High-flame-retardant expanded graphite polymer-containing foam wave-absorbing material

Abstract

Belonging to the technical field of wave-absorbing materials, the invention provides a high flame retardant expanded graphite-containing polymer foam wave-absorbing material, which comprises, by volume, 0-90 parts of conductive polymer foam particles and 10-100 parts of expanded graphite-containing composite resin foam particles. The expanded graphite-containing composite resin foam particles comprise 30-95 parts of conductive polymer foam particles and 5-70 parts of coating paint. The phosphorus-based flame retardant and the expanded graphite have a synergistic effect in the polymer foam wave-absorbing material, and the expanded graphite coats the surfaces of conductive polymer foam particles in an adhesive form to obtain composite resin foam particles containing the expanded graphite; the expanded graphite-containing composite resin foam particles and the conductive polymer foam particles are mixed according to a certain proportion and are molded into the wave-absorbing material, so that the problem that the oxygen index required by a polymer foam wave-absorbing material for an electromagnetic wave darkroom is larger than 28, particularly the oxygen index is larger than 32, and the indexes 1, 2 and 3 of the NRL Report 8093 standard are met is solved.

Description

Technical Field

[0001] This invention relates to the field of flame retardant technology for polymer foam microwave absorbing materials with improved oxygen index, and to a polypropylene foam and polyethylene foam microwave absorbing material that meets the oxygen index requirement of greater than 28, and more particularly to a polypropylene foam that meets the oxygen index requirement of greater than 32 and meets the first, second, and third indicators of NRL Report 8093 standard, as well as the preparation method thereof. Background Technology

[0002] In the application fields of electromagnetic anechoic chambers and military-grade absorbing materials, the requirements for the flame retardant performance and internal environment of electromagnetic anechoic chambers are very high. At present, traditional electromagnetic anechoic chambers are still mainly based on sponge absorbing materials. Sponges have a high water absorption rate and a limited service life. Foamed polystyrene absorbing materials are not environmentally friendly due to the fact that styrene is carcinogenic and belongs to the International Agency for Research on Cancer (IARC) Group 2B. New foamed polypropylene and foamed polyethylene absorbing materials have attracted market attention and application due to their cleanliness, environmental friendliness and low water absorption rate. The only drawback is their poor flame retardant performance.

[0003] According to the requirements of the Technical Specification for Electromagnetic Anechoic Chamber Engineering (GB50826-2012), the oxygen index of the absorbing material in the electromagnetic anechoic chamber must be greater than 28. In many research institutes and high-power testing applications, customers require the oxygen index to be greater than 32. Furthermore, the US Navy standard NRL Report 8093 requires the absorbing material for electromagnetic anechoic chambers to meet the requirements of items 1, 2, and 3 of this standard.

[0004] Currently available polypropylene foam absorbing materials rarely meet these two standards. Patent number 201710970210.7, entitled "A Method for Preparing Highly Conductive and Highly Flame-Retardant Polypropylene Foamed Beads," states that in the examples, a board with a resistance of 800-1350 ohms can achieve an oxygen index of 28. While increasing the pressing density of the prepared board samples can slightly improve the oxygen index, it is impossible to achieve an oxygen index greater than 32 when producing pyramidal absorbing materials. In the patent embodiment .7, the planar absorbing material exhibits a certain absorption effect at specific frequencies of 200MHz-1.2GHz. Based on the impedance matching design of the absorbing material, satisfying 201710970210.7, the patent embodiment requires at least a 500-700mm high pyramidal absorbing material, or a soft magnetic ferrite sheet absorbing material, and only the top few frequencies of the resonant peak on the 200MHz-1.2GHz absorption rate curve are sufficient to achieve this. Although 201710970210... The thickness of the sheet material in the 7th embodiment is not described, but regardless of the thickness of the single-layer resistive loss type sheet material, it cannot meet the absorption rate of 28 at 200 MHz and 69 at 400 MHz. The same is true for other embodiments. Obviously, this patented method can only be applied to specific frequencies and cannot meet the requirements for wide-band absorption rate and flame retardancy of pyramidal absorbing materials for electromagnetic anechoic chambers. In the patent application with patent number CN202110787462.2, the absorption rate of the single-layer sheet material at 200 MHz is ≥37. Similarly, the principle is that no single-layer resistive loss type absorbing material sheet material of any thickness can achieve such an index. Therefore, this patented method can only be applied to specific frequencies. If pyramidal absorbing materials are to be produced, the oxygen index cannot reach 28, let alone greater than 32. Therefore, this method also cannot meet the requirements for wide-band absorption rate and flame retardancy of pyramidal absorbing materials for electromagnetic anechoic chambers. Foamed polypropylene or polyethylene foam microwave absorbing materials are made by mixing conductive agents, flame retardants, and nucleating agents and then granulating them. The foaming process is carried out using supercritical carbon dioxide. To prioritize the absorption of electromagnetic waves, a relatively large amount of conductive agent needs to be added. If the material can still be molded by a foam molding machine after foaming, the proportion of flame retardant that can be added is relatively small. Therefore, it is difficult for the oxygen index of the final polypropylene or polyethylene foam microwave absorbing material to be higher than 28. Summary of the Invention

[0005] The purpose of this invention is to overcome and supplement the shortcomings of the existing technology, and to provide a high flame-retardant polymer foam absorbing material containing expanded graphite. Due to the property that expanded graphite expands several times when heated, it isolates oxygen after expansion, making it an excellent flame-retardant material. However, the particles of expanded graphite are generally relatively large and cannot be melted into polymers and then foamed like other powder materials. Therefore, highly filled expanded graphite is coated onto the surface of polypropylene or polyethylene foam particles in the form of an adhesive. Due to the poor adhesion of the adhesive to the surface of polypropylene or polyethylene foam particles, the prepared conductive polymer foam particles are pretreated with an adhesion promoter and then mixed with expanded graphite-containing composite resin foam particles to prepare a polymer foam absorbing material for electromagnetic anechoic chambers. The material requires an oxygen index greater than 28, especially an oxygen index greater than 32, and meets the 1st, 2nd, and 3rd indicators of the NRLReport 8093 standard.

