Flexible electromagnetic shielding material and preparation method thereof

Abstract

The invention provides a flexible electromagnetic shielding material and a preparation method thereof. The electromagnetic shielding material sequentially comprises a first conductive layer, a composite wave absorbing layer and a second conductive layer. The first conductive layer and the second conductive layer each include a thermoplastic resin and a conductive material. The composite wave absorbing layer is formed by mixing at least 80-110 parts by weight of epoxy resin, 20-30 parts by weight of filler and 5-10 parts by weight of a rare earth-based metal organic framework, the electromagnetic shielding material is arranged to be of a layered structure, when electromagnetic waves make contact with the first conductive layer, induction current can be generated on the surface of the first conductive layer, the electromagnetic waves are reflected through the high conductivity of the induction current, and therefore the electromagnetic shielding material can be used for shielding the electromagnetic waves. Meanwhile, unreflected electromagnetic waves can enter the composite wave absorbing layer in a transmission manner, the rare earth-based metal organic framework can generate a polarization phenomenon under the action of the electromagnetic waves and further absorb the electromagnetic waves, and when the electromagnetic waves are not completely absorbed in the composite wave absorbing layer, the second conductive layer can further reflect the electromagnetic waves and the electromagnetic waves are absorbed by the composite wave absorbing layer.

Description

Technical Field

[0001] This application relates to the technical field of electromagnetic shielding, and particularly relates to a flexible electromagnetic shielding material and a preparation method thereof. Background Art

[0002] With the rapid development of electronic technology, the problem of electromagnetic radiation has become increasingly serious, which has an adverse impact on the normal operation of electronic devices and human health. Traditional electromagnetic shielding materials are mostly metal materials. Although they have good shielding effects, they have disadvantages such as large mass, poor flexibility, and difficulty in processing. In recent years, with the rapid development of emerging fields such as wearable devices and flexible electronic devices, the demand for flexible electromagnetic shielding materials has been increasing.

[0003] Currently, the flexible electromagnetic shielding materials on the market mainly achieve the electromagnetic shielding function by adding metal particles or conductive polymers into the polymer matrix. However, these materials still have great limitations in terms of flexibility and shielding efficiency. Summary of the Invention

[0004] In order to solve at least one of the above technical problems, an embodiment of this application provides a flexible electromagnetic shielding material.

[0005] In addition, an embodiment of this application also provides a preparation method of a flexible electromagnetic shielding material.

[0006] An embodiment of this application provides a flexible electromagnetic shielding material. The flexible electromagnetic shielding material has a layered structure and sequentially includes a first conductive layer, a composite wave-absorbing layer, and a second conductive layer. Both the first conductive layer and the second conductive layer include a thermoplastic resin and a conductive material. The composite wave-absorbing layer is composed of, by weight, at least 80 - 110 parts of epoxy resin, 20 - 30 parts of filler, and 5 - 10 parts of rare earth metal-organic framework mixed together.

[0007] In some embodiments of this application, the composite wave-absorbing layer is composed of the following components by weight: 80 - 110 parts of epoxy resin, 20 - 30 parts of filler, 5 - 10 parts of rare earth metal-organic framework, 1 - 2 parts of dispersant, 0.1 - 0.8 parts of acid anhydride curing accelerator, and 80 - 90 parts of acid anhydride curing agent.

[0008] In some embodiments of this application, in the rare earth metal-organic framework, the rare earth element includes one or both of yttrium and gadolinium.

[0009] In some embodiments of this application, the molar ratio of yttrium to gadolinium is 2:3.

[0010] In some embodiments of the present application, the method for preparing the rare earth metal-organic framework comprises the following steps: Weigh 2,5-dihydroxyterephthalic acid, yttrium nitrate hexahydrate, and gadolinium nitrate hexahydrate, place them in a reaction vessel, add an N,N-dimethylformamide solution to dissolve, ultrasonically disperse at room temperature for 50 min to 60 min, transfer to a reaction kettle, and react at a temperature of 120 °C to 150 °C for 12 h to 24 h to obtain a mixed solution. Add ethanol to the mixed solution and centrifuge at a rate of 10000 r / min to 12000 r / min for 5 min to 8 min to precipitate the product in the mixed solution, perform centrifugal separation, repeatedly wash the precipitated product with an N,N-dimethylformamide solution and an absolute ethanol solution until the supernatant is colorless, filter, and vacuum dry at 65 °C to 75 °C for 12 h to 24 h to obtain the rare earth metal-organic framework.

