Multi-layer coextruded flame-retardant polyolefin foam particle wave-absorbing material and preparation method therefor

Abstract

Provided in the present invention are a multi-layer coextruded flame-retardant polyolefin foam particle wave-absorbing material and a preparation method therefor. In the method, polyolefin resins having different properties are compounded, and a polyolefin resin A is highly filled with a flame retardant filler to form a flame-retardant polyolefin layer that serves as an inner layer of a particle; a polyolefin resin B is filled with a conductive agent to form a conductive polyolefin layer that serves as an outer layer of the particle; the compounded polyolefin resin particle is foamed under foaming conditions for the polyolefin resin B, thereby preparing a polyolefin resin wave-absorbing foam particle having an unfoamed or slightly foamed inner layer and a foamed outer layer; and then a wave-absorbing material is prepared by means of a foam forming process. Therefore, the flame retardance, environmental protection properties and economic value of a thermoplastic foam wave-absorbing material are improved, the production efficiency is improved, and good application prospects are exhibited.

Description

A multilayer co-extruded flame-retardant polyolefin foam particle microwave absorbing material and its preparation method Technical Field

[0001] This invention relates to the field of polyolefin foam microwave absorbing materials, specifically to a multilayer co-extruded flame-retardant polyolefin foam particle microwave absorbing material and its preparation method. Background Technology

[0002] In the field of microwave absorbing materials, most polyolefin foam microwave absorbing materials have the advantages of low water absorption, high hardness, odorless and environmentally friendly properties, and stable performance over long-term use. They are the main replacement products for sponge-type microwave absorbing materials in the future. However, when used in electromagnetic anechoic chambers, poor flame retardant performance is a major drawback of polyolefin foam microwave absorbing materials.

[0003] Common polyolefin foam microwave absorbing materials include polystyrene foam, polypropylene, and polyethylene foam. Polystyrene foam uses pentane as a blowing agent, which has a strong pungent odor at high temperatures and poor mechanical properties, resulting in a high damage rate during use. Its temperature resistance is generally worse than that of polypropylene or polyethylene. Therefore, carbon dioxide-foamed polypropylene or polyethylene foam has higher mechanical properties and temperature resistance than polystyrene foam.

[0004] Currently, commercially available foamed polypropylene or polyethylene microwave absorbing materials are made by mixing polypropylene or polyethylene resin, microwave absorbing agents, and flame retardants, granulating the mixture, and then preparing it through a carbon dioxide foaming process. To ensure microwave absorption performance, a larger amount of microwave absorbing agent needs to be added, which necessitates a reduction in the proportion of flame retardant. This affects the flame retardant performance, and ultimately, the oxygen index of the resulting polypropylene or polyethylene foam microwave absorbing material cannot meet the requirement of a greater than 28 in the "Technical Specification for Electromagnetic Anechoic Chamber Engineering GB50826-2012".

[0005] In existing technologies, adhesives are typically mixed with flame retardants to prepare polypropylene or polyethylene microwave absorbing foam particles, so that the microwave absorbing and flame retardant coating is coated on the surface of the polymer foam particles. However, in this production process, dissolving the adhesive requires the addition of organic solvents. The use of general organic solvents may pose significant safety hazards and is not environmentally friendly, which can have a certain impact on the physical and mental health of the operators. Moreover, the process is complex, requiring extrusion, granulation, foaming, coating, and molding. Since the polymer foam has a large volume, the operation becomes more difficult, often resulting in low production efficiency.