[0006] The technical solution adopted in this invention is: A highly flame-retardant wave-absorbing material containing expanded graphite polymer foam, wherein, by volume, it comprises 0-90 parts of conductive polymer foam particles and 10-100 parts of expanded graphite composite resin foam particles.

[0007] Preferably, the high flame-retardant expanded graphite polymer foam absorbing material comprises, by weight, 30-95 parts of conductive polymer foam particles and 5-70 parts of coating material, wherein the coating material comprises 5-50 parts of adhesive, 1-30 parts of conductive agent, 0-20 parts of first flame retardant, 20-95 parts of expanded graphite, and 13-300 parts of solvent.

[0008] Preferably, the high flame-retardant expanded graphite polymer foam absorbing material comprises: the conductive polymer foam particles selected from polypropylene foam particles and polyethylene foam particles, wherein each polypropylene foam particle or polyethylene foam particle includes at least one polypropylene resin or polyethylene resin, and each polypropylene resin or polyethylene resin comprises the following components by weight: 45-95 parts of polypropylene or polyethylene, 5-35 parts of conductive agent, 1-10 parts of dispersant, 1-20 parts of first flame retardant, and 0.1-1 parts of nucleating agent.

[0009] Preferably, in the high flame-retardant expanded graphite polymer foam microwave absorbing material: the conductive agent is selected from one or more of carbon black, graphite, graphene, carbon nanotubes or metal powder; the first flame retardant is selected from one or more of coated red phosphorus, ammonium polyphosphate, aluminum dihydrogen phosphate, pentaerythritol phosphate, and brominated polystyrene.

[0010] Preferably, in the high flame-retardant expanded graphite polymer foam microwave absorbing material, the dispersant is selected from one or two of PE wax and triphenyl phosphate.

[0011] Preferably, in the high flame-retardant expanded graphite polymer foam microwave absorbing material, the adhesive is selected from one or more of phenolic resin, acrylic resin, ethylene-acrylic acid copolymer, polyvinyl acetate, polyurethane adhesive and silicone resin.

[0012] Preferably, in the high flame-retardant expanded graphite polymer foam absorbing material, the outer surface of the conductive polymer foam particles is further coated with an adhesion promoter, and the mass ratio of the conductive polymer foam particles to the adhesion promoter is 80-99:1-20.

[0013] Preferably, in the high flame-retardant expanded graphite polymer foam absorbing material, the adhesion promoter is a chlorinated polypropylene solution.

[0014] A method for preparing a highly flame-retardant expanded graphite polymer foam microwave absorbing material, comprising the following steps: Step S1: By weight, 45-95 parts of polypropylene or polyethylene, 5-35 parts of conductive agent, 1-10 parts of dispersant, and 1-20 parts of first flame retardant are mixed evenly to obtain a first mixture. Then, the first mixture is mixed in an internal mixer and extruded through a twin-screw extruder to obtain at least 0.3-2 mg of polypropylene resin or polyethylene resin. Step S2: Add at least one polypropylene resin or polyethylene resin from step S1 to a high-pressure reactor, and add water and surfactant to the high-pressure reactor. Then start stirring, heat and introduce nitrogen or carbon dioxide. Continue heating until the melting point of the polypropylene resin or polyethylene resin is reached, and release the material to foam and obtain polypropylene foam particles or polyethylene foam particles, thus obtaining conductive polymer foam particles. Step S3: By weight, add 5-50 parts of adhesive, 1-30 parts of conductive agent, 1-30 parts of first flame retardant, 20-95 parts of expanded graphite and 13-300 parts of solvent to a disperser and disperse evenly to obtain a coating. Step S4: By weight, add 30-95 parts of conductive polymer foam particles from step S3 to a mixing tank, then add 5-70 parts of coating material from S4 and stir to obtain a second mixture. Add the second mixture to a drying device to dry and sieve to obtain foam particles containing expanded graphite composite resin. Step S5: By volume, 0-90 parts of conductive polymer foam particles and 10-100 parts of expanded graphite composite resin foam particles are mixed to obtain a third mixture, and then the third mixture is molded to obtain a microwave absorbing material.

[0015] A method for preparing a highly flame-retardant expanded graphite polymer foam microwave absorbing material, comprising the following steps by weight: Step S1: By weight, 45-95 parts of polypropylene or polyethylene, 5-35 parts of conductive agent, 1-10 parts of dispersant, and 1-20 parts of first flame retardant are mixed evenly to obtain a first mixture. Then, the first mixture is mixed in an internal mixer and extruded through a twin-screw extruder to obtain at least 0.3-2 mg of polypropylene resin or polyethylene resin. Step S2: Add at least one polypropylene resin or polyethylene resin from step S1 to a high-pressure reactor, and add water and surfactant to the high-pressure reactor. Then start stirring, heat and introduce nitrogen or carbon dioxide. Continue heating until the melting point of polypropylene resin or polyethylene resin is reached, and release the material to foam and obtain polypropylene foam particles and polyethylene foam particles, thus obtaining conductive polymer foam particles. Step S3: By weight, mix 80-99 parts of conductive polymer foam particles from step S2 with 1-20 parts of adhesion promoter and add them to an air-drying device to dry, thereby obtaining composite foam particles; Step S4: By weight, add 5-50 parts of adhesive, 1-30 parts of conductive agent, 1-30 parts of first flame retardant, 20-95 parts of expanded graphite and 13-300 parts of solvent to a disperser and disperse evenly to obtain a coating. Step S5: By weight, add 30-95 parts of the composite foam particles from step S3 to a mixing tank, then add 5-70 parts of the coating material from S4 and stir to obtain a second mixture. Add the second mixture to a drying equipment to dry, and sieve to obtain foam particles containing expanded graphite composite resin. Step S6: Mix 0-90 parts of composite foam particles and 10-100 parts of expanded graphite composite resin foam particles by volume to obtain a third mixture, and then mold the third mixture to obtain a microwave absorbing material.