[0011] In some embodiments of the present application, the filler is hollow silica powder and flaky boron nitride.

[0012] In some embodiments of the present application, the thermoplastic resin is thermoplastic polyurethane elastomer or polyamide. The conductive material is any one of carbon fiber, graphite, acetylene black, and Ketjen black; wherein the mass ratio of the conductive material to the thermoplastic resin is 7-9:2.

[0013] The embodiment of the present application also provides a method for preparing a flexible electromagnetic shielding material, comprising the following steps: Mix epoxy resin, filler, and rare earth metal-organic framework and perform molding by pressing to prepare a composite wave-absorbing layer. Mix the conductive material and the thermoplastic resin, dry and then perform molding by pressing to obtain a first conductive layer and a second conductive layer respectively. Stack the first conductive layer, the composite wave-absorbing layer, and the second conductive layer in sequence, and perform hot pressing to obtain the flexible electromagnetic shielding material.

[0014] In some embodiments of the present application, the method for preparing the composite wave-absorbing layer specifically comprises the following steps: Mix epoxy resin, filler, and rare earth metal-organic framework for 1 h to 3 h until evenly mixed to obtain a premix. Stir and disperse the premix and the dispersant for 5 min to 10 min, take out and perform vacuum degassing for 10 min to obtain a first mixture. Mix the anhydride curing accelerator and the anhydride curing agent for 10 min to 20 min to obtain a second mixture. Shear and mix the first mixture and the second mixture at 30 °C to 50 °C for 8 min to 15 min to obtain a blend solution, then dry under vacuum and perform molding by pressing to obtain the composite wave-absorbing layer.

[0015] In some embodiments of the present application, the hot pressing temperature is 150 °C to 175 °C, and the hot pressing time is 15 min to 30 min.

[0016] Compared with the prior art, the flexible electromagnetic shielding material provided by the embodiments of the present application has good flexibility and mechanical properties by setting the electromagnetic shielding material as a layered structure, including a composite wave-absorbing layer and conductive layers disposed on both sides of the composite wave-absorbing layer. When electromagnetic waves contact the first conductive layer, induced currents will be generated on the surface of the first conductive layer, and the electromagnetic waves will be reflected through its high conductivity. However, there will also be unreflected electromagnetic waves transmitted into the composite wave-absorbing layer. Under the action of electromagnetic waves, the rare earth metal-organic framework will undergo polarization, generating dielectric loss and converting the electromagnetic wave energy into heat energy to further absorb the electromagnetic waves. When the electromagnetic waves are not completely absorbed in the composite wave-absorbing layer, the second conductive layer will further reflect the electromagnetic waves, which will be absorbed by the composite wave-absorbing layer again, so as to achieve the characteristics of reflection-absorption-re-reflection absorption. Brief Description of the Drawings

[0017] Figure 1 It is a cross-sectional SEM image of the flexible electromagnetic shielding material prepared in Embodiment 1 of the present application. Detailed Embodiments

[0018] The following will clearly and completely describe the technical solutions in the embodiments of the present application with reference to the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments.

[0019] With the development of electronic information technology and the wide application of wireless information transmission, people are increasingly relying on various devices that transmit electromagnetic waves. The electromagnetic pollution hazards generated by these electronic devices during use cannot be ignored. The electromagnetic pollution caused by electromagnetic waves is mainly reflected in two aspects: one is harming human health. If people live in an environment polluted by electromagnetic waves for a long time, it will have certain negative impacts on the nervous system and endocrine system; on the other hand, it is interfering with the normal functions of electronic devices. Electromagnetic waves with similar frequencies will cause signal interference to the operation of surrounding electronic devices and communication systems, affecting the devices' reception and transmission of communication signals, and even causing information security problems. Therefore, developing electromagnetic protection materials with high electromagnetic wave shielding performance to strengthen human protection and alleviate the mutual interference of various radiation sources has important research significance and application prospects.

