A high flame-retardant polymer foam wave-absorbing material containing expanded graphite and a preparation method thereof

Abstract

The application provides a kind of high flame-retardant polymer foam wave-absorbing material containing expanded graphite and a preparation method thereof, and belongs to the technical field of wave-absorbing material.The wave-absorbing material includes 0-90 parts of conductive polymer foam particles and 10-100 parts of expanded graphite-containing composite resin foam particles by volume fraction.The expanded graphite-containing composite resin foam particles include 30-95 parts of conductive polymer foam particles and 5-70 parts of coating paint.The phosphorus flame retardant and expanded graphite have a synergistic effect in the polymer foam wave-absorbing material.The expanded graphite is coated on the surface of the conductive polymer foam particles in the form of adhesive to obtain the expanded graphite-containing composite resin foam particles.The expanded graphite-containing composite resin foam particles and the conductive polymer foam particles are mixed in a certain proportion to form the wave-absorbing material by molding.The polymer foam wave-absorbing material for electromagnetic wave darkroom meets the requirements of oxygen index greater than 28, especially an oxygen index greater than 32, and meets the 1, 2 and 3 indicators of the NRL Report 8093 standard.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of improving the oxygen index of polymer foam wave-absorbing materials, and is a polypropylene foam and polyethylene foam wave-absorbing material and a preparation method thereof, which meet the requirements of an oxygen index greater than 28, in particular, an oxygen index greater than 32 and the 1, 2 and 3 indicators of the NRL Report 8093 standard. BACKGROUND

[0002] In the application field of electromagnetic wave darkrooms and military wave-absorbing materials, the requirements for the flame-retardant performance of electromagnetic wave darkrooms and the internal environment of darkrooms are very high. At present, the traditional electromagnetic wave darkrooms are mainly sponge wave-absorbing materials. The sponge has a high water absorption rate and a limited service life. The foamed polystyrene material wave-absorbing material has carcinogenicity due to the carcinogenicity of styrene, belongs to the 2B class of the International Agency for Research on Cancer (IARC), and is not environmentally friendly to the indoor environment of the built electromagnetic wave darkroom. The new foamed polypropylene and foamed polyethylene wave-absorbing materials are clean, environmentally friendly and have a low water absorption rate, and thus are concerned and applied by the market. The only disadvantage is poor flame retardant performance.

[0003] According to the requirements in the electromagnetic wave darkroom engineering technical specification (GB50826-2012), the oxygen index of the wave-absorbing material in the electromagnetic wave darkroom must be greater than 28. Many scientific research institutes and high-power test sites require that the oxygen index must be greater than 32 in actual use. In addition, the American Navy Standard NRL Report 8093 requires that the wave-absorbing material of the electromagnetic wave darkroom must meet the 1, 2 and 3 indicators of this standard.

[0004] The polypropylene foam wave-absorbing material on the market is difficult to meet these two standards. The patent with the patent number 201710970210.7 and the patent name of a preparation method of high-conductive and high-flame-retardant polypropylene foaming beads can achieve an oxygen index of 28 for a plate with a resistance of 800-1350 ohms. The prepared plate sample can improve the pressing density when pressing the plate, which helps to improve the oxygen index. However, if an angle pyramid wave-absorbing material is produced, the oxygen index cannot reach 28, and it is impossible to reach an oxygen index value greater than 32. The flat plate wave-absorbing material in the patent example of 201710970210.7 has a certain absorption effect at a specific frequency of 200M-1.2G. According to the impedance matching design of wave-absorbing materials, it meets the conditions of-28 dB absorption rate at 200M in the patent example of 201710970210.7. At least 500-700 mm high angle pyramid wave-absorbing materials or soft magnetic ferrite sheet wave-absorbing materials with only a few frequency points at the top of the resonance peak on the absorption rate curve of 200M-1.2G are required to achieve this. Although the plate thickness is not described in the example of 201710970210.7, no matter what the thickness of the single-layer resistance loss type plate is, it cannot meet the absorption rate of 28 at 200M and 69 at 400M. The same applies to other examples. Obviously, this patent method can only be applied to a specific frequency and cannot meet the requirements of wide frequency band absorption rate and flame retardation of angle pyramid wave-absorbing materials used in electromagnetic wave darkrooms. The patent with the patent application number CN202110787462.2 also uses a single-layer plate wave-absorbing material in the example, which has an absorption rate of ≥37 at 200M. The same principle is that no matter what the thickness of the single-layer resistance loss type wave-absorbing material plate is, it cannot achieve such indicators. Therefore, this patent method can only be applied to a specific frequency. If an angle pyramid wave-absorbing material is produced, the oxygen index cannot reach 28, and the oxygen index cannot reach more than 32. Therefore, this method cannot meet the requirements of wide frequency band absorption rate and flame retardation of angle pyramid wave-absorbing materials used in electromagnetic wave darkrooms.

[0005] Foamed polypropylene or polyethylene foam wave-absorbing materials are mixed with conductive agents, flame retardants, and nucleating agents after granulation, and then foamed through a supercritical carbon dioxide foaming process. To preferentially meet the absorption of electromagnetic waves, a large amount of conductive agent needs to be added. If a material that can be molded after foaming is to be produced, the proportion of flame retardants that can be added is relatively small. Therefore, the oxygen index of the finally produced polypropylene or polyethylene foam wave-absorbing material is difficult to be higher than 28. SUMMARY

[0006] The present application aims at overcoming, supplementing the deficiencies existing in the prior art, and providing a high flame-retardant expanded graphite-containing polymer foam wave-absorbing material and a preparation method thereof. Due to the characteristic that expanded graphite expands several times when heated and insulates oxygen after expansion, it is a very good flame-retardant material. However, the particles of general expanded graphite are relatively large, and cannot be fused in the polymer for foaming like other powder materials. Therefore, the high-filled expanded graphite is coated on the surface of polypropylene or polyethylene foam particles in the form of adhesive. Due to the poor adhesion of the adhesive on the surface of the polypropylene or polyethylene foam particles, the prepared conductive polymer foam particles are pretreated with an adhesion promoter and then mixed with the expanded graphite-containing composite resin foam particles to prepare a polymer foam wave-absorbing material for electromagnetic wave darkroom, which requires an oxygen index greater than 28, especially an oxygen index greater than 32, and meets the 1, 2, and 3 indicators of the NRL Report 8093 standard.