[0016] Advantages of this invention: (1) In this invention, red phosphorus and expanded graphite work synergistically. As a flame retardant, red phosphorus burns upon heating to produce phosphorus pentoxide. When the polymer burns at high temperatures, the expanded graphite adsorbs the sulfuric acid compounds in the graphite intercalation and begins to decompose. The expanded graphite flakes begin to expand vertically, expanding to hundreds of times their original size to form graphite worms. The sulfuric acid compounds decompose to produce SO2, CO2, and water. The combustion of polypropylene or polyethylene produces CO2 and water. The phosphorus pentoxide generated by the combustion of red phosphorus captures the decomposition reaction of sulfuric acid compounds in the expanded graphite intercalation and the water vapor molecules generated by the combustion of the polymer, forming phosphate hydrate H3PO4. During the combustion of the polymer, the expanded graphite expands to form graphite worms that interweave and support the surface of the burning polymer. The polymer surface is catalyzed by the acid formed by the combustion of red phosphorus to form carbon, and the inorganic powders such as carbon black and graphite in the polymer are combined with the phosphate hydrate H3PO4 formed by the combustion of red phosphorus. The thick oily substance binds these substances together to form a three-dimensional and dense oxygen-isolated structure. Due to the interlacing support of expanded graphite worms on the polymer surface and the thick oily substance of phosphate hydrate H3PO4 formed by the combustion of red phosphorus to form an oxygen-isolated three-dimensional structure, this synergistic effect can effectively prevent the entry of oxygen, thereby improving the flame retardant effect of the microwave absorbing material. The polymer foam microwave absorbing material for electromagnetic anechoic chambers requires an oxygen index greater than 28. The microwave absorbing material of this invention can actually achieve an oxygen index greater than 32, which meets the 1st, 2nd and 3rd indicators of the NRL Report 8093 standard.

[0017] (2) The present invention uses a pressure mixer to mix the conductive agent and various powder additives and then granulate them. Due to the accurate formula ratio, the stability and batch consistency of the dielectric constant and loss tangent of the electromagnetic parameters of the absorbing material can be ensured.

[0018] (3) The first flame retardant in the coating of the present invention is 0-20 parts. Since expanded graphite is conductive, it is necessary to adjust the electromagnetic parameters of each layer of material when producing multi-layer composite microwave absorbing materials. When producing high-frequency, ultra-high-frequency or terahertz microwave absorbing materials, it is necessary to adjust the electromagnetic parameters and reduce the proportion of expanded graphite. The non-conductive first flame retardant replaces part of the expanded graphite to adjust the electromagnetic parameters. Although the flame retardant effect is not as good as that of high-filled expanded graphite, it can still meet the flame retardant requirement of the basic flame retardant index of microwave absorbing materials being greater than 28.

[0019] (4) The high flame-retardant polymer foam absorbing material containing expanded graphite of the present invention can replace the traditional polyurethane sponge absorbing material. After the electromagnetic anechoic chamber is dismantled, the polymer foam absorbing material can be recycled, melted and reprocessed into other plastic products. The traditional polyurethane sponge absorbing material can only be incinerated after it is dismantled and scrapped. The polymer foam absorbing material of the present invention can reduce environmental pollution. Its low water absorption rate and high oxygen index characteristics can also be applied in outdoor and military camouflage fields, and its application prospects are relatively broad. Attached Figure Description

[0020] Figure 1 SEM images of the expanded graphite composite resin foam particles prepared in Examples 1-9.

[0021] Figure 2 The images shown are SEM images of the expanded graphite composite resin foam particles prepared in Examples 1-9 after being cut open.

[0022] Figure 3 This is a schematic diagram of the structure of the 500 high-absorbing pyramidal microwave absorbing material prepared in Examples 1-9 and Comparative Examples 1-2 of the present invention. Detailed Implementation

[0023] The present invention will be further described below with reference to specific accompanying drawings and embodiments.

[0024] Example 1 A method for preparing a highly flame-retardant microwave absorbing material containing expanded graphite polymer foam includes the following steps: Step S1: By weight, 62.9 parts of polypropylene, 25 parts of conductive carbon black, 2 parts of PE wax, 0.1 parts of zinc borate, and 8 parts of coated red phosphorus are mixed evenly to obtain a first mixture. Then, the first mixture is mixed in an internal mixer and extruded through a twin-screw extruder. The twin-screw extruder has eleven stages, and the extrusion temperatures are as follows: stage 1 140°C, stage 2 150°C, stage 3 165°C, stage 4 175°C, stage 5 180°C, stage 6 185°C, stage 7 192°C, stage 8 185°C, stage 9 192°C, stage 10 185°C, stage 11 192°C, and die 210°C, to obtain 0.3-2 mg of polypropylene resin. Step S2: Add the polypropylene resin obtained in Step S1 to a high-pressure reactor. Add water equal to twice the weight of the added polypropylene resin and sodium dodecylbenzenesulfonate at 3% of the weight of the added water to the high-pressure reactor. Start stirring, heat, and introduce carbon dioxide. The pressure is 3.5 MPa, and the reactor temperature is 140℃. Discharge the material, foam it, wash it, and dry it to obtain a density of 45 kg / m³. 3 Polypropylene foam particles were used to obtain conductive polymer foam particles. Step S3: By weight, add 28 parts of polyvinyl acetate, 5 parts of conductive carbon black, 67 parts of 80-mesh expanded graphite and 60 parts of ethanol to a disperser and disperse evenly. The disperser speed is 1500 rpm and the dispersion time is 30 minutes to obtain the coating. Step S4: By weight, add 20kg of conductive polymer foam particles obtained in step S3 to a mixing tank, then add 30kg of coating material and stir to obtain a second mixture. The stirring speed is 80 rpm. The second mixture is then added to a drying device for drying. The main shaft crushing motor is set to 30 rpm. The drying and crushing process takes 20 minutes. The mixture is then sieved to obtain foam particles containing expanded graphite composite resin. Step S5: By volume, the 50L conductive polymer foam particles and 50L expanded graphite composite resin foam particles obtained in step S3 are mixed to obtain a third mixture, and then the third mixture is molded to obtain a 500 high-angle pyramidal microwave absorbing material.