[0006] Typically, the coating of microwave-absorbing and flame-retardant materials does not significantly increase the volume of the foam particles. Ignoring the weight of the coating, calculations show that when mixing polypropylene or polyethylene microwave-absorbing foam particles with the coating in a 2-cubic-meter mixing tank, the mixing, discharging, and drying process takes approximately one hour per tank. Since only 50%-70% of the space in a 2-cubic-meter mixing tank can be utilized, only about 40-56 kg of foam particles with a density of 40 kg / cubic meter can be produced per hour. Only with continuous 24-hour production could approximately 1120 kg of foam particles be produced per day. Generally, the average consumption of microwave-absorbing material for microwave anechoic chambers is about 15 kg per square meter. A large microwave anechoic chamber of 10,000 square meters requires approximately 150 tons of absorbing material. Even with 24-hour continuous production, producing 150 tons of material would take 133 days. Many customers with large anechoic chambers have very strict deadline requirements, and the production efficiency described above is simply insufficient to meet the needs of large-scale market applications. Summary of the Invention

[0007] To address the insufficient production capacity of existing technologies, the inventors have developed a high-capacity, high-efficiency production process for producing polyolefin foam particle microwave absorbing materials with high flame retardant properties and high economic value through continuous research and development. This process utilizes composite polyolefin resins with different properties. A flame retardant filler is highly filled into polyolefin resin A as the inner layer of the particles, while a conductive agent is filled into polyolefin resin B as the outer layer. The composite polyolefin particles are placed in a reactor and foamed under the foaming conditions of polyolefin resin B, resulting in polyolefin resin microwave absorbing foam particles with a non-foamed or micro-foamed inner layer and a normally foamed outer layer. These particles are then processed using a foam molding process to produce polyolefin resin foam microwave absorbing materials. Because each microwave absorbing foam particle uniformly contains a significant proportion of flame retardant B, the molded microwave absorbing material is evenly distributed across each area, thereby further improving the flame retardancy, environmental friendliness, and economic value of thermoplastic foam microwave absorbing materials while also increasing production efficiency.

[0008] Specifically, the technical solution adopted in this invention is as follows:

[0009] This invention provides a multilayer co-extruded flame-retardant polyolefin foam particle microwave absorbing material, characterized in that: each foam particle has a layered structure, comprising a flame-retardant polyolefin layer and a conductive polyolefin layer, wherein the flame-retardant polyolefin layer is the inner layer and the conductive polyolefin layer is the outer layer; or the flame-retardant polyolefin layer is the middle layer, and the conductive polyolefin layer is both the inner and outer layers; or the flame-retardant polyolefin layer is the outer layer and the conductive polyolefin layer is the inner layer. Preferably, the flame-retardant polyolefin layer is the inner layer and the conductive polyolefin layer is the outer layer.

[0010] Furthermore, in the multilayer co-extruded flame-retardant polyolefin foam particle microwave absorbing material of the present invention, the flame-retardant polyolefin comprises, by weight: 20-80 parts of polyolefin resin A, 20-80 parts of flame retardant A, 0-15 parts of conductive masterbatch, and 0-20 parts of functional additive masterbatch A, which are mixed and then extruded into flame-retardant polyolefin masterbatch A particles. Preferably, the conductive masterbatch comprises 0.1-15 parts and the functional additive masterbatch A comprises 0.1-20 parts.

[0011] Furthermore, in the multilayer co-extruded flame-retardant polyolefin foam particle microwave absorbing material of the present invention, the flame retardant A is selected from one or more of organic flame retardants, inorganic flame retardants, ammonium salt flame retardants, organosilicon flame retardants, and halogen flame retardants.

[0012] Preferably, the organic flame retardant in flame retardant A is selected from one or more of pentaerythritol phosphate, triphenyl phosphate, resorcinol bis(diphenyl phosphate), bisphenol A bis(diphenyl phosphate), trimethyl phosphate, triethyl phosphate, trihexyl phosphate, triphenyl phosphate, monoammonium phosphate, ammonium dihydrogen phosphate, tributyl phosphate, trioctyl phosphate, tributoxyethyl phosphate, diethyl ethylphosphonate, dimethyl phosphate, alkyl diarylphosphonate, 2-ethylhexyl diphenyl phosphate, and dimer aryl phosphate; the inorganic flame retardant is selected from... The flame retardant is selected from one or more of the following: magnesium hydroxide, red phosphorus, coated red phosphorus, aluminum dihydrogen phosphate, expanded graphite, aluminum hydroxide, borate, aluminum oxalate, zinc sulfide, and zinc borate; the ammonium salt flame retardant is selected from one or more of the following: ammonium polyphosphate, ammonium sulfate, and melamine polyphosphate; the organosilicon flame retardant is selected from one or more of the following: organosilicon and polysiloxane; the halogenated flame retardant is selected from one or more of the following: tetrabromobisphenol A, decabromodiphenyl ethane, octabromodiphenyl ether, chlorinated paraffin, brominated polystyrene, tribromophenol, brominated epoxy flame retardant, and chlorinated polyolefin resin.

