A flexible electromagnetic shielding material and a method for manufacturing the same

Abstract

The application provides a flexible electromagnetic shielding material and a preparation method thereof. The electromagnetic shielding material comprises 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 comprise a thermoplastic resin and a conductive material. The composite wave-absorbing layer comprises at least 80-110 parts by weight of an epoxy resin, 20-30 parts by weight of a filler and 5-10 parts by weight of a rare earth metal organic framework. The electromagnetic shielding material is provided in a layered structure. When electromagnetic waves contact the first conductive layer, induced current is generated on the surface of the first conductive layer, the electromagnetic waves are reflected through the high conductivity, but at the same time, the electromagnetic waves that are not reflected are transmitted into the composite wave-absorbing layer. The rare earth metal organic framework is polarized under the action of the electromagnetic waves, further absorbing the electromagnetic waves. When the electromagnetic waves are not completely absorbed in the composite wave-absorbing layer, the second conductive layer further reflects the electromagnetic waves, which are further absorbed by the composite wave-absorbing layer.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of electromagnetic shielding, in particular to a flexible electromagnetic shielding material and a preparation method thereof. BACKGROUND

[0002] With the rapid development of electronic technology, the problem of electromagnetic radiation is becoming increasingly serious, which has an adverse effect on the normal operation of electronic devices and human health. Traditional electromagnetic shielding materials are mostly metal materials, which have good shielding effect, but have the disadvantages of 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 is increasing.

[0003] The flexible electromagnetic shielding materials on the market at present mainly realize electromagnetic shielding function by adding metal particles or conductive polymers in a polymer matrix. However, these materials still have great limitations in flexibility and shielding effectiveness. SUMMARY

[0004] In order to solve at least one of the above technical problems, the embodiments of the present application provide a flexible electromagnetic shielding material.

[0005] In addition, the embodiments of the present application also provide a preparation method of the flexible electromagnetic shielding material.

[0006] The embodiments of the present application provide a flexible electromagnetic shielding material, which has a layered structure and comprises 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 comprise a thermoplastic resin and a conductive material. The composite wave-absorbing layer comprises, by weight fraction, 80-110 parts of epoxy resin, 20-30 parts of filler, and 5-10 parts of rare earth metal-organic framework.

[0007] In some embodiments of the present application, the composite wave-absorbing layer comprises, by weight fraction, 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 the present application, the rare earth element in the rare earth metal-organic framework comprises one or both of yttrium and gadolinium.

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

[0010] In some embodiments of the present application, the preparation method of the rare earth metal organic framework comprises the following steps: weighing 2,5-dihydroxyterephthalic acid, yttrium nitrate hexahydrate and gadolinium nitrate hexahydrate, and placing them in a reaction container; dissolving with N,N-dimethylformamide solution; ultrasonic dispersion at room temperature for 50 min to 60 min; transferring to a reaction kettle; reacting at a temperature of 120 ℃ to 150 ℃ for 12 h to 24 h to obtain a mixed solution. Ethanol is added to the mixed solution, and the product in the mixed solution is precipitated by centrifugation at a speed of 10,000 r / min to 12,000 r / min for 5 min to 8 min. After centrifugal separation, the precipitated product is repeatedly washed with N,N-dimethylformamide solution and anhydrous ethanol solution until the supernatant is colorless. Filtration and vacuum drying at 65 ℃ to 75 ℃ for 12 h to 24 h 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 a thermoplastic polyurethane elastomer or a 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 present application also provides a preparation method of a flexible electromagnetic shielding material, comprising the following steps: mixing an epoxy resin, a filler and a rare earth metal organic framework to mold a composite wave-absorbing layer. Mixing a conductive material and a thermoplastic resin, and molding after drying to obtain a first conductive layer and a second conductive layer, respectively. Stacking the first conductive layer, the composite wave-absorbing layer and the second conductive layer in sequence, and hot-pressing to obtain the flexible electromagnetic shielding material.

