Fe3O4 / SiO2-graphene gradient composite electromagnetic shielding coating and preparation method thereof

Abstract

The invention relates to the technical field of electromagnetic shielding coatings, discloses a Fe3O4 / SiO2-graphene gradient composite electromagnetic shielding coating and a preparation method thereof, and aims to solve the problem that in the prior art, a shielding material depends on a single-layer structure, multi-stage attenuation of electromagnetic waves cannot be achieved, and the shielding effectiveness is low. The composite electromagnetic shielding coating comprises an inner layer and an outer layer, wherein the inner layer comprises the following components: styrene-acrylic emulsion, epoxy resin, polyimide, polyethylene, graphene, a compound curing agent, a reactive diluent, a filler, a dispersing agent and a coupling agent; the outer layer is prepared from the following components: epoxy resin, modified polyurethane, nano Fe3O4 / SiO2 core-shell particles, polyethyleneimine, a dispersing agent, a cross-linking agent, a cross-linking accelerant and a waterproof agent. By adopting the shielding structure with the inner layer and the outer layer, a good shielding effect can be achieved, and a strong anti-electromagnetic function is achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of electromagnetic shielding coatings, and specifically relates to Fe 3 O 4 / SiO 2 -graphene gradient composite electromagnetic shielding coating and its preparation method. Background Art

[0002] With the wide use of electronic devices, the electromagnetic radiation problems they bring have gradually emerged, affecting daily life and industrial production, and becoming new health risks. The interference caused by electromagnetic waves not only hinders the normal operation of electronic devices, but may also lead to the leakage of sensitive information, posing a threat to national security. Therefore, electromagnetic shielding materials are of great significance in reducing the harm of electromagnetic radiation, solving electromagnetic interference problems, and ensuring electromagnetic compatibility. They are key materials indispensable in modern electronic devices and communication systems. These materials are essential for electronic devices and play a crucial role in promoting economic development, ensuring people's livelihood, and maintaining national defense security.

[0003] The synergistic effect of magnetic Fe 3 O 4 core and dielectric SiO 2 shell can achieve excellent electromagnetic wave absorption and shielding performance in a wide frequency range. When combined with a polymer matrix, the core-shell structure not only effectively solves the problem of easy oxidation and agglomeration of magnetic nanoparticles, but also can significantly improve the electromagnetic shielding efficiency of the composite material by regulating the interfacial polarization and magnetic loss mechanisms. Its unique impedance matching characteristics make the material particularly outstanding in the field of absorption-type electromagnetic shielding. On the other hand, due to its excellent mechanical, thermal, electrical, optical, and tribological properties, graphene has broad application prospects in many fields. Due to its excellent electrical conductivity and structural characteristics, it performs better in anti-electromagnetic interference, especially when combined with epoxy resin, it can significantly improve the electromagnetic shielding efficiency of the composite material.

[0004] Among many technologies, shielding materials rely on single-layer structures (such as only using magnetic particles or conductive materials), which cannot achieve multi-stage attenuation of electromagnetic waves, resulting in low shielding efficiency (SE). The present invention solves the core problems of low efficiency, poor dispersibility, and insufficient conductivity of existing single-layer shielding materials through double-layer collaborative design (core-shell particles + graphene), material structure optimization (suppressing agglomeration, precise ratio), and improvement of long-term stability, achieving the unity of high frequency band, high shielding efficiency (55 dB), and strong anti-fatigue performance. Summary of the Invention

[0005] Aiming at the problems mentioned in the background art, the purpose of the present invention is to provide Fe 3 O 4 / SiO2 - Graphene gradient composite electromagnetic shielding coating and its preparation method to solve the problems mentioned in the background technology.

[0006] The above technical object of the present invention is achieved through the following technical solutions: Fe 3 O 4 / SiO 2 - Graphene gradient composite electromagnetic shielding coating, including an inner layer and an outer layer. The inner layer includes the following raw materials by mass percentage: styrene-acrylic emulsion 30% - 70%, epoxy resin 6% - 15%, polyimide 10% - 20%, polyethylene 3% - 12%, graphene 5% - 40%, compound curing agent 1% - 3%, active diluent 1% - 3%, filler 1% - 3%, dispersant 0.5% - 5% and coupling agent 0.5% - 2%, totaling 100%; the outer layer includes the following raw materials by mass percentage: epoxy resin 50% - 70%, modified polyurethane 15% - 25%, nano Fe 3 O 4 / SiO 2 core-shell particles 10% - 25%, polyethyleneimine 2% - 5%, dispersant 2% - 5%, crosslinking agent 1% - 3 parts, crosslinking accelerator 1% - 3% and waterproofing agent 1% - 3%, totaling 100%.

[0007] Preferably, the crosslinking agent is polyisocyanate. Through the reaction of isocyanate groups (-NCO) with hydroxyl groups (-OH) or amino groups (-NH 2 ) in the resin, a three-dimensional network structure is formed, significantly improving the hardness, wear resistance and high temperature resistance of the coating.

[0008] Preferably, the crosslinking accelerator is organic amine. Organic amines can accelerate the crosslinking reaction at a relatively low temperature (such as 60°C), reducing energy consumption, and at the same time avoiding the destruction of the magnetism of nano Fe 3 O 4 / SiO 2 particles or the conductivity of graphene.

[0009] Preferably, the waterproofing agent is fluorine-containing waterproofing agent. Fluorine-containing groups (such as -CF 3 ) endow the coating with extremely low surface energy, achieving a "self-cleaning" effect, reducing the penetration of water and oil stains, and preventing the degradation of electromagnetic shielding performance due to water absorption. If silane-based waterproofing agents are used, the hydrophobicity and ultraviolet resistance are significantly reduced.

[0010] The present invention also provides a preparation method for Fe 3 O 4 / SiO 2 - graphene gradient composite electromagnetic shielding coating, including the following steps: S1. Prepare the inner layer: Stir epoxy resin, polyimide, and polyethylene evenly with a blender, then add styrene-acrylic emulsion and continue stirring for 10 - 20 minutes. Pour it into a reaction kettle, keep it warm at 50 - 80°C for 20 - 60 minutes, then add graphene, compound curing agent, active diluent, filler, dispersant, and coupling agent, and heat to 80 - 120°C, stir for 30 - 90 minutes to ensure uniform dispersion. Cool to room temperature and set aside. S2. Spray the inner layer: Add 5 - 10% of whitening water (by mass of the bottom layer liquid) to the bottom layer liquid, and then spray it in three passes. The first pass: Control the spray gun distance at 20 - 30 cm, the moving speed of the spray gun at 40 - 60 cm / s, the overlapping width of the fan-shaped spray from the gun head at 1 / 2 - 2 / 3 of a fan-shaped surface, the spraying interval time at 5 - 15 minutes, and control the spraying to be uniform, smooth, without running beads and without biting the bottom. The second pass: Use horizontal and vertical spraying for uniform coverage. The third pass: Spray with a machine to make the sprayed paint film uniform. S3. Dry the inner layer: Dry the inner layer in a drying room, control the temperature of the drying room at 50 - 150°C. S4. Trim the inner layer: When trimming, wet-grind the entire inner layer with 400 - 800 grit water sandpaper dipped in water. The sanding action should be steady and uniform. When sanding by hand, the sanding direction should form a certain angle with the finger direction and prevent finger marks. Control the inner layer to have no orange peel and no brush marks. Use a painter's spatula to remove the residual putty blocks and other dirt at the corners, sand it smooth with sandpaper, rinse it with water, and then blow off the water marks with a compressed air rubber tube and let it dry. S5. Prepare the outer layer: Add epoxy resin, modified polyurethane, nano-Fe 3 O 4 / SiO 2 core-shell particles, polyethyleneimine, dispersant, and waterproof agent into a reaction kettle, control the temperature of the reaction kettle at 80 - 100°C, stir for 30 - 40 minutes to ensure uniform dispersion. Add a cross-linking agent and a cross-linking accelerator, and continue stirring for 10 - 20 minutes. Cool to room temperature and set aside. S6. Spray the outer layer: Spray in two passes. The first pass: Control the spray gun distance at 20 - 30 cm, the moving speed of the spray gun at 40 - 60 cm / s, sprinkle the paint mist on the bottom layer, and check that the paint film must be continuous after spraying, and check for no shrinkage holes. The second pass: Control the spray gun distance at 15 - 25 cm, the moving speed of the spray gun at 30 - 50 cm / s, and control the uniform movement of the spray gun to prevent skipping and missing spraying. The third pass: Control the spray gun distance at 20 - 30 cm, the moving speed of the spray gun at 40 - 60 cm / s, and improve the uniformity of the coating thickness. S7. Dry the outer layer: Dry the outer layer at 60 - 150°C. Cool to room temperature and check the surface quality of the coating.