[0020] Currently, the most widely used electromagnetic shielding materials are mainly carbon materials, graphene, etc. These materials themselves have low strength and lack flexibility, and cannot meet the mechanical strength requirements of one-dimensional conductive fibers in practical applications. To solve the problems of poor flexibility and low shielding efficiency of existing electromagnetic shielding materials.

[0021] The inventors of this application have found through research that MOF materials have tunable nanostructures and abundant porosities. The derived electromagnetic wave absorbing materials usually exhibit excellent electrical conductivity, magnetism, and sufficient defect sites and interfacial structures, thus showing unique advantages in impedance matching and microwave loss. Under the action of electromagnetic waves, organic ligands and metal ions will undergo polarization phenomena, generating dielectric loss and converting electromagnetic wave energy into heat energy. In particular, rare-earth-based metal-organic framework materials have a large number of open metal sites, a high specific surface area, and large voids, which can further improve the wave absorption performance.

[0022] Therefore, the embodiments of this application aim to provide a flexible electromagnetic shielding material. The electromagnetic shielding material has a layered structure and sequentially includes a first conductive layer, a composite wave absorbing layer, and a second conductive layer. Both the first conductive layer and the second conductive layer include a thermoplastic resin and a conductive material. The composite wave absorbing layer is composed of, by weight, at least 80 to 110 parts of epoxy resin, 20 to 30 parts of filler, and 5 to 10 parts of rare-earth-based metal-organic framework mixed together. By setting the electromagnetic shielding material to have a layered structure, including a composite wave absorbing layer and conductive layers disposed on both sides of the composite wave absorbing layer, it has good flexibility and mechanical properties. When electromagnetic waves contact the first conductive layer, an induced current will be generated on the surface of the first conductive layer, and the electromagnetic waves will be reflected through its high electrical conductivity. However, there will also be unreflected electromagnetic waves that transmit into the composite wave absorbing layer. Under the action of electromagnetic waves, the rare-earth-based metal-organic framework will undergo polarization phenomena, generating dielectric loss and converting electromagnetic wave energy into heat energy to further absorb electromagnetic waves. When the electromagnetic waves are not completely absorbed in the composite wave absorbing layer, the second conductive layer will further reflect the electromagnetic waves, which will be absorbed by the composite wave absorbing layer again, so as to achieve the characteristics of reflection-absorption-re-reflection absorption.

[0023] In an embodiment of this application, the composite wave absorbing layer is composed of the following components by weight: 80 to 110 parts of epoxy resin, 20 to 30 parts of filler, 5 to 10 parts of rare-earth-based metal-organic framework, 1 to 2 parts of dispersant, 0.1 to 0.8 parts of acid anhydride curing accelerator, and 80 to 90 parts of acid anhydride curing agent.

[0024] In some embodiments, the acid anhydride curing accelerator can be one or more of tetrabutylammonium bromide, tetrabutylammonium bromide, and triphenylphosphine.

[0025] In some embodiments, the acid anhydride curing agent can be one or more of acid anhydride curing agents such as methylhexahydrophthalic anhydride, methyltetrahydrophthalic anhydride, hexahydrophthalic anhydride, and tetrahydrophthalic anhydride.

[0026] In one embodiment of the present application, in the rare earth metal-organic framework, the rare earth elements include one or both of yttrium and gadolinium. The hierarchical pore structure of rare earth elements yttrium and gadolinium can increase the multiple reflection paths of electromagnetic waves. Combining with the interlayer dielectric polarization effect of flaky boron nitride and hollow silica powder, wide-band electromagnetic wave absorption is achieved. The 4f electron transition of rare earth element gadolinium can also enhance magnetic loss. The electronic configuration of yttrium element enables it to form strong coordination bonds with organic ligands, and the conjugated π-electron system of the MOF framework enhances dielectric polarization, resulting in significant dielectric loss for high-frequency electromagnetic waves. At the same time, gadolinium element has a high spin magnetic moment, which generates strong eddy current loss and natural resonance in the alternating electromagnetic field, effectively absorbing low-frequency interference. Optionally, the combined use of yttrium and gadolinium can balance the ratio of dielectric loss and magnetic loss, especially significantly improving the adaptability in complex electromagnetic environments.