[0014] In some embodiments of the present application, the preparation method of the composite wave-absorbing layer specifically comprises the following steps: mixing an epoxy resin, a filler and a rare earth metal organic framework for 1 h to 3 h to obtain a premix. Stirring and dispersing the premix and a dispersing agent for 5 min to 10 min, and taking out for vacuum degassing for 10 min to obtain a first mixture. Mixing an acid anhydride curing accelerator and an acid anhydride curing agent for 10 min to 20 min to obtain a second mixture. Shearing and mixing the first mixture and the second mixture at 30 ℃ to 50 ℃ for 8 min to 15 min to obtain a blended solution, and then drying under vacuum to mold the composite wave-absorbing layer.

[0015] In some embodiments of the present application, the hot-pressing temperature is 150 ℃ to 175 ℃, 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 embodiment of the 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 arranged on both sides of the composite wave-absorbing layer. When electromagnetic waves contact the first conductive layer, induced current is generated on the surface of the first conductive layer, and the electromagnetic waves are reflected by the high electrical conductivity of the first conductive layer. However, at the same time, electromagnetic waves that are not reflected will be transmitted into the composite wave-absorbing layer. The rare earth metal organic framework will polarize under the action of electromagnetic waves, generate dielectric loss, convert electromagnetic wave energy into heat energy, and further absorb electromagnetic waves. When 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 reabsorbed by the composite wave-absorbing layer, to achieve the characteristics of reflection-absorption-re-reflection absorption. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 FIG. 1 is a cross-sectional SEM image of the flexible electromagnetic shielding material prepared in Embodiment 1 of the application. DETAILED DESCRIPTION

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

[0019] With the development of electronic information technology and the widespread application of wireless information transmission, people are increasingly dependent on various electromagnetic wave transmission devices. The electromagnetic pollution caused by electromagnetic waves is mainly manifested in two aspects: one is the harm to human health. People living in an environment polluted by electromagnetic waves for a long time will have a certain negative impact on the nervous system and endocrine system. The other aspect is the interference with the normal function of electronic devices. Electromagnetic waves with similar frequencies can cause signal interference to the operation of surrounding electronic devices and communication systems, affect the reception and transmission of communication signals, and even cause information security problems. Therefore, it is of great research significance and application prospect to develop electromagnetic shielding materials with high electromagnetic wave shielding performance to strengthen human body protection and alleviate mutual interference of various radiation sources.

[0020] The most widely used electromagnetic shielding materials at present are carbon materials and graphene. However, these materials have low strength and lack flexibility, which cannot guarantee the mechanical strength requirements of one-dimensional conductive fibers in actual applications. In order to solve the problem of poor flexibility and low shielding efficiency of existing electromagnetic shielding materials.

[0021] The present application inventors have found that MOF materials have tunable nanostructures and abundant porosity. The derived electromagnetic wave absorbing materials generally exhibit excellent electrical conductivity, magnetism, and sufficient defect sites and interface structures, thereby exhibiting unique advantages in impedance matching and microwave loss. The organic ligand and metal ion will undergo polarization under the action of electromagnetic waves, producing 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, and have a high specific surface area and large voids, which can further improve the wave absorption performance.

[0022] To this end, the embodiments of the present application provide a flexible electromagnetic shielding material, which is a layered structure and 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 comprise a thermoplastic resin and a conductive material. The composite wave absorbing layer comprises, by weight fraction, at least 80-110 parts of an epoxy resin, 20-30 parts of a filler, and 5-10 parts of a rare earth-based metal organic framework. By setting the electromagnetic shielding material as a layered structure, including the composite wave absorbing layer and the conductive layers arranged on both sides of the composite wave absorbing layer, good flexibility and mechanical properties are obtained. When electromagnetic waves contact the first conductive layer, induced current will be generated on the surface of the first conductive layer, which reflects electromagnetic waves through its high electrical conductivity. However, unreflected electromagnetic waves will also be transmitted into the composite wave absorbing layer. The rare earth-based metal organic framework will undergo polarization under the action of electromagnetic waves, producing dielectric loss and converting electromagnetic wave energy into heat energy, thereby further absorbing electromagnetic waves. When electromagnetic waves are not completely absorbed in the composite wave absorbing layer, the second conductive layer will further reflect electromagnetic waves, which will be reabsorbed by the composite wave absorbing layer, thereby achieving the characteristics of reflection-absorption-re-reflection absorption.