[0007] The technical scheme adopted by the present application is:

[0008] A high flame-retardant expanded graphite-containing polymer foam wave-absorbing material, wherein: the conductive polymer foam particles are 0-90 parts by volume, and the expanded graphite-containing composite resin foam particles are 10-100 parts by volume.

[0009] Preferably, the high flame-retardant expanded graphite-containing polymer foam wave-absorbing material, wherein: the expanded graphite-containing composite resin foam particles include conductive polymer foam particles 30-95 parts by weight and coating paint 5-70 parts by weight, the coating paint includes adhesive 5-50 parts by weight, conductive agent 1-30 parts by weight, first flame retardant 0-20 parts by weight, expanded graphite 20-95 parts by weight, and solvent 13-300 parts by weight.

[0010] Preferably, the high flame-retardant expanded graphite-containing polymer foam wave-absorbing material, wherein: the conductive polymer foam particles are selected from one of polypropylene foam particles and polyethylene foam particles, and the polypropylene foam particles or polyethylene foam particles each include at least one polypropylene resin or polyethylene resin, each of the polypropylene resin or polyethylene resin includes the following components by weight: polypropylene or polyethylene 45-95 parts, conductive agent 5-35 parts, dispersing agent 1-10 parts, first flame retardant 1-20 parts, and nucleating agent 0.1-1 parts.

[0011] Preferably, the high flame-retardant expanded graphite-containing polymer foam wave-absorbing material, wherein: the conductive agent is selected from one or more of carbon black, graphite, graphene, carbon nanotube, or metal powder; and the first flame retardant is selected from one or more of coated red phosphorus, ammonium polyphosphate, aluminum dihydrogen phosphate, pentaerythritol phosphate, and brominated polystyrene.

[0012] Preferably, the high flame-retardant expanded graphite-containing polymer foam wave-absorbing material, wherein: the dispersing agent is selected from one or both of PE wax and triphenyl phosphate.

[0013] Preferably, the high flame-retardant expanded graphite-containing polymer foam wave-absorbing material, wherein: the adhesive is selected from one or more of phenolic, acrylic resin, ethylene acrylic acid copolymer, polyvinyl acetate, polyurethane adhesive and silicone resin.

[0014] Preferably, the high flame-retardant expanded graphite-containing polymer foam wave-absorbing material, wherein: 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.

[0015] Preferably, the high flame-retardant expanded graphite-containing polymer foam wave-absorbing material, wherein: the adhesion promoter is a chlorinated polypropylene solution.

[0016] A method for preparing a high flame-retardant expanded graphite-containing polymer foam wave-absorbing material, comprising the following steps:

[0017] Step S1: uniformly mix 45-95 parts by weight of polypropylene or polyethylene, 5-35 parts by weight of a conductive agent, 1-10 parts by weight of a dispersing agent, and 1-20 parts by weight of a first flame retardant to obtain a first mixture, then mix the first mixture with a banbury mixer and extrude it through a twin-screw extruder to obtain at least one 0.3-2 mg polypropylene resin or polyethylene resin;

[0018] Step S2: add the at least one polypropylene resin or polyethylene resin of step S1 to a high-pressure reaction kettle, and add water and a surfactant to the high-pressure reaction kettle, then start stirring, heat and introduce nitrogen or carbon dioxide, continue heating until the polypropylene resin or polyethylene resin reaches its melting point, and release the material to foam to obtain polypropylene foam particles or polyethylene foam particles, thereby obtaining conductive polymer foam particles;

[0019] Step S3: uniformly disperse 5-50 parts by weight of an adhesive, 1-30 parts by weight of a conductive agent, 1-30 parts by weight of a first flame retardant, 20-95 parts by weight of expanded graphite, and 13-300 parts by weight of a solvent in a dispersing machine to obtain a coating material;

[0020] Step S4: add 30-95 parts by weight of the conductive polymer foam particles of step S3 to a stirring barrel, then add 5-70 parts by weight of the coating material of S4 and stir to obtain a second mixture, and dry the second mixture in a drying device and sieve to obtain expanded graphite-containing composite resin foam particles;

[0021] Step S5: 0-90 parts by volume of the conductive polymer foam particles and 10-100 parts by volume of the expanded graphite-containing composite resin foam particles are mixed to obtain a third mixture, and then the third mixture is shaped to obtain the wave-absorbing material.

[0022] A preparation method of a high-flame-retardant expanded graphite-containing polymer foam wave-absorbing material, wherein: by weight parts, comprising the following steps:

[0023] Step S1: 45-95 parts by weight of polypropylene or polyethylene, 5-35 parts by weight of a conductive agent, 1-10 parts by weight of a dispersing agent, and 1-20 parts by weight of a first flame retardant are uniformly mixed to obtain a first mixture, and then the first mixture is mixed by a banbury mixer and then extruded by a double-screw extruder to obtain at least one kind of 0.3-2 mg polypropylene resin or polyethylene resin;

[0024] Step S2: The at least one kind of polypropylene resin or polyethylene resin in step S1 is added to a high-pressure reaction kettle, water and a surfactant are added to the high-pressure reaction kettle, then stirring is started, heating is started and nitrogen or carbon dioxide is introduced, and the material is foamed by continuously heating to the melting point of the polypropylene resin or polyethylene resin to obtain polypropylene foam particles or polyethylene foam particles, thereby obtaining conductive polymer foam particles;

