A foamed metal-based wave-absorbing and shielding sheet and a method for preparing the same

Abstract

The application relates to the technical field of wave-absorbing shielding materials, in particular to a preparation method of a foamed metal-based wave-absorbing shielding sheet, which comprises the following steps: metal foil shielding layer pretreatment: the upper and lower surfaces of the metal foil shielding layer are dip-coated with a conductive adhesive layer; preparation of a master batch of a wave-absorbing foaming layer: a thermoplastic resin matrix, an elastomer, a magnetic loss type wave-absorbing filler, an electric loss type wave-absorbing filler, an antioxidant, a crosslinking agent, a compatibilizer and a lubricant are weighed, granulated after stirring and then a master batch of the wave-absorbing foaming layer is obtained; co-extrusion of the metal foil shielding layer and the master batch of the wave-absorbing foaming layer into a film: the metal foil shielding layer passes through a die head of a co-extrusion calender, the master batch of the wave-absorbing foaming layer is added into the co-extrusion calender, a CO2 and N2 mixture is injected into a cylinder of the co-extrusion calender, the mixture is extruded after mixing and then is coated on the metal foil shielding layer, and finally the finished product is obtained after pressure reduction and removal of CO2; the foamed metal-based wave-absorbing shielding sheet prepared by the method has the characteristics of light weight, lightness, good foaming effect and green environmental protection.

Description

Technical Field

[0001] This invention relates to the field of microwave absorbing and shielding materials, specifically to a foamed metal-based microwave absorbing and shielding sheet and its preparation method. Background Technology

[0002] Electromagnetic shielding materials are used to reduce electromagnetic wave reflection and absorb electromagnetic energy, and they have significant applications and implications in modern technology. With the widespread use of electronic devices, electromagnetic interference (EMI) has become increasingly serious. EMI shielding materials can effectively reduce EMI between devices and improve electromagnetic compatibility. In the military field, they are widely used in stealth technology to reduce the likelihood of detection by minimizing radar wave reflection, thereby improving the concealment of military equipment. In wireless communication systems, foamed EMI shielding materials are used to reduce interference between antennas, improve signal transmission quality, and enhance system stability.

[0003] Foamed absorbing structures can reduce the overall density of materials, improve electromagnetic wave attenuation and absorption performance, and broaden the overall absorption frequency band. Introducing foamed absorbing structures into absorbing materials can achieve good compatibility between impedance matching and absorption performance, reducing electromagnetic wave echo reflections on the material's outer surface. Chinese patent CN115838493A discloses a porous structure shielding absorbing composite material and its preparation method. This application describes a process where composite absorbing foaming powder is physically deposited onto a self-made nanotube continuous network aggregate, rolled into a film, and then vacuum heat-treated to stack and foam to form hollow micropores. The porous structure shielding absorbing composite material obtained by this application has high shielding efficiency. However, this process may result in poor foaming effect due to filler sedimentation, and it involves many steps, resulting in high cost. Furthermore, it is difficult to obtain foamed sheets with a thickness of less than 1 mm, limiting its application scenarios. Summary of the Invention

[0004] In view of this, a foamed metal-based microwave absorbing shielding sheet with good foaming effect, thinness, light weight, and environmental friendliness is provided, as well as its preparation method.