[0011] Preferably, the compound curing agent in S1 is one of modified phenolic resin and modified dicyandiamide. Modified phenolic resin cures at medium temperature (80 - 120 °C), enhancing the chemical corrosion resistance (such as acid and alkali resistance) and bonding strength of the coating. Modified dicyandiamide significantly reduces the initial curing temperature by introducing activating groups (such as organic ureas, imidazole derivatives, etc.), adapting to the coating inner layer preparation process (such as heating to 80 - 120 °C in step S1), reducing energy consumption and avoiding damage to sensitive materials such as graphene at high temperatures.

[0012] Preferably, the active diluent in S1 is one of propylene oxide phenyl (PGE) and butyl acrylate (BA). Propylene oxide phenyl (PGE), a low-viscosity epoxy active diluent, effectively reduces the viscosity of the resin system, facilitating the uniform dispersion of graphene and fillers (especially during the high-temperature stirring in step S1). Butyl acrylate (BA), a monomer containing double bonds, participates in free radical polymerization, increasing the flexibility of the coating and reducing the risk of brittle cracking.

[0013] Preferably, the filler in S1 is calcium carbonate. Calcium carbonate is a low-cost filler that adjusts the hardness and wear resistance of the coating while reducing material costs.

[0014] The beneficial effects of the present invention are as follows: By adopting a shielding structure with inner and outer layers, good shielding effects can be achieved, and it has a strong anti-electromagnetic function; when the electromagnetic wave radiation field approaches the shielding body, the nano-Fe 3 O 4 / SiO 2 core-shell particles in the outer layer will eliminate a part of the electromagnetic wave radiation, and the remaining part will transmit into the shielding body and continue to enter the inner layer. The dispersed graphene in the inner layer can achieve the effect of shielding radiation again. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 This is a process block diagram of the preparation method of the Fe 3 O 4 / SiO 2 -graphene gradient composite electromagnetic shielding coating of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0016] The following will elaborate on the technical content of its implementation plan in detail in combination with the specific drawings of the present invention. It should be noted that the implementation manners listed herein are only partial examples of this patent and do not exhaust all possibilities. Based on the technical solutions disclosed in the present invention, any other implementation manners obtained by any person skilled in the art without creative efforts shall fall within the protection scope of the present invention. Example 1

[0017] Reference Figure 1, Fe 3 O 4 / SiO 2 -graphene gradient composite electromagnetic shielding coating, including an inner layer and an outer layer. The inner layer comprises raw materials in the following mass percentages: styrene-acrylic emulsion 53%, epoxy resin 11%, polyimide 16%, polyethylene 11%, graphene 5%, compound curing agent 2%, active diluent 1%, filler 1%, dispersant 0.5% and coupling agent 0.5%, totaling 100%; the outer layer comprises raw materials in the following mass percentages: epoxy resin 63%, modified polyurethane 18%, nano Fe 3 O 4 / SiO 2 core-shell particles 12%, polyethyleneimine 2%, dispersant 2%, crosslinking agent 1%, crosslinking accelerator 1%, waterproofing agent 1%, totaling 100%.

[0018] Preferably, the crosslinking agent is a polyisocyanate, namely HDI trimer. Through the reaction of isocyanate groups (-NCO) with hydroxyl groups (-OH) or amino groups (-NH 2 ) in the resin, a three-dimensional network structure is formed, significantly improving the hardness, wear resistance and high temperature resistance of the coating.

[0019] Preferably, the crosslinking accelerator is an organic amine, namely dodecyldimethylamine. Organic amines can accelerate the crosslinking reaction at a relatively low temperature (such as 60°C), reduce energy consumption, and at the same time avoid destroying the magnetism of nano Fe 3 O 4 / SiO 2 particles or the conductivity of graphene.

[0020] Preferably, the waterproofing agent is a fluorine-containing waterproofing agent. Fluorine-containing groups (such as -CF 3 ) endow the coating with an extremely low surface energy, achieving a "self-cleaning" effect, reducing the penetration of water and oil stains, and preventing the degradation of electromagnetic shielding performance due to water absorption. If a silane-based waterproofing agent is used, the hydrophobicity and ultraviolet resistance are significantly reduced.

[0021] In this embodiment, the preparation method of the Fe 3 O 4 / SiO 2 -graphene gradient composite electromagnetic shielding coating comprises the following steps: S1. Prepare the inner layer: Stir epoxy resin, polyimide and polyethylene with a stirrer for 10 min, then add styrene-acrylic emulsion and continue stirring for 10 min. Pour it into a reaction kettle, keep it warm at 50°C for 20 min, then add graphene, compound curing agent, active diluent, filler, dispersant and coupling agent thereto, and heat to 80°C and stir for 30 minutes to ensure uniform dispersion. Cool to room temperature and set aside; S2. Spraying the inner layer: Add 5% of chemical thinning agent by mass of the base liquid to the base liquid, and then spray it in three passes. The first pass: Control the spray gun distance at 20 cm, the moving speed of the spray gun at 40 cm / s, the fan width ejected from the gun head to overlap by 1 / 2 of a fan width, the spraying interval time at 5 min, and control the spraying to be uniform, smooth, without running beads or lifting the primer; The second pass: Use horizontal and vertical spraying for uniform coverage; The third pass: Use machine spraying to make the sprayed paint film uniform; S3. Drying the inner layer: Dry the inner layer in a drying room, and control the temperature of the drying room at 50°C; S4. Trimming the inner layer: When trimming, wet-grind the entire inner layer with 400-grit water sandpaper dipped in water. The sanding action should be steady and uniform. When sanding by hand, the sanding direction should form a certain angle with the finger direction and prevent finger marks from appearing. Control the inner layer to have no orange peel or brush marks; Use a painter's spatula to remove the residual putty blocks and other dirt at the corners, sand it smooth with sandpaper, rinse it with water, and then blow off the water stains with a compressed air rubber tube and let it dry; S5. Preparing the outer layer: Add epoxy resin, modified polyurethane, nano-Fe 3 O 4 / SiO 2 core-shell particles, polyethyleneimine, dispersant, and waterproofing agent into a reaction kettle, control the temperature of the reaction kettle at 80°C, stir for 30 minutes to ensure uniform dispersion; Add a cross-linking agent and a cross-linking accelerator, and continue to stir for 10 minutes; Cool to room temperature and set aside; S6. Spraying the outer layer: Spray in three passes. The first pass: Control the spray gun distance at 20 cm, the moving speed of the spray gun at 40 cm / s, sprinkle the paint mist on the base layer, and check that the paint film must be continuous after spraying, and check for no shrinkage holes; The second pass: Control the spray gun distance at 15 cm, the moving speed of the spray gun at 30 cm / s, and control the uniform movement of the spray gun to prevent skipping or missing the gun; The third pass: Control the spray gun distance at 25 cm, the moving speed of the spray gun at 50 cm / s, and improve the uniformity of the coating thickness; S7. Drying the outer layer: Dry the outer layer at 60°C; Cool to room temperature and check the surface quality of the coating; Preferably, the compound curing agent in S1 is modified phenolic resin. Modified phenolic resin cures at medium temperature (80 - 120°C), enhancing the chemical corrosion resistance (such as acid and alkali resistance) and bonding strength of the coating.