[0027] In one embodiment of the present application, the molar ratio of yttrium to gadolinium is 2:3. When the molar ratio is 2:3, the lattice constant of the MOF has the highest matching degree with the ligand size, the crystal defect density is reduced, and the attenuation of the wave absorption performance caused by lattice stress is avoided. The incorporation of yttrium inhibits the magnetic domain aggregation of gadolinium ions, prevents impedance mismatch caused by excessive magnetic loss in the wave absorption layer, and improves the incident efficiency of electromagnetic waves.

[0028] In one embodiment of the present application, the preparation method of the rare earth metal-organic framework includes the following steps: Step S101: Weigh 2,5-dihydroxyterephthalic acid, yttrium nitrate hexahydrate, and gadolinium nitrate hexahydrate and place them in a reaction vessel. Add N,N-dimethylformamide solution to dissolve, ultrasonically disperse at room temperature for 50 min to 60 min, transfer to a reaction kettle, and react at a temperature of 120 °C to 150 °C for 12 h to 24 h to obtain a mixed solution. The use of the multidentate ligand 2,5-dihydroxyterephthalic acid has high structural stability and a large specific surface area for the MOF synthesized by multiple metals. Gadolinium provides magnetic loss, and yttrium provides dielectric loss, synergistically covering a wider frequency band.

[0029] In some embodiments, the mass ratio of the multidentate ligand 2,5-dihydroxyterephthalic acid to yttrium nitrate hexahydrate and gadolinium nitrate hexahydrate is 13:4.

[0030] In some embodiments, the reaction temperature can be 120 °C, 130 °C, 140 °C, 150 °C or any value within the range composed of any two of the above values. The reaction time can be 12 h, 14 h, 16 h, 20 h, 22 h, 24 h or any value within the range composed of any two of the above values.

[0031] Step S102: Add ethanol to the mixed solution and centrifuge at a rate of 10,000 r / min to 12,000 r / min for 5 min to 8 min to precipitate the product in the mixed solution. Then perform centrifugal separation, repeatedly wash the precipitated product with N,N-dimethylformamide solution and anhydrous ethanol solution until the supernatant is colorless, filter, and vacuum dry at 65 °C to 75 °C for 12 h to 24 h to obtain the rare earth metal-organic framework.

[0032] In some embodiments, the centrifugation rate can be 10,000 r / min, 11,000 r / min, 12,000 r / min, or any value within the range composed of any two of the above values. The centrifugation time can be 5 min, 6 min, 7 min, 8 min, or any value within the range composed of any two of the above values.

[0033] In one embodiment of the present application, the filler is hollow silica powder and flaky boron nitride. In some embodiments, the mass ratio of hollow silica powder to flaky boron nitride is 1:3. By using the combination of hollow silica powder and flaky boron nitride, the hollow structure of the hollow silica powder can reduce stress concentration in the resin matrix, and the flaky boron nitride can conduct heat synergistically with the hollow silica powder after blending, quickly dissipating heat energy.

[0034] In one embodiment of the present application, the thermoplastic resin is thermoplastic polyurethane elastomer or polyamide. The conductive material is any one of carbon fiber, graphite, acetylene black, and Ketjen black; wherein the mass ratio of the conductive material to the thermoplastic resin is 7-9:2. In some embodiments, the mass ratio of the conductive material to the thermoplastic resin can be 7:2, 4:1, 9:2, or any value within the range composed of any two of the above values. More preferably, the mass ratio of the conductive material to the thermoplastic resin is 7:2. More preferably, carbon fiber and thermoplastic polyurethane elastomer are used.

[0035] In some embodiments, the dispersant is POSS glycidyl ether oxypropyl cyclotetrasiloxane, with a viscosity of 4500 cps to 4800 cps; a density of 1.25 g / ml to 1.30 g / ml; a molecular weight of 1337.88; and an epoxy equivalent of 167. It is an organic-inorganic hybrid cage-shaped polyhedral oligomeric silsesquioxane with a molecular size of 1.5 nm. As an efficient dispersant for nanomaterials, it enhances the dispersibility of nanomaterials and is purchased from Xi'an Ruixi Biotechnology Co., Ltd. On the other hand, the inventors found that selecting POSS glycidyl ether oxypropyl cyclotetrasiloxane can not only promote the dispersion of MOF and the filler, but also increase the bonding strength between the conductive layer and the epoxy resin during the hot pressing process.