[0005] To achieve the above objectives, the technical solution adopted by this invention is as follows: A method for preparing a foamed metal-based microwave absorbing shielding sheet includes the following steps: Pretreatment of metal foil shielding layer: The upper and lower surfaces of the metal foil shielding layer are subjected to plasma surface treatment respectively, and the upper and lower surfaces of the metal foil shielding layer are then coated with a conductive adhesive layer. Preparation of masterbatch for microwave absorbing foam layer: Weigh 10-15 parts by weight of thermoplastic resin matrix, 2-8 parts of elastomer, 3-7 parts of magnetic loss type microwave absorbing filler, 1-4 parts of electrical loss type microwave absorbing filler, 0-0.5 parts of antioxidant, 0-0.5 parts of crosslinking agent, 0-0.5 parts of compatibilizer and 0-1 parts of lubricant, stir mechanically until uniform, and then granulate using a three-screw extruder to obtain the masterbatch for microwave absorbing foam layer; Co-extrusion film formation of masterbatch for metal foil shielding layer and microwave absorbing foam layer: The metal foil shielding layer with conductive adhesive layer is passed through the die of co-extrusion calender by a traction machine. The masterbatch for microwave absorbing foam layer is added to the co-extrusion calender through a hopper. A supercritical CO2 and N2 mixture is injected into the barrel above the barrel before the shearing section of the co-extrusion calender by a high-pressure pump. The power of the melt metering pump after the extruder barrel is adjusted so that the supercritical CO2 and N2 mixture and the masterbatch for microwave absorbing foam layer are mixed at a predetermined pressure in the barrel. After mixing, it is extruded through the die of co-extrusion calender and coated onto the metal foil shielding layer. The pressure is reduced to remove CO2, resulting in a foamed metal-based microwave absorbing shielding sheet coated with two layers of microwave absorbing foam.

[0006] Preferably, the amount of supercritical CO2 and N2 mixture added is controlled at 0.5-15 phr, and the supercritical CO2 and N2 mixture and the masterbatch of the microwave absorbing foam layer are mixed at a pressure of 1-10 MPa in the barrel.

[0007] Preferably, the masterbatch for preparing the microwave absorbing foam layer further includes, by weight, 0-0.5 parts of antioxidant, 0-0.5 parts of crosslinking agent, 0-0.5 parts of compatibilizer, and 0-1 parts of lubricant.

[0008] Preferably, the microwave absorbing foam layer comprises a thermoplastic resin matrix, an elastomer, microwave absorbing fillers, and additives. More preferably, the thermoplastic resin matrix comprises one or a mixture of two or more of polypropylene, polycarbonate, polyarylene ether nitrile, polyphenylene sulfide, thermoplastic polyphenylene ether, polyimide, polyetherimide, polyamide, and polyarylene ether ketone; the elastomer comprises one or a mixture of two or more of thermoplastic polyurethane, thermoplastic silicone elastomer, SEBS, polyolefin rubber, polyolefin elastomer, and EPDM rubber; and the additives comprise at least one of antioxidants, lubricants, compatibilizers, and crosslinking agents.

[0009] More preferably, the microwave absorbing filler includes magnetic loss type microwave absorbing filler and electrical loss type microwave absorbing filler, and the mass ratio of the magnetic loss type microwave absorbing filler to the electrical loss type microwave absorbing filler is 5:1 to 5:3. The magnetic loss type microwave absorbing filler includes one or more of neodymium iron boron, iron silicon aluminum, carbonyl iron powder, nano ferrite, and cobalt iron alloy. The electrical loss type microwave absorbing filler includes one or more of graphene, carbon nanotubes, carbon nanofibers, acetylene black, and zinc oxide.

[0010] Preferably, the metal foil shielding layer includes one of copper foil, aluminum foil, and nickel foil.

[0011] Preferably, the conductive adhesive layer comprises, by weight, 15-30 parts of acrylate prepolymer, 5-10 parts of unsaturated olefin prepolymer, 0-10 parts of unsaturated organosilicon, 25-40 parts of copper powder, 0.5-3 parts of initiator, and 15-40 parts of ethyl acetate.

[0012] A foamed metal-based microwave absorbing shielding sheet, comprising a metal foil shielding layer, a microwave absorbing foaming layer, and a conductive adhesive layer, wherein the microwave absorbing foaming layer is attached to the upper and lower surfaces of the metal foil shielding layer through the conductive adhesive layer.

[0013] Preferably, the thickness of the single layer of the microwave absorbing foam layer is 50um to 800um, the thickness of the metal foil shielding layer is 35 to 100um, and the thickness of the conductive adhesive layer is 2 to 100um.