[0013] On the other hand, in the multilayer co-extruded flame-retardant polyolefin foam particle microwave absorbing material of the present invention, the conductive polyolefin comprises, by weight, 2-98 parts of polyolefin resin B, 1-20 parts of flame retardant masterbatch B, 0.1-95 parts of conductive masterbatch, and 1-20 parts of functional additive masterbatch B.

[0014] Furthermore, in the multilayer co-extruded flame-retardant polyolefin foam particle microwave absorbing material of the present invention, the flame-retardant masterbatch B comprises a polyolefin resin and a flame retardant B. The polyolefin resin is selected from one or more of polypropylene, polyethylene, ethylene-acrylic acid copolymer, and chlorinated polyolefin resin; the flame retardant B is selected from one or more of organic flame retardants, inorganic flame retardants, ammonium salt flame retardants, organosilicon flame retardants, and halogenated flame retardants. After mixing, the flame-retardant masterbatch B particles are obtained by extrusion.

[0015] Preferably, the organic flame retardant in flame retardant B is selected from one or more of pentaerythritol phosphate, triphenyl phosphate, resorcinol bis(diphenyl phosphate), bisphenol A bis(diphenyl phosphate), trimethyl phosphate, triethyl phosphate, trihexyl phosphate, triphenyl phosphate, monoammonium phosphate, ammonium dihydrogen phosphate, tributyl phosphate, trioctyl phosphate, tributoxyethyl phosphate, diethyl ethylphosphonate, dimethyl phosphate, alkyl diarylphosphonate, 2-ethylhexyl diphenyl phosphate, and dimer aryl phosphate; the inorganic flame retardant is selected from... The flame retardant is selected from one or more of the following: magnesium hydroxide, red phosphorus, coated red phosphorus, aluminum dihydrogen phosphate, expanded graphite, aluminum hydroxide, borate, aluminum oxalate, zinc sulfide, and zinc borate; the ammonium salt flame retardant is selected from one or more of the following: ammonium polyphosphate, ammonium sulfate, and melamine polyphosphate; the organosilicon flame retardant is selected from one or more of the following: organosilicon and polysiloxane; the halogenated flame retardant is selected from one or more of the following: tetrabromobisphenol A, decabromodiphenyl ethane, octabromodiphenyl ether, chlorinated paraffin, brominated polystyrene, tribromophenol, brominated epoxy flame retardant, and chlorinated polyolefin resin.

[0016] Furthermore, in the multilayer co-extruded flame-retardant polyolefin foam particle microwave absorbing material of the present invention, the flame retardant B accounts for 1-20% of the total mass of the conductive polyolefin by mass percentage, preferably 2-15%, preferably 3-10%, and more preferably 5-8%.

[0017] Furthermore, in the multilayer co-extruded flame-retardant polyolefin foam particle microwave absorbing material of the present invention, the polyolefin resin A and polyolefin resin B are polyolefin resins, preferably one or more of polypropylene resin, polyethylene resin, maleic anhydride-grafted polypropylene, ethylene-acrylic acid copolymer, and chlorinated polyolefin resin. Preferably, polyolefin resin A is a polymer that only softens at a certain temperature and does not foam or is difficult to foam. When polyolefin resin B reaches its melting point, polyolefin resin A has not reached its melting point or is close to melting; or when the reactor begins to discharge and cool, polyolefin resin A has not begun to crystallize or its crystallization rate is less than that of polyolefin resin B.

