology, the present invention provides a flexible resistive film based on polyvinylidene fluoride (PVDF), its preparation method and application, the purpose of which is to propose a resistive film with simple, accurate, controllable, uniform and stable sheet resistance for use in the electromagnetic wave field.

[0005] To achieve the above objectives, according to one aspect of the present invention, a flexible resistive film based on polyvinylidene fluoride (PVDF) is provided, comprising PVDF and a conductive agent; wherein, PVDF, as a binder, exists in a uniformly mixed form with the conductive agent, and is obtained by film formation through a dispersion; the mass ratio of PVDF to the conductive agent and the film thickness of the resistive film together determine the final sheet resistance of the resistive film.

[0006] Furthermore, the conductive agent is conductive carbon black.

[0007] According to another aspect of the present invention, a method for preparing the flexible resistive film as described above is provided, comprising: (1) Add polyvinylidene fluoride (PVDF) to an organic solvent and mix thoroughly to obtain a uniform and transparent dispersion; (2) Add the conductive agent to the transparent dispersion to obtain a conductive dispersion; (3) The conductive dispersion is prepared into a wet film on a carrier substrate, and after drying and curing, a flexible resistive film based on PVDF is obtained.

[0008] Furthermore, the organic solvent is one or more of N,N-dimethylformamide, N-methylpyrrolidone, or acetone; the conductive dispersion is prepared on the carrier material by means of blade coating, casting, spin coating, or spray coating.

[0009] Furthermore, the drying temperature was set to 40 ℃-50 ℃, and the drying time was set to 10 h-12 h.

[0010] Furthermore, step (1) includes: (11) Preparation of PVDF dispersion by low-speed pre-dispersion: PVDF is added to a reaction vessel containing organic solvent and pre-mixed at low speed; if PVDF agglomeration occurs, it is manually crushed in a dust-free environment using a crushing tool, and intermittent wall scraping is used to remove the agglomeration; the low speed is to prevent powder from flying and to remove air inside the powder particles and between the agglomerates, and to prevent the formation of a bubble layer on the liquid surface. (12) Disperse the low-speed pre-dispersed liquid at a medium speed. If there is PVDF agglomeration, crush it manually with a crushing tool in a dust-free environment and use intermittent wall scraping to remove the agglomeration. The medium speed is required to break up the clumps formed by the powder absorbing water and expanding, and to evenly disperse the local thick material to all corners of the container. (13) The medium-speed dispersion is subjected to high-speed enhanced dispersion until the dispersion becomes a transparent and colorless liquid.

[0011] Furthermore, step (2) is performed in the same manner as step (1) to achieve the addition of the conductive agent.

[0012] Furthermore, before performing step (3), the method also includes: measuring the fineness of the dispersion; when the fineness does not reach the preset threshold, using the low speed, medium speed and high speed of step (2), stirring again in stages until the fineness reaches the preset threshold, wherein the preset threshold is determined by the actual required surface roughness and sheet resistance uniformity of the resistive film.

[0013] According to another aspect of the present invention, the application of the flexible resistive film as described above in a resistive film absorber or electromagnetic shielding structure is provided.

[0014] Furthermore, when the flexible resistive film is used in the resistive film absorber, the mass ratio of the conductive agent is 1% to 80% of the mass of PVDF; the thickness of the resistive film is 10 μm to 500 μm; and the sheet resistance of the resistive film is 1 to 1000 Ω / sq. Preferably, the mass ratio of the conductive agent is 10% to 40% of the mass of PVDF. Based on this, the thickness of the resistive film is determined by the required absorption performance.

[0015] In summary, compared with the prior art, the technical solutions conceived by this invention have the following main advantages: 1. This invention proposes a flexible resistive film based on polyvinylidene fluoride (PVDF). PVDF possesses strong chemical resistance and excellent mechanical properties. Furthermore, since PVDF and the conductive agent exist in a uniform mixture and are obtained through film formation via dispersion, screen printing can be avoided. The resistive film can be prepared using a blade coating method, which is simple and ensures uniformity of the resistive film. The sheet resistance of the resistive film is determined by the mass ratio of PVDF and the conductive agent, as well as the film thickness, resulting in stable, precise, and controllable sheet resistance. When applied, for example, to microwave absorbers, this ensures stable and predictable electromagnetic response characteristics across a wide frequency range. Therefore, it enables efficient and stable preparation of microwave absorbers with high absorption performance, significantly improving product yield and batch consistency, and providing a reliable material basis for the application of high-performance electromagnetic shielding and protection technologies. Therefore, this invention utilizes the high-bond-energy carbon-fluorine bond structure in PVDF polymers, combined with conductive agents, to prepare a resistive film with high physical and chemical stability and uniform and controllable sheet resistance. Under extremely harsh service environments, this material can still maintain stable dielectric parameters and wave absorption performance, significantly extending the engineering service life of electromagnetic protection and reducing maintenance costs.