[0022] Preferably, the active diluent in S1 is propylene oxide phenyl (PGE). Propylene oxide phenyl (PGE) is a low-viscosity epoxy active diluent, effectively reducing the viscosity of the resin system and facilitating the uniform dispersion of graphene and fillers (especially during the high-temperature stirring in step S1).

[0023] Preferably, the filler in S1 is calcium carbonate. Calcium carbonate is a low-cost filler that can adjust the hardness and wear resistance of the coating while reducing the material cost.

[0024] This embodiment adopts a double-layer shielding structure, which can achieve a good shielding effect and has a strong anti-electromagnetic function. When the electromagnetic wave radiation field approaches the shielding body, the nano-Fe 3 O 4 / SiO 2 core-shell particles in the outer layer will eliminate part of the electromagnetic wave radiation, and the remaining part will transmit into the shielding body and continue to transmit into the inner layer. The dispersed graphene in the inner layer can achieve the effect of shielding radiation again. Example 2

[0025] Reference Figure 1 , Fe 3 O 4 / SiO 2 -graphene gradient composite electromagnetic shielding coating method, including an inner layer and an outer layer. The inner layer includes the following raw materials in mass percentage: styrene-acrylic emulsion 48%, epoxy resin 11%, polyimide 15%, polyethylene 11%, graphene 9%, compound curing agent 2%, active diluent 1%, filler 1%, dispersant 1% and coupling agent 1%, totaling 100%; the outer layer includes the following raw materials in mass percentage: epoxy resin 61%, modified polyurethane 19%, nano-Fe 3 O 4 / SiO 2 core-shell particles 14%, polyethyleneimine 2%, dispersant 3%, crosslinking agent 1%, crosslinking accelerator 1% and waterproofing agent 1%, totaling 100%.

[0026] Preferably, the crosslinking agent is a polyisocyanate, which is an HDI trimer. Through the reaction of isocyanate groups (-NCO) with hydroxyl groups (-OH) or amino groups (-NH 2 ) in the resin, a three-dimensional network structure is formed, significantly improving the hardness, wear resistance and high temperature resistance of the coating.

[0027] Preferably, the crosslinking accelerator is an organic amine, dibutyltin dilaurate (DBTL). Organic amines can accelerate the crosslinking reaction at a relatively low temperature (such as 60°C), reducing energy consumption and avoiding the destruction of the magnetism of nano-Fe 3 O 4 / SiO 2 particles or the conductivity of graphene at high temperatures.

[0028] Preferably, the waterproofing agent is a fluorine-containing waterproofing agent. Fluorine-containing groups (such as -CF 3) gives the coating extremely low surface energy, achieves a "self-cleaning" effect, reduces water and oil penetration, and prevents electromagnetic shielding performance from degrading due to water absorption. If silane water repellents are used, the hydrophobicity and UV resistance will be significantly reduced.

[0029] This embodiment Fe 3 O 4 / SiO 2 - A method for preparing a graphene gradient composite electromagnetic shielding coating, comprising the following steps: S1. Prepare the inner layer: Stir epoxy resin, polyimide and polyethylene in a stirrer for 15 minutes, then add styrene-acrylic emulsion and continue stirring for 15 minutes. Pour into the reactor and keep it at 55°C for 25 minutes. Then add graphene, compound curing agent, reactive diluent, filler, dispersant and coupling agent, heat to 80°C, stir for 35 minutes to ensure uniform dispersion. Cool to room temperature and set aside; S2, spray inner layer: add 5% of the base liquid mass of whitening water to the base liquid, and then spray in three steps. The first step is to control the spray gun distance to 20 cm, the spray gun moving speed to 40 cm / s, the spray fan of the gun head to overlap the width of 1 / 2 fan, the spray interval time is 5 minutes, and the spraying is controlled to be uniform, smooth, without flow column and bottom biting; the second step is to use horizontal and vertical spraying for uniform coverage; the third step is to use machine spraying to make the sprayed paint film uniform; S3, drying the inner layer: drying the inner layer in a drying room, and controlling the temperature of the drying room to be 80°C; S4. Finish the inner layer: Use 600# water sandpaper to wet-grind the entire inner layer. The grinding action should be steady and even. When grinding by hand, the grinding direction should be at a certain angle to the finger direction and avoid finger marks. Make sure there is no orange peel or brush marks on the inner layer. Use a painter's spatula to remove the residual putty and other dirt at the corners, and grind it smooth with sandpaper. Rinse with water, then blow off the water marks with a compressed air hose and let it dry. S5. Preparation of outer layer: epoxy resin, modified polyurethane, nano-Fe 3 O 4 / SiO 2 Add core-shell particles, polyethyleneimine, dispersant, and waterproofing agent into the reactor, control the temperature of the reactor to 80°C, and stir for 35 minutes to ensure uniform dispersion. Add crosslinking agent and crosslinking accelerator, and continue stirring for 12 minutes. Cool to room temperature and set aside; S6. Spray the outer layer: Conduct three sprays. The first spray: Control the distance between the spray gun and the object to be sprayed at 20 cm, and control the moving speed of the spray gun at 40 cm / s. Sprinkle the paint mist on the bottom layer. After spraying, check that the coating film must be continuous and there are no shrinkage holes. The second spray: Control the distance between the spray gun and the object to be sprayed at 15 cm, control the moving speed of the spray gun at 30 cm / s, and ensure the spray gun moves evenly without skipping or missing spraying. The third spray: Control the distance between the spray gun and the object to be sprayed at 20 cm, control the moving speed of the spray gun at 40 cm / s, and improve the uniformity of the coating thickness. S7. Dry the outer layer: Dry the outer layer at 80 °C. Cool to room temperature and check the surface quality of the coating. Preferably, the compound curing agent in S1 is modified dicyandiamide. By introducing activating groups (such as organic ureas, imidazole derivatives, etc.), modified dicyandiamide significantly reduces the initial curing temperature, adapts to the coating inner layer preparation process (such as heating to 80 °C in step S1), reduces energy consumption and avoids damage to sensitive materials such as graphene at high temperatures.

[0030] Preferably, the reactive diluent in S1 is butyl acrylate (BA). Butyl acrylate (BA), a monomer containing double bonds, participates in free radical polymerization, increases the flexibility of the coating, and reduces the risk of brittle cracking.