[0025] Example 2 A method for preparing a highly flame-retardant microwave absorbing material containing expanded graphite polymer foam includes the following steps: Step S1: By weight, 62.9 parts of polypropylene, 25 parts of conductive carbon black, 2 parts of PE wax, 0.1 parts of zinc borate, and 8 parts of coated red phosphorus are mixed evenly to obtain a first mixture. Then, the first mixture is mixed in an internal mixer and extruded through a twin-screw extruder. The twin-screw extruder has eleven stages, and the extrusion temperatures are as follows: stage 1 140°C, stage 2 150°C, stage 3 165°C, stage 4 175°C, stage 5 180°C, stage 6 185°C, stage 7 192°C, stage 8 185°C, stage 9 192°C, stage 10 185°C, stage 11 192°C, and die 210°C, to obtain 0.3-2 mg of polypropylene resin. Step S2: Add the polypropylene resin obtained in Step S1 to a high-pressure reactor. Add water equal to twice the weight of the added polypropylene resin and sodium dodecylbenzenesulfonate at 3% of the weight of the added water to the high-pressure reactor. Start stirring, heat, and introduce carbon dioxide. The pressure is 3.5 MPa, and the reactor temperature is 140℃. Discharge the material, foam it, wash it, and dry it to obtain a density of 45 kg / m³. 3 Polypropylene foam particles were used to obtain conductive polymer foam particles. Step S3: By weight, add 28 parts of polyvinyl acetate, 5 parts of conductive carbon black, 67 parts of 80-mesh expanded graphite and 60 parts of ethanol to a disperser and disperse evenly. The disperser speed is 1500 rpm and the dispersion time is 30 minutes to obtain the coating. Step S4: By weight, add 20kg of conductive polymer foam particles obtained in step S3 to a mixing tank, then add 30kg of coating material and stir to obtain a second mixture. The stirring speed is 80 rpm. The second mixture is then added to a drying device for drying. The main shaft crushing motor is set to 30 rpm. The drying and crushing process takes 20 minutes. The mixture is then sieved to obtain foam particles containing expanded graphite composite resin. Step S5: By volume, the 60L conductive polymer foam particles and 40L expanded graphite composite resin foam particles obtained in step S3 are mixed to obtain a third mixture, and then the third mixture is molded to obtain a 500 high-angle pyramidal microwave absorbing material.

[0026] Example 3 A method for preparing a highly flame-retardant microwave absorbing material containing expanded graphite polymer foam includes the following steps: Step S1: By weight, 62.9 parts of polypropylene, 25 parts of conductive carbon black, 2 parts of PE wax, 0.1 parts of zinc borate, and 8 parts of coated red phosphorus are mixed evenly to obtain a first mixture. Then, the first mixture is mixed in an internal mixer and extruded through a twin-screw extruder. The twin-screw extruder has eleven stages, and the extrusion temperatures are as follows: stage 1 140°C, stage 2 150°C, stage 3 165°C, stage 4 175°C, stage 5 180°C, stage 6 185°C, stage 7 192°C, stage 8 185°C, stage 9 192°C, stage 10 185°C, stage 11 192°C, and die 210°C, to obtain 0.3-2 mg of polypropylene resin. Step S2: Add the polypropylene resin obtained in Step S1 to a high-pressure reactor. Add water equal to twice the weight of the added polypropylene resin and sodium dodecylbenzenesulfonate at 3% of the weight of the added water to the high-pressure reactor. Start stirring, heat, and introduce carbon dioxide. The pressure is 3.5 MPa, and the reactor temperature is 140℃. Discharge the material, foam it, wash it, and dry it to obtain a density of 45 kg / m³. 3 Polypropylene foam particles were used to obtain conductive polymer foam particles. Step S3: By weight, add 28 parts of polyvinyl acetate, 5 parts of conductive carbon black, 67 parts of 80-mesh expanded graphite and 60 parts of ethanol to a disperser and disperse evenly. The disperser speed is 1500 rpm and the dispersion time is 30 minutes to obtain the coating. Step S4: By weight, add 20kg of conductive polymer foam particles obtained in step S3 to a mixing tank, then add 30kg of coating material and stir to obtain a second mixture. The stirring speed is 80 rpm. The second mixture is then added to a drying device for drying. The main shaft crushing motor is set to 30 rpm. The drying and crushing process takes 20 minutes. The mixture is then sieved to obtain foam particles containing expanded graphite composite resin. Step S5: By volume, mix the 70L conductive polymer foam particles and 30L expanded graphite composite resin foam particles obtained in step S3 to obtain a third mixture, and then mold the third mixture to obtain a 500 high-angle pyramidal microwave absorbing material.