[0016] 2. This invention also proposes a method for preparing a flexible resistive film. First, a uniformly mixed conductive dispersion containing PVDF and a conductive agent is prepared. This conductive dispersion can be coated onto a carrier substrate using, for example, a blade coating method. The sheet resistance of the resistive film is controlled by the ratio of the conductive agent to the PVDF components and the coating thickness. Due to the use of a uniformly dispersed functional dispersion and a precise coating / film-forming process, the dispersion can be cured using a low-temperature, slow-drying process to form a resistive film with uniform thickness, a smooth surface, and good continuity. It is less prone to pinholes, cracks, or localized build-up, and exhibits high interfacial bonding strength between the PVDF and the carrier substrate. This resistive film maintains stable dielectric parameters and absorption performance even under extremely harsh service environments. Therefore, this method is simple to implement, lower in cost, and allows for more controllable film thickness.

[0017] 3. Regarding the preparation method, this invention proposes a three-stage dispersion method to prepare the dispersion, which fully ensures the uniform mixing of PVDF and conductive agent, thereby ensuring the quality of the resistive film.

[0018] 4. This invention also proposes the application of flexible resistive films in resistive film absorbers or electromagnetic shielding structures, which can significantly improve the functional accuracy, manufacturing cost, and extend the service life of materials in electromagnetic fields such as electromagnetic shielding, while reducing maintenance costs. Attached Figure Description

[0019] Figure 1 This is a structural diagram of a flexible resistive film provided in an embodiment of the present invention.

[0020] Figure 2 A process flow diagram of the flexible resistive film preparation method provided in the embodiments of the present invention.

[0021] Figure 3 This is a schematic diagram of the resistive film composite absorber structure provided in an embodiment of the present invention.

[0022] Figure 4 The diagram shows the test results of the microwave absorption performance of the absorber, as provided in Example 1 of the present invention.

[0023] Figure 5 Example 5 provided in this embodiment of the invention shows the test results of the microwave absorption performance of the absorber. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be 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 illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0025] Example 1 A flexible resistive film based on polyvinylidene fluoride (PVDF) includes PVDF and a conductive agent; wherein, PVDF acts as a binder and exists in a uniformly mixed form with the conductive agent, and is obtained by film formation through a dispersion; the mass ratio of PVDF to the conductive agent and the film thickness of the resistive film together determine the final sheet resistance of the resistive film.

[0026] This embodiment presents a flexible resistive film, which is mainly composed of polyvinylidene fluoride (PVDF) and a conductive agent. PVDF possesses extremely strong chemical resistance and excellent mechanical properties. The superior corrosion resistance of PVDF is primarily attributed to two physicochemical characteristics at the molecular level: Carbon-fluorine bond (CF bond): Fluorine atoms are extremely electronegative, and the carbon-fluorine bond is one of the strongest known organic covalent bonds. This makes the PVDF molecular chain extremely stable, and it is very difficult for conventional chemical substances to break this chemical bond and destroy the main chain; High crystallinity: PVDF is a semi-crystalline polymer (crystallization can reach about 50%). The high density of crystalline regions forms a tight physical barrier, making it extremely difficult for chemical molecules (even tiny water molecules or chloride ions) to penetrate into the material.

[0027] In practical engineering applications, PVDF exhibits near-absolute immunity to the following chemicals at room temperature or even higher temperatures (up to 150°C): inorganic acids, strong oxidants, halogens and organic solvents, ultraviolet light and nuclear radiation.