[0018] Furthermore, in the multilayer co-extruded flame-retardant polyolefin foam particle microwave absorbing material of the present invention, the conductive masterbatch includes polypropylene resin or polyethylene resin, microwave absorbing powder, and / or dispersing lubricant. The microwave absorbing powder is selected from one or more of carbon black, graphite, carbon nanotubes, graphene, and carbon fiber. The dispersing lubricant is selected from one or more of polyethylene wax, polypropylene wax, and polytetrafluoroethylene wax.

[0019] Furthermore, in the multilayer co-extruded flame-retardant polyolefin foam particle microwave absorbing material of the present invention, the functional additive masterbatch A comprises a polyolefin resin and functional additives. The polyolefin resin is selected from one or more of polypropylene, polyethylene, maleic anhydride-grafted polypropylene, ethylene-acrylic acid copolymer, and chlorinated polyolefin resin. The functional additives are selected from one or more of antioxidants, ultraviolet absorbers, antibacterial agents, dispersants, lubricants, and molecular weight regulators. The functional additive masterbatch B comprises a polyolefin resin and functional additives. The polyolefin resin is selected from one or more of polypropylene, polyethylene, maleic anhydride-grafted polypropylene, ethylene-acrylic acid copolymer, and chlorinated polyolefin resin. The functional additives are selected from one or more of nucleating agents, antioxidants, ultraviolet absorbers, antibacterial agents, and dispersants.

[0020] Another aspect of this invention is to provide a method for preparing a multilayer co-extruded flame-retardant polyolefin foam particle microwave absorbing material, comprising the following steps:

[0021] Step S1: Mix polyolefin resin A, flame retardant A, conductive masterbatch, and functional additive masterbatch A, and then extrude them into flame retardant masterbatch A granules.

[0022] Step S2: Add the flame-retardant masterbatch A particles obtained in step S1 to extruder A. Simultaneously, add polyolefin resin B, flame-retardant masterbatch B, conductive masterbatch, and functional additive masterbatch B to extruder B. Extrude through a two-layer co-extrusion die to obtain columnar or ring-shaped polyolefin resin microwave-absorbing particles with an inner layer of flame-retardant polyolefin and an outer layer of conductive polyolefin, or vice versa.

[0023] By replacing the extruder die with a 3-layer co-extrusion die, columnar or ring-shaped polyolefin resin microwave absorbing particles with flame-retardant polyolefin as the middle layer and conductive polyolefin as the inner and outer layers are obtained.

[0024] Step S3: Add the polyolefin resin microwave absorbing particles obtained in step S2 to the reactor, add water and dispersant to the high-pressure reactor, stir, heat and pressurize by introducing carbon dioxide, continue heating until the melting point of the conductive polyolefin layer resin is reached, and release the material to obtain polyolefin microwave absorbing foam particles with flame retardant polyolefin layer that does not foam or micro-foams while conductive polyolefin layer foams.

[0025] Step S4: The polyolefin absorbing foam particles obtained in step S3 are molded into absorbing materials.

[0026] Furthermore, in step S1 of the preparation method of the multilayer co-extruded flame-retardant polyolefin foam particle microwave absorbing material of the present invention, the polyolefin resin A is 20-80 parts, the flame retardant A is 20-80 parts, the conductive masterbatch is 0.1-15 parts, and the functional auxiliary agent masterbatch A is 0.1-20 parts.

[0027] Furthermore, in step S2 of the preparation method of the multilayer co-extruded flame-retardant polyolefin foam particle microwave absorbing material of the present invention, the polyolefin resin B is 2-98 parts, the flame retardant masterbatch B is 1-20 parts, the conductive masterbatch is 0.1-95 parts, and the functional additive masterbatch B is 1-20 parts.