[0025] Step S3: 80-99 parts by weight of the conductive polymer foam particles in step S2 are mixed with 1-20 parts by weight of an adhesion promoter and dried in a drying device to obtain composite foam particles;

[0026] Step S4: 5-50 parts by weight of an adhesive, 1-30 parts by weight of a conductive agent, 1-30 parts by weight of a first flame retardant, 20-95 parts by weight of expanded graphite, and 13-300 parts by weight of a solvent are uniformly dispersed in a dispersing machine to obtain a coating material;

[0027] Step S5: 30-95 parts by weight of the composite foam particles in step S3 are added to a stirring barrel, then 5-70 parts by weight of the coating material in S4 is added and stirred to obtain a second mixture, and the second mixture is dried in a drying device and sieved to obtain expanded graphite-containing composite resin foam particles;

[0028] Step S6: 0-90 parts by volume of the composite foam particles and 10-100 parts by volume of the expanded graphite-containing composite resin foam particles are mixed to obtain a third mixture, and then the third mixture is shaped to obtain the wave-absorbing material.

[0029] Advantages of the present application:

[0030] (1) The present application uses red phosphorus and expanded graphite in synergistic effect as a flame retardant. Red phosphorus burns to produce diphosphorus pentoxide when heated. Expanded graphite begins to decompose the intercalated sulfuric acid compound when the polymer is burned at high temperature. The expanded graphite flakes begin to expand in the vertical direction, expand to hundreds of times their original size, and form graphite worms. The decomposition of the intercalated sulfuric acid compound produces SO2, CO2 and water. The polymer burns to produce CO2 and water. The diphosphorus pentoxide produced by the burning of red phosphorus captures the water vapor molecules produced by the decomposition of the intercalated sulfuric acid compound and the burning of the polymer, forming phosphoric acid crystal hydrate H3PO4. The expanded graphite expands to form graphite worms that are supported on the surface of the burning polymer. The polymer surface is catalyzed by the acid formed by the burning of red phosphorus to form carbon. The inorganic powders such as carbon black and graphite in the polymer are connected together by the thick, oily phosphoric acid crystal hydrate H3PO4 to form a three-dimensional and dense oxygen barrier structure. Due to the interlaced support of the expanded graphite worms on the surface of the polymer and the formation of the three-dimensional oxygen barrier structure by the thick, oily phosphoric acid crystal hydrate H3PO4, the synergistic effect effectively prevents the entry of oxygen, thereby improving the flame retardant effect of the wave-absorbing material. The polymer foam wave-absorbing material used in electromagnetic darkrooms requires an oxygen index of greater than 28. The actual oxygen index of the wave-absorbing material of the present application can reach greater than 32, meeting the 1, 2 and 3 indicators of the NRL Report 8093 standard.

[0031] (2) The present application uses a pressurized internal mixer to mix the conductive agent and various powder additives, and then granulates. Due to the accurate formula ratio, the dielectric constant and loss tangent performance indicators of the wave-absorbing material are stable and consistent in batches.

[0032] (3) The first flame retardant in the coating is 0-20 parts. Due to the conductivity of expanded graphite, the electromagnetic parameters of each layer of material need to be adjusted when producing multi-layer composite wave-absorbing materials. When producing high-frequency, ultra-high frequency or terahertz wave-absorbing materials, the electromagnetic parameters need to be adjusted. The proportion of expanded graphite is reduced. The 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 also meet the basic flame retardant requirements of the wave-absorbing material, with an oxygen index of greater than 28.

[0033] (4) The high-flame-retardant expanded graphite-containing polymer foam wave-absorbing material of the present application can replace traditional polyurethane sponge wave-absorbing materials. After the electromagnetic darkroom is dismantled, the polymer foam wave-absorbing material can be recycled and melted to be processed into other plastic products. Traditional polyurethane sponge wave-absorbing materials can only be incinerated after being dismantled and scrapped. The polymer foam wave-absorbing material of the present application can reduce environmental pollution. The low water absorption and high oxygen index characteristics can also be applied in outdoor and military camouflage fields, and have a wide application prospect. BRIEF DESCRIPTION OF DRAWINGS

[0034] Figure 1 SEM images of the expanded graphite-containing composite resin foam particles prepared in Examples 1-9.

[0035] Figure 2 SEM images of the expanded graphite-containing composite resin foam particles prepared in Examples 1-9.

[0036] Figure 3 Structural schematic diagram of the 500 high-absorption-angle pyramid wave-absorbing material prepared in Examples 1-9 and Comparative Examples 1-2. DETAILED DESCRIPTION

[0037] The present application will be further described below in conjunction with specific drawings and examples.

[0038] Example 1

[0039] A preparation method of a high-flame-retardant expanded graphite-containing polymer foam wave-absorbing material, comprising the following steps:

[0040] Step S1: 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 uniformly to obtain a first mixture, and then the first mixture is mixed with a banbury mixer and then extruded through a double-screw extruder. The double-screw extruder has a total of eleven sections, and the extrusion temperatures are 140° for the first section, 150° for the second section, 165° for the third section, 175° for the fourth section, 180° for the fifth section, 185° for the sixth section, 192° for the seventh section, 185° for the eighth section, 192° for the ninth section, 185° for the tenth section, and 192° for the eleventh section. The die temperature is 210°, and 0.3-2 mg of polypropylene resin is obtained;

[0041] Step S2: The polypropylene resin obtained in step S1 is added to a high-pressure reaction kettle, and water with a weight of 2 times the weight of the polypropylene resin and sodium dodecylbenzenesulfonate with a weight of 3% of the weight of the added water are added to the high-pressure reaction kettle. Stirring is started, heating is performed, and carbon dioxide is introduced. The pressure is 3.5 MPa, and the reaction kettle temperature is 140°C. The released material is foamed, washed, and dried to obtain polypropylene foam particles with a density of 45 kg / m 3 , and conductive polymer foam particles are obtained;

[0042] Step S3: 28 parts of polyvinyl acetate, 5 parts of conductive carbon black, 67 parts of 80-mesh expanded graphite, and 60 parts of ethanol are added to a dispersing machine and dispersed uniformly at a speed of 1500 revolutions per minute for 30 minutes to obtain a coating material;

[0043] Step S4: 20 kg of the conductive polymer foam particles obtained in step S3 are added into a stirring barrel, followed by adding 30 kg of the coating material and stirring to obtain a second mixture, the stirring speed is 80 rpm, and the second mixture is added into a air drying device for drying, the main shaft crushing motor is set to 30 rpm, the drying and crushing is performed for 20 minutes, and the expanded graphite composite resin foam particles are obtained by sieving;

[0044] Step S5: 50 L of the conductive polymer foam particles obtained in step S3 and 50 L of the expanded graphite composite resin foam particles are mixed to obtain a third mixture, and then the third mixture is shaped to obtain 500 high-angle pyramid wave-absorbing materials.