[0014] Compared with the prior art, the present invention has at least the following beneficial effects: First, this invention patent adopts a composite method of absorbing foam layer and metal foil shielding layer. The absorbing foam layer has good electromagnetic wave loss performance, while the metal foil shielding layer provides good electromagnetic shielding performance. It can effectively prevent excessive electromagnetic radiation from being reflected into the electronic components, which would cause signal distortion. Overall, it maintains good shielding performance while also taking into account electromagnetic wave absorption performance.

[0015] Secondly, compared with traditional polyurethane foam absorbing materials and multilayer PMI absorbing foam, this invention uses supercritical fluid extrusion foaming process. Supercritical CO2 is used as a foaming agent, which is non-toxic and recyclable, avoiding the environmental pollution problems of traditional organic foaming agents. It is green and environmentally friendly. Its foaming material has a fine and uniformly distributed cell structure, which can improve the foaming uniformity and foaming effect.

[0016] Third, the supercritical CO2 foaming technology can precisely control the microporous structure, and can prepare thin foam sheets with a thickness of only 50μm per layer. When combined with a metal foil shielding layer, it can achieve product thinning and lightness. At the same time, it can achieve a lower density compared with organic microwave absorbing foam materials, making the product lighter. Furthermore, by adjusting process parameters (pressure, temperature, fluid injection volume), a series of products with different absorption frequency bands and impedance matching can be achieved. Attached Figure Description

[0017] Figure 1 This is a schematic diagram illustrating the steps of a method for preparing a foamed metal-based microwave absorbing shielding sheet according to an embodiment of the present invention.

[0018] Figure 2 This is a schematic diagram of the cross-sectional structure of a foamed metal-based microwave absorbing shielding sheet according to an embodiment of the present invention.

[0019] Figure 2middle, 1. Wave-absorbing foam layer; 2. Metal foil shielding layer; 3. Conductive adhesive layer. Detailed Implementation

[0020] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0021] Please see Figures 1 to 2 This illustration shows an embodiment of the present invention providing a foamed metal-based microwave absorbing shielding sheet and its preparation method.

[0022] One embodiment of the present invention provides a foamed metal-based microwave absorbing shielding sheet, which includes a metal foil shielding layer 2, a microwave absorbing foam layer 1, and a conductive adhesive layer 3. The microwave absorbing foam layer 1 is attached to the upper and lower surfaces of the metal foil shielding layer 2 through the conductive adhesive layer 3. Specifically, the microwave absorbing foam layer 1 includes a thermoplastic resin matrix, an elastomer, microwave absorbing fillers, and additives.

[0023] More specifically, the thermoplastic resin matrix includes one or a mixture of two or more of polypropylene, polycarbonate, polyarylene ether nitrile, polyphenylene sulfide, thermoplastic polyphenylene ether, polyimide, polyetherimide, polyamide, and polyarylene ether ketone. More specifically, the elastomer includes one or a mixture of two or more of thermoplastic polyurethane, thermoplastic silicone elastomer, SEBS, polyolefin rubber, polyolefin elastomer, and EPDM rubber.

[0024] More specifically, the microwave absorbing filler includes magnetic loss type microwave absorbing filler and electrical loss type microwave absorbing filler, and the mass ratio of the magnetic loss type microwave absorbing filler to the electrical loss type microwave absorbing filler is 5:1 to 5:3. The magnetic loss type microwave absorbing filler includes one or more of neodymium iron boron, iron silicon aluminum, carbonyl iron powder, nano ferrite, and cobalt iron alloy. The electrical loss type microwave absorbing filler includes one or more of graphene, carbon nanotubes, carbon nanofibers, acetylene black, and zinc oxide.

[0025] More specifically, the additives include at least one of antioxidants, lubricants, compatibilizers, and crosslinking agents.

[0026] Specifically, the metal foil shielding layer 2 includes one of copper foil, aluminum foil, and nickel foil.

[0027] Specifically, the conductive adhesive layer 3 comprises, by weight, 15-30 parts of acrylate prepolymer, 5-10 parts of unsaturated olefin prepolymer, 0-10 parts of unsaturated organosilicon, 25-40 parts of copper powder, 0.5-3 parts of initiator, and 15-40 parts of ethyl acetate.