[0028] Since PVDF and the conductive agent exist in a uniform mixture and are obtained through film formation via dispersion, screen printing can be avoided. The resistive film can be prepared using a blade coating method, which is simple and ensures uniformity of the resistive film. Furthermore, the sheet resistance of the resistive film is determined by the mass ratio of PVDF and the conductive agent, as well as the film thickness, resulting in stable, precise, and controllable sheet resistance. In applications such as microwave absorbers, this ensures stable and predictable electromagnetic response characteristics across a wide frequency range. Therefore, it enables efficient and stable preparation of microwave absorbers with high absorption performance, significantly improving product yield and batch consistency, and providing a reliable material basis for the application of high-performance electromagnetic shielding and protection technologies.

[0029] In this embodiment, the sheet resistance of the resistive film depends on the mass ratio of polyvinylidene fluoride (PVDF) and the conductive agent, as well as the film thickness. Since the resistive film is formed through a dispersion liquid, the film thickness is controllable, and the film formation method does not affect the sheet resistance. Compared to existing resistive film compositions and structures, the resistive film proposed in this embodiment allows for convenient and effective adjustment of the sheet resistance, thereby meeting various microwave absorption design requirements. Compared to traditional metamaterial absorbers, the sheet resistance of the resistive film is precisely controllable and can be prepared using a blade coating method, with film thicknesses in the micrometer range. Compared to nanometers, it offers better durability (e.g., wear resistance). Compared to screen printing, it eliminates the need for custom stencils, making production simpler, lower in cost, and with more controllable film thickness. Furthermore, the PVDF resistive film is flexible, making it suitable for large-scale applications in flexible electronic devices.

[0030] Therefore, in summary, this embodiment utilizes the high-bond-energy carbon-fluorine bond structure in PVDF polymer, combined with a conductive agent, to prepare a resistive film with high physicochemical stability and uniform and controllable sheet resistance. Even under extremely harsh service environments, this material can maintain stable dielectric parameters and absorption performance, significantly extending the engineering service life of electromagnetic protection and reducing maintenance costs. The film structure is as follows: Figure 1 As shown.

[0031] The conductive agent can be conductive carbon black, carbon nanotubes and / or graphene. Considering that the conductive agent is used for resistive films, conductive carbon black, which has poor conductivity, can be selected as the conductive agent as a preferred embodiment.

[0032] Example 2 A method for preparing a flexible resistive film as described in Example 1, such as... Figure 2 As shown, it includes: (1) Add polyvinylidene fluoride (PVDF) to an organic solvent and mix thoroughly to obtain a uniform and transparent dispersion; (2) Add the conductive agent to the transparent dispersion to obtain a conductive dispersion; (3) The conductive dispersion is prepared on a carrier substrate, dried and cured to obtain a PVDF-based resistive film.

[0033] It should be noted that the method in this embodiment can be performed at room temperature, and the introduction of moisture should be avoided.

[0034] This embodiment prepares a uniformly mixed conductive dispersion containing PVDF and a conductive agent. The conductive dispersion can be coated onto a carrier substrate using, for example, a blade coating method. The sheet resistance of the resistive film is controlled by the ratio of the conductive agent to the PVDF components and the coating thickness. Due to the use of a uniformly dispersed functional dispersion and a precise coating / film-forming process, the slurry is cured using a low-temperature, slow-drying process to form a resistive film with uniform thickness, a smooth surface, and good continuity. It is less prone to pinholes, cracks, or localized build-up, and exhibits high interfacial bonding strength between the PVDF and the carrier substrate. Even under extremely harsh service environments, this resistive film maintains stable dielectric parameters and microwave absorption performance. The preparation method of this embodiment is rationally designed, highly scalable, and can meet actual operating conditions, showing broad application prospects. Therefore, the raw materials for producing composite microwave absorbing materials using resistive films are readily available, the production process is simple, the cost is low, and it can be mass-produced. The preparation method of this embodiment can achieve controllable, efficient, and stable sheet resistance preparation of microwave absorbers.

[0035] Generally, the aforementioned carrier substrate requires a material with a low dielectric constant, such as polyimide film (PI film), glass fiber reinforced epoxy resin board (FR-4), and / or polyethylene terephthalate plastic film (PET film).