[0045] Example 2

[0046] A preparation method of a high-flame-retardant expanded graphite-containing polymer foam wave-absorbing material, comprising the following steps:

[0047] Step S1: 62.9 parts of polypropylene, 25 parts of conductive carbon black, 2 parts of PE wax, 0.1 part of zinc borate, and 8 parts of coated red phosphorus are uniformly mixed to obtain a first mixture, and then the first mixture is mixed by a banbury mixer and then extruded by a double-screw extruder, the double-screw extruder has eleven sections, and the extrusion temperatures are 140°, 150°, 165°, 175°, 180°, 185°, 192°, 185°, 192°, 185°, and 192°, respectively, the die head is 210°, and 0.3-2 mg of polypropylene resin is obtained;

[0048] Step S2: The polypropylene resin obtained in step S1 is added into a high-pressure reaction kettle, water with a weight of 2 times the weight of the polypropylene resin and sodium dodecylbenzenesulfonate with a weight of 3% of the weight of the water are added into the high-pressure reaction kettle, stirring is started, heating is performed, and carbon dioxide is introduced, the pressure is 3.5 MPa, the reaction kettle temperature is 140°C, and the released material is foamed, cleaned, and dried to obtain polypropylene foam particles with a density of 45 kg / m 3 ;

[0049] Step S3: 28 parts of polyvinyl acetate, 5 parts of conductive carbon black, 67 parts of 80-mesh expanded graphite, and 60 parts of ethanol are added into a disperser and uniformly dispersed, the disperser speed is 1500 rpm, and the dispersion is performed for 30 minutes to obtain a coating material;

[0050] Step S4: 20 kg of the conductive polymer foam particles obtained in step S3 are added into a stirring barrel, followed by adding 30 kg of the coating paint and stirring to obtain a second mixture, the stirring speed is 80 rpm, and the second mixture is added into a air drying device for drying, the main shaft crushing motor is set to 30 rpm, the drying and crushing is performed for 20 minutes, and the expanded graphite composite resin foam particles are obtained by sieving;

[0051] Step S5: 60 L of the conductive polymer foam particles obtained in step S3 and 40 L of the expanded graphite composite resin foam particles are mixed to obtain a third mixture, and then the third mixture is shaped to obtain 500 high-angle pyramid wave-absorbing materials.

[0052] Example 3

[0053] A preparation method of a high-flame-retardant expanded graphite-containing polymer foam wave-absorbing material, comprising the following steps:

[0054] Step S1: 62.9 parts of polypropylene, 25 parts of conductive carbon black, 2 parts of PE wax, 0.1 part of zinc borate, 8 parts of coated red phosphorus are uniformly mixed to obtain a first mixture, and then the first mixture is mixed by a banbury mixer and then extruded by a double-screw extruder, the double-screw extruder has eleven sections, and the extrusion temperatures are 140°, 150°, 165°, 175°, 180°, 185°, 192°, 185°, 192°, 185°, and 192° in sequence, the die head is 210°, and 0.3-2 mg of polypropylene resin is obtained;

[0055] Step S2: The polypropylene resin obtained in step S1 is added into a high-pressure reaction kettle, water with a weight of 2 times the weight of the polypropylene resin and sodium dodecylbenzenesulfonate with a weight of 3% of the weight of the added water are added into the high-pressure reaction kettle, stirring is started, heating is performed, and carbon dioxide is introduced, the pressure is 3.5 mpa, the reaction kettle temperature is 140°, and the polypropylene foam particles with a density of 45 kg / m 3 are obtained by foaming, cleaning, and drying the discharged material, and the conductive polymer foam particles are obtained;

[0056] Step S3: 28 parts of polyvinyl acetate, 5 parts of conductive carbon black, 67 parts of 80-mesh expanded graphite, and 60 parts of ethanol are added into a disperser and uniformly dispersed, the disperser speed is 1500 rpm, and the dispersion is performed for 30 minutes to obtain a coating paint;

[0057] Step S4: 20 kg of the conductive polymer foam particles obtained in step S3 are added into a stirring barrel, followed by adding 30 kg of the coating paint and stirring to obtain a second mixture, the stirring speed is 80 rpm, and the second mixture is added into a air drying device for drying, the main shaft crushing motor is set to 30 rpm, the drying and crushing is performed for 20 minutes, and the expanded graphite composite resin foam particles are obtained by sieving;

[0058] Step S5: 70 L of the conductive polymer foam particles obtained in step S3 and 30 L of the expanded graphite composite resin foam particles are mixed to obtain a third mixture, and then the third mixture is shaped to obtain 500 high-angle pyramid wave-absorbing materials.