[0028] Furthermore, in step S2 of the preparation method of the multilayer co-extruded flame-retardant polyolefin foam particle microwave absorbing material of the present invention, the obtained columnar or ring-shaped polyolefin resin microwave absorbing particles are 0.5-6 mg.

[0029] Furthermore, in step S3 of the preparation method of the multilayer co-extruded flame-retardant polyolefin foam particle microwave absorbing material of the present invention, the stirring speed is 300 r / min, the heating is continued until the melting point of the conductive polypropylene resin layer is 145°C, and the carbon dioxide pressure is 3.5 MPa.

[0030] The advantages and beneficial effects of this invention are as follows:

[0031] (1) The process of this invention is simple. The microwave absorbing material that meets the highest level of UL94-V0 flame retardant standard can be obtained by direct extrusion, granulation, foaming and molding, without the need for a coating process. At the same time, it has no impact on the mechanical properties of the microwave absorbing material.

[0032] (2) Each microwave absorbing foam particle prepared by the present invention contains a large proportion of flame retardant B, which works synergistically with flame retardant A. The flame retardant can be evenly distributed in each area of ​​the molded microwave absorbing material, and the product can have better flame retardant effect both inside and outside. While improving the flame retardant performance, it will not affect the surface appearance of the microwave absorbing material.

[0033] (3) The process of this invention is environmentally friendly. No solvent is required in the production process. It is harmless to the physical and mental health of workers, has no impact on the air quality of the built darkroom, and does not pollute the environment.

[0034] (4) The present invention has high production efficiency. One extruder can produce 6 tons of polyolefin resin microwave absorbing particles every 24 hours, and one 2.5 cubic meter high pressure reactor can foam 6 tons of polyolefin microwave absorbing foam particles every 24 hours. It only takes 25 days to produce 150 tons of microwave absorbing material required for a 10,000 square meter large darkroom, which greatly shortens the project period.

[0035] (5) This invention has high economic value, and its production capacity can be increased by about 4 to 5 times per day compared with the coating process. Moreover, it does not require the use of organic solvents, which greatly reduces production costs. Attached Figure Description

[0036] Figure 1 is a scanning electron microscope (SEM) image of the foam particles prepared in Example 1, with an inner layer of flame-retardant polypropylene and an outer layer of conductive polypropylene, after being cut open.

[0037] Figure 2 is a photograph of the foam absorbing particles prepared in Example 1, with an inner layer of flame-retardant polypropylene and an outer layer of conductive polypropylene.

[0038] Figure 3 is a comparison of the reflectance of the samples prepared by the examples and comparative examples. Detailed Implementation

[0039] The present invention will be further described below with reference to the accompanying drawings and preferred embodiments. It should be understood that the preferred embodiments described herein are for illustration and explanation only and are not intended to limit the present invention. The raw materials, masterbatches, reagents, etc. used in the following embodiments are all commercially available or conventionally obtainable.

[0040] As described in this invention, the polyolefin resin of this invention comprises polyolefins with different melting points obtained by mixing one or more polyolefins. The polyolefin resin includes, but is not limited to, polypropylene, polyethylene, maleic anhydride-grafted polypropylene, ethylene-acrylic acid copolymer resin, chlorinated polyolefin resin, etc.