[0023] In an embodiment of the present application, the composite wave absorbing layer comprises, by weight fraction, 80-110 parts of an epoxy resin, 20-30 parts of a filler, 5-10 parts of a rare earth-based metal organic framework, 1-2 parts of a dispersing agent, 0.1-0.8 parts of an acid anhydride curing accelerator, and 80-90 parts of an acid anhydride curing agent.

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

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

[0026] In an embodiment of the present application, the rare earth metal organic framework, the rare earth element includes one or both of yttrium and gadolinium. The hierarchical pore structure of the rare earth elements yttrium and gadolinium can increase the multiple reflection path of electromagnetic waves, and in combination with the interlayer dielectric polarization effect of the flaky boron nitride and the hollow silicon micropowder, the wide-band electromagnetic wave absorption is realized. The 4f electron transition of the rare earth element gadolinium can also enhance the magnetic loss. The electronic configuration of the yttrium element enables it to form a strong coordination bond with the organic ligand, and the conjugated pi electron system of the MOF skeleton enhances the dielectric polarization, which produces significant dielectric loss to high-frequency electromagnetic waves. At the same time, the gadolinium element has a high spin magnetic moment, which produces 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 proportion of dielectric loss and magnetic loss, and the adaptability in complex electromagnetic environments is significantly improved.

[0027] In an 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 wave absorption performance attenuation caused by lattice stress is avoided. The incorporation of yttrium suppresses the magnetic domain aggregation of gadolinium ions, prevents impedance mismatching of the wave absorption layer caused by excessive magnetic loss, and improves the electromagnetic wave incidence efficiency.

[0028] In an embodiment of the present application, the preparation method of the rare earth metal organic framework comprises the following steps:

[0029] Step S101, weigh 2,5-dihydroxyterephthalic acid, yttrium nitrate hexahydrate and gadolinium nitrate hexahydrate into a reaction container, add N,N-dimethylformamide solution for dissolution, ultrasonic dispersion at room temperature for 50 min to 60 min, transfer to a reaction kettle, and react at a temperature of 120 DEG C to 150 DEG C for 12 h to 24 h to obtain a mixed solution. The multi-dentate ligand 2,5-dihydroxyterephthalic acid has high structural stability and large specific surface area for the synthesis of MOF of multiple metals, gadolinium provides magnetic loss, yttrium provides dielectric loss, and wider frequency bands are covered in cooperation.

[0030] In some embodiments, the mass ratio of the multi-dentate ligand 2,5-dihydroxyterephthalic acid to yttrium nitrate hexahydrate and gadolinium nitrate hexahydrate is 13:4.

[0031] In some embodiments, the reaction temperature can be 120 DEG C, 130 DEG C, 140 DEG C, 150 DEG 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.

[0032] Step S102, adding ethanol to the mixed solution to precipitate the product in the mixed solution at a speed of 10000 r / min to 12000 r / min for 5 min to 8 min, centrifugal separation, repeatedly washing the precipitated product with N,N dimethylformamide solution and anhydrous ethanol solution until the supernatant is colorless, filtering, and vacuum drying at 65 ℃ to 75 ℃ for 12 h to 24 h to obtain the rare earth metal organic framework.

[0033] In some embodiments, the centrifugal speed can be 10000 r / min, 11000 r / min, 12000 r / min, or any value within the range composed of any two of the above values. The centrifugal 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.

[0034] In an 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 and flaky boron nitride is 1:3. By using hollow silica powder and flaky boron nitride together, the hollow structure of hollow silica powder can reduce stress concentration in the resin matrix, and flaky boron nitride can synergistically conduct heat after being blended with hollow silica powder, quickly dissipating thermal energy.

[0035] In an 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.