[0059] Example 4

[0060] A preparation method of a high-flame-retardant expanded graphite-containing polymer foam wave-absorbing material, comprising the following steps:

[0061] Step S1: 62.9 parts of polypropylene, 25 parts of conductive carbon black, 2 parts of PE wax, 0.1 part of zinc borate, and 8 parts of coated red phosphorus are uniformly mixed to obtain a first mixture, and then the first mixture is mixed by a banbury mixer and then extruded by a double-screw extruder, the double-screw extruder has eleven sections, and the extrusion temperatures are 140°, 150°, 165°, 175°, 180°, 185°, 192°, 185°, 192°, 185°, and 192°, respectively, the die head is 210°, and 0.3-2 mg of polypropylene resin is obtained;

[0062] Step S2: The polypropylene resin obtained in step S1 is added into a high-pressure reaction kettle, water with a weight of 2 times the weight of the polypropylene resin and sodium dodecylbenzenesulfonate with a weight of 3% of the weight of the water are added into the high-pressure reaction kettle, stirring is started, heating is performed, and carbon dioxide is introduced, the pressure is 3.5 MPa, the reaction kettle temperature is 140°C, and the polypropylene foam particles with a density of 45 kg / m 3 are obtained by foaming, cleaning, and drying the discharged material, and the conductive polymer foam particles are obtained;

[0063] Step S3: 28 parts of polyvinyl acetate, 5 parts of conductive carbon black, 67 parts of 80-mesh expanded graphite, and 60 parts of ethanol are added into a disperser and uniformly dispersed, the disperser speed is 1500 rpm, and the dispersion is performed for 30 minutes to obtain a coating paint;

[0064] Step S4: 20 kg of the conductive polymer foam particles obtained in step S3 are added into a stirring barrel, followed by adding 30 kg of the coating material and stirring to obtain a second mixture, the stirring speed is 80 rpm, and the second mixture is added into a air drying device for drying, the main shaft crushing motor is set to 30 rpm, the drying and crushing is performed for 20 minutes, and the expanded graphite composite resin foam particles are obtained by sieving;

[0065] Step S5: 80 L of the conductive polymer foam particles obtained in step S3 and 20 L of the expanded graphite composite resin foam particles are mixed to obtain a third mixture, and then the third mixture is shaped to obtain 500 high-angle pyramid wave-absorbing materials.

[0066] Example 5

[0067] A preparation method of a high-flame-retardant expanded graphite-containing polymer foam wave-absorbing material, comprising the following steps:

[0068] Step S1: 62.9 parts of polypropylene, 25 parts of conductive carbon black, 2 parts of PE wax, 0.1 part of zinc borate, and 8 parts of coated red phosphorus are uniformly mixed to obtain a first mixture, and then the first mixture is mixed by a banbury mixer and then extruded by a double-screw extruder, the double-screw extruder has eleven sections, and the extrusion temperatures are 140°, 150°, 165°, 175°, 180°, 185°, 192°, 185°, 192°, 185°, and 192°, respectively, the die head is 210°, and 0.3-2 mg of polypropylene resin is obtained;

[0069] Step S2: The polypropylene resin obtained in step S1 is added into a high-pressure reaction kettle, water with a weight of 2 times the weight of the polypropylene resin and sodium dodecylbenzenesulfonate with a weight of 3% of the weight of the water are added into the high-pressure reaction kettle, stirring is started, heating is performed, and carbon dioxide is introduced, the pressure is 3.5 mpa, the reaction kettle temperature is 140°, and the polypropylene foam particles with a density of 45 kg / m 3 are obtained, and the conductive polymer foam particles are obtained;

[0070] Step S3: 28 parts of polyvinyl acetate, 5 parts of conductive carbon black, 67 parts of 80-mesh expanded graphite, and 60 parts of ethanol are added into a disperser and uniformly dispersed, the disperser speed is 1500 rpm, and the dispersion is performed for 30 minutes to obtain a coating material;

[0071] Step S4: 20 kg of the conductive polymer foam particles obtained in step S3 are added into a stirring barrel, followed by adding 30 kg of the coating material and stirring to obtain a second mixture, the stirring speed is 80 rpm, and the second mixture is added into a air drying device for drying, the main shaft crushing motor is set to 30 rpm, the drying and crushing is performed for 20 minutes, and the expanded graphite composite resin foam particles are obtained by sieving;

[0072] Step S5: 90 L of the conductive polymer foam particles obtained in step S3 and 10 L of the expanded graphite composite resin foam particles are mixed to obtain a third mixture, and then the third mixture is shaped to obtain 500 high-angle pyramid wave-absorbing materials.

[0073] Example 6

[0074] A preparation method of a high-flame-retardant expanded graphite-containing polymer foam wave-absorbing material, comprising the following steps:

[0075] Step S1: 62.9 parts of polypropylene, 25 parts of conductive carbon black, 2 parts of PE wax, 0.1 part of zinc borate, and 8 parts of coated red phosphorus are uniformly mixed to obtain a first mixture, and then the first mixture is mixed by a banbury mixer and then extruded by a double-screw extruder, the double-screw extruder has eleven sections, and the extrusion temperatures are 140°, 150°, 165°, 175°, 180°, 185°, 192°, 185°, 192°, 185°, and 192°, respectively, the die head is 210°, and 0.3-2 mg of polypropylene resin is obtained;

[0076] Step S2: The polypropylene resin obtained in step S1 is added into a high-pressure reaction kettle, water with a weight of 2 times the weight of the polypropylene resin and sodium dodecylbenzenesulfonate with a weight of 3% of the weight of the water are added into the high-pressure reaction kettle, stirring is started, heating is performed, and carbon dioxide is introduced, the pressure is 3.5 MPa, the reaction kettle temperature is 140°C, and the released material is foamed, cleaned, and dried to obtain polypropylene foam particles with a density of 45 kg / m 3 ;

[0077] Step S3: 28 parts of polyvinyl acetate, 5 parts of conductive carbon black, 67 parts of 80-mesh expanded graphite, and 60 parts of ethanol are added into a disperser and uniformly dispersed, the disperser speed is 1500 rpm, and the dispersion is performed for 30 minutes to obtain a coating material;

[0078] Step S4: 20 kg of the conductive polymer foam particles obtained in step S3 are added into a stirring barrel, followed by adding 30 kg of the coating material and stirring to obtain a second mixture, the stirring speed is 80 rpm, and the second mixture is added into a air drying device for drying, the main shaft crushing motor is set to 30 rpm, the drying and crushing is performed for 20 minutes, and the expanded graphite composite resin foam particles are obtained by sieving;

[0079] Step S5: 40 L of the conductive polymer foam particles obtained in step S3 and 60 L of the expanded graphite composite resin foam particles are mixed to obtain a third mixture, and then the third mixture is shaped to obtain 500 high-angle pyramid wave-absorbing materials.