[0036] Compared with the prior art, the flexible electromagnetic shielding material provided by the embodiments of the present application has the following beneficial effects: By setting the electromagnetic shielding material as a layered structure, including a composite wave-absorbing layer and conductive layers disposed on both sides of the composite wave-absorbing layer, it has good flexibility and mechanical properties. When electromagnetic waves contact the first conductive layer, induced currents will be generated on the surface of the first conductive layer, and electromagnetic waves will be reflected through its high conductivity. However, there will also be unreflected electromagnetic waves transmitted into the composite wave-absorbing layer. Under the action of electromagnetic waves, the rare earth metal-organic framework will undergo polarization, generating dielectric loss and converting the electromagnetic wave energy into heat energy to further absorb electromagnetic waves. When the electromagnetic waves are not completely absorbed in the composite wave-absorbing layer, the second conductive layer will further reflect the electromagnetic waves, which will be absorbed by the composite wave-absorbing layer again, so as to achieve the characteristics of reflection-absorption-re-reflection absorption.

[0037] The preparation method of the flexible electromagnetic shielding material provided by the embodiments of the present application specifically includes the following steps: Step S1: Mix epoxy resin, filler, and rare earth metal-organic framework and mold them into a composite wave-absorbing layer.

[0038] In an embodiment of the present application, the preparation method of the composite wave-absorbing layer in step S1 specifically includes the following steps: Step S101: Mix epoxy resin, filler, and rare earth metal-organic framework for 1 h to 3 h until evenly mixed to obtain a premix.

[0039] Step S102: Stir and disperse the premix and the dispersant for 5 min to 10 min, take it out and perform vacuum degassing for 10 min to obtain a first mixture.

[0040] Step S103: Mix the acid anhydride curing accelerator and the acid anhydride curing agent for 10 min to 20 min to obtain a second mixture. Shear-mix the first mixture and the second mixture at 30 °C to 50 °C for 8 min to 15 min to obtain a blend solution, and then dry it under vacuum and mold it to obtain a composite wave-absorbing layer.

[0041] In an embodiment of the present application, the hot pressing temperature is 150 °C to 175 °C, and the hot pressing time is 15 min to 30 min.

[0042] Step S2: Mix the conductive material and the thermoplastic resin, dry them and then mold them to obtain the first conductive layer and the second conductive layer respectively.

[0043] Step S3: Stack the first conductive layer, the composite wave-absorbing layer, and the second conductive layer in sequence, and hot press them to obtain a flexible electromagnetic shielding material.

[0044] Compared with the prior art, the preparation method of the flexible electromagnetic shielding material provided by the embodiments of the present application first prepares a composite wave-absorbing layer, and then hot-presses a first conductive layer and a second conductive layer on both sides of the composite wave-absorbing layer respectively. During the hot-pressing process, the thermoplastic resin softens and undergoes interfacial diffusion in the epoxy resin layer to form an interpenetrating network, improving the strength of the interlayer bonding. The hot-pressing time ensures sufficient cross-linking between the resins without thermal decomposition, and the overall thickness of the material is controllable.

[0045] The foregoing flexible electromagnetic shielding material is further described below through specific embodiments.

[0046] Example 1 Step 1: Weigh 100 parts of epoxy resin, 23 parts of hollow silica powder and flaky boron nitride with a mass ratio of 1:3, and 8 parts of rare earth metal-organic framework by weight, stir and mix for 1 h until evenly mixed to obtain a premix. Add 1 part of dispersant POSS glycidyl ether oxypropyl cyclotetrasiloxane to the premix, stir and disperse the premix and the dispersant for 10 min, take it out and perform vacuum degassing for 10 min to obtain a first mixture. Mix 0.7 part of tetramethylammonium bromide and 80 parts of methylhexahydrophthalic anhydride for 15 min to obtain a second mixture. Shear-mix the first mixture and the second mixture at 50 °C for 15 min to obtain a blend solution, and then dry it under vacuum and mold it to obtain a composite wave-absorbing layer.