[0036] In some embodiments, the dispersant is POSS glycidyl ether oxypropyl cyclotetrasiloxane, the viscosity is 4500 cps to 4800 cps; the density is 1.25 g / ml to 1.30 g / ml; the molecular weight is 1337.88; and the epoxy equivalent weight is 167. It is an organic and inorganic hybrid cage-like polysilsesquioxane with a molecular size of 1.5 nm, which is used as a high-efficiency dispersant for nanomaterials to enhance the dispersibility of nanomaterials, and is purchased from Xi'an Rixi Biological Technology Co., Ltd. On the other hand, the inventors found that the use of POSS glycidyl ether oxypropyl cyclotetrasiloxane can not only promote the dispersion of MOF and filler, but also increase the bonding strength between the conductive layer and the epoxy resin during hot pressing.

[0037] 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 arranged on both sides of the composite wave-absorbing layer, good flexibility and mechanical properties are obtained; when electromagnetic waves contact the first conductive layer, induced current is generated on the surface of the first conductive layer, and the electromagnetic waves are reflected by the high electrical conductivity, but at the same time, electromagnetic waves that are not reflected are transmitted into the composite wave-absorbing layer; under the action of electromagnetic waves, the rare earth metal organic framework will undergo polarization, generate dielectric loss, and convert electromagnetic wave energy into heat energy, 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 reabsorbed by the composite wave-absorbing layer, to achieve the characteristics of reflection-absorption-re-reflection absorption.

[0038] The preparation method of the flexible electromagnetic shielding material provided by the embodiments of the present application specifically includes the following steps:

[0039] Step S1, a composite wave-absorbing layer is prepared by mixing an epoxy resin, a filler and a rare earth metal organic framework and then being molded.

[0040] In an embodiment of the present application, the preparation method of the composite wave-absorbing layer in step S1 specifically includes the following steps:

[0041] Step S101, the epoxy resin, the filler and the rare earth metal organic framework are mixed for 1 h to 3 h to be uniformly mixed, to obtain a premix.

[0042] Step S102, the premix and a dispersant are stirred and dispersed for 5 min to 10 min, and after being taken out, vacuum debubbling is performed for 10 min, to obtain a first mixture.

[0043] Step S103, an acid anhydride curing accelerator and an acid anhydride curing agent are mixed for 10 min to 20 min to obtain a second mixture; the first mixture and the second mixture are sheared and mixed at 30 ℃ to 50 ℃ for 8 min to 15 min to obtain a blended solution, which is then dried under vacuum, molded and formed, to obtain the composite wave-absorbing layer.

[0044] In an embodiment of the present application, the hot pressing temperature is 150 ℃ to 175 ℃, and the hot pressing time is 15 min to 30 min.

[0045] Step S2, a conductive material and a thermoplastic resin are mixed, dried and then molded, to respectively obtain a first conductive layer and a second conductive layer.

[0046] Step S3, the first conductive layer, the composite wave-absorbing layer and the second conductive layer are stacked in sequence, and hot pressing is performed, to obtain the flexible electromagnetic shielding material.

[0047] Compared with the prior art, the preparation method of the flexible electromagnetic shielding material provided by the embodiment of the application prepares a composite wave-absorbing layer first, and then hot-presses a first conductive layer and a second conductive layer on two sides of the composite wave-absorbing layer respectively. During the hot-pressing process, the thermoplastic resin softens and interfacial diffusion occurs in the epoxy resin layer to form an interpenetrating network, thereby improving 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.

[0048] The foregoing flexible electromagnetic shielding material is further described below through specific embodiments.

[0049] Embodiment 1

[0050] Step 1: 100 parts of epoxy resin, 23 parts of hollow silicon micro-powder and flaky boron nitride with a mass ratio of 1:3, and 8 parts of rare earth metal organic framework were weighed by weight, stirred and mixed for 1 h to obtain a premix. 1 part of a dispersant POSS glycidyl ether oxypropyl cyclotetrasiloxane was added to the premix, and the premix and the dispersant were stirred and dispersed for 10 min. After taking out, vacuum degassing was performed for 10 min to obtain a first mixture. 0.7 parts of tetramethylammonium bromide and 80 parts of methylhexahydrophthalic anhydride were mixed for 15 min to obtain a second mixture. The first mixture and the second mixture were sheared and mixed at 50 ℃ for 15 min to obtain a blended solution, which was then dried under vacuum and molded to obtain a composite wave-absorbing layer.

