Double-layer electromagnetic shielding material and preparation method thereof

Abstract

The invention discloses a double-layer electromagnetic shielding material which comprises an electromagnetic wave absorption foaming layer and an electromagnetic wave shielding layer, the electromagnetic wave absorption foaming layer comprises the following components in percentage by weight: 4-49 wt% of carbon nanotubes, 0-41 wt% of iron powder and 50-95 wt% of polyvinylidene fluoride, and the porosity of the electromagnetic wave absorption foaming layer is 55-95%; the electromagnetic wave shielding layer is prepared from the following components in percentage by weight: 30 to 70 weight percent of carbon nanotubes and 30 to 70 weight percent of polyvinylidene fluoride; the thickness of the electromagnetic wave absorption foaming layer ranges from 1 mm to 10 mm, and the thickness of the electromagnetic wave shielding layer ranges from 0.3 mm to 5 mm. According to the double-layer electromagnetic shielding material, through the synergistic effect of the electromagnetic wave absorption foaming layer and the high-conductivity electromagnetic wave shielding layer, an absorption-reflection-reabsorption electromagnetic energy dissipation path is achieved, and therefore the excellent high-broadband electromagnetic shielding performance and the low-reflection characteristic are obtained.

Description

Technical Field

[0001] The present invention relates to the technical field of polymer materials, and particularly to a double-layer electromagnetic shielding material and a preparation method thereof. Background Art

[0002] With the rapid development of wireless communication technology, the impact of electromagnetic interference (EMI) on electronic devices, communication systems and even human health has become increasingly serious. To address this challenge, it is particularly important to develop efficient, broadband and lightweight electromagnetic shielding materials. However, traditional electromagnetic shielding materials mainly rely on the reflection mechanism, resulting in secondary reflection pollution of electromagnetic waves, thus affecting the stability of devices and the surrounding environment. Traditional electromagnetic shielding materials mainly adopt a single-layer structure, and these materials have the following problems when dealing with electromagnetic waves: First, the absorption ability of single-layer materials is relatively limited, especially in the wide frequency band or high frequency band, and their shielding effect will be significantly reduced; Second, the reflectivity of the materials is relatively strong, causing secondary reflection of electromagnetic waves and bringing additional electromagnetic pollution. Therefore, the application of single-layer shielding materials in the field of high-efficiency electromagnetic interference shielding is greatly limited.

[0003] To solve these problems, researchers have tried to develop multi-layer composite material structures. By combining different functional layers (such as absorption layer and shielding layer), multiple absorption and reflection mechanisms can be realized, thus effectively improving the electromagnetic wave absorption ability and shielding effect of the materials. For example, some studies have proposed multi-layer composite materials made of conductive fillers such as carbon nanotubes (CNT) and graphene, and these materials can improve the electromagnetic wave absorption performance of the materials, but still face the problems of limited absorption bandwidth and large reflection loss.

[0004] In addition, composite materials based on foam structures have also received attention in the field of electromagnetic wave absorption. Microporous foam materials can not only reduce the volume of the materials, but also increase the scattering and reflection effects inside the materials, thus enhancing their electromagnetic wave absorption ability. In particular, the supercritical CO2 foam technology has been widely used in the preparation of composite materials because it can optimize the electromagnetic wave absorption characteristics of the materials by adjusting the porosity and microstructure of the foam. However, the electromagnetic properties of the foam structure are still limited by the uniform dispersion of the fillers and the damage of the conductive network when there are too many pores.

[0005] It can be seen that the main challenges faced by existing electromagnetic shielding materials lie in how to balance the absorption and reflection performance, overcome the electromagnetic pollution problem caused by high reflection, and improve the absorption ability in the wide frequency band. In addition, how to optimize the overall performance of the materials by using multi-layer composite structures, especially to achieve efficient electromagnetic wave absorption while keeping the materials lightweight, is still a research hotspot in this field. Summary of the Invention

[0006] An object of the present invention is to provide a double-layer electromagnetic wave shielding material with high broadband electromagnetic wave absorption ability and less reflection, and a preparation method thereof.

[0007] The present invention is achieved by the following technical solutions: A double-layer electromagnetic shielding material includes an electromagnetic wave absorption foam layer and an electromagnetic wave shielding layer. The electromagnetic wave absorption foam layer, by weight percentage, includes 4-49 wt% carbon nanotubes, 0-41 wt% iron powder, 50-95 wt% polyvinylidene fluoride. And the porosity of the electromagnetic wave absorption foam layer is 55-95%; the electromagnetic wave shielding layer, by weight percentage, includes 30-70 wt% carbon nanotubes, 30-70 wt% polyvinylidene fluoride; the thickness range of the electromagnetic wave absorption foam layer is 1-10 mm, and the thickness of the electromagnetic wave shielding layer is 0.3-5 mm.