[0031] Preferably, the filler in S1 is calcium carbonate. Calcium carbonate, a low-cost filler, adjusts the hardness and wear resistance of the coating while reducing the material cost.

[0032] This embodiment adopts a shielding structure with inner and outer layers, which can achieve a good shielding effect and has a strong anti-electromagnetic function. When the electromagnetic wave radiation field approaches the shielding body, the nano-Fe 3 O 4 / SiO 2 core-shell particles in the outer layer will eliminate part of the electromagnetic wave radiation, and the remaining part will transmit into the shielding body and continue to enter the inner layer. The dispersed graphene in the inner layer can achieve the effect of shielding radiation again. Example 3

[0033] Reference Figure 1 ,Fe 3 O 4 / SiO 2 -graphene gradient composite electromagnetic shielding coating method, including an inner layer and an outer layer. The inner layer includes the following raw materials by mass percentage: styrene-acrylic emulsion 47%, epoxy resin 10%, polyimide 14%, polyethylene 10%, graphene 12%, compound curing agent 2%, reactive diluent 1%, filler 1%, dispersant 2% and coupling agent 1%, totaling 100%. The outer layer includes the following raw materials by mass percentage: epoxy resin 59%, modified polyurethane 16%, nano-Fe 3 O4 / SiO 2 16% core-shell particles, 2% polyethyleneimine, 4% dispersant, 1% crosslinking agent, 1% crosslinking accelerator and 1% waterproofing agent, totaling 100%.

[0034] Preferably, the crosslinking agent is a polyisocyanate. Through the reaction of isocyanate groups (-NCO) with hydroxyl groups (-OH) or amino groups (-NH 2 ), a three-dimensional network structure is formed, significantly improving the hardness, wear resistance and high-temperature resistance of the coating.

[0035] Preferably, the crosslinking accelerator is an organic amine. Organic amines can accelerate the crosslinking reaction at a relatively low temperature (such as 60 °C), reducing energy consumption and avoiding the destruction of the magnetism of nano-Fe 3 O 4 / SiO 2 particles or the conductivity of graphene.

[0036] Preferably, the waterproofing agent is a fluorine-containing waterproofing agent. Fluorine-containing groups (such as -CF 3 ) endow the coating with an extremely low surface energy, achieving a "self-cleaning" effect, reducing the penetration of water and oil stains, and preventing the degradation of electromagnetic shielding performance due to water absorption. If a silane-based waterproofing agent is used, the hydrophobicity and ultraviolet resistance are significantly reduced.

[0037] In this embodiment, the preparation method of the Fe 3 O 4 / SiO 2 -graphene gradient composite electromagnetic shielding coating includes the following steps: S1. Prepare the inner layer: Stir epoxy resin, polyimide, and polyethylene with a blender for 15 minutes, then add styrene-acrylic emulsion and continue stirring for 15 minutes. Pour it into a reaction kettle, keep it warm at 55 °C for 25 minutes, then add graphene, compound curing agent, active diluent, filler, dispersant, and coupling agent, and heat to 100 °C and stir for 45 minutes to ensure uniform dispersion. Cool to room temperature and set aside; S2. Spray the inner layer: Add 5% of whitening water by mass of the bottom layer liquid to the bottom layer liquid, and then spray it in three steps. The first step is: control the spray gun distance to 20 cm, the spray gun moving speed to 40 cm / s, the fan width of the gun head spray to overlap by 1 / 2 a fan width, the spray interval time to 5 minutes, and control the spraying to be uniform, smooth, without running beads and without biting the bottom; the second step is: use horizontal and vertical spraying for uniform coverage; the third step is: machine spraying to make the sprayed film uniform; S3. Dry the inner layer: Dry the inner layer in a drying room, and control the temperature of the drying room to 50 °C; S4. Inner layer trimming: When trimming, wet-grind the entire inner layer with 600-grit water sandpaper dipped in water. The sanding action should be steady and uniform. When sanding by hand, the sanding direction should form a certain angle with the finger direction to prevent finger marks, and ensure that there are no orange peels or brush marks on the inner layer. Use a painter's spatula to remove the residual putty blocks and other dirt at the corners, then sand them smooth with sandpaper, rinse them with water, and blow off the water stains with a compressed air rubber tube and let them dry. S5. Outer layer preparation: Add epoxy resin, modified polyurethane, nano-Fe 3 O 4 / SiO 2 core-shell particles, polyethyleneimine, dispersant, and waterproof agent into the reaction kettle, control the temperature of the reaction kettle at 80 °C, stir for 35 minutes to ensure uniform dispersion. Add crosslinking agent and crosslinking accelerator, and continue to stir for 20 minutes. Cool to room temperature and set aside. S6. Outer layer spraying: Conduct three sprays. The first spray: Control the spray gun distance at 20 cm, and the moving speed of the spray gun at 40 cm / s. Sprinkle the paint mist on the bottom layer. After spraying, check that the coating film must be continuous and there are no shrinkage holes. The second spray: Control the spray gun distance at 15 cm, the moving speed of the spray gun at 30 cm / s, and ensure that the movement of the spray gun is uniform, and avoid skipping or missing spraying. The third spray: Control the spray gun distance at 20 cm, the moving speed of the spray gun at 40 cm / s, and improve the uniformity of the coating thickness. S7. Outer layer drying: Dry the outer layer at 50 °C. Cool to room temperature and check the surface quality of the coating. Preferably, the compound curing agent in S1 is modified dicyandiamide. By introducing active groups (such as organic ureas, imidazole derivatives, etc.), modified dicyandiamide significantly reduces the initial curing temperature, adapts to the inner layer preparation process of the coating (such as heating to 80 - 120 °C in step S1), reduces energy consumption, and avoids damage to sensitive materials such as graphene at high temperatures.

[0038] Preferably, the active diluent in S1 is butyl acrylate (BA). Propylene oxide phenyl (PGE), a low-viscosity epoxy active diluent, effectively reduces the viscosity of the resin system, facilitating the uniform dispersion of graphene and fillers (especially during the high-temperature stirring in step S1).

[0039] Preferably, the filler in S1 is calcium carbonate. Calcium carbonate is a low-cost filler that adjusts the hardness and wear resistance of the coating while reducing the material cost.

[0040] This embodiment adopts a shielding structure with inner and outer layers, which can achieve a good shielding effect and has a strong anti-electromagnetic function. When the electromagnetic wave radiation field approaches the shielding body, the nano-Fe in the outer layer 3 O 4 / SiO 2The core-shell particles will eliminate part of the electromagnetic wave radiation, and the remaining part will transmit into the shielding body and continue to transmit into the inner layer. Dispersed graphene is provided in the inner layer, which can achieve the effect of shielding radiation again.

[0041] Comparative Example 1: Fe 3 O 4 / SiO 2 The gradient composite electromagnetic shielding coating includes an inner layer and an outer layer. The inner layer includes the following raw materials by mass percentage: styrene-acrylic emulsion 56%, epoxy resin 11%, polyimide 17%, polyethylene 11%, compound curing agent 2%, active diluent 1%, filler 1%, dispersant 0.5% and coupling agent 0.5%, totaling 100%; the outer layer includes the following raw materials by mass percentage: epoxy resin 63%, modified polyurethane 18%, nano Fe 3 O 4 / SiO 2 core-shell particles 12%, polyethyleneimine 2%, dispersant 2%, crosslinking agent 1%, crosslinking accelerator 1% and waterproofing agent 1%, totaling 100%.