[0041] As described in this invention, the flame retardant A in the flame-retardant polyolefin of this invention includes, but is not limited to: organic flame retardants, inorganic flame retardants, ammonium salt flame retardants, organosilicon flame retardants, halogenated flame retardants, etc. Specifically, organic flame retardants include, but are not limited to, pentaerythritol phosphate, triphenyl phosphate, resorcinol bis(diphenyl phosphate), bisphenol A bis(diphenyl phosphate), trimethyl phosphate, triethyl phosphate, trihexyl phosphate, triphenyl phosphate, monoammonium phosphate, ammonium dihydrogen phosphate, tributyl phosphate, trioctyl phosphate, tributoxyethyl phosphate, diethyl ethylphosphonate, dimethyl phosphate, alkyl diarylphosphonate, 2-ethylhexyl diphenyl phosphate, dimer aryl phosphate, etc.; inorganic flame retardants include, but are not limited to: pentaerythritol phosphate, triphenyl phosphate, resorcinol bis(diphenyl phosphate), bis(diethyl ethyl phosphonate), diethyl ethylphosphonate, dimethyl phosphate, alkyl diarylphosphonate, 2-ethylhexyl diphenyl phosphate, dimer aryl phosphate, etc.; inorganic flame retardants include, but are not limited to: pentaerythritol phosphate, triphenyl phosphate, resorcinol bis(diphenyl phosphate), diethyl phosphonate, diethyl alkyl diarylphosphonate, diethylhexyl diphenyl phosphate, dimer aryl phosphate, etc. The preferred flame retardants are one or more of magnesium hydroxide, red phosphorus, coated red phosphorus, aluminum dihydrogen phosphate, expanded graphite, aluminum hydroxide, borate, aluminum oxalate, zinc sulfide, and zinc borate; the preferred ammonium salt flame retardants are ammonium polyphosphate, ammonium sulfate, melamine polyphosphate, etc.; the preferred organosilicon flame retardants are organosilicone, polysiloxane, etc.; the preferred halogenated flame retardants are tetrabromobisphenol A, decabromodiphenyl ethane, octabromodiphenyl ether, chlorinated paraffin, brominated polystyrene, tribromophenol, brominated epoxy flame retardants, chlorinated polyolefin resin, etc.

[0042] As described in this invention, the flame retardant B in the conductive polyolefin of this invention includes, but is not limited to, organic flame retardants, inorganic flame retardants, ammonium salt flame retardants, organosilicon flame retardants, halogenated flame retardants, etc. Specifically, the preferred organic flame retardants are pentaerythritol phosphate, triphenyl phosphate, resorcinol bis(diphenyl phosphate), bisphenol A bis(diphenyl phosphate), trimethyl phosphate, triethyl phosphate, trihexyl phosphate, triphenyl phosphate, monoammonium phosphate, ammonium dihydrogen phosphate, tributyl phosphate, trioctyl phosphate, tributoxyethyl phosphate, diethyl ethylphosphonate, dimethyl phosphate, alkyl diarylphosphonate, 2-ethylhexyl diphenyl phosphate, dimer aryl phosphate, etc. Inorganic flame retardants are preferably magnesium hydroxide, red phosphorus, coated red phosphorus, aluminum dihydrogen phosphate, expanded graphite, aluminum hydroxide, borate, aluminum oxalate, zinc sulfide, zinc borate, etc.; ammonium salt flame retardants are preferably ammonium polyphosphate, ammonium sulfate, melamine polyphosphate, etc.; organosilicon flame retardants are preferably organosilicone, polysiloxane, etc.; halogenated flame retardants are preferably tetrabromobisphenol A, decabromodiphenyl ethane, octabromodiphenyl ether, chlorinated paraffin, brominated polystyrene, tribromophenol, brominated epoxy flame retardants, chlorinated polyolefin resin, etc.

[0043] As described in this invention, the functional additive A in the functional additive masterbatch A includes, but is not limited to: antioxidants, ultraviolet absorbers, antibacterial agents, dispersants, lubricants, molecular weight regulators, etc.; the functional additive B in the functional additive masterbatch B includes, but is not limited to: nucleating agents, antioxidants, ultraviolet absorbers, antibacterial agents, dispersants, lubricants, etc.; the molecular weight regulator includes, but is not limited to: benzoyl peroxide, di-tert-butyl peroxide, methyl ethyl ketone peroxide, 2,5-dimethyl-2,5-di(tert-butylperoxide)hexane, etc.

[0044] As described in this invention, the microwave absorbing powder in the conductive masterbatch includes, but is not limited to, carbon black, graphite, carbon nanotubes, graphene, carbon fibers, etc. The dispersing lubricant includes, but is not limited to, polyethylene wax, polypropylene wax, polytetrafluoroethylene wax, etc.