[0008] Preferably, the electromagnetic wave absorption foam layer is composed of 3-7 wt% carbon nanotubes, 25-35 wt% iron powder, 60-70 wt% polyvinylidene fluoride. And by controlling the foaming process, the porosity of the electromagnetic wave absorption foam layer is adjusted to 70-80%. At the same time, when the thickness of the foam layer is adjusted to 1-3 mm and the thickness of the electromagnetic wave shielding layer is 0.7-1.3 mm, it has significantly higher electromagnetic wave shielding ability.

[0009] Preferably, the electromagnetic wave shielding layer, by weight percentage, includes 45-55 wt% carbon nanotubes, 45-55 wt% polyvinylidene fluoride.

[0010] The average diameter of the carbon nanotubes is 0.5-20 nm, and the average length is 0.5 μm -20 μm.

[0011] The carbon nanotubes can be one or a mixture of multi-walled carbon nanotubes or single-walled carbon nanotubes.

[0012] The average particle size range of the iron powder is 10-20 μm.

[0013] Preferably, the melting temperature of polyvinylidene fluoride is 140-145 °C. Polyvinylidene fluoride with a preferred melting temperature is easier to control the foaming degree and porosity.

[0014] In the technical solution of the present invention, without changing the high broadband electromagnetic shielding performance and low reflection characteristics, those skilled in the art of the present invention can modify the double-layer electromagnetic shielding material according to actual needs. By weight percentage, the electromagnetic wave absorption foam layer can also add 0-0.5 wt% of silica; by weight percentage, the electromagnetic wave shielding layer can also add 0-0.5 wt% of silane coupling agent.

[0015] A preparation method of a double-layer electromagnetic shielding material includes the following steps: Electromagnetic wave absorption foam layer: Weigh each component, mix them evenly, prepare the composite material by melt blending method, and then foam it by supercritical carbon dioxide or supercritical nitrogen foaming method to obtain the electromagnetic wave absorption foam layer material; specifically, the processing temperature range of the above melt blending method is 190 - 210 °C; the operation steps of the above supercritical carbon dioxide or supercritical nitrogen foaming method are: saturation pressure 12 - 15 Mpa and saturation time (i.e., supercritical fluid infiltration time) 10 - 60 min, the saturation temperature range is set at 100 - 140 °C, then release the pressure to atmospheric pressure, and then cool and shape.

[0016] To improve the dispersion of carbon nanotubes and iron powder in polyvinylidene fluoride, the carbon nanotubes and iron powder can be first dispersed in a solvent (such as DMF, etc.), the polyvinylidene fluoride can be dissolved in a solvent (such as DMF, etc.), then the two are mixed and stirred evenly, then injected into water to precipitate particles, and then melt blending is carried out.

[0017] Electromagnetic wave shielding layer: Weigh each component, mix them evenly, and prepare the electromagnetic wave shielding layer by hot pressing; then laminate the electromagnetic wave absorption foam layer and the electromagnetic wave shielding layer to obtain a double-layer electromagnetic shielding material.

[0018] The lamination mentioned above is bonding or hot pressing.

[0019] The present invention has the following beneficial effects compared with the prior art: The present invention combines dielectric and magnetic fillers through a microporous structure, adjusts the porosity size, optimizes the impedance matching, significantly improves the electromagnetic wave absorption ability of the foam layer, the reflection loss is as low as -60 dB, and the effective absorption bandwidth reaches 3.7 GHz. The present invention also lies in the design of a double-layer structure with a specific thickness, which enables electromagnetic waves to experience multiple reflections and absorptions inside the material, reduces surface reflection, the reflection coefficient is lower than 0.4, and excellent shielding performance mainly based on absorption is obtained, with a high A / R ratio. It can be seen that through the synergistic effect of the double-layer electromagnetic shielding material of the electromagnetic wave absorption foam layer and the highly conductive electromagnetic wave shielding layer, an electromagnetic energy dissipation path of absorption - reflection - reabsorption is realized, thereby obtaining excellent high broadband electromagnetic shielding performance and low reflection characteristics. Description of the Drawings

[0020] Figure 1 : SEM photos of the foam layer of the double-layer electromagnetic shielding materials of Examples 1 - 3 with different porosities.

[0021] Figure 2 : Variation of electromagnetic parameters (dielectric and magnetic permeability constants) with porosity for the non-foamed foam layer and the foam layers of Examples 1 - 3 with different void fractions.

[0022] Figure 3:Structure diagram of the double - layer electromagnetic wave shielding material of the present invention and schematic diagram of electromagnetic shielding performance test results. Detailed implementation manners

[0023] The present invention will be described in detail below in conjunction with specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but do not limit the present invention in any form. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made. These all belong to the protection scope of the present invention.