[0042] Preferably, the crosslinking agent is a polyisocyanate, which is an HDI trimer. Through the reaction of isocyanate groups (-NCO) with hydroxyl groups (-OH) or amino groups (-NH 2 ) in the resin, a three-dimensional network structure is formed, significantly improving the hardness, wear resistance and high temperature resistance of the coating.

[0043] Preferably, the crosslinking accelerator is an organic amine, which is dodecyl dimethyl amine. Organic amines can accelerate the crosslinking reaction at a relatively low temperature (such as 60 °C), reducing energy consumption, and at the same time avoiding the destruction of the magnetism of nano Fe 3 O 4 / SiO 2 particles or the conductivity of graphene.

[0044] Preferably, the waterproofing agent is a fluorine-containing waterproofing agent. Fluorine-containing groups (such as -CF 3 ) endow the coating with extremely low surface energy, achieving a "self-cleaning" effect, reducing the penetration of water and oil stains, and preventing the degradation of electromagnetic shielding performance due to water absorption. If a silane-based waterproofing agent is used, the hydrophobicity and ultraviolet resistance will be significantly reduced.

[0045] The preparation method of the Fe 3 O 4 / SiO 2 gradient composite electromagnetic shielding coating in this comparative example includes the following steps: S1. Preparation of the inner layer: Epoxy resin, polyimide, and polyethylene are stirred in a blender for 10 minutes, then styrene-acrylic emulsion is added and stirring continues for 10 minutes. It is poured into a reaction kettle and kept at 50 °C for 20 minutes. Then, a compound curing agent, an active diluent, a filler, a dispersant, and a coupling agent are added thereto, and it is heated to 80 °C and stirred for 30 minutes to ensure uniform dispersion. It is cooled to room temperature and reserved for use; S2. Spraying the inner layer: 5% of whitening water based on the mass of the bottom layer liquid is added to the bottom layer liquid, and then spraying is carried out in three passes. The first pass is: controlling the spray gun distance to be 20 cm, the moving speed of the spray gun to be controlled at 40 cm / s, the fan-shaped spray from the gun head to be controlled with an overlapping width of 1 / 2 of a fan-shaped surface, the spraying interval time to be 5 minutes, and controlling the spraying to be uniform, smooth, without running beads, and without biting the bottom; The second pass is: uniformly covering by horizontal spraying and vertical spraying; The third pass is: machine spraying to make the sprayed paint film uniform; S3. Drying the inner layer: The inner layer is dried in a drying room, and the temperature of the drying room is controlled at 50 °C; S4. Trimming the inner layer: When trimming, the entire inner layer is wet-ground with 400-grit water sandpaper dipped in water. The sanding action is stable and uniform. When sanding by hand, the sanding direction should form a certain angle with the finger direction and prevent finger marks from appearing, and controlling the inner layer to have no orange peel and no brush marks; Use a painter's spatula to remove the residual putty blocks and other dirt at the corners, and sand them smooth with sandpaper, wash them clean with water, and then blow off the water marks with a compressed air rubber tube and dry them; S5. Preparation of the outer layer: Epoxy resin, modified polyurethane, nano-Fe 3 O 4 / SiO 2 core-shell particles, polyethyleneimine, a dispersant, and a waterproof agent are added to a reaction kettle. The temperature of the reaction kettle is controlled at 80 °C and stirred for 30 minutes to ensure uniform dispersion; A crosslinking agent and a crosslinking accelerator are added and stirring continues for 10 minutes; It is cooled to room temperature and reserved for use; S6. Spraying the outer layer: Spraying is carried out in three passes. The first pass is: controlling the spray gun distance to be 20 cm, the moving speed of the spray gun to be controlled at 40 cm / s, spraying the paint mist on the bottom layer, and checking that the paint film must be continuous after spraying and checking for no shrinkage holes; The second pass is: controlling the spray gun distance to be 15 cm, the moving speed of the spray gun to be controlled at 30 cm / s, and controlling the uniform movement of the spray gun to prevent skipping and leaking of the gun; The third pass is: controlling the spray gun distance to be 25 cm, the moving speed of the spray gun to be controlled at 50 cm / s, and improving the uniformity of the coating thickness; S7. Drying the outer layer: The outer layer is dried at 60 °C; It is cooled to room temperature and the surface quality of the coating is checked; Preferably, the compound curing agent in S1 is modified phenolic resin. Modified phenolic resin cures at medium temperature (80 - 120 °C), enhancing the chemical corrosion resistance (such as acid and alkali resistance) and bonding strength of the coating.

[0046] Preferably, the reactive diluent in S1 is propylene oxide phenyl (PGE). Propylene oxide phenyl (PGE), a low-viscosity epoxy reactive diluent, effectively reduces the viscosity of the resin system, facilitating the uniform dispersion of graphene and fillers (especially during the high-temperature stirring in step S1).

[0047] Preferably, the filler in S1 is calcium carbonate. Calcium carbonate is a low-cost filler that adjusts the hardness and wear resistance of the coating while reducing the material cost.

[0048] This comparative example uses a two-layer shielding structure and fails to achieve a good shielding effect; due to the lack of a three-dimensional conductive network constructed by graphene in the inner layer, electromagnetic waves only pass through the nano Fe 3 O 4 / SiO 2 core-shell particles are attenuated once, and the shielding effectiveness (SE) decreases by about 35% compared to Example 1 (measured from 50 dB to 32 dB), and the electromagnetic fatigue resistance of the coating is significantly reduced.

[0049] Comparative Example 2 Nano Fe 3 O 4 —Graphene gradient composite electromagnetic shielding coating, including an inner layer and an outer layer. The inner layer includes the following mass percentages: styrene-acrylic emulsion 48%, epoxy resin 11%, polyimide 15%, polyethylene 11%, graphene 9%, compound curing agent 2%, reactive diluent 1%, filler 1%, dispersant 1%, and coupling agent 1%, totaling 100%; the outer layer includes the following mass percentages: epoxy resin 61%, modified polyurethane 19%, nano Fe 3 O 4 particles 14%, polyethyleneimine 2%, dispersant 3%, crosslinking agent 1%, crosslinking accelerator 1%, and waterproofing agent 1%, totaling 100%.

[0050] Preferably, the crosslinking agent is polyisocyanate, which is HDI trimer. Through the reaction of isocyanate groups (-NCO) with hydroxyl groups (-OH) or amino groups (-NH 2 ) in the resin, a three-dimensional network structure is formed, significantly improving the hardness, wear resistance, and high-temperature resistance of the coating.

[0051] Preferably, the crosslinking accelerator is an organic amine, dibutyltin dilaurate (DBTL). Organic amines can accelerate the crosslinking reaction at a relatively low temperature (such as 60 °C), reducing energy consumption, and at the same time avoiding the destruction of the magnetism of nano Fe 3 O 4 / SiO 2 particles or the conductivity of graphene.

[0052] Preferably, the waterproofing agent is a fluorine-containing waterproofing agent. The fluorine-containing group (such as -CF 3 ) gives the coating an extremely low surface energy, achieving a "self-cleaning" effect, reducing the penetration of water and oil stains, and preventing the degradation of electromagnetic shielding performance due to water absorption. If a silane-based waterproofing agent is used, the hydrophobicity and ultraviolet resistance are significantly reduced.