[0045] Example 1:

[0046] A multilayer co-extruded flame-retardant polypropylene foam particle microwave absorbing material, the preparation method of which includes the following steps:

[0047] Step S1: By weight, mix 48 parts of polypropylene resin A, 50 parts of expanded graphite, 1 part of conductive masterbatch, and 1 part of functional additive masterbatch A, and then extrude them into flame retardant masterbatch A granules.

[0048] Step S2: By weight, 60 parts of the flame retardant masterbatch A obtained in step S1 are added to extruder A as the inner layer; 19 parts of polypropylene resin B, 15 parts of flame retardant masterbatch B, 70 parts of conductive masterbatch, and 1 part of functional additive masterbatch B are added to extruder B as the outer layer. The flame retardant masterbatch B is formulated with 45 parts polypropylene, 50 parts coated red phosphorus, and 5 parts PE wax, mixed and extruded. Calculations show that the coated red phosphorus in the flame retardant B accounts for 7.5% of the total mass of the conductive polypropylene; 0.5-6 mg columnar polypropylene resin microwave absorbing particles with an inner layer of flame retardant polypropylene and an outer layer of conductive polypropylene are obtained by extrusion through a two-layer co-extrusion die.

[0049] Step S3: Add the polypropylene resin microwave absorbing particles obtained in step S2 to the reactor. Add water and sodium dodecylbenzenesulfonate (3.5% by weight of water) as a dispersant to the high-pressure reactor. Start stirring at a speed of 300 r / min. Heat and gradually introduce carbon dioxide. Continue heating until the melting point of the outer conductive polypropylene resin layer reaches 145°C and the carbon dioxide pressure reaches 3.5 MPa. Release the material and wash to obtain polypropylene foam microwave absorbing particles with a non-foamed inner layer and a foamed outer layer.

[0050] Step S4: The polypropylene foam absorbing particles obtained in step S3 are molded to obtain polypropylene foam absorbing material.

[0051] Note:

[0052] 1. The flame-retardant polypropylene layer is a mixture, and its melting point is 150°C as measured by differential scanning calorimetry (DSC); the conductive polypropylene layer is also a mixture, and its melting point is 145°C as measured by DSC.

[0053] 2. The functional additive masterbatch A is prepared by mixing 70 parts polypropylene resin and 30 parts polyethylene (PE) wax (a dispersing lubricant) using an extruder. The functional additive masterbatch B is prepared by mixing 70 parts polypropylene resin, 10 parts zinc borate (a nucleating agent), 5 parts BASF antioxidant (model 1010), and 1 part PE wax (a dispersing lubricant) using an extruder.

[0054] 3. The conductive masterbatch is prepared by mixing 77 parts polypropylene resin, 3 parts PE wax (dispersing lubricant), and 20 parts carbon black (specific surface area greater than 1000) in an internal mixer and then extruding.

[0055] Example 2

[0056] A multilayer co-extruded flame-retardant polypropylene foam particle microwave absorbing material, the preparation method of which includes the following steps:

[0057] Step S1: By weight, 24 parts of maleic anhydride-grafted polypropylene resin and 24 parts of ethylene-acrylic acid copolymer, totaling 46 parts, are used as polyolefin resin A, along with 50 parts of expanded graphite, 1 part of conductive masterbatch, and 3 parts of functional additive masterbatch A. The mixture is then extruded to form flame retardant masterbatch A granules.

[0058] Step S2: By weight, 14 parts of polypropylene resin B, 15 parts of flame retardant masterbatch B, 70 parts of conductive masterbatch, and 1 part of functional additive masterbatch B are mixed and added to extruder A as the inner layer. The flame retardant masterbatch B is formulated with 45 parts of polypropylene, 50 parts of coated red phosphorus, and 5 parts of PE wax. After mixing, it is formed by extrusion. 60 parts of flame retardant masterbatch A particles obtained in step S1 are added to extruder B as the outer layer. Through a two-layer co-extrusion die, 0.5-6 mg of columnar polypropylene resin microwave absorbing particles with conductive polypropylene as the inner layer and flame retardant polypropylene as the outer layer are obtained.