[0051] Step 2: carbon fibers and thermoplastic polyurethane were mixed in a mass ratio of 7:2, dried, and then molded to obtain a first conductive layer and a second conductive layer, respectively.

[0052] Step 3: the first conductive layer, the composite wave-absorbing layer, and the second conductive layer were stacked in sequence, and hot-pressed to obtain a flexible electromagnetic shielding material, with a hot-pressing temperature of 150 ℃ and a hot-pressing time of 15 min.

[0053] Embodiment 2

[0054] Step 1: 110 parts of epoxy resin, 20 parts of hollow silicon micro-powder and flaky boron nitride with a mass ratio of 1:3, and 10 parts of rare earth metal organic framework were weighed by weight, stirred and mixed for 1 h to obtain a premix. 2 parts of a dispersant POSS glycidyl ether oxypropyl cyclotetrasiloxane were added to the premix, and the premix and the dispersant were stirred and dispersed for 10 min. After taking out, vacuum degassing was performed for 10 min to obtain a first mixture. 0.7 parts of tetramethylammonium bromide and 80 parts of methylhexahydrophthalic anhydride were mixed for 15 min to obtain a second mixture. The first mixture and the second mixture were sheared and mixed at 50 ℃ for 15 min to obtain a blended solution, which was then dried under vacuum and molded to obtain a composite wave-absorbing layer.

[0055] Step 2, mixing carbon fiber and thermoplastic polyurethane in a mass ratio of 7:2, drying and then molding to obtain the first conductive layer and the second conductive layer respectively;

[0056] Step 3, stacking the first conductive layer, the composite wave-absorbing layer and the second conductive layer in sequence, hot-pressing molding, the hot-pressing temperature is 150 ℃, the hot-pressing time is 15 min, to obtain the flexible electromagnetic shielding material.

[0057] Example 3

[0058] Step 1, weighing 80 parts of epoxy resin, 20 parts of hollow silica powder and flaky boron nitride in a mass ratio of 1:3 and 5 parts of rare earth metal organic framework, stirring and mixing for 1 h to obtain a premix. Adding 1 part of dispersant POSS glycidyl ether oxypropyl cyclotetrasiloxane to the premix, stirring and dispersing the premix and the dispersant for 10 min, then taking out and vacuum degassing for 10 min to obtain a first mixture. Mixing 0.7 parts of tetramethylammonium bromide and 80 parts of methylhexahydrophthalic anhydride for 15 min to obtain a second mixture. Shearing mixing the first mixture and the second mixture at 50 ℃ for 15 min to obtain a blending solution, then drying under vacuum and molding to obtain the composite wave-absorbing layer.

[0059] Step 2, mixing carbon fiber and thermoplastic polyurethane in a mass ratio of 7:2, drying and then molding to obtain the first conductive layer and the second conductive layer respectively;

[0060] Step 3, stacking the first conductive layer, the composite wave-absorbing layer and the second conductive layer in sequence, hot-pressing molding, the hot-pressing temperature is 150 ℃, the hot-pressing time is 15 min, to obtain the flexible electromagnetic shielding material.

[0061] Comparative Example 1

[0062] The specific process of the preparation process refers to Example 1, the difference is that no rare earth metal organic framework is added in step 1, and the preparation method of the rest of the electromagnetic shielding material is basically the same as that of Example 1, which will not be described in detail here.

[0063] Comparative Example 2

[0064] 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 method of the rest of the electromagnetic shielding material is basically the same as that of Example 1, which will not be described in detail here.

[0065] Comparative Example 3

[0066] 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 method of the rest of the electromagnetic shielding material is basically the same as that of Example 1, which will not be described in detail here.

[0067] Comparative Example 4

[0068] The preparation process is similar to that of Example 1, except that steps 2 and 3 are not performed, and the preparation method of the electromagnetic shielding material is basically the same as that of Example 1, which is not described in detail here.