[0053] This preparation method of the nano-Fe 3 O 4 —graphene gradient composite electromagnetic shielding coating includes the following steps: S1. Prepare the inner layer: Stir epoxy resin, polyimide, and polyethylene in a blender for 15 min, then add styrene-acrylic emulsion and continue stirring for 15 min. Pour it into a reaction kettle and keep it warm at 55 °C for 25 min. Then add graphene, compound curing agent, active diluent, filler, dispersant, and coupling agent, and heat to 80 °C and stir for 35 minutes to ensure uniform dispersion. Cool to room temperature and set aside; S2. Spray the inner layer: Add 5% of whitening water by mass of the bottom layer liquid to the bottom layer liquid, and then spray it in three passes. The first pass: Control the spray gun distance to be 20 cm, the moving speed of the spray gun to be controlled at 40 cm / s, the fan-shaped spray from the gun head to be controlled with an overlapping width of 1 / 2 a fan-shaped surface, the spraying interval time to be 5 min, and control the spraying to be uniform, smooth, without running beads, and without biting the bottom; The second pass: Use horizontal and vertical spraying for uniform coverage; The third pass: Use machine spraying to make the sprayed paint film uniform; S3. Dry the inner layer: Dry the inner layer in a drying room, and control the temperature of the drying room to be 80 °C; S4. Trim the inner layer: When trimming, wet-grind the entire inner layer with 600-grit water sandpaper dipped in water. The sanding action should be stable and uniform. When sanding by hand, the sanding direction should form a certain angle with the finger direction and prevent finger marks from appearing, and control the inner layer without orange peel and brush marks; Use a painter's spatula to remove the residual putty blocks and other dirt at the corners, sand them smooth with sandpaper, rinse them with water, and then blow off the water stains with a compressed air rubber tube and dry them; S5. Prepare the outer layer: Add epoxy resin, modified polyurethane, nano-Fe 3 O 4 particles, polyethyleneimine, dispersant, and waterproofing agent into the reaction kettle, control the temperature of the reaction kettle to be 80 °C, and stir for 35 minutes to ensure uniform dispersion. Add crosslinking agent and crosslinking accelerator, and continue stirring for 12 minutes. Cool to room temperature and set aside; S6. Spray the outer layer: Conduct three sprays. The first spray: Control the distance between the spray gun and the object to be sprayed at 20 cm, and control the moving speed of the spray gun at 40 cm / s. Spray the paint mist on the bottom layer. After spraying, check that the coating film must be continuous and there are no shrinkage holes. The second spray: Control the distance between the spray gun and the object to be sprayed at 15 cm, control the moving speed of the spray gun at 30 cm / s, and ensure that the movement of the spray gun is uniform, without skipping or missing spraying. The third spray: Control the distance between the spray gun and the object to be sprayed at 20 cm, control the moving speed of the spray gun at 40 cm / s, and improve the uniformity of the coating thickness. S7. Dry the outer layer: Dry the outer layer at 80 °C. Cool it to room temperature and check the surface quality of the coating. Preferably, the compound curing agent in S1 is modified dicyandiamide. By introducing active groups (such as organic ureas, imidazole derivatives, etc.), modified dicyandiamide significantly reduces the initial curing temperature, adapts to the preparation process of the inner layer of the coating (such as heating to 80 °C in step S1), reduces energy consumption, and avoids damage to sensitive materials such as graphene at high temperatures.

[0054] Preferably, the active diluent in S1 is butyl acrylate (BA). Butyl acrylate (BA), a monomer containing double bonds, participates in free radical polymerization, increases the flexibility of the coating, and reduces the risk of brittle cracking.

[0055] Preferably, the filler in S1 is calcium carbonate. Calcium carbonate is a low-cost filler that can adjust the hardness and wear resistance of the coating while reducing the material cost.

[0056] This comparative example uses a two-layer shielding structure and fails to achieve a good shielding effect. Due to the agglomeration of ordinary Fe 3 O 4 particles, the dispersion is uneven, the shielding efficiency of the coating fluctuates greatly in the 10 - 1000 MHz frequency band, the average SE value is reduced by 42% compared with Example 2 (from 55 dB to 32 dB), and the adhesion of the coating decreases by 30%.

[0057] Comparative Example 3 Fe 3 O 4 / SiO 2 - graphene gradient composite electromagnetic shielding coating, including an inner layer and an outer layer. The inner layer includes the following mass percentages: styrene-acrylic emulsion 51%, epoxy resin 10%, polyimide 16%, polyethylene 12%, graphene 4%, compound curing agent 2%, active diluent 1%, filler 1%, dispersant 2%, and coupling agent 1%, totaling 100%. The outer layer includes the following mass percentages: epoxy resin 59%, modified polyurethane 16%, nano Fe 3 O 4 / SiO 2Core-shell particles: 16%, polyethyleneimine: 2%, dispersant: 4%, crosslinking agent: 1%, crosslinking accelerator: 1%, and waterproofing agent: 1%, totaling 100%.

[0058] Preferably, the crosslinking agent is a polyisocyanate. Through the reaction of isocyanate groups (-NCO) with hydroxyl groups (-OH) or amino groups (-NH 2 ) in the resin, a three-dimensional network structure is formed, significantly improving the hardness, wear resistance, and high-temperature resistance of the coating.

[0059] Preferably, the crosslinking accelerator is an organic amine. Organic amines can accelerate the crosslinking reaction at a relatively low temperature (such as 60 °C), reducing energy consumption, and at the same time avoiding the destruction of the magnetism of nano-Fe 3 O 4 / SiO 2 particles or the conductivity of graphene.

[0060] Preferably, the waterproofing agent is a fluorine-containing waterproofing agent. Fluorine-containing groups (such as -CF 3 ) endow the coating with an extremely low surface energy, achieving a "self-cleaning" effect, reducing the penetration of water and oil stains, and preventing the degradation of electromagnetic shielding performance due to water absorption. If a silane-based waterproofing agent is used, the hydrophobicity and ultraviolet resistance are significantly reduced.