[0036] As a preferred embodiment, the organic solvent is one or more of N,N-dimethylformamide, N-methylpyrrolidone, or acetone; the conductive dispersion is prepared on the carrier material by means of blade coating, casting, spin coating, or spray coating. Among them, blade coating can precisely control the wet film thickness (usually between tens and hundreds of micrometers) by adjusting the micrometer-level scale of the blade; spin coating involves dropping a small amount of slurry onto the center of the substrate and using the centrifugal force generated by the high-speed rotation of the spin coater (usually several thousand rpm) to spread the slurry evenly, which is suitable for extremely thin and uniform films; spray coating is used when the substrate is not planar or needs to be coated on a large area of ​​flexible substrate, so it can be coated on three-dimensional curved surfaces, rough surfaces, or porous materials.

[0037] As a preferred embodiment, the drying temperature is set to 40 ℃-50 ℃ and the time is set to 10 h-12 h.

[0038] As a preferred embodiment, step (1) includes: (11) Pre-dispersion of PVDF at low speed: Add PVDF to a reaction vessel containing organic solvent, for example, start a vacuum stirrer to pre-mix at low speed; if there is PVDF agglomeration, crush it manually with a crushing tool in a dust-free environment (manual crushing can prevent the machine stirring from causing the agglomeration to grow larger and larger), and use intermittent wall scraping to remove agglomeration to improve dispersion uniformity; low speed is required to: prevent flying powder and dry powder from floating, and remove air inside the powder particles and between agglomerates to prevent the formation of bubble layer on the liquid surface; (12) Disperse the low-speed pre-dispersed liquid at medium speed. If there is PVDF agglomeration, crush it manually with a crushing tool in a dust-free environment and use intermittent wall scraping to remove the agglomeration and improve the dispersion uniformity. Medium speed satisfies the following: break up the clumps formed by the powder absorbing water and expanding, and evenly disperse the local thick material to all corners of the container. (13) The medium-speed dispersion is subjected to high-speed enhanced dispersion until the dispersion becomes a transparent and colorless liquid.

[0039] As a further preferred embodiment, step (2) can be performed in the same manner as step (1) to achieve the addition of the conductive agent.

[0040] This embodiment takes into account that if high-speed stirring is started directly without using a step-by-step speed increase, it will lead to powder splashing, air entrapment (a large number of bubbles), and the formation of "dry core" clumps that cannot be broken up inside.

[0041] The first stage is low-speed stirring, the core tasks of which are wetting and degassing.

[0042] Preventing "Flying Powder": Powders like conductive carbon black, carbon nanotubes, or PVDF have extremely low bulk density (very light). If stirred directly at high speeds, the powder will be lifted and dispersed into the air, causing material loss, imbalanced formulations, and potentially posing an inhalation hazard. Initial Wetting: At low speeds, the powder is slowly drawn into a polar solvent (such as NMP). The solvent has sufficient time to penetrate the pores of the powder through capillary action. Air Removal: A large amount of air is trapped within the powder particles and between aggregates. Low-speed stirring allows the solvent to slowly expel this air, preventing the formation of a thick "bubble layer" on the surface.

[0043] The second stage is medium-speed mixing, the core task of which is macroscopic mixing and breaking up of lumps.

[0044] When the powder has been completely immersed in the liquid and no dry powder is floating, increase the speed to medium.

[0045] Breaking up large clumps: After the initial wetting stage, the powder absorbs water, swells, and clumps together into large "dough lumps" (similar to the state when water is first added to dough). The moderate eddies generated by medium-speed stirring tear these clumps apart into smaller flocs. Establishing macroscopic homogeneity: Evenly dispersing locally thick material throughout the container ensures that the concentration and viscosity of the entire system are initially consistent.

[0046] The third stage: high-speed stirring, the core task of which is high-shear micro-dispersion.

[0047] Once the slurry appears macroscopically homogeneous, high-speed stirring is initiated. This provides powerful shear force: Nanoscale conductive fillers (such as carbon nanotubes and carbon black) possess extremely strong van der Waals forces, causing them to clump together tightly like chewing gum. Only the powerful mechanical shearing generated by high-speed rotation can forcefully "tear apart" these microscopic aggregates. This disrupts polymer chain entanglement: For high-molecular polymers like PVDF, high-speed stirring helps the polymer molecular chains further expand and untangle in the solvent, allowing for complete dissolution. Finally, it forms a stable conductive network: Only at this stage will the conductive filler be truly stripped into single strands or a few layers, uniformly suspended within the PVDF gel network, forming a long-lasting and stable dispersion.