[0028] Specifically, the thickness of the single layer of the microwave absorbing foam layer 1 is 50um~800um, the thickness of the metal foil shielding layer 2 is 35~100um, and the thickness of the conductive adhesive layer 3 is 2~100um.

[0029] Another aspect of the present invention provides a method for preparing a foamed metal-based microwave absorbing shielding sheet, comprising the following steps: Pretreatment of metal foil shielding layer 2: The upper and lower surfaces of the metal foil shielding layer 2 are subjected to plasma surface treatment respectively, and the upper and lower surfaces of the metal foil shielding layer 2 are then coated with conductive adhesive layer 3. Preparation of masterbatch for microwave absorbing foam layer 1: Weigh 10-15 parts by weight of thermoplastic resin matrix, 2-8 parts by weight of elastomer, 3-7 parts by weight of magnetic loss type microwave absorbing filler, 1-4 parts by weight of electrical loss type microwave absorbing filler, 0-0.5 parts by weight of antioxidant, 0-0.5 parts by weight of crosslinking agent, 0-0.5 parts by weight of compatibilizer and 0-1 parts by weight of lubricant, stir mechanically until uniform, and then granulate using a three-screw extruder to obtain masterbatch for microwave absorbing foam layer 1; The masterbatch of metal foil shielding layer 2 and microwave absorbing foam layer 1 is co-extruded into a film: the metal foil shielding layer 2, which is coated with conductive adhesive layer 3, is passed through the die of co-extrusion calender by a traction machine. The masterbatch of microwave absorbing foam layer 1 is added to the co-extrusion calender through a hopper. A supercritical CO2 and N2 mixture is injected into the barrel above the barrel before the shearing section of the co-extrusion calender by a high-pressure pump. The amount of supercritical CO2 and N2 mixture added is controlled at 0.5-15 phr. The power of the melt metering pump after the extruder barrel is adjusted so that the supercritical CO2 and N2 mixture and the masterbatch of microwave absorbing foam layer 1 are mixed at a pressure of 1~10 MPa in the barrel. After mixing, it is extruded through the die of co-extrusion calender and coated onto the metal foil shielding layer 2. The pressure is reduced to remove CO2 and obtain a foamed metal-based microwave absorbing shielding sheet coated with two layers of microwave absorbing foam layer 1.

[0030] Specifically, the two steps of pretreatment of metal foil shielding layer 2 and preparation of masterbatch for microwave absorbing foam layer 1 are independent and have no order of precedence. They can be carried out simultaneously. If they are carried out in steps, the pretreatment of metal foil shielding layer 2 can be carried out first, or the masterbatch for microwave absorbing foam layer 1 can be prepared first. After the pretreatment of metal foil shielding layer 2 and the preparation of masterbatch for microwave absorbing foam layer 1 are completed, the metal foil shielding layer 2 and the masterbatch for microwave absorbing foam layer 1 are co-extruded into a film.

[0031] The following examples illustrate the steps of preparing foamed metal-based microwave absorbing shielding sheets, as well as the performance of the prepared sheets.

[0032] In all embodiments, the preparation of conductive adhesive layer 3 requires weighing 10 parts of methyl acrylate, 5 parts of methyl methacrylate, 10 parts of ethyl acrylate, 8 parts of styrene, 5 parts of vinyl silane, 30 parts of copper powder, 1.5 parts of azobisisobutyronitrile, and 20 parts of ethyl acetate.

[0033] Example 1 (1) Copper foil pretreatment A 30µm thick, 400mm wide rolled copper foil roll undergoes two plasma surface treatments on both the top and bottom surfaces using a plasma nano-coating machine. After treatment, a conductive adhesive layer is applied by a metal foil wire rod coating machine, followed by a semi-curing treatment at 80℃. Each area passes through a heating chamber for 3 minutes.