[0080] Example 7

[0081] A preparation method of a high-flame-retardant expanded graphite-containing polymer foam wave-absorbing material, comprising the following steps:

[0082] Step S1: 62.9 parts of polypropylene, 25 parts of conductive carbon black, 2 parts of PE wax, 0.1 part of zinc borate, and 8 parts of coated red phosphorus are uniformly mixed to obtain a first mixture, and then the first mixture is mixed by a banbury mixer and then extruded by a double-screw extruder, the double-screw extruder has eleven sections, and the extrusion temperatures are 140°, 150°, 165°, 175°, 180°, 185°, 192°, 185°, 192°, 185°, and 192°, respectively, the die head is 210°, and 0.3-2 mg of polypropylene resin is obtained;

[0083] Step S2: The polypropylene resin obtained in step S1 is added into a high-pressure reaction kettle, water with a weight of 2 times the weight of the polypropylene resin and sodium dodecylbenzenesulfonate with a weight of 3% of the weight of the water are added into the high-pressure reaction kettle, stirring is started, heating is performed, and carbon dioxide is introduced, the pressure is 3.5 mpa, the reaction kettle temperature is 140°, and the polypropylene foam particles with a density of 45 kg / m 3 are obtained, and the conductive polymer foam particles are obtained;

[0084] Step S3: 28 parts of polyvinyl acetate, 5 parts of conductive carbon black, 67 parts of 80-mesh expanded graphite, and 60 parts of ethanol are added into a disperser and uniformly dispersed, the disperser speed is 1500 rpm, and the dispersion is performed for 30 minutes to obtain a coating material;

[0085] Step S4: 20 kg of the conductive polymer foam particles obtained in step S3 are added into a stirring barrel, then 30 kg of the coating material is added and stirred to obtain a second mixture, the stirring speed is 80 rpm, and the second mixture is added into a air drying device for drying, the main shaft crushing motor is set to 30 rpm, the drying and crushing is performed for 20 minutes, and the expanded graphite composite resin foam particles are obtained by sieving;

[0086] Step S5: 100 L of the expanded graphite composite resin foam particles obtained in step S4 are formed to obtain 500 high-angle pyramid wave-absorbing materials.

[0087] Example 8

[0088] A preparation method of a high-flame-retardant expanded graphite-containing polymer foam wave-absorbing material, comprising the following steps:

[0089] Step S1: 62.9 parts of polypropylene, 25 parts of conductive carbon black, 2 parts of PE wax, 0.1 part of zinc borate, and 8 parts of ammonium polyphosphate are uniformly mixed to obtain a first mixture, then the first mixture is mixed by a banbury mixer and then extruded by a double-screw extruder, the double-screw extruder has eleven sections, and the extrusion temperatures of the eleven sections are 140°, 150°, 165°, 175°, 180°, 185°, 192°, 185°, 192°, 185°, and 192°, respectively, the die head is 210°, and 0.3-2 mg of polypropylene resin is obtained;

[0090] Step S2: The polypropylene resin obtained in step S1 is added into a high-pressure reaction kettle, water with a weight of 2 times that of the polypropylene resin and sodium dodecylbenzenesulfonate with a weight of 3% of the added water are added into the high-pressure reaction kettle, stirring is started, heating is performed, and carbon dioxide is introduced, the pressure is 3.5 MPa, the reaction kettle temperature is 140°, and the polypropylene foam particles with a density of 45 kg / m 3 are obtained by foaming, cleaning, and drying the discharged material, and the conductive polymer foam particles are obtained;

[0091] Step S3: 28 parts of polyvinyl acetate, 5 parts of conductive carbon black, 67 parts of 80-mesh expanded graphite, and 60 parts of ethanol are uniformly dispersed in a disperser at a speed of 1500 rpm for 30 minutes to obtain a coating material;

[0092] Step S4: 20 kg of the conductive polymer foam particles obtained in step S3 are added into a stirring barrel, then 30 kg of the coating material is added and stirred to obtain a second mixture, the stirring speed is 80 rpm, and the second mixture is added into a air drying device for drying, the main shaft crushing motor is set to 30 rpm, the drying and crushing is performed for 20 minutes, and the expanded graphite composite resin foam particles are obtained by sieving;

[0093] Step S5: 50L of the conductive polymer foam particles obtained in step S3 and 50L of the expanded graphite-containing composite resin foam particles are mixed to obtain a third mixture, and then the third mixture is shaped to obtain 500 high-angle pyramid wave-absorbing materials.

[0094] Example 9

[0095] A preparation method of a high-flame-retardant expanded graphite-containing polymer foam wave-absorbing material, comprising the following steps:

[0096] Step S1: 62.9 parts of polypropylene, 25 parts of conductive carbon black, 2 parts of PE wax, 0.1 part of zinc borate, and 8 parts of coated red phosphorus are uniformly mixed to obtain a first mixture, and then the first mixture is mixed by a banbury mixer and then extruded by a double-screw extruder, the double-screw extruder has eleven sections, and the extrusion temperatures are 140°, 150°, 165°, 175°, 180°, 185°, 192°, 185°, 192°, 185°, and 192°, respectively, the die temperature is 210°, and 0.3-2mg of polypropylene resin is obtained;