[0048] As a further preferred embodiment, before performing step (3), the method further includes: measuring the fineness of the dispersion; when the fineness does not reach the preset threshold, using the low speed, medium speed and high speed of step (2), stirring again in stages until the fineness reaches the preset threshold, and finally obtaining a PVDF dispersion with good stability and no obvious agglomeration and bubbles. The preset threshold is determined by the actual required surface roughness and sheet resistance uniformity of the resistive film.

[0049] It should be noted that during the dispersion process, the system temperature gradually rises due to mechanical stirring and shearing. When the dispersion temperature reaches the preset upper limit (e.g., 40–70°C), the stirring speed is paused or reduced (e.g., reduced to 100–300 rpm or stopped completely), and the system is allowed to stand or slowly stirred under vacuum for 5–60 minutes to allow the system to cool naturally or the jacketed circulating water cooling system to return to the target temperature range (e.g., 20–40°C). After the temperature drops, the set speed is resumed to continue the dispersion operation. Temperature control prevents the introduction of moisture.

[0050] In the subsequent step (3), for example, the carrier substrate can be placed on a flatbed coater, the blade distance can be adjusted, and the dispersion can be evenly dripped into the blade using a dropper. The flatbed coater can be turned on and the speed can be adjusted to, for example, 50 mm / s to obtain a wet resistive film. The obtained wet resistive film can be transferred to a blower dryer, placed flat, and the blower dryer can be started. The temperature setting is preferably 40-50 ℃ and the time setting is preferably 10-12 h to complete the preparation of the resistive film. When the blade distance is 50-500 μm and the speed is 50 mm / s, the thickness of the resistive film is about 10 μm.

[0051] Example 3 An application of the flexible resistive film as described in Embodiment 1 in a resistive film absorber or electromagnetic shielding structure. That is, the resistive film functional layer in the absorber adopts the resistive film as described in Embodiment 1 above, or the resistive film of Embodiment 1 itself is used as an electromagnetic shielding structure.

[0052] In microwave absorber design applications, optionally, by weight, 4-8 parts of polyvinylidene fluoride (PVDF), 90-95 parts of organic solvent, and 1-2 parts of conductive agent. As a preferred embodiment, when the resistive film is used in a resistive film absorber, the conductive agent accounts for 5%-80% of the PVDF mass; the resistive film thickness is 10 μm to 500 μm; and the sheet resistance of the resistive film is 1-1000 Ω / sq. Preferably, the conductive agent accounts for 10%-40% of the PVDF mass, and the thickness of the resistive film is determined by the desired microwave absorption performance.

[0053] In this field, the general requirement for resistive film absorbers is a sheet resistance of approximately 300-400 Ω / sq, with 377 Ω / sq achieving perfect matching with free space and maximizing absorption. Based on experimental experience, this embodiment uses a conductive agent mass fraction of approximately 10-40%. The sheet resistance of the resistive film can be adjusted by changing the film thickness to achieve the desired absorption performance.

[0054] For electromagnetic shielding, the sheet resistance is usually much lower than 377 Ω / sq, and the lower the better (the stronger the metallicity). Impedance matching is not required. In this case, the sheet resistance of the resistive film can be reduced by increasing the content of conductive agent and increasing the thickness of the resistive film, thus achieving electromagnetic shielding.

[0055] For example, such as Figure 3 As shown, the absorber structure comprises three layers, from top to bottom: a flexible resistive film, a dielectric layer, and a perfect electrical conductor (PEC) sheet. The dielectric layer includes aramid honeycomb or PMI foam, and the PEC sheet includes a metal plate or a highly conductive carbon fiber plate. The fabrication method includes the following steps: cutting the resistive film into specific sizes and shapes according to actual needs, aligning and bonding it with the dielectric layer and PEC sheet of specific sizes; and then performing vacuum hot-pressing composite bonding on the bonded assembly, for example, with the pressure controlled at 0.3... 0.5MPa, temperature 60 80℃, for 120 seconds It takes 180 minutes for the absorbing material to bond firmly.

[0056] The relevant technical solutions are the same as above, and will not be repeated here.