[0061] This comparative example Fe 3 O 4 / SiO 2 - graphene gradient composite electromagnetic shielding coating preparation method, including the following steps: S1. Prepare the inner layer: Stir epoxy resin, polyimide, and polyethylene with a stirrer for 15 min, then add styrene-acrylic emulsion and continue stirring for 15 min. Pour it into the reaction kettle, keep it warm at 55 °C for 25 min, then add graphene, compound curing agent, active diluent, filler, dispersant, and coupling agent, and heat to 100 °C and stir for 45 minutes to ensure uniform dispersion. Cool to room temperature and set aside; S2. Spray the inner layer: Add 5% of whitening water by mass of the bottom layer liquid to the bottom layer liquid, and then spray it in three steps. The first step is: control the spray gun distance to 20 cm, the spray gun moving speed is controlled at 40 cm / s, the fan-shaped spray from the gun head is controlled to overlap by 1 / 2 of the fan width, the spraying interval time is 5 min, and control the spraying to be uniform, smooth, without dripping and without biting the bottom; the second step is: use horizontal spraying and vertical spraying for uniform coverage; the third step is: machine spraying to make the sprayed paint film uniform; S3. Dry the inner layer: Dry the inner layer in a drying room, control the temperature of the drying room to 50 °C; S4. Inner layer trimming: When trimming, wet-grind the entire inner layer with 600-grit wet sandpaper dipped in water. The sanding action should be steady and uniform. When sanding by hand, the sanding direction should form a certain angle with the finger direction to prevent finger marks, and ensure that there are no orange peels or brush marks on the inner layer. Use a painter's spatula to remove the residual putty blocks and other dirt at the corners, sand them smooth with sandpaper, rinse them clean with water, then blow off the water stains with a compressed air rubber tube and let them dry. S5. Outer layer preparation: Add epoxy resin, modified polyurethane, nano-Fe 3 O 4 / SiO 2 core-shell particles, polyethyleneimine, dispersant, and waterproof agent into the reaction kettle, control the temperature of the reaction kettle at 80 °C, stir for 35 minutes to ensure uniform dispersion. Add cross-linking agent and cross-linking accelerator, and continue to stir for 20 minutes. Cool to room temperature and set aside. S6. Outer layer spraying: Conduct three sprays. The first spray: Control the spray gun distance at 20 cm, and the moving speed of the spray gun at 40 cm / s. Sprinkle the paint mist on the bottom layer. After spraying, check that the coating film must be continuous and there are no shrinkage holes. The second spray: Control the spray gun distance at 15 cm, the moving speed of the spray gun at 30 cm / s, and ensure that the movement of the spray gun is uniform, and prevent skipping or missing spraying. The third spray: Control the spray gun distance at 20 cm, the moving speed of the spray gun at 40 cm / s, to improve the uniformity of the coating thickness. S7. Outer layer drying: Dry the outer layer at 50 °C. Cool to room temperature and check the surface quality of the coating. Preferably, the compound curing agent in S1 is modified dicyandiamide. By introducing activation groups (such as organic ureas, imidazole derivatives, etc.), modified dicyandiamide significantly reduces the initial curing temperature, adapts to the inner layer preparation process of the coating (such as heating to 80 °C in step S1), reduces energy consumption and avoids damage to sensitive materials such as graphene at high temperatures.

[0062] Preferably, the active diluent in S1 is butyl acrylate (BA). Butyl acrylate (BA) is a monomer containing double bonds, which participates in free radical polymerization, increases the flexibility of the coating, and reduces the risk of brittle cracking.

[0063] Preferably, the filler in S1 is calcium carbonate. Calcium carbonate is a low-cost filler that can adjust the hardness and wear resistance of the coating while reducing the material cost.

[0064] This comparative example uses a two-layer shielding structure and fails to achieve a good shielding effect. When the electromagnetic wave radiation field approaches the shielding body, the nano-Fe in the outer layer 3 O 4 / SiO 2The core-shell particles will eliminate a part of the electromagnetic wave radiation, and the remaining part will transmit into the shielding body and continue to transmit into the inner layer. Since the proportion of graphene set in the inner layer is too small, the conductivity is reduced, and the electromagnetic shielding performance (especially the reflection loss) is significantly decreased.

[0065] In summary, the following data results are obtained: Example 1: This example adopts a two-layer shielding structure, which can achieve a good shielding effect; the inner layer and the outer layer can achieve a good shielding effect and have a strong anti-electromagnetic function; when the electromagnetic wave radiation field approaches the shielding body, the nano-Fe in the outer layer 3 O 4 / SiO 2 The core-shell particles will eliminate a part of the electromagnetic wave radiation, and the remaining part will transmit into the shielding body and continue to transmit into the inner layer. The dispersed graphene set in the inner layer can achieve the effect of shielding radiation again.

[0066] Example 2: This example adopts a two-layer shielding structure, which can achieve a good shielding effect; the inner layer and the outer layer can achieve a good shielding effect and have a strong anti-electromagnetic function; when the electromagnetic wave radiation field approaches the shielding body, the nano-Fe in the outer layer 3 O 4 / SiO 2 The core-shell particles will eliminate a part of the electromagnetic wave radiation, and the remaining part will transmit into the shielding body and continue to transmit into the inner layer. The dispersed graphene set in the inner layer can achieve the effect of shielding radiation again.

[0067] Example 3: This example adopts a two-layer shielding structure, which can achieve a good shielding effect; the inner layer and the outer layer can achieve a good shielding effect and have a strong anti-electromagnetic function; when the electromagnetic wave radiation field approaches the shielding body, the nano-Fe in the outer layer 3 O 4 / SiO 2 The core-shell particles will eliminate a part of the electromagnetic wave radiation, and the remaining part will transmit into the shielding body and continue to transmit into the inner layer. The dispersed graphene set in the inner layer can achieve the effect of shielding radiation again.

[0068] Comparative Example 1: This comparative example adopts a two-layer shielding structure and fails to achieve a good shielding effect; due to the lack of a three-dimensional conductive network constructed by graphene in the inner layer, the electromagnetic wave only passes through the nano-Fe 3 O 4 / SiO 2 The core-shell particles are attenuated once, and the shielding effectiveness (SE) is decreased by about 35% compared with Example 1 (measured from 50 dB to 32 dB), and the anti-electromagnetic fatigue of the coating is significantly reduced.

[0069] Comparative Example 2: This comparative example adopted a two-layer shielding structure and failed to achieve a good shielding effect; due to the agglomeration of ordinary Fe 3 O 4 particles resulting in uneven dispersion, the shielding efficiency of the coating fluctuated more in the 10 - 1000 MHz frequency band, the average SE value decreased by 42% compared with Example 2 (from 55 dB to 32 dB), and the adhesion of the coating decreased by 30%.

[0070] Comparative Example 3: This comparative example adopted a two-layer shielding structure and failed to achieve a good shielding effect; when the electromagnetic wave radiation field approaches the shielding body, the nano-Fe 3 O 4 / SiO 2 core-shell particles in the outer layer will eliminate a part of the electromagnetic wave radiation, and the remaining part will transmit into the shielding body and continue to transmit into the inner layer. Since the proportion of graphene set in the inner layer is too small, the conductivity decreases, and the electromagnetic shielding performance (especially the reflection loss) decreases significantly; the shielding performance test results of Examples 1 - 3 and Comparative Examples 1 - 3 are shown in Table 1 below: Table 1: Category Structural Composition Core Difference / Problem Change in Shielding Effectiveness (SE) Change in Shielding Effectiveness (SE) Example 1 <![CDATA[Outer layer: Nano-Fe 3 O 4 / SiO 2 Inner layer of core-shell particles: Dispersed graphene]]> Double-Layer Cooperative Shielding (Core-Shell Particles + Graphene) Shielding Effectiveness 50 dB Excellent Anti-Electromagnetic Fatigue Resistance Example 2 <![CDATA[Outer layer: Nano-Fe 3 O 4 / SiO 2 Inner layer of core-shell particles: Dispersed graphene]]> Core-Shell Structure Improves Dispersibility Shielding Effectiveness 55 dB Stable Coating Adhesion Example 3 <![CDATA[Outer layer: Nano-Fe 3 O 4 / SiO 2 Inner layer of core-shell particles: Dispersed graphene]]> Optimized Graphene Content Excellent Reflection Loss Performance Good Electrical Conductivity Comparative Example 1 <![CDATA[Outer layer: Nano-Fe 3 O 4 / SiO 2 Inner layer of core-shell particles: Without graphene]]> Lack of Graphene Inner Layer SE Decreases by 35% (50 dB → 32 dB) Significant Reduction in Anti-Electromagnetic Fatigue Resistance Comparative Example 2 <![CDATA[Outer layer: ordinary Fe 3 O 4 particles (non-core-shell) Inner layer: dispersed graphene]]> <![CDATA[Fe 3 O 4 Uneven dispersion of particle agglomeration]]> Average SE Decreases by 42% (55 dB → 32 dB) Adhesion Decreases by 30%, and Band Shielding Efficiency Fluctuation Increases Comparative Example 3 <![CDATA[Outer layer: Nano-Fe 3 O 4 / SiO 2 Inner layer of core-shell particles: Low-content graphene]]> Insufficient Proportion of Inner-Layer Graphene Significant Decrease in Reflection Loss (Overall SE Decrease) Reduced Electrical Conductivity Through experiments, it was found that the present invention achieved multi-stage electromagnetic attenuation through the double-layer synergistic effect of the outer-layer nano-Fe 3 O 4 / SiO 2 core-shell particles and the inner-layer graphene. The experimental data showed that the shielding effectiveness (SE) of Example 1 reached 50 dB, which was about 35% higher than that of Comparative Example 1 (SE = 32 dB), verifying the key role of the inner-layer graphene in constructing the gradient composite shielding structure. By comparing Example 2 with Comparative Example 2, it was found that the use of nano-Fe 3 O 4 / SiO 2 core-shell particles could increase the shielding effectiveness by 42% (55 dB vs 32 dB). Its core-shell structure effectively solved the problem of Fe 3 O 4 particle agglomeration, reduced the SE fluctuation of the coating in the 10 - 1000 MHz wide frequency band, and at the same time increased the adhesion by 30%. The inner-layer graphene generated reflection loss by constructing a conductive network. When the content of graphene was insufficient (Comparative Example 3), the reflection loss performance decreased significantly. The optimized graphene dispersion system in the examples could achieve secondary attenuation of electromagnetic waves after penetrating the outer layer, increasing the overall shielding effectiveness by about 35% (compared with Comparative Example 1).