[0034] (2) Preparation of microwave absorbing foam masterbatch Weigh out 10 parts by weight of polypropylene, 2 parts of EPDM rubber, 5 parts of carbonyl iron powder, 2 parts of acetylene black, 0.1 parts of benzoyl peroxide, 0.2 parts of dibenzothiazole disulfide, and 0.5 parts of Irganox 1076 antioxidant. After mechanically mixing evenly, granulate using a three-screw extruder to obtain the microwave absorbing foam masterbatch for the microwave absorbing foam layer. The temperature of the plasticizing section of the barrel is 180℃.

[0035] (3) Co-extrusion of copper foil and microwave absorbing foam masterbatch into film The metal foil shielding layer coated with a high-temperature conductive adhesive layer is passed through a specially made impregnation mold of a polymer extrusion calender by a traction machine. The microwave absorbing foaming masterbatch prepared in step (2) is added to the polymer casting machine through a hopper. A supercritical CO2 and N2 mixture with a mass ratio of 3:1 is injected into the barrel above the barrel before the shearing section of the co-extrusion calender through a high-pressure pump. The injection volume of supercritical fluid is controlled to be about 5 phr by adjusting the feeding speed and the supercritical fluid injection speed. The power of the melt metering pump after the extruder barrel is adjusted so that the supercritical fluid mixture and the microwave absorbing foaming masterbatch are mixed at a pressure of about 7 MPa in the barrel. The mixture is then extruded into the mold and coated with metal foil. The temperature of the extrusion plasticizing section is 190℃ and the die head temperature is 180℃. After exiting the mold, the metal foil is immediately formed by a 40℃ pressure roller. After exiting the roller, the remaining CO2 escapes, resulting in a copper foil foamed microwave absorbing sheet with two layers of fine-pore microwave absorbing foaming layers, with a thickness of 1.4 mm. The sheet exhibits a reflection loss of less than -10dB within the 10.1-14.9 GHz range, an effective absorption bandwidth of 4.8 GHz, an optimal reflection loss of -47.4dB, and an overall shielding effectiveness of 74dB.

[0036] Example 2 (1) Copper foil pretreatment A 30µm thick, 400mm wide rolled copper foil roll undergoes two plasma surface treatments on both the top and bottom surfaces using a plasma nano-coating machine. After treatment, a conductive adhesive layer is applied by a metal foil wire rod coating machine, followed by a semi-curing treatment at 80℃. Each area passes through a heating chamber for 3 minutes.

[0037] (2) Preparation of microwave absorbing foam masterbatch Weigh out 10 parts by weight of polypropylene, 2 parts of EPDM rubber, 5 parts of carbonyl iron powder, 2 parts of acetylene black, 0.1 parts of benzoyl peroxide, 0.2 parts of dibenzothiazole disulfide, and 0.5 parts of Irganox 1076 antioxidant. After mechanically mixing evenly, granulate using a three-screw extruder to obtain the microwave absorbing foam masterbatch for the microwave absorbing foam layer. The temperature of the plasticizing section of the barrel is 180℃.

[0038] (3) Co-extrusion of copper foil and microwave absorbing foam masterbatch into film The metal foil shielding layer coated with a high-temperature conductive adhesive layer is passed through a specially made impregnation mold of a polymer extrusion calender by a traction machine. The microwave absorbing foaming masterbatch prepared in step (2) is added to the polymer casting machine through a hopper. A supercritical CO2 and N2 mixture with a mass ratio of 3:1 is injected into the barrel above the barrel before the shearing section of the co-extrusion calender through a high-pressure pump. The injection volume of supercritical fluid is controlled to be about 1 phr by adjusting the feeding speed and the supercritical fluid injection speed. The power of the melt metering pump after the extruder barrel is adjusted so that the supercritical fluid mixture and the microwave absorbing foaming masterbatch are mixed at a pressure of about 5 MPa in the barrel. The mixture is then extruded into the mold and coated with metal foil. The temperature of the extrusion plasticizing section is 190℃ and the die temperature is 180℃. After exiting the mold, the metal foil is immediately formed by a 40℃ pressure roller. After exiting the roller, the remaining CO2 escapes, resulting in a copper foil foamed microwave absorbing sheet with two layers of fine-pore microwave absorbing foaming layers, with a thickness of 1.4 mm. The sheet exhibits a reflection loss of less than -10dB in the 7.6-12.1 GHz range, an effective absorption bandwidth of 4.5 GHz, an optimal reflection loss of -29.5dB, and an overall shielding effectiveness of 65dB.