[0057] In microwave absorption theory, to achieve maximum absorption in a specific frequency band (such as the X-band or Ku-band), the thickness of the resistive film must precisely match a quarter wavelength of the electromagnetic wave corresponding to that frequency. Even a difference of tens of micrometers in thickness can cause a severe shift in the absorption peak or a precipitous drop in performance. Traditional spraying or dip coating methods struggle to precisely control film thickness, while blade coating / spreading methods, through precise adjustment of the blade gap, can achieve extremely high-precision micron-level wet film thickness control. This invention utilizes the precise thickness measurement characteristics of coating and other processes to achieve "on-demand customization" of the resistive film thickness according to the requirements of different target microwave frequency bands, thereby ensuring a perfect match between the intrinsic impedance of the film and the free-space impedance, and minimizing the microwave reflectivity of the specific frequency band.

[0058] This invention induces carbon-based fillers to form an anisotropic conductive network with a specific orientation within a PVDF matrix by precisely controlling the coating speed and shear rate. This ordered microstructure enables more efficient and strong electromagnetic coupling with incident microwaves of specific polarization directions, significantly enhancing dipole polarization loss and interfacial polarization loss. Furthermore, this invention employs an automated casting coating process, completely eliminating thickness gradients and surface orange peel defects easily generated by traditional film deposition processes. Its excellent macroscopic electromagnetic uniformity effectively suppresses incoherent scattering of high-frequency microwaves at the interface, ensuring electromagnetic performance stability in large-area applications.

[0059] Many high-level microwave materials can only be fabricated into coin-sized samples in the laboratory, while practical microwave applications (such as large radar dome anti-interference layers, electromagnetic shielding wallpaper for computer rooms, and flexible wearable antennas) require continuous, large-area thin films. The laboratory-scale coating method is fundamentally consistent with industrial-grade roll-to-roll slot-die coating or microgravure coating. The method provided by this invention not only possesses excellent electromagnetic properties but also overcomes the production bottleneck of traditional complex processes. This fabrication process does not require expensive high-vacuum equipment, is environmentally friendly, and can be directly transferred to continuous roll-to-roll industrial production lines, paving the way for the commercial mass production of large-area flexible microwave shielding / absorbing materials.

[0060] To better illustrate the present invention, the following examples are provided: Example 1 A PVDF flexible resistive film is prepared by the following method: (1) Raw material preparation: including carrier material and functional dispersion material. The carrier material is FR-4 membrane with a length of 300 mm, a width of 300 mm, and a thickness of 0.15 mm; the functional dispersion material is as follows: weigh 15 g of polyvinylidene fluoride, 300 ml of N-methylpyrrolidone, and 3.75 g of carbon black; (2) Preparation of dispersion: Polyvinylidene fluoride was poured into N-methylpyrrolidone and premixed at low speed using a vacuum stirrer with a vacuum degree of [missing information]. 0.08 MPa, preferred rotation speed 600 rpm, time 300 s; inspect the dispersion, crush the PVDF agglomerates with a stirring rod; start the vacuum mixer at medium speed, with a vacuum degree of [missing value]. 0.1 MPa, preferably 1000 rpm, time 600 s; inspect the dispersion, crush the PVDF agglomerates with a stirring rod; start the vacuum mixer at medium speed, vacuum degree is [missing information]. 0.1 MPa, preferably 2000 rpm, time 1200 s; a transparent and colorless dispersion was obtained; (3) Preparation of conductive dispersion: Pour carbon black into the dispersion of (2), and premix it at low speed using a vacuum mixer with a vacuum degree of [missing information]. 0.08 MPa, preferred rotation speed 600 rpm, time 300 s; start the vacuum mixer at medium speed, vacuum degree is... 0.1 MPa, preferably 1000 rpm, time 600 s; start the vacuum mixer at medium speed, vacuum degree is A uniform conductive dispersion was obtained by applying pressure of 0.1 MPa, preferably at a rotation speed of 2000 rpm, for a time of 1200 s. (4) Resistor film coating: Clean the surface of the above carrier material with alcohol cotton to ensure that there are no impurities on the surface and enhance the bonding force with the conductive material in the future; place it in an automatic coating machine and turn on the vacuum adsorption function; adjust the thickness of the scraper to 200 μm, and evenly drip the conductive dispersion liquid in (3) onto the back of the scraper to ensure that there is conductive dispersion liquid in every gap of the scraper; select a coating machine with a speed of 50 mm / s and a coating length of 300 mm, and turn on the coating machine; (5) Drying the resistive film: After (4) is completed, gently remove the scraper and slowly and evenly move the obtained resistive film into the blower dryer. Set the temperature of the blower dryer to 40 ℃ and the time to 12 h. The temperature and time can fluctuate within a suitable range. (6) Absorber preparation method: The resistive film obtained in (5) is cut into lengths of 250 mm and widths according to actual needs, and then PMI foam layer and copper plate are bonded to it. The thickness of PMI foam is 7 mm and the thickness of copper plate is 1 mm. The whole assembly is vacuum hot-pressed and composited, with the pressure controlled at 0.3 ppm. 0.5 MPa, temperature 60 80℃, for 120 seconds 180 minutes are required for the absorbing material to bond firmly. The die-cut absorbing material can be directly bonded to metal surfaces or the inner layer of composite structures, suitable for electromagnetic interference suppression in aviation, aerospace, and communication equipment. Absorption performance test results are as follows: Figure 4 As shown.