[0097] Step S2: The polypropylene resin obtained in step S1 is added to a high-pressure reaction kettle, water with a weight of 2 times the weight of the polypropylene resin and sodium dodecylbenzenesulfonate with a weight of 3% of the weight of the added water are added to the high-pressure reaction kettle, stirring is started, heating is performed, and carbon dioxide is introduced, the pressure is 3.5mpa, the reaction kettle temperature is 140℃, and the released material is foamed, cleaned, and dried to obtain polypropylene foam particles with a density of 45kg / m 3 The conductive polymer foam particles are obtained;

[0098] Step S3: 99 parts of the conductive polymer foam particles of step S2 and 1 part of an adhesion promoter are mixed and added to a drying device to obtain composite foam particles;

[0099] Step S4: 28 parts of polyvinyl acetate, 5 parts of conductive carbon black, 67 parts of 80-mesh expanded graphite, and 60 parts of ethanol are uniformly dispersed in a dispersing machine at a speed of 1500 revolutions per minute for 30 minutes to obtain a coating material;

[0100] Step S5: 20kg of the composite foam particles obtained in step S3 are added to a stirring barrel, then 30kg of the coating material is added and stirred to obtain a second mixture at a stirring speed of 80 revolutions per minute, and then the second mixture is added to a drying device for drying, a main shaft crushing motor is set to 30 revolutions per minute, and drying and crushing are performed for 20 minutes, and then sieving is performed to obtain expanded graphite-containing composite resin foam particles;

[0101] Step S6: 100L of the expanded graphite composite resin foam particles obtained in step S5 are formed to obtain 500 high-angle pyramid wave-absorbing materials.

[0102] Comparative Example 1

[0103] A preparation method of a high-flame-retardant expanded graphite-containing polymer foam wave-absorbing material, comprising the following steps:

[0104] Step S1: 64.9 parts of polypropylene, 25 parts of conductive carbon black, 2 parts of PE wax, 0.1 part of zinc borate, and 8 parts of coated red phosphorus are uniformly mixed to obtain a first mixture, and then the first mixture is mixed by a banbury mixer and then extruded by a double-screw extruder. The double-screw extruder has eleven sections, and the extrusion temperatures are 140° for the first section, 150° for the second section, 165° for the third section, 175° for the fourth section, 180° for the fifth section, 185° for the sixth section, 192°C for the seventh section, 185° for the eighth section, 192° for the ninth section, 185° for the tenth section, and 192° for the eleventh section. The die temperature is 210°, and 0.3-2mg of polypropylene resin is obtained;

[0105] Step S2: The polypropylene resin is added to a high-pressure reaction kettle, 2 times the weight of the added polypropylene resin of water and 3% of the weight of the added water of sodium dodecylbenzenesulfonate are added to the high-pressure reaction kettle, stirring is started, heating is performed, and carbon dioxide is introduced. The pressure is 3.5mpa, the reaction kettle temperature is 140°C, and the released material is foamed, cleaned, and dried to obtain conductive polypropylene foam particles with a density of 45kg / m 3 ;

[0106] Step S3: 100L of the conductive polypropylene foam particles are formed to obtain 500 high-wave-absorbing angle pyramid wave-absorbing materials.

[0107] Comparative Example 2

[0108] A preparation method of a high-flame-retardant expanded graphite-containing polymer foam wave-absorbing material, comprising the following steps:

[0109] Step S1: 64.9 parts of polypropylene, 25 parts of conductive carbon black, 2 parts of PE wax, 0.1 part of zinc borate, and 8 parts of coated red phosphorus are uniformly mixed to obtain a first mixture, and then the first mixture is mixed by a banbury mixer and then extruded by a double-screw extruder. The double-screw extruder has eleven sections, and the extrusion temperatures are 140° for the first section, 150° for the second section, 165° for the third section, 175° for the fourth section, 180° for the fifth section, 185° for the sixth section, 192°C for the seventh section, 185° for the eighth section, 192° for the ninth section, 185° for the tenth section, and 192° for the eleventh section. The die temperature is 210°, and 0.3-2mg of polypropylene resin is obtained;

[0110] Step S2: The polypropylene resin obtained in step S1 is added to a high-pressure reaction kettle, and water in an amount of 2 times the weight of the polypropylene resin and sodium dodecylbenzenesulfonate in an amount of 3% of the weight of the water are added to the high-pressure reaction kettle. Stirring is started, heating is performed, and carbon dioxide is introduced. The pressure is 3.5 MPa, and the temperature of the reaction kettle is 140°C. The material is discharged, foamed, cleaned, and dried to obtain polypropylene foam particles with a density of 45 kg / m 3 to obtain conductive polymer foam particles;

[0111] Step S3: 28 parts of polyvinyl acetate, 5 parts of conductive carbon black, 67 parts of 80-mesh expanded graphite, and 60 parts of ethanol are added to a dispersing machine and uniformly dispersed at a speed of 1500 revolutions per minute for 30 minutes to obtain a coating material.

[0112] Step S4: 20 kg of the conductive polymer foam particles obtained in step S3 are added to a stirring barrel, followed by the addition of 30 kg of the coating material and stirring to obtain a second mixture at a stirring speed of 80 revolutions per minute. The second mixture is then dried in a drying device with a main shaft crushing motor set at 30 revolutions per minute for 20 minutes, and sieving is performed to obtain expanded graphite-containing composite resin foam particles.

[0113] Step S5: 50 L of the conductive polymer foam particles obtained in step S3 and 50 L of the expanded graphite-containing composite resin foam particles are mixed to obtain a third mixture. The third mixture is then molded to obtain 500 high-angle pyramid wave-absorbing materials.

[0114] The wave-absorbing materials of Examples 1-9 and Comparative Examples 1-2 are tested, and the test results are shown in Table 1.