[0069] Comparative Example 5

[0070] The preparation process is similar to that of Example 1, except that only the first conductive layer is stacked in step 3, and the preparation method of the electromagnetic shielding material is basically the same as that of Example 1, which is not described in detail here.

[0071] Comparative Example 6

[0072] The preparation process is similar to that of Example 1, except that the hot pressing temperature in step 3 is 200°C, and the preparation method of the electromagnetic shielding material is basically the same as that of Example 1, which is not described in detail here.

[0073] Comparative Example 7

[0074] The preparation process is similar to that of Example 1, except that the hot pressing temperature in step 3 is 140°C, and the preparation method of the electromagnetic shielding material is basically the same as that of Example 1, which is not described in detail here.

[0075] Comparative Example 8

[0076] The preparation process is similar to that of Example 1, except that no dispersing agent is added in step 1, and the preparation method of the electromagnetic shielding material is basically the same as that of Example 1, which is not described in detail here.

[0077] The flexible electromagnetic shielding materials obtained in Examples 1-3 and Comparative Examples 1-8 are tested for performance, and the test results are shown in Table 1. For example, the scanning electron microscope results of the flexible electromagnetic shielding material in Example 1 are shown in FIG. 1. Figure 1

[0078] Table 1

[0079]

[0080] ​According to the data in Table 1, the electromagnetic shielding effectiveness in Example 1 reaches 97, and the thermal conductivity reaches 2.8, which benefits from the addition of the rare earth metal organic framework, and the reflection-absorption-re-reflection effect with the first and second conductive layers is achieved, establishing an electromagnetic multiple loss mechanism. The flexible matrix of the thermoplastic polyurethane and the dispersant in the epoxy resin enhance the interfacial bonding ability between the conductive layer and the composite wave-absorbing layer in the hot pressing process. In Comparative Example 1, due to the absence of MOF, the electromagnetic wave only relies on the reflection of the conductive layer, and the penetration at high frequency increases significantly. In Comparative Example 2, the agglomeration of MOF leads to the blockage of the pores in the wave-absorbing layer, and the shielding effectiveness decreases. The agglomerated MOF also destroys the crosslinked network of the resin, resulting in poor mechanical properties. In Comparative Example 3, the addition amount of MOF is insufficient, leading to weakened wave-absorbing ability. In Comparative Example 4, there is no conductive layer, and only the wave-absorbing ability can be relied on, resulting in low shielding effectiveness. In Comparative Example 5, there is only one conductive layer, and after reflection-absorption, part of the electromagnetic wave still penetrates. In Comparative Example 6, the material degrades at high temperature during hot pressing, resulting in poor interlayer bonding. In Comparative Example 7, the low temperature does not fuse, and the interface is easy to separate. In Comparative Example 8, no dispersant is added, and a small amount of MOF and filler agglomerate, resulting in weak interlayer bonding and reduced tensile strength.

[0081] It can be seen that, by setting the electromagnetic shielding material as a layered structure, including a composite wave-absorbing layer and conductive layers arranged on both sides of the composite wave-absorbing layer, the present application has good flexibility and mechanical properties. When the electromagnetic wave contacts the first conductive layer, an induced current is generated on the surface of the first conductive layer, which reflects the electromagnetic wave through its high electrical conductivity. However, at the same time, the electromagnetic wave that has not been reflected will transmit into the composite wave-absorbing layer. The rare earth metal organic framework will polarize under the action of the electromagnetic wave, resulting in dielectric loss and converting the electromagnetic wave energy into heat energy, further absorbing the electromagnetic wave. When the electromagnetic wave is not completely absorbed in the composite wave-absorbing layer, the second conductive layer will further reflect the electromagnetic wave, which is then re-absorbed by the composite wave-absorbing layer, achieving 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 and second conductive layers on both sides of the composite wave-absorbing layer. During the hot pressing process, the thermoplastic resin softens and interfacial diffusion occurs in the epoxy resin layer, forming an interpenetrating network and improving the interlayer bonding strength. The hot pressing time ensures that the resins are fully crosslinked without thermal decomposition, and the overall thickness of the material is controllable.