[0047] Step 2: Mix carbon fiber and thermoplastic polyurethane according to a mass ratio of 7:2, dry and mold them respectively to obtain a first conductive layer and a second conductive layer; Step 3: Stack the first conductive layer, the composite wave-absorbing layer, and the second conductive layer in sequence, and hot-press them. The hot-pressing temperature is 150 °C and the hot-pressing time is 15 min to obtain a flexible electromagnetic shielding material.

[0048] Example 2 Step 1: Weigh 110 parts of epoxy resin, 20 parts of hollow silica powder and flaky boron nitride with a mass ratio of 1:3, and 10 parts of rare earth metal-organic framework by weight, stir and mix for 1 h until evenly mixed to obtain a premix. Add 2 parts of dispersant POSS glycidyl ether oxypropyl cyclotetrasiloxane to the premix, stir and disperse the premix and the dispersant for 10 min, take it out and perform vacuum degassing for 10 min to obtain a first mixture. Mix 0.7 part of tetramethylammonium bromide and 80 parts of methylhexahydrophthalic anhydride for 15 min to obtain a second mixture. Shear-mix the first mixture and the second mixture at 50 °C for 15 min to obtain a blend solution, and then dry it under vacuum and mold it to obtain a composite wave-absorbing layer.

[0049] Step 2: Mix carbon fiber and thermoplastic polyurethane according to a mass ratio of 7:2, dry and mold them respectively to obtain a first conductive layer and a second conductive layer; Step 3: Stack the first conductive layer, the composite wave-absorbing layer, and the second conductive layer in sequence, and perform hot pressing to form. The hot pressing temperature is 150 °C, and the hot pressing time is 15 min to obtain a flexible electromagnetic shielding material.

[0050] Example 3 Step 1: Weigh 80 parts of epoxy resin, 20 parts of hollow silica powder and flaky boron nitride with a mass ratio of 1:3, and 5 parts of rare earth metal-organic framework by weight, stir and mix for 1 h until evenly mixed to obtain a premix. Add 1 part of dispersant POSS glycidyl ether oxypropyl cyclotetrasiloxane to the premix, stir and disperse the premix and the dispersant for 10 min, take it out and perform vacuum degassing for 10 min to obtain a first mixture. Mix 0.7 parts of tetramethylammonium bromide and 80 parts of methylhexahydrophthalic anhydride for 15 min to obtain a second mixture. Shear and mix the first mixture and the second mixture at 50 °C for 15 min to obtain a blend solution, and then dry it under vacuum and perform molding by pressing to obtain a composite wave-absorbing layer.

[0051] Step 2: Mix carbon fiber and thermoplastic polyurethane in a mass ratio of 7:2, dry and then perform molding by pressing to obtain the first conductive layer and the second conductive layer respectively; Step 3: Stack the first conductive layer, the composite wave-absorbing layer, and the second conductive layer in sequence, and perform hot pressing to form. The hot pressing temperature is 150 °C, and the hot pressing time is 15 min to obtain a flexible electromagnetic shielding material.

[0052] Comparative Example 1 The specific process of the preparation process refers to Example 1, the difference is that the rare earth metal-organic framework is not added in Step 1, and the preparation methods of the remaining electromagnetic shielding materials are basically the same as those in Example 1, and will not be elaborated here.

[0053] Comparative Example 2 The specific process of the preparation process refers to Example 1, the difference is that the addition amount of the rare earth metal-organic framework in Step 1 is 15 parts, and the preparation methods of the remaining electromagnetic shielding materials are basically the same as those in Example 1, and will not be elaborated here.

[0054] Comparative Example 3 The specific process of the preparation process refers to Example 1, the difference is that the addition amount of the rare earth metal-organic framework in Step 1 is 3 parts, and the preparation methods of the remaining electromagnetic shielding materials are basically the same as those in Example 1, and will not be elaborated here.

[0055] Comparative Example 4 The specific process of the preparation process refers to Example 1, the difference is that Steps 2 and 3 are not carried out, and the preparation methods of the remaining electromagnetic shielding materials are basically the same as those in Example 1, and will not be elaborated here.

[0056] Comparative Example 5 The specific process of the preparation process refers to Example 1, except that only the first conductive layer is stacked in Step 3, and the preparation methods of the remaining electromagnetic shielding materials are basically the same as those in Example 1, and will not be elaborated here.