[0115] Table 1

[0116]

[0117] As can be seen from Table 1, Comparative Example 1 does not add expanded graphite but only adds 8% red phosphorus flame retardant. The measured oxygen index is 25. If more red phosphorus flame retardant is added, the oxygen index will not increase much. If more than 15% of red phosphorus is added, the oxygen index will actually decrease. Red phosphorus is a particularly flammable material, which poses a safety hazard to the extrusion granulation process and equipment. Comparative Example 2 does not add red phosphorus flame retardant, and the foamed polypropylene particles do not contain red phosphorus flame retardant. It is found through testing that the oxygen index of the particles without red phosphorus flame retardant is only 24. The reason is that without the participation of red phosphorus in the polymer combustion reaction, a dense and film-forming three-dimensional network structure is not formed on the surface of the polymer, and oxygen can still enter through the gaps in the expanded graphite worms, which is not conducive to improving the oxygen index.

[0118] Example 1 is a polypropylene resin foam particle containing red phosphorus flame retardant and a composite resin foam particle containing expanded graphite, which is tested for comprehensive performance evaluation at a ratio of 50:50, and the oxygen index can reach 32.5, but due to the surface coating of the composite resin foam particle containing expanded graphite, the coating has poor compatibility and adhesion with the polypropylene foam, which is equivalent to the composite resin foam particle containing expanded graphite being sandwiched between the conductive polymer foam particles, and due to the excessive mixing, the conductive polymer foam particles cannot be continuously welded, so through the physical property test, the elongation at break is 0.5%, which can be known from the following other examples; Example 7 is a wave-absorbing material made of a composite resin foam particle containing expanded graphite alone, and the oxygen index can be as high as 33.5, and since the adhesion of the adhesive is better than the adhesion of the adhesive to the surface of the polymer resin, the mechanical properties of Example 7 are better than those of the composite resin foam particle containing expanded graphite in multiple proportions; Example 8 is a polyphosphoric acid ammonium (APP) instead of a red phosphorus flame retardant, which is compared with Example 1, and since the APP has a low phosphorus content, it can help improve the oxygen index, but through the test, it is found that the effect of improving the oxygen index is not as good as that of the expanded graphite and red phosphorus flame retardant.

[0119] Example 9 is a surface treatment of the conductive polymer foam particle with an adhesion promoter and then coated with a coating containing expanded graphite, and through the test, it is found that the mechanical properties are greatly improved compared with the mechanical properties of Example 7 with a full coating of expanded graphite coating.

[0120] By comparing Example 5 with Comparative Example 1, only 20 parts of the composite resin foam particle containing expanded graphite mixed with the conductive polymer foam particle can meet the requirement of the oxygen index greater than 28 in the electromagnetic wave darkroom engineering technical specification (GB50826-2012), and Examples 7 and 9 can meet the requirements of most customers for the oxygen index greater than 32.

[0121] From Figure 1 It can be seen that the expanded graphite is uniformly attached to the surface of the polypropylene foam particle, Figure 2 The polypropylene particle is cut and scanned by a scanning electron microscope, and a small amount of nucleating agent can increase the pore size of the foam and improve the surface roughness of the foam particle to help improve the adhesion.

[0122] Finally, it should be noted that the above specific embodiments are only used to illustrate the technical solutions of the present application and are not limiting, and although the present application has been described in detail with reference to the examples, those skilled in the art should understand that the technical solutions of the present application can be modified or replaced by equivalents without departing from the spirit and scope of the present application, and they should be covered in the scope of the claims of the present application.

Claims

1. A method for preparing a highly flame-retardant microwave absorbing material containing expanded graphite polymer foam, characterized in that, Includes the following steps: Step S1: By weight, 45-95 parts of polypropylene, 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 one 0.3-2 mg polypropylene resin. Step S2: Add at least one polypropylene resin obtained in step S1 to a high-pressure reactor, and add water and surfactant to the reactor. Then start stirring, heat, and introduce nitrogen or carbon dioxide. Continue heating until the polypropylene resin reaches its melting point, then release the material to foam and obtain a density of 45 kg / m³. 3 Polypropylene foam particles were used to obtain 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 obtained in step S2 to a mixing tank, then add 5-70 parts of coating material obtained in step S3 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 obtained in step S2 and 10-100 parts of expanded graphite composite resin foam particles obtained in step S4 are mixed to obtain a third mixture, and then the third mixture is molded to obtain a microwave absorbing material. The first flame retardant is selected from one or more of coated red phosphorus, ammonium polyphosphate, aluminum dihydrogen phosphate, pentaerythritol phosphate, and brominated polystyrene.

2. A method for preparing a highly flame-retardant microwave absorbing material containing expanded graphite polymer foam, characterized in that, Includes the following steps: Step S1: By weight, 45-95 parts of polypropylene, 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 one 0.3-2 mg polypropylene resin. Step S2: Add at least one polypropylene resin obtained in step S1 to a high-pressure reactor, and add water and surfactant to the reactor. Then start stirring, heat, and introduce nitrogen or carbon dioxide. Continue heating until the polypropylene resin reaches its melting point, then release the material to foam and obtain a density of 45 kg / m³. 3 Polypropylene foam particles were used to obtain conductive polymer foam particles. Step S3: By weight, mix 80-99 parts of conductive polymer foam particles obtained in 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 obtained in step S3 to a mixing tank, then add 5-70 parts of the coating material obtained in step S4 and stir to obtain a second mixture. Add the second mixture to a drying equipment to dry and sieve to obtain expanded graphite composite resin foam particles. Step S6: By volume, 0-90 parts of composite foam particles obtained in step S3 and 10-100 parts of expanded graphite composite resin foam particles obtained in step S5 are mixed to obtain a third mixture, and then the third mixture is molded to obtain a microwave absorbing material. The first flame retardant is selected from one or more of coated red phosphorus, ammonium polyphosphate, aluminum dihydrogen phosphate, pentaerythritol phosphate, and brominated polystyrene; the adhesion promoter is a chlorinated polypropylene solution.

3. The method for preparing 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.

4. The method for preparing 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.

5. The method for preparing 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.

6. The microwave absorbing material prepared by the method for preparing the high flame retardant expanded graphite polymer foam microwave absorbing material according to claim 1 or 2.

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