[0039] Example 3 (1) Preparation of microwave absorbing foam masterbatch Weigh out 10 parts by weight of polypropylene, 2 parts of EPDM rubber, 5 parts of carbonyl iron powder, 2 parts of acetylene black, 0.1 parts of benzoyl peroxide, 0.2 parts of dibenzothiazole disulfide, and 0.5 parts of Irganox 1076 antioxidant. After mechanically mixing evenly, granulate using a three-screw extruder to obtain the microwave absorbing foam masterbatch for the microwave absorbing foam layer. The temperature of the plasticizing section of the barrel is 180℃.

[0040] (2) Extrusion film formation of microwave absorbing foaming masterbatch Microwave-absorbing foam masterbatch is fed into a polymer casting machine via a hopper. A supercritical CO2 to N2 mixture (3:1 mass ratio) is injected into the barrel above the shearing section of the co-extrusion calender via a high-pressure pump. The injection rate of the supercritical fluid is controlled to approximately 5 phr by adjusting the feeding speed and the supercritical fluid injection speed. The power of the melt metering pump after the extruder barrel is adjusted to maintain a pressure of approximately 7 MPa between the supercritical fluid mixture and the microwave-absorbing foam masterbatch within the barrel. The extrusion plasticizing section temperature is 190℃, and the die temperature is 180℃. Immediately after demolding, the material is rolled using a 40℃ pressure roller. Residual CO2 escapes after exiting the roller, resulting in a 1.4mm thick, foil-free, fine-pore microwave-absorbing foam sheet. This sheet exhibits a reflection loss of less than -10dB in the 11.1-14.2 GHz range, an effective absorption bandwidth of 3.1 GHz, an optimal reflection loss of -51.6dB, and an overall shielding effectiveness of 40dB.

[0041] Example 4 (1) Copper foil pretreatment A 30µm thick, 400mm wide rolled copper foil roll undergoes two plasma surface treatments on both the top and bottom surfaces using a plasma nano-coating machine. After treatment, a conductive adhesive layer is applied by a metal foil wire rod coating machine, followed by a semi-curing treatment at 80℃. Each area passes through a heating chamber for 5 minutes.

[0042] (2) Preparation of microwave absorbing foam masterbatch Weigh out 8 parts PA6, 4 parts POE rubber, 5 parts carbonyl iron powder, 2 parts acetylene black, 0.1 parts silica, 0.2 parts PE wax, and 0.5 parts Irganox 1076 antioxidant by weight. After mechanically mixing evenly, granulate using a three-screw extruder to obtain the microwave absorbing foam masterbatch for the microwave absorbing foam layer. The temperature of the plasticizing section of the barrel is 240℃.

[0043] (3) Co-extrusion of copper foil and microwave absorbing foam masterbatch into film The metal foil shielding layer coated with a high-temperature conductive adhesive layer is passed through a specially made impregnation mold of a polymer extrusion calender by a traction machine. The microwave absorbing foaming masterbatch prepared in step (2) is added to the polymer casting machine through a hopper. A supercritical CO2 and N2 mixture with a mass ratio of 3:1 is injected into the barrel above the barrel before the shearing section of the co-extrusion calender through a high-pressure pump. The injection volume of supercritical fluid is controlled to be about 5 phr by adjusting the feeding speed and the supercritical fluid injection speed. The power of the melt metering pump after the extruder barrel is adjusted so that the supercritical fluid mixture and the microwave absorbing foaming masterbatch are mixed at a pressure of about 7 MPa in the barrel. The mixture is then extruded into the mold and coated onto the metal foil. The temperature of the extrusion plasticizing section is 240℃ and the temperature of the die head is 245℃. After exiting the mold, the metal foil is immediately formed by a 60℃ pressure roller. After exiting the roller, the remaining CO2 escapes, resulting in a copper foil foamed microwave absorbing sheet with two layers of fine-pore microwave absorbing foaming layers, with a thickness of 1.4 mm. The sheet exhibits a reflection loss of less than -10dB in the 9.5-13.6 GHz range, an effective absorption bandwidth of 4.1 GHz, an optimal reflection loss of -42.5dB, and an overall shielding effectiveness of 76dB.