[0071] Under the premise of keeping the coating thickness ≤ 0.5 mm, the double-layer shielding system of the present invention achieves a shielding effectiveness of ≥ 50 dB in a wide frequency band (10 - 1000 MHz), and at the same time has excellent mechanical properties (the adhesion is increased by 30%) and long-term stability, meeting the requirements of modern electronic devices for lightweight and highly reliable electromagnetic protection.

[0072] Although the embodiments of the present invention have been shown and described, those of ordinary skill in the art can understand that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principle and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. Fe3O4 / SiO2-graphene gradient composite electromagnetic shielding coating, characterized in that: The invention comprises an inner layer and an outer layer, wherein the inner layer comprises the following raw materials in mass percentage: 30% to 70% styrene-acrylic emulsion, 6% to 15% epoxy resin, 10% to 20% polyimide, 3% to 12% polyethylene, 5% to 40% graphene, 1% to 3% compound curing agent, 1% to 3% active diluent, 1% to 3% filler, 0.5% to 5% dispersant and 0.5% to 2% coupling agent, which total 100%; the outer layer comprises the following raw materials in mass percentage: 50% to 70% epoxy resin, 15% to 25% modified polyurethane, 10% to 25% nano Fe3O4 / SiO2 core-shell particles, 2% to 5% polyethyleneimine, 2% to 5% dispersant, 1% to 3% cross-linking agent, 1% to 3% cross-linking accelerator and 1% to 3% waterproofing agent, which total 100%.

2. The Fe3O4 / SiO2-graphene gradient composite electromagnetic shielding coating according to claim 1, characterized in that: The crosslinking agent is polyisocyanate.

3. The Fe3O4 / SiO2-graphene gradient composite electromagnetic shielding coating according to claim 1, characterized in that: The cross-linking accelerator is an organic amine.

4. The Fe3O4 / SiO2-graphene gradient composite electromagnetic shielding coating according to claim 1, characterized in that: The waterproofing agent is a fluorine-containing waterproofing agent.

5. A method for preparing a Fe3O4 / SiO2-graphene gradient composite electromagnetic shielding coating according to any one of claims 1 to 4, characterized in that: The specific steps include: S1. Preparation of inner layer: After epoxy resin, polyimide and polyethylene are uniformly stirred in a stirrer, styrene-acrylic emulsion is added and stirring is continued for 10 to 20 minutes; the mixture is poured into a reactor and kept warm at 50 to 80° C. for 20 to 60 minutes, and then graphene, compound curing agent, reactive diluent, filler, dispersant and coupling agent are added thereto, and the mixture is heated to 80 to 120° C. and stirred for 30 to 90 minutes to ensure uniform dispersion; the mixture is cooled to room temperature and set aside; S2, spray inner layer: add 5-10% of the base liquid mass of whitening water to the base liquid, and then spray in three steps. The first step is to control the spray gun distance to 20-30 cm, the spray gun moving speed to 40-60 cm / s, the spray fan of the gun head to overlap the width of 1 / 2-2 / 3 of the fan, the spraying interval time is 5-15min, and the spraying is controlled to be uniform, smooth, without flow column and without biting the bottom; the second step is to use horizontal and vertical spraying for uniform coverage; the third step is to use machine spraying to make the sprayed paint film uniform; S3, drying the inner layer: drying the inner layer in a drying room, controlling the temperature of the drying room to be 50-150°C; S4. Finish the inner layer: Use 400-800# water sandpaper to wet-grind the entire inner layer. The grinding action should be steady and even. When grinding by hand, the grinding direction should be at a certain angle to the finger direction and avoid finger marks. Make sure there is no orange peel or brush marks on the inner layer. Use a painter's spatula to remove residual putty and other dirt at the corners, and grind it smooth with sandpaper. Rinse with water, then use a compressed air hose to blow off the water marks and let it dry. S5, prepare the outer layer: add epoxy resin, modified polyurethane, nano Fe3O4 / SiO2 core-shell particles, polyethyleneimine, dispersant, and waterproofing agent into a reactor, control the temperature of the reactor to 80-100°C, stir for 30-40 minutes to ensure uniform dispersion; add a crosslinking agent and a crosslinking accelerator, continue stirring for 10-20 minutes; cool to room temperature and set aside; S6. Spray the outer layer: perform two spraying steps. The first step is to control the spray gun distance to 20-30 cm, the spray gun moving speed to 40-60 cm / s, and spray the paint mist on the bottom layer. After spraying, check that the coating film must be connected into one piece and check that there are no shrinkage holes. The second step is to control the spray gun distance to 15-25 cm, the spray gun moving speed to 30-50 cm / s, and control the spray gun to move evenly, and avoid the phenomenon of jumping or missing the gun. The third step is to control the spray gun distance to 20-30 cm, the spray gun moving speed to 40-60 cm / s, and improve the uniformity of coating thickness. S7. Dry the outer layer: dry the outer layer at 60-150°C; cool to room temperature and check the surface quality of the coating.

6. The method for preparing the Fe3O4 / SiO2-graphene gradient composite electromagnetic shielding coating according to claim 5, characterized in that: The compound curing agent in S1 is one of modified phenolic resin and modified dicyandiamide.

7. The method for preparing the Fe3O4 / SiO2-graphene gradient composite electromagnetic shielding coating according to claim 5, characterized in that: The active diluent in S1 is one of propylene oxide phenyl and butyl acrylate.

8. The method for preparing the Fe3O4 / SiO2-graphene gradient composite electromagnetic shielding coating according to claim 5, characterized in that: The filler in S1 is calcium carbonate.

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