[0027] Example 4 A method for preparing a highly flame-retardant microwave absorbing material containing expanded graphite polymer foam includes the following steps: Step S1: By weight, 62.9 parts of polypropylene, 25 parts of conductive carbon black, 2 parts of PE wax, 0.1 parts of zinc borate, and 8 parts of coated red phosphorus are mixed evenly to obtain a first mixture. Then, the first mixture is mixed in an internal mixer and extruded through a twin-screw extruder. The twin-screw extruder has eleven stages, and the extrusion temperatures are as follows: stage 1 140°C, stage 2 150°C, stage 3 165°C, stage 4 175°C, stage 5 180°C, stage 6 185°C, stage 7 192°C, stage 8 185°C, stage 9 192°C, stage 10 185°C, stage 11 192°C, and die 210°C, to obtain 0.3-2 mg of polypropylene resin. Step S2: Add the polypropylene resin obtained in Step S1 to a high-pressure reactor. Add water equal to twice the weight of the added polypropylene resin and sodium dodecylbenzenesulfonate at 3% of the weight of the added water to the high-pressure reactor. Start stirring, heat, and introduce carbon dioxide. The pressure is 3.5 MPa, and the reactor temperature is 140℃. Discharge the material, foam it, wash it, and dry it to obtain a density of 45 kg / m³. 3 Polypropylene foam particles were used to obtain conductive polymer foam particles. Step S3: Add 28 parts of polyvinyl acetate, 5 parts of conductive carbon black, 67 parts of 80-mesh expanded graphite and 60 parts of ethanol to a disperser and disperse evenly. The disperser speed is 1500 rpm and the dispersion time is 30 minutes to obtain the coating. Step S4: By weight, add 20kg of conductive polymer foam particles obtained in step S3 to a mixing tank, then add 30kg of coating material and stir to obtain a second mixture. The stirring speed is 80 rpm. The second mixture is then added to a drying device for drying. The main shaft crushing motor is set to 30 rpm. The drying and crushing process takes 20 minutes. The mixture is then sieved to obtain foam particles containing expanded graphite composite resin. Step S5: By volume, mix 80L of conductive polymer foam particles and 20L of expanded graphite composite resin foam particles obtained in step S3 to obtain a third mixture, and then mold the third mixture to obtain a 500 high-angle pyramidal microwave absorbing material.

[0028] Example 5 A method for preparing a highly flame-retardant microwave absorbing material containing expanded graphite polymer foam includes the following steps: Step S1: By weight, 62.9 parts of polypropylene, 25 parts of conductive carbon black, 2 parts of PE wax, 0.1 parts of zinc borate, and 8 parts of coated red phosphorus are mixed evenly to obtain a first mixture. Then, the first mixture is mixed in an internal mixer and extruded through a twin-screw extruder. The twin-screw extruder has eleven stages, and the extrusion temperatures are as follows: stage 1 140°C, stage 2 150°C, stage 3 165°C, stage 4 175°C, stage 5 180°C, stage 6 185°C, stage 7 192°C, stage 8 185°C, stage 9 192°C, stage 10 185°C, stage 11 192°C, and die 210°C, to obtain 0.3-2 mg of polypropylene resin. Step S2: Add the polypropylene resin obtained in Step S1 to a high-pressure reactor. Add water equal to twice the weight of the added polypropylene resin and sodium dodecylbenzenesulfonate at 3% of the weight of the added water to the high-pressure reactor. Start stirring, heat, and introduce carbon dioxide. The pressure is 3.5 MPa, and the reactor temperature is 140℃. Discharge the material, foam it, wash it, and dry it to obtain a density of 45 kg / m³. 3 Polypropylene foam particles were used to obtain conductive polymer foam particles. Step S3: By weight, add 28 parts of polyvinyl acetate, 5 parts of conductive carbon black, 67 parts of 80-mesh expanded graphite and 60 parts of ethanol to a disperser and disperse evenly. The disperser speed is 1500 rpm and the dispersion time is 30 minutes to obtain the coating. Step S4: By weight, add 20kg of conductive polymer foam particles obtained in step S3 to a mixing tank, then add 30kg of coating material and stir to obtain a second mixture. The stirring speed is 80 rpm. The second mixture is then added to a drying device for drying. The main shaft crushing motor is set to 30 rpm. The drying and crushing process takes 20 minutes. The mixture is then sieved to obtain foam particles containing expanded graphite composite resin. Step S5: By volume, mix the 90L conductive polymer foam particles and 10L expanded graphite composite resin foam particles obtained in step S3 to obtain a third mixture, and then mold the third mixture to obtain a 500 high-angle pyramidal microwave absorbing material.

[0029] Example 6 A method for preparing a highly flame-retardant microwave absorbing material containing expanded graphite polymer foam includes the following steps: Step S1: By weight, 62.9 parts of polypropylene, 25 parts of conductive carbon black, 2 parts of PE wax, 0.1 parts of zinc borate, and 8 parts of coated red phosphorus are mixed evenly to obtain a first mixture. Then, the first mixture is mixed in an internal mixer and extruded through a twin-screw extruder. The twin-screw extruder has eleven stages, and the extrusion temperatures are as follows: stage 1 140°C, stage 2 150°C, stage 3 165°C, stage 4 175°C, stage 5 180°C, stage 6 185°C, stage 7 192°C, stage 8 185°C, stage 9 192°C, stage 10 185°C, stage 11 192°C, and die 210°C, to obtain 0.3-2 mg of polypropylene resin. Step S2: Add the polypropylene resin obtained in Step S1 to a high-pressure reactor. Add water equal to twice the weight of the added polypropylene resin and sodium dodecylbenzenesulfonate at 3% of the weight of the added water to the high-pressure reactor. Start stirring, heat, and introduce carbon dioxide. The pressure is 3.5 MPa, and the reactor temperature is 140℃. Discharge the material, foam it, wash it, and dry it to obtain a density of 45 kg / m³. 3 Polypropylene foam particles were used to obtain conductive polymer foam particles. Step S3: By weight, add 28 parts of polyvinyl acetate, 5 parts of conductive carbon black, 67 parts of 80-mesh expanded graphite and 60 parts of ethanol to a disperser and disperse evenly. The disperser speed is 1500 rpm and the dispersion time is 30 minutes to obtain the coating. Step S4: By weight, add 20kg of conductive polymer foam particles obtained in step S3 to a mixing tank, then add 30kg of coating material and stir to obtain a second mixture. The stirring speed is 80 rpm. The second mixture is then added to a drying device for drying. The main shaft crushing motor is set to 30 rpm. The drying and crushing process takes 20 minutes. The mixture is then sieved to obtain foam particles containing expanded graphite composite resin. Step S5: By volume, the 40L conductive polymer foam particles and 60L expanded graphite composite resin foam particles obtained in step S3 are mixed to obtain a third mixture, and then the third mixture is molded to obtain a 500 high-angle pyramidal microwave absorbing material.