[0082] It can be understood that the above embodiments are only exemplary embodiments adopted to illustrate the principles of the present application, and the present application is not limited thereto. Various modifications and improvements can be made by those of ordinary skill in the art without departing from the spirit and essence of the present application, and these modifications and improvements are also considered within the protection scope of the present application.

Claims

1. A flexible electromagnetic shielding material, characterized by, The flexible electromagnetic shielding material is a layered structure, sequentially comprising a first conductive layer, a composite wave-absorbing layer and a second conductive layer; The first conductive layer and the second conductive layer both comprise a thermoplastic resin and a conductive material; The composite wave-absorbing layer is composed of, by weight fraction, 80-110 parts of an epoxy resin, 20-30 parts of a filler, 5-10 parts of a rare earth metal organic framework, 1-2 parts of a dispersant, 0.1-0.8 parts of an acid anhydride curing accelerator and 80-90 parts of an acid anhydride curing agent, wherein in the rare earth metal organic framework, the rare earth elements are yttrium and gadolinium in a molar ratio of 2:3, the filler is hollow silica powder and flaky boron nitride, and the dispersant is POSS glycidyl ether oxypropyl cyclotetrasiloxane.

2. The flexible electromagnetic shielding material of claim 1, wherein, The preparation method of the rare earth metal organic framework comprises the following steps: 2,5-dihydroxyterephthalic acid, yttrium nitrate hexahydrate and gadolinium nitrate hexahydrate are weighed and placed in a reaction container, dissolved in N,N-dimethylformamide solution, ultrasonically dispersed at room temperature for 50-60 minutes, transferred to a reaction kettle, reacted at a temperature of 120-150 DEG C for 12-24 hours to obtain a mixed solution; Ethanol is added to the mixed solution, centrifuged at a speed of 10,000-12,000 r / min for 5-8 minutes to precipitate the product in the mixed solution, centrifuged, and the precipitated product is repeatedly washed with N,N-dimethylformamide solution and anhydrous ethanol solution until the supernatant is colorless, filtered, vacuum dried at 65-75 DEG C for 12-24 hours to obtain the rare earth metal organic framework.

3. The flexible electromagnetic shielding material of claim 1, wherein, The thermoplastic resin is a thermoplastic polyurethane elastomer or a 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.

4. A method for producing the flexible electromagnetic shielding material according to any one of claims 1 to 3, characterized by, The method comprises the following steps: The epoxy resin, the filler and the rare earth metal organic framework are mixed and molded to prepare the composite wave-absorbing layer; The conductive material and the thermoplastic resin are mixed and molded after drying to obtain the first conductive layer and the second conductive layer respectively; The first conductive layer, the composite wave-absorbing layer and the second conductive layer are stacked in sequence and hot-pressed to obtain the flexible electromagnetic shielding material.

5. The method of claim 4, wherein the flexible electromagnetic shielding material is prepared by the steps of: (a) preparing a mixture of a conductive material and a resin; (b) coating the mixture on a substrate; (c) drying the mixture; and (d) removing the substrate. The preparation method of the composite wave-absorbing layer comprises the following steps: The epoxy resin, the filler and the rare earth metal organic framework are mixed uniformly for 1-3 hours to obtain a premix; The premix and the dispersant are stirred and dispersed for 5-10 minutes, and then vacuum degassed for 10 minutes after being taken out to obtain a first mixture; The acid anhydride curing accelerator and the acid anhydride curing agent are mixed for 10-20 minutes to obtain a second mixture; The first mixture and the second mixture are sheared and mixed at 30-50 DEG C for 8-15 minutes to obtain a blended solution, which is then dried under vacuum and molded to obtain the composite wave-absorbing layer.

6. The method of claim 4, wherein the flexible electromagnetic shielding material is prepared by the steps of: (a) preparing a mixture of a conductive material and a resin; (b) coating the mixture on a substrate; (c) drying the mixture; and (d) removing the substrate. The temperature of the hot pressing is 150-175 DEG C, and the time of the hot pressing is 15-30 minutes.

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