[0061] The microwave absorbing material prepared by this example, compared with existing resistive film structures and compositions, requires no additional adhesive layer, reduces the number of interfaces, and lowers the risk of interlayer debonding. Furthermore, it allows for flexible combination of films with different sheet resistances and shapes with foam substrates of varying thicknesses to create microwave absorbing materials that meet different operational requirements, enabling precise control of specific frequency bands and satisfying the electromagnetic compatibility needs of diverse application scenarios.

[0062] Example 2 The difference between Example 2 and Example 1 is that the coating thickness of the flexible resistive film is different. In this example, the scraper thickness (i.e., wet film thickness) is set to 300 μm.

[0063] Example 3 The difference between Example 3 and Example 1 is that the coating thickness of the flexible resistive film is different. In this example, the scraper thickness (i.e., wet film thickness) is set to 400 μm.

[0064] Example 4 The difference between Example 4 and Example 1 lies in the formulation of the functional dispersion material, as described below: Weigh 14 g of polyvinylidene fluoride, 250 ml of N-methylpyrrolidone, and 6 g of carbon black.

[0065] Example 5 The difference between Example 5 and Example 4 lies in the thickness of the flexible resistive film coating; Example 5 has a coating thickness (i.e., wet film thickness) of 50 μm. The microwave absorption performance test results are as follows: Figure 5 As shown.

[0066] Example 6 The difference between Example 6 and Example 4 is that the coating thickness of the flexible resistive film is different. The coating thickness (i.e., wet film thickness) of the resistive film is 100 μm.

[0067] Example 7 The difference between Example 7 and Example 4 is that the coating thickness of the flexible resistive film is different. The coating thickness (i.e., wet film thickness) of the resistive film is 300 μm.

[0068] Example 8 The difference between Example 8 and Example 4 is that the coating thickness of the flexible resistive film is different. The coating thickness (i.e., wet film thickness) of the resistive film is 500 μm.

[0069] The difference between Example 9 and Example 1 lies in the formulation of the functional dispersion material, as described below: Weigh 19 g of polyvinylidene fluoride, 250 ml of N-methylpyrrolidone, and 1 g of carbon black.

[0070] The difference between Example 10 and Example 1 lies in the formulation of the functional dispersion material, as described below: Weigh 17 g of polyvinylidene fluoride, 250 ml of N-methylpyrrolidone, and 3 g of carbon black.

[0071] The difference between Example 11 and Example 1 lies in the formulation of the functional dispersion material, as described below: Weigh 16 g of polyvinylidene fluoride, 250 ml of N-methylpyrrolidone, and 4 g of carbon black.

[0072] Table 1. Membrane thickness and sheet resistance test data

[0073] As shown in Table 1 above, Example 1 3 or 4-8 reveal the significant effect of wet film thickness on the sheet resistance of flexible resistive films. Sheet resistance is negatively correlated with wet film thickness, which enables continuous control and simple preparation of gradient resistive films.

[0074] Comparative examples 1, 2, 4, and 7 further reveal the significant influence of the mass ratio of PVDF to conductive agent on the sheet resistance of the resistive film. The sheet resistance is positively correlated with the above mass ratio, which enables continuous control and simple preparation of gradient resistive films.

[0075] Thickness measurements were taken for Examples 1-8. The overall thickness of the resistive film and the carrier substrate was 155-170 μm, and the thickness of the carrier substrate was 150 μm. By controlling the thickness of the PVDF resistive film within the range of 5 μm to 20 μm, the film has better flexibility and bendability, significantly improving resistance stability, while reducing material costs and enhancing the miniaturization and integration capabilities of the device.