[0030] Example 7 A method for preparing a highly flame-retardant microwave absorbing material containing expanded graphite polymer foam includes the following steps: Step S1: By weight, 62.9 parts of polypropylene, 25 parts of conductive carbon black, 2 parts of PE wax, 0.1 parts of zinc borate, and 8 parts of coated red phosphorus are mixed evenly to obtain a first mixture. Then, the first mixture is mixed in an internal mixer and extruded through a twin-screw extruder. The twin-screw extruder has eleven stages, and the extrusion temperatures are as follows: stage 1 140°C, stage 2 150°C, stage 3 165°C, stage 4 175°C, stage 5 180°C, stage 6 185°C, stage 7 192°C, stage 8 185°C, stage 9 192°C, stage 10 185°C, stage 11 192°C, and die 210°C, to obtain 0.3-2 mg of polypropylene resin. Step S2: Add the polypropylene resin obtained in Step S1 to a high-pressure reactor. Add water equal to twice the weight of the added polypropylene resin and sodium dodecylbenzenesulfonate at 3% of the weight of the added water to the high-pressure reactor. Start stirring, heat, and introduce carbon dioxide. The pressure is 3.5 MPa, and the reactor temperature is 140℃. Discharge the material, foam it, wash it, and dry it to obtain a density of 45 kg / m³. 3 Polypropylene foam particles were used to obtain conductive polymer foam particles. Step S3: By weight, add 28 parts of polyvinyl acetate, 5 parts of conductive carbon black, 67 parts of 80-mesh expanded graphite and 60 parts of ethanol to a disperser and disperse evenly. The disperser speed is 1500 rpm and the dispersion time is 30 minutes to obtain the coating. Step S4: By weight, add 20kg of conductive polymer foam particles obtained in step S3 to a mixing tank, then add 30kg of coating material and stir to obtain a second mixture. The stirring speed is 80 rpm. The second mixture is then added to a drying device for drying. The main shaft crushing motor is set to 30 rpm. The drying and crushing process takes 20 minutes. The mixture is then sieved to obtain foam particles containing expanded graphite composite resin. Step S5: The 100L expanded graphite composite resin foam particles obtained in step S4 are molded to obtain a 500 high-angle cone microwave absorbing material.

[0031] Example 8 A method for preparing a highly flame-retardant microwave absorbing material containing expanded graphite polymer foam includes the following steps: Step S1: By weight, 62.9 parts of polypropylene, 25 parts of conductive carbon black, 2 parts of PE wax, 0.1 parts of zinc borate, and 8 parts of ammonium polyphosphate are mixed evenly to obtain a first mixture. Then, the first mixture is mixed in an internal mixer and extruded through a twin-screw extruder. The twin-screw extruder has eleven stages, and the extrusion temperatures are as follows: stage 1 140°C, stage 2 150°C, stage 3 165°C, stage 4 175°C, stage 5 180°C, stage 6 185°C, stage 7 192°C, stage 8 185°C, stage 9 192°C, stage 10 185°C, stage 11 192°C, and die 210°C, to obtain 0.3-2 mg of polypropylene resin. Step S2: Add the polypropylene resin obtained in Step S1 to a high-pressure reactor. Add water equal to twice the weight of the added polypropylene resin and sodium dodecylbenzenesulfonate at 3% of the weight of the added water to the high-pressure reactor. Start stirring, heat, and introduce carbon dioxide. The pressure is 3.5 MPa, and the reactor temperature is 140℃. Discharge the material, foam it, wash it, and dry it to obtain a density of 45 kg / m³. 3 Polypropylene foam particles were used to obtain conductive polymer foam particles. Step S3: By weight, add 28 parts of polyvinyl acetate, 5 parts of conductive carbon black, 67 parts of 80-mesh expanded graphite and 60 parts of ethanol to a disperser and disperse evenly. The disperser speed is 1500 rpm and the dispersion time is 30 minutes to obtain the coating. Step S4: By weight, add 20kg of conductive polymer foam particles obtained in step S3 to a mixing tank, then add 30kg of coating material and stir to obtain a second mixture. The stirring speed is 80 rpm. The second mixture is then added to a drying device for drying. The main shaft crushing motor is set to 30 rpm. The drying and crushing process takes 20 minutes. The mixture is then sieved to obtain foam particles containing expanded graphite composite resin. Step S5: By volume, the 50L conductive polymer foam particles and 50L expanded graphite composite resin foam particles obtained in step S3 are mixed to obtain a third mixture, and then the third mixture is molded to obtain a 500 high-angle pyramidal microwave absorbing material.