[0044] Example 5 (1) Copper foil pretreatment A 30µm thick, 400mm wide rolled copper foil roll undergoes two plasma surface treatments on both the top and bottom surfaces using a plasma nano-coating machine. After treatment, a conductive adhesive layer is applied by a metal foil wire rod coating machine, followed by a semi-curing treatment at 80℃. Each area passes through a heating chamber for 5 minutes.

[0045] (2) Preparation of microwave absorbing foam masterbatch Weigh out 8 parts PA6, 4 parts POE rubber, 5 parts carbonyl iron powder, 2 parts acetylene black, 0.1 parts silica, 0.2 parts PE wax, and 0.5 parts Irganox 1076 antioxidant by weight, mix them evenly by mechanical mixing, and then granulate them using a three-screw extruder to obtain the microwave absorbing foam masterbatch for the microwave absorbing foam layer.

[0046] (3) Co-extrusion of copper foil and microwave absorbing foam masterbatch into film The metal foil shielding layer coated with a high-temperature conductive adhesive layer is passed through a specially made impregnation mold of a polymer extrusion calender by a traction machine. The microwave absorbing foaming masterbatch prepared in step (2) is added to the polymer casting machine through a hopper. A supercritical CO2 and N2 mixture with a mass ratio of 3:1 is injected above the barrel before the shearing section of the extrusion calender barrel by a high-pressure pump. The injection volume of supercritical fluid is controlled to be about 1 phr by adjusting the feeding speed and the supercritical fluid injection speed. The power of the melt metering pump after the extruder barrel is adjusted so that the supercritical fluid mixture and the microwave absorbing foaming masterbatch are mixed at a pressure of about 7 MPa in the barrel. The mixture is then extruded into the mold and coated with metal foil. The temperature of the extrusion plasticizing section is 240℃ and the temperature of the die head is 245℃. After exiting the mold, the metal foil is immediately formed by a pressure roller with a certain temperature. After exiting the roller, the remaining CO2 escapes, resulting in a copper foil foamed microwave absorbing sheet with two layers of fine-pore microwave absorbing foaming layers, with a thickness of 1.4 mm. The sheet exhibits a reflection loss of less than -10dB in the 7.9-13.2 GHz range, an effective absorption bandwidth of 5.3 GHz, an optimal reflection loss of -31.2dB, and an overall shielding effectiveness of 68dB.

[0047] Table 1. Effective absorption band, effective absorption bandwidth, optimal reflection loss value, and shielding effectiveness of Examples 1 to 5

[0048] It should be noted that the present invention is not limited to the above-described embodiments. Based on the inventive spirit of the present invention, those skilled in the art can make other changes, and these changes made in accordance with the inventive spirit of the present invention should be included within the scope of protection claimed by the present invention.