[0076] Therefore, this invention enables efficient and stable preparation of resistive films, significantly improving their absorption performance; it ensures stable sheet resistance of the resistive film while providing strong interlayer adhesion. Simultaneously, it ensures stable and predictable electromagnetic response characteristics of the absorbing material within the absorption frequency band, significantly improving product yield and batch consistency. By controlling the proportion of conductive material components and the wet film thickness, the conductivity and adhesion of the resistive film are effectively adjusted. Low-temperature, long-term drying ensures the stability of the resistive film. No additional adhesive coating or activation treatment is required during preparation, simplifying the process and improving production efficiency and yield.

[0077] Those skilled in the art will readily understand 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 within the scope of protection of the present invention.

Claims

1. A flexible resistive film based on polyvinylidene fluoride (PVDF), characterized in that, As an electromagnetic functional material, it includes polyvinylidene fluoride (PVDF) and a conductive agent; wherein, PVDF acts as a binder and exists in a uniformly mixed form with the conductive agent, and is obtained by forming a film through a uniformly dispersed liquid; the mass ratio of PVDF and the conductive agent, as well as the film thickness of the resistive film, together determine the final sheet resistance of the resistive film.

2. The flexible resistive film as described in claim 1, characterized in that, The conductive agent is conductive carbon black.

3. A method for preparing a flexible resistive film as described in claim 1 or 2, characterized in that, include: (1) Add polyvinylidene fluoride (PVDF) to an organic solvent and mix thoroughly to obtain a uniform and transparent dispersion; (2) Add the conductive agent to the transparent dispersion to obtain a uniform conductive dispersion; (3) The conductive dispersion is prepared into a wet film on a carrier substrate, and after drying and curing, a flexible resistive film based on PVDF is obtained.

4. The preparation method according to claim 3, characterized in that, The organic solvent is one or more of N,N-dimethylformamide, N-methylpyrrolidone, or acetone; the conductive dispersion is prepared into a wet film on the carrier substrate by means of blade coating, casting, spin coating, or spray coating.

5. The preparation method according to claim 3, characterized in that, The drying temperature was set to 40 ℃-50 ℃, and the drying time was set to 10h-12h.

6. The preparation method according to claim 3, characterized in that, Step (1) includes: (11) Preparation of PVDF dispersion by low-speed pre-dispersion: PVDF is added to a reaction vessel containing organic solvent and pre-mixed at low speed; if PVDF agglomeration occurs, it is manually crushed in a dust-free environment using a crushing tool, and intermittent wall scraping is used to remove the agglomeration; the low speed is to prevent powder from flying and to remove air inside the powder particles and between the agglomerates, and to prevent the formation of a bubble layer on the liquid surface. (12) Disperse the low-speed pre-dispersed liquid at a medium speed. If there is PVDF agglomeration, crush it manually with a crushing tool in a dust-free environment and use intermittent wall scraping to remove the agglomeration. The medium speed is required to break up the clumps formed by the powder absorbing water and expanding, and to evenly disperse the local thick material to all corners of the container. (13) The dispersion of medium speed is subjected to high speed enhanced dispersion until the dispersion becomes a transparent and colorless liquid, wherein the high speed satisfies: higher than the medium speed described in step (12).

7. The preparation method according to claim 6, characterized in that, Step (2) is performed in the same manner as step (1) to add the conductive agent.

8. The preparation method according to claim 7, characterized in that, Before performing step (3), the method further includes: measuring the fineness of the dispersion; when the fineness does not reach the preset threshold, using the low speed, medium speed and high speed of step (2), stirring again in stages until the fineness reaches the preset threshold, wherein the preset threshold is determined by the actual required surface roughness and sheet resistance uniformity of the resistive film.

9. The application of a flexible resistive film as described in claim 1 or 2 in a resistive film absorber or electromagnetic shielding structure.

10. The application as described in claim 9, characterized in that, When a flexible resistive film is used in a resistive film absorber, the mass ratio of the conductive agent is 5% to 80% of the mass of PVDF; the thickness of the resistive film is 10 μm to 500 μm; and the sheet resistance of the resistive film is 1 to 1000 Ω / sq. Preferably, the mass ratio of the conductive agent is 10% to 40% of the mass of PVDF. Based on this, the thickness of the resistive film is determined by the required absorption performance.

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