[0024] The raw materials used in the present invention are as follows: Polyvinylidene fluoride: The melting temperature is about 143 °C, the grade is Kynar Flex 2800 - 00, and the manufacturer is Arkema; Carbon nanotubes: The average diameter is about 9.5 nm, the average length is about 1.5 μm, multi - wall carbon nanotubes, purchased from Nanocyl SA; Iron powder: The average particle size is about 15 μm, and the manufacturer is Zhengzhou Qishan New Material Technology Research Institute; Adhesive: 502 glue, a glue with α - cyanoacrylate ethyl ester as the main component.

[0025] Preparation method of the double - layer electromagnetic shielding material in the embodiment: For the electromagnetic wave absorption foam layer: Weigh each component, then mix them evenly and prepare the composite material by the melt - blending method (the equipment is a twin - screw extruder, Xplore Instruments B.V., Sittard, the temperature is 200 °C), and then foam by the supercritical carbon dioxide foaming method, control the foaming degree to obtain the electromagnetic wave absorption foam layer material; For the electromagnetic wave shielding layer: Weigh each component, then mix them evenly and prepare the electromagnetic wave shielding layer by hot - pressing molding (the equipment is a flat vulcanizer, the hot - pressing temperature is 200 °C); Then bond the electromagnetic wave absorption foam layer and the electromagnetic wave shielding layer to obtain the double - layer electromagnetic shielding material.

[0026] Each test method: (1) Porosity: Test using an instrument (electronic density meter DH - 300, DahoMeter) (water displacement method).

[0027] (2) Shielding performance and reflection loss: Test using an instrument (vector network analyzer keysight 5227B) (calculate electromagnetic absorption and shielding through S - parameters and electromagnetic parameters).

[0028] Table 1: Parameters and test results of the double - layer electromagnetic wave shielding material in the embodiment

[0029] As can be seen from Examples 1-3, by adjusting the porosity of the foam layer and controlling the thickness range of the foam layer through process adjustment, the electromagnetic wave absorption and shielding performance of Example 2 is significantly better. It can be seen that the design of the double-layer structure with an optimized thickness enables electromagnetic waves to experience multiple reflections and absorptions inside the material, reducing surface reflection and obtaining excellent shielding performance mainly based on absorption, with a high A / R ratio.

Claims

1. A double-layer electromagnetic shielding material, comprising an electromagnetic wave absorbing foaming layer and an electromagnetic wave shielding layer, characterized in that: The electromagnetic wave absorbing foaming layer comprises 4-49wt% carbon nanotubes, 0-41wt% iron powder, and 50-95wt% polyvinylidene fluoride by weight percentage, and the porosity of the electromagnetic wave absorbing foaming layer is 55-95%; the electromagnetic wave shielding layer comprises 30-70wt% carbon nanotubes and 30-70wt% polyvinylidene fluoride by weight percentage; the thickness range of the electromagnetic wave absorbing foaming layer is 1-10mm, and the thickness of the electromagnetic wave shielding layer is 0.3-5mm.

2. The double-layer electromagnetic shielding material according to claim 1, characterized in that: The electromagnetic wave absorbing foaming layer comprises 3-7wt% carbon nanotubes, 25-35wt% iron powder, and 60-70wt% polyvinylidene fluoride by weight percentage. The porosity of the electromagnetic wave absorbing foaming layer is 70-80%, the thickness range is 1-3 mm, and the thickness of the electromagnetic wave shielding layer is 0.7-1.3 mm.

3. The double-layer electromagnetic shielding material according to claim 1, characterized in that: The carbon nanotubes have an average diameter of 0.5-20 nm and an average length of 0.5 μm-20 μm.

4. The double-layer electromagnetic shielding material according to claim 1, characterized in that: The average particle size of the iron powder is in the range of 10-20 μm.

5. The double-layer electromagnetic shielding material according to claim 1, characterized in that: The melting temperature of the polyvinylidene fluoride is 140-145°C.

6. The double-layer electromagnetic shielding material according to claim 1, characterized in that: Measured by weight percentage, the electromagnetic wave absorbing foaming layer further comprises 0-0.5wt% of silicon dioxide; and measured by weight percentage, the electromagnetic wave shielding layer further comprises 0-0.5wt% of a silane coupling agent.

7. The method for preparing the double-layer electromagnetic shielding material according to any one of claims 1 to 6, characterized in that: The following steps are involved: Electromagnetic wave absorbing foaming layer: weigh each component, mix them evenly, and prepare a composite material by a melt blending method, and then foam them by a supercritical carbon dioxide foaming method to obtain an electromagnetic wave absorbing foaming layer material; Electromagnetic wave shielding layer: weigh each component, mix them evenly and then prepare the electromagnetic wave shielding layer by hot pressing; The electromagnetic wave absorbing foaming layer and the electromagnetic wave shielding layer are then laminated to obtain a double-layer electromagnetic shielding material.

8. The method for preparing a double-layer electromagnetic shielding material according to claim 7, characterized in that: The lamination is bonding or hot pressing.

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