[0032] Example 9 A method for preparing a highly flame-retardant microwave absorbing material containing expanded graphite polymer foam includes the following steps: Step S1: By weight, 62.9 parts of polypropylene, 25 parts of conductive carbon black, 2 parts of PE wax, 0.1 parts of zinc borate, and 8 parts of coated red phosphorus are mixed evenly to obtain a first mixture. Then, the first mixture is mixed in an internal mixer and extruded through a twin-screw extruder. The twin-screw extruder has eleven stages, and the extrusion temperatures are as follows: stage 1 140°C, stage 2 150°C, stage 3 165°C, stage 4 175°C, stage 5 180°C, stage 6 185°C, stage 7 192°C, stage 8 185°C, stage 9 192°C, stage 10 185°C, stage 11 192°C, and die 210°C, to obtain 0.3-2 mg of polypropylene resin. Step S2: Add the polypropylene resin obtained in Step S1 to a high-pressure reactor. Add water equal to twice the weight of the added polypropylene resin and sodium dodecylbenzenesulfonate at 3% of the weight of the added water to the high-pressure reactor. Start stirring, heat, and introduce carbon dioxide. The pressure is 3.5 MPa, and the reactor temperature is 140℃. Discharge the material, foam it, wash it, and dry it to obtain a density of 45 kg / m³. 3 Polypropylene foam particles were used to obtain conductive polymer foam particles. Step S3: By weight, mix 99 parts of conductive polymer foam particles from step S2 with 1 part of adhesion promoter and add them to a drying equipment to dry, thereby obtaining composite foam particles. Step S4: By weight, add 28 parts of polyvinyl acetate, 5 parts of conductive carbon black, 67 parts of 80-mesh expanded graphite and 60 parts of ethanol to a disperser and disperse evenly. The disperser speed is 1500 rpm and the dispersion time is 30 minutes to obtain the coating. Step S5: By weight, add 20kg of composite foam particles obtained in step S3 to a mixing tank, then add 30kg of coating material and stir to obtain a second mixture. The stirring speed is 80 rpm. The second mixture is then added to a drying device for drying. The main shaft crushing motor is set to 30 rpm. The drying and crushing process takes 20 minutes. The mixture is then sieved to obtain foam particles containing expanded graphite composite resin. Step S6: The 100L expanded graphite composite resin foam particles obtained in step S5 are molded into a 500 high-angle cone microwave absorbing material.

[0033] Comparative Example 1 A method for preparing a highly flame-retardant microwave absorbing material containing expanded graphite polymer foam includes the following steps: Step S1: By weight, 64.9 parts of polypropylene, 25 parts of conductive carbon black, 2 parts of PE wax, 0.1 parts of zinc borate, and 8 parts of coated red phosphorus are mixed evenly to obtain a first mixture. Then, the first mixture is mixed in an internal mixer and extruded through a twin-screw extruder. The twin-screw extruder has eleven stages, and the extrusion temperatures are as follows: stage 1 140°C, stage 2 150°C, stage 3 165°C, stage 4 175°C, stage 5 180°C, stage 6 185°C, stage 7 192°C, stage 8 185°C, stage 9 192°C, stage 10 185°C, stage 11 192°C, and die 210°C, to obtain 0.3-2 mg of polypropylene resin. Step S2: Add polypropylene resin to a high-pressure reactor, then add water equal to twice the weight of the added polypropylene resin and sodium dodecylbenzenesulfonate at 3% of the weight of the added water. Start stirring, heat, and introduce carbon dioxide. The pressure is 3.5 MPa, and the reactor temperature is 140℃. Discharge the material, foam it, wash it, and dry it to obtain a density of 45 kg / m³. 3 Conductive polypropylene foam particles; Step S3: 100L of conductive polypropylene foam particles are molded to obtain 500 high-absorption cone microwave absorbing material.

[0034] Comparative Example 2 A method for preparing a highly flame-retardant microwave absorbing material containing expanded graphite polymer foam includes the following steps: Step S1: By weight, 70.9 parts polypropylene, 25 parts conductive carbon black, 2 parts PE wax, and 0.1 parts zinc borate are mixed evenly to obtain a first mixture. The first mixture is then mixed in an internal mixer and extruded through a twin-screw extruder. The twin-screw extruder has eleven stages, and the extrusion temperatures are as follows: stage 1 140°C, stage 2 150°C, stage 3 165°C, stage 4 175°C, stage 5 180°C, stage 6 185°C, stage 7 192°C, stage 8 185°C, stage 9 192°C, stage 10 185°C, stage 11 192°C, and die 210°C, to obtain 0.3-2 mg of polypropylene resin. Step S2: Add the polypropylene resin obtained in Step S1 to a high-pressure reactor. Add water equal to twice the weight of the added polypropylene resin and sodium dodecylbenzenesulfonate at 3% of the weight of the added water to the high-pressure reactor. Start stirring, heat, and introduce carbon dioxide. The pressure is 3.5 MPa, and the reactor temperature is 140℃. Discharge the material, foam it, wash it, and dry it to obtain a density of 45 kg / m³. 3 Polypropylene foam particles were used to obtain conductive polymer foam particles. Step S3: By weight, add 28 parts of polyvinyl acetate, 5 parts of conductive carbon black, 67 parts of 80-mesh expanded graphite and 60 parts of ethanol to a disperser and disperse evenly. The disperser speed is 1500 rpm and the dispersion time is 30 minutes to obtain the coating. Step S4: By weight, add 20kg of conductive polymer foam particles obtained in step S3 to a mixing tank, then add 30kg of coating material and stir to obtain a second mixture. The stirring speed is 80 rpm. The second mixture is then added to a drying device for drying. The main shaft crushing motor is set to 30 rpm. The drying and crushing process takes 20 minutes. The mixture is then sieved to obtain foam particles containing expanded graphite composite resin. Step S5: By volume, the 50L conductive polymer foam particles and 50L expanded graphite composite resin foam particles obtained in step S3 are mixed to obtain a third mixture, and then the third mixture is molded to obtain a 500 high-angle pyramidal microwave absorbing material.

[0035] The microwave absorbing materials of Examples 1-9 and Comparative Examples 1-2 were tested, and the test results are shown in Table 1.