Claims

1. A method for preparing a foamed metal-based microwave absorbing shielding sheet, characterized in that, The method for preparing the foamed metal-based microwave absorbing shielding sheet includes the following steps: Pretreatment of metal foil shielding layer: The upper and lower surfaces of the metal foil shielding layer are subjected to plasma surface treatment respectively, and the upper and lower surfaces of the metal foil shielding layer are then coated with a conductive adhesive layer. Preparation of masterbatch for microwave absorbing foam layer: Weigh 10-15 parts by weight of thermoplastic resin matrix, 2-8 parts by weight of elastomer, 3-7 parts by weight of magnetic loss type microwave absorbing filler, 1-4 parts by weight of electrical loss type microwave absorbing filler, 0-0.5 parts by weight of antioxidant, 0-0.5 parts by weight of crosslinking agent, 0-0.5 parts by weight of compatibilizer and 0-1 parts by weight of lubricant, stir mechanically until uniform, and then granulate using a three-screw extruder to obtain masterbatch for microwave absorbing foam layer; Co-extrusion film of masterbatch for metal foil shielding layer and microwave absorbing foam layer: The metal foil shielding layer with conductive adhesive layer is passed through the die of co-extrusion mill by traction machine. The masterbatch for microwave absorbing foam layer is added to co-extrusion mill through hopper. A supercritical CO2 and N2 mixture is injected into the top of the barrel before the shearing section of the barrel of co-extrusion mill by high pressure pump. The power of melt metering pump after the extruder barrel is adjusted so that the supercritical CO2 and N2 mixture and the masterbatch for microwave absorbing foam layer are mixed at a predetermined pressure in the barrel. After mixing, it is extruded through the die of co-extrusion mill and coated onto the metal foil shielding layer. The pressure is reduced to obtain a foamed metal-based microwave absorbing shielding sheet coated with upper and lower microwave absorbing foam layers. The microwave absorbing filler includes magnetic loss type microwave absorbing filler and electrical loss type microwave absorbing filler, and the mass ratio of the magnetic loss type microwave absorbing filler to the electrical loss type microwave absorbing filler is 5:1 to 5:

3. The magnetic loss type microwave absorbing filler includes one or more of neodymium iron boron, iron silicon aluminum, carbonyl iron powder, nano ferrite, and cobalt iron alloy. The electrical loss type microwave absorbing filler includes one or more of graphene, carbon nanotubes, carbon nanofibers, acetylene black, and zinc oxide. The conductive adhesive layer comprises, by weight, 15-30 parts of acrylate prepolymer, 5-10 parts of unsaturated olefin prepolymer, 0-10 parts of unsaturated organosilicon, 25-40 parts of copper powder, 0.5-3 parts of initiator, and 15-40 parts of ethyl acetate.

2. The method for preparing a foamed metal-based microwave absorbing shielding sheet as described in claim 1, characterized in that, The amount of supercritical CO2 and N2 mixture added is controlled at 0.5-15 phr, and the supercritical CO2 and N2 mixture and the masterbatch of the microwave absorbing foam layer are mixed in the barrel at a pressure of 1-10 MPa.

3. The method for preparing a foamed metal-based microwave absorbing shielding sheet as described in claim 1, characterized in that, The thermoplastic resin matrix includes one or a mixture of two or more of polypropylene, polycarbonate, polyarylether nitrile, polyphenylene sulfide, thermoplastic polyphenylene ether, polyimide, polyetherimide, polyamide, and polyarylether ketone; the elastomer includes one or a mixture of two or more of thermoplastic polyurethane, thermoplastic silicone elastomer, SEBS, polyolefin rubber, polyolefin elastomer, and EPDM rubber.

4. The method for preparing a foamed metal-based microwave absorbing shielding sheet as described in claim 1, characterized in that, The metal foil shielding layer includes one of copper foil, aluminum foil, and nickel foil.

5. A foamed metal-based microwave absorbing shielding sheet, prepared by the method according to any one of claims 1-4, characterized in that, The foamed metal-based microwave absorbing shielding sheet includes a metal foil shielding layer, a microwave absorbing foaming layer, and a conductive adhesive layer. The microwave absorbing foaming layer is attached to the upper and lower surfaces of the metal foil shielding layer through the conductive adhesive layer.

6. The foamed metal-based microwave absorbing shielding sheet as described in claim 5, characterized in that, The thickness of the single layer of the microwave absorbing foam layer is 50µm~800µm, the thickness of the metal foil shielding layer is 35~100µm, and the thickness of the conductive adhesive layer is 2~100µm.

Images