[0057] Comparative Example 6 The specific process of the preparation process refers to Example 1, except that the hot pressing temperature in Step 3 is 200 °C, and the preparation methods of the remaining electromagnetic shielding materials are basically the same as those in Example 1, and will not be elaborated here.

[0058] Comparative Example 7 The specific process of the preparation process refers to Example 1, except that the hot pressing temperature in Step 3 is 140 °C, and the preparation methods of the remaining electromagnetic shielding materials are basically the same as those in Example 1, and will not be elaborated here.

[0059] Comparative Example 8 The specific process of the preparation process refers to Example 1, except that no dispersant is added in Step 1, and the preparation methods of the remaining electromagnetic shielding materials are basically the same as those in Example 1, and will not be elaborated here.

[0060] The performance of the flexible electromagnetic shielding materials obtained in Examples 1-3 and Comparative Examples 1-8 was tested, and the test results are shown in Table 1. Exemplarily, the scanning electron microscope results of the flexible electromagnetic shielding material in Example 1 are as Figure 1 shown.

[0061] Table 1 It can be seen from the data in Table 1 that the electromagnetic shielding effectiveness in Example 1 reaches 97, and the thermal conductivity reaches 2.8. Thanks to the addition of rare earth metal organic frameworks, and the effect of reflection-absorption-re-reflection is achieved with the first and second conductive layers, establishing an electromagnetic multi-loss mechanism. The flexible matrix of thermoplastic polyurethane and the dispersant in epoxy resin synergistically enhance the interfacial bonding ability between the conductive layer and the composite absorbing layer during the hot pressing process. In Comparative Example 1, due to the lack of MOF, electromagnetic waves only rely on the reflection of the conductive layer, and the penetration in the high-frequency band increases significantly. In Comparative Example 2, due to the excessive amount of MOF resulting in agglomeration, the pores inside the absorbing layer are blocked, the shielding effectiveness decreases, and the agglomerated MOF destroys the resin cross-linking network, resulting in poor mechanical properties. In Comparative Example 3, due to insufficient addition of MOF, the wave absorption ability is weakened. In Comparative Example 4, there is no conductive layer, and only the wave absorption ability can be relied on, resulting in a low shielding effectiveness. In Comparative Example 5, there is only one conductive layer, and after reflection-absorption, there are still some electromagnetic waves penetrating. In Comparative Example 6, the material is degraded due to high hot pressing temperature, resulting in poor interlayer bonding. In Comparative Example 7, it is not fused at low temperature, and the interface delaminates and is easy to separate. In Comparative Example 8, no dispersant is added, there is a small amount of agglomeration of MOF and fillers, the interlayer bonding force is weak, and the tensile strength decreases.

[0062] It can be seen that in this application, by setting the electromagnetic shielding material as a layered structure, including a composite wave-absorbing layer and conductive layers disposed on both sides of the composite wave-absorbing layer, it has good flexibility and mechanical properties. When electromagnetic waves contact the first conductive layer, induced currents will be generated on the surface of the first conductive layer, and electromagnetic waves are reflected through its high conductivity. However, at the same time, there will also be unreflected electromagnetic waves transmitted into the composite wave-absorbing layer. Under the action of electromagnetic waves, the rare earth metal-organic framework will undergo polarization, generating dielectric loss, converting the electromagnetic wave energy into heat energy, and further absorbing electromagnetic waves. When the electromagnetic waves are not completely absorbed in the composite wave-absorbing layer, the second conductive layer will further reflect the electromagnetic waves, which are then absorbed by the composite wave-absorbing layer again, so as to achieve the characteristics of reflection-absorption-re-reflection absorption. The provided preparation method first prepares the composite wave-absorbing layer, and then hot-presses the first conductive layer and the second conductive layer on both sides of the composite wave-absorbing layer respectively. During the hot-pressing process, the thermoplastic resin softens and undergoes interfacial diffusion in the epoxy resin layer, forming an interpenetrating network, which improves the strength of the interlayer bonding. The hot-pressing time ensures that the resins are fully crosslinked without thermal decomposition, and the overall thickness of the material is controllable.