[0036] Table 1

[0037] As shown in Table 1, Comparative Example 1, without the addition of expanded graphite and only with the addition of 8% red phosphorus flame retardant, had an oxygen index of 25. Increasing the proportion of red phosphorus flame retardant further did not significantly improve the oxygen index. Adding more than 15% red phosphorus had the opposite effect, as red phosphorus is a highly flammable material and poses a safety hazard to the extrusion granulation process and equipment. Comparative Example 2, without the addition of red phosphorus flame retardant, produced foamed polypropylene granules. Testing revealed that the oxygen index without red phosphorus flame retardant was only 24. This is because no red phosphorus participated in the polymer combustion reaction, and a dense, film-forming three-dimensional network structure was not formed on the polymer surface. Oxygen could still enter through the worm-like gaps in the expanded graphite, thus not significantly improving the oxygen index.

[0038] Example 1 involved a 50:50 mixture of polypropylene resin foam particles containing red phosphorus flame retardant and composite resin foam particles containing expanded graphite. The overall performance was evaluated, and the oxygen index reached 32.5. However, due to the coating layer on the surface of the composite resin foam particles containing expanded graphite, the coating had weak compatibility and adhesion with the polypropylene foam. Essentially, the expanded graphite composite resin foam particles were sandwiched between the conductive polymer foam particles. Excessive mixing prevented the conductive polymer foam particles from being continuously fused together. Therefore, physical property testing revealed an elongation at break of 0.5%. Further tests in subsequent examples... Examples show that: Example 7 is a microwave absorbing material made solely of expanded graphite composite resin foam particles, with an oxygen index as high as 33.5. Because the adhesion between adhesives is better than the adhesion between adhesives and polymer resin surfaces, Example 7 has better mechanical properties than the mechanical properties of a mixture containing expanded graphite composite resin foam particles in multiple proportions. Example 8 uses ammonium polyphosphate (APP) to replace red phosphorus flame retardant. Compared with Example 1, since APP has a lower phosphorus content, it helps to improve the oxygen index to some extent. However, through testing, it was found that its effect on improving the oxygen index is not as good as the effect of the combination of expanded graphite and red phosphorus flame retardant. Example 9 involved treating the surface of conductive polymer foam particles with an adhesion promoter and then coating it with a layer of expanded graphite. Tests showed that the mechanical properties were significantly improved compared to the fully coated expanded graphite layer in Example 7.

[0039] By comparing Example 5 with Comparative Example 1, it is found that mixing 20 parts of expanded graphite composite resin foam particles with conductive polymer foam particles can meet the requirement of an oxygen index greater than 28 in the Technical Specification for Electromagnetic Anechoic Chamber Engineering (GB50826-2012). Examples 7 and 9 can even meet the actual use requirements of most customers for an oxygen index greater than 32.

[0040] from Figure 1It can be seen that expanded graphite is uniformly adhered to the surface of polypropylene foam particles. Figure 2 The image shows a cut scanning electron microscope image of polypropylene particles. Adding less nucleating agent can increase the pore size of the foam and improve the surface roughness of the foam particles, thus helping to improve adhesion.

[0041] Finally, it should be noted that the above specific embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to examples, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A highly flame-retardant wave-absorbing material containing expanded graphite polymer foam, characterized in that: By volume, it includes 0-90 parts of conductive polymer foam particles and 10-100 parts of expanded graphite composite resin foam particles. The conductive polymer foam particles are selected from polypropylene foam particles and polyethylene foam particles. Each polypropylene foam particle or polyethylene foam particle includes at least one polypropylene resin or polyethylene resin. By weight, each polypropylene resin or polyethylene resin includes the following components: 45-95 parts of polypropylene or polyethylene, 5-35 parts of conductive agent, 1-10 parts of dispersant, 1-20 parts of first flame retardant, and 0.1-1 parts of nucleating agent.

2. The high flame-retardant expanded graphite polymer foam microwave absorbing material according to claim 1, characterized in that: By weight, the expanded graphite composite resin foam particles comprise 30-95 parts of conductive polymer foam particles and 5-70 parts of coating material, wherein the coating material comprises 5-50 parts of adhesive, 1-30 parts of conductive agent, 0-20 parts of first flame retardant, 20-95 parts of expanded graphite, and 13-300 parts of solvent.

3. The high flame-retardant expanded graphite polymer foam microwave absorbing material according to claim 1 or 2, characterized in that: When the conductive polymer foam particles are polypropylene foam particles, their density is 45 kg / m³. 3 .

4. The high flame-retardant expanded graphite polymer foam microwave absorbing material according to claim 1 or 2, characterized in that: The conductive agent is selected from one or more of carbon black, graphite, graphene, and carbon nanotubes; the first flame retardant is selected from one or more of coated red phosphorus, ammonium polyphosphate, aluminum dihydrogen phosphate, pentaerythritol phosphate, and brominated polystyrene.

5. The high flame-retardant expanded graphite polymer foam microwave absorbing material according to claim 1 or 2, characterized in that: The dispersant is selected from one or both of PE wax and triphenyl phosphate.

6. The high flame-retardant expanded graphite polymer foam microwave absorbing material according to claim 1 or 2, characterized in that: The adhesive is selected from one or more of phenolic resins, acrylic resins, ethylene-acrylic acid copolymers, polyvinyl acetate, polyurethane adhesives, and silicone resins.

7. The high flame-retardant expanded graphite polymer foam microwave absorbing material according to claim 1 or 2, characterized in that: The outer surface of the conductive polymer foam particles is also coated with an adhesion promoter, and the mass ratio of the conductive polymer foam particles to the adhesion promoter is 80-99:1-20.

8. The high flame-retardant expanded graphite polymer foam microwave absorbing material according to claim 7, characterized in that: The adhesion promoter is a chlorinated polypropylene solution.

9. The high flame-retardant expanded graphite polymer foam microwave absorbing material according to claim 1 or 2, characterized in that, The conductive agent is carbon black, the dispersant is PE wax, the first flame retardant is coated red phosphorus, the nucleating agent is zinc borate, the adhesive is polyvinyl acetate, and the solvent is ethanol; the microwave absorbing material is obtained by molding a mixture of the conductive polymer foam particles and the foam particles containing expanded graphite composite resin.

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