[0063] It can be understood that the above embodiments are merely exemplary embodiments adopted to illustrate the principle of the present invention. However, the present invention is not limited thereto. For those of ordinary skill in the art, various modifications and improvements can be made without departing from the spirit and essence of the present invention, and these modifications and improvements are also regarded as the protection scope of the present invention.

Claims

1. A flexible electromagnetic shielding material, characterized in that: The flexible electromagnetic shielding material is a layered structure, which includes a first conductive layer, a composite wave absorbing layer, and a second conductive layer in sequence; The first conductive layer and the second conductive layer both include a thermoplastic resin and a conductive material; The composite wave absorbing layer comprises, by weight, at least 80 to 110 parts of epoxy resin, 20 to 30 parts of filler and 5 to 10 parts of rare earth-based metal organic framework.

2. The flexible electromagnetic shielding material according to claim 1, characterized in that: The composite wave absorbing layer is composed of the following components by weight: 80-110 parts of epoxy resin, 20-30 parts of filler, 5-10 parts of rare earth metal organic framework, 1-2 parts of dispersant, 0.1-0.8 parts of anhydride curing accelerator and 80-90 parts of anhydride curing agent.

3. The flexible electromagnetic shielding material according to claim 1 or 2, characterized in that: In the rare earth-based metal organic framework, the rare earth element includes one or both of yttrium and gadolinium.

4. The flexible electromagnetic shielding material according to claim 3, characterized in that: The molar ratio of yttrium to gadolinium is 2:

3.

5. The flexible electromagnetic shielding material according to claim 1, characterized in that: The method for preparing the rare earth-based metal organic framework comprises the following steps: Weigh 2,5-dihydroxyterephthalic acid, yttrium nitrate hexahydrate and gadolinium nitrate hexahydrate into a reaction container, add N,N-dimethylformamide solution to dissolve, ultrasonically disperse at room temperature for 50 min to 60 min, transfer to a reaction kettle, react at a temperature of 120° C. to 150° C. for 12 h to 24 h to obtain a mixed solution; Add ethanol to the mixed solution and centrifuge at a rate of 10000 r / min to 12000 r / min for 5 min to 8 min to precipitate the product in the mixed solution, centrifuge and separate, repeatedly wash the precipitated product with N,N dimethylformamide solution and anhydrous ethanol solution until the supernatant is colorless, filter, and vacuum dry at 65° C. to 75° C. for 12 h to 24 h to obtain a rare earth-based metal organic framework.

6. The flexible electromagnetic shielding material according to claim 1, characterized in that: The fillers are hollow silicon micropowder and flaky boron nitride.

7. The flexible electromagnetic shielding material according to claim 1, characterized in that: The thermoplastic resin is a thermoplastic polyurethane elastomer or polyamide; The conductive material is any one of carbon fiber, graphite, acetylene black and Ketjen black; wherein the mass ratio of the conductive material to the thermoplastic resin is 7-9:

2.

8. A method for preparing a flexible electromagnetic shielding material, characterized in that: The following steps are involved: The composite wave absorbing layer is prepared by compression molding of mixed epoxy resin, filler and rare earth-based metal organic framework; Mixing a conductive material and a thermoplastic resin, drying and compression molding, and obtaining a first conductive layer and a second conductive layer respectively; The first conductive layer, the composite absorbing layer and the second conductive layer are stacked in sequence and hot-pressed to obtain a flexible electromagnetic shielding material.

9. The method for preparing a flexible electromagnetic shielding material according to claim 8, characterized in that: The method for preparing the composite absorbing layer specifically comprises the following steps: Mixing the epoxy resin, the filler and the rare earth metal organic framework for 1 to 3 hours until the mixture is uniform, thereby obtaining a premix; Stir and disperse the premix and the dispersant for 5 to 10 minutes, take out and perform vacuum degassing for 10 minutes to obtain a first mixture; Mixing the anhydride curing accelerator and the anhydride curing agent for 10 to 20 minutes to obtain a second mixture; The first mixture and the second mixture are shear-mixed at 30° C. to 50° C. for 8 to 15 minutes to obtain a blended solution, which is then dried under vacuum and compression molded to obtain a composite absorbing layer.

10. The method for preparing a flexible electromagnetic shielding material according to claim 8, characterized in that: The hot pressing temperature is 150° C. to 175° C., and the hot pressing time is 15 min to 30 min.

Images