[0059] Step S3: Add the polyolefin resin microwave absorbing particles obtained in step S2 to the reactor. Add water and sodium dodecylbenzenesulfonate (3.5% by weight of water) as a dispersant to the high-pressure reactor. Start stirring at a speed of 300 r / min. Heat and gradually introduce carbon dioxide. Continue heating until the inner conductive polypropylene resin layer reaches a melting point of 145°C and the carbon dioxide pressure reaches 3.5 MPa. Release the material and wash to obtain polypropylene foam microwave absorbing particles with foamed inner layer and non-foamed outer layer.

[0060] Step S4: The polypropylene foam absorbing particles obtained in step S3 are molded to obtain polypropylene foam absorbing material.

[0061] Note:

[0062] 1. The flame-retardant polypropylene layer is a mixture, and its melting point is 146°C as measured by DSC; the conductive polypropylene layer is also a mixture, and its melting point is 145°C as measured by DSC.

[0063] 2. The functional additive masterbatch A is prepared by mixing 70 parts of polypropylene resin, 29.5 parts of dispersing lubricant PE wax, and 0.5% of molecular weight regulator di-tert-butyl peroxide through an extruder; the functional additive masterbatch B is prepared by mixing 70 parts of polypropylene resin, 10 parts of nucleating agent zinc borate, 5 parts of BASF antioxidant of type 1010, and 1 part of dispersing lubricant PE wax through an extruder.

[0064] Example 3

[0065] A multilayer co-extruded flame-retardant polypropylene foam particle microwave absorbing material, the preparation method of which includes the following steps:

[0066] Step S1: By weight, 24 parts of maleic anhydride-grafted polypropylene resin and 24 parts of ethylene-acrylic acid copolymer, totaling 46 parts, are used as polypropylene resin, along with 50 parts of expanded graphite, 1 part of conductive masterbatch, and 3 parts of functional additive masterbatch A. The mixture is then extruded to form flame retardant masterbatch A particles.

[0067] Step S2: By weight, 19 parts of polypropylene resin B, 10 parts of flame retardant masterbatch B, 70 parts of conductive masterbatch, and 1 part of functional additive masterbatch B are mixed and added to extruder A as the inner and outer layers, respectively. The flame retardant masterbatch B has the following composition: 45 parts of polypropylene, 50 parts of coated red phosphorus, and 5 parts of PE wax. After mixing, it is obtained by extrusion. 60 parts of the flame retardant masterbatch A particles obtained in step S1 are added to extruder B as the intermediate layer. The particles are extruded through a 3-layer co-extrusion die to obtain 0.5-6 mg columnar polypropylene resin microwave absorbing particles with conductive polypropylene for the inner and outer layers and flame retardant polypropylene for the intermediate layer.

[0068] Step S3: Add the polypropylene resin microwave absorbing particles obtained in step S2 to the reactor. Add water and sodium dodecylbenzenesulfonate (3.5% by weight of water) as a dispersant to the high-pressure reactor. Start stirring at a speed of 300 r / min. Heat and gradually introduce carbon dioxide. Continue heating until the melting point of the conductive polypropylene resin in the inner and outer layers reaches 145°C. Release the material and wash to obtain polypropylene foam microwave absorbing particles with foamed inner and outer layers but no foamed middle layer.

[0069] Step S4: The polyolefin foam absorbing particles obtained in step S3 are molded into polypropylene foam absorbing material.

[0070] Note:

[0071] 1. The flame-retardant polypropylene layer is a mixture, and its melting point is 146°C as measured by DSC; the conductive polypropylene layer is also a mixture, and its melting point is 145°C as measured by DSC.

[0072] 2. The functional additive masterbatch A is prepared by mixing 70 parts of polypropylene resin, 25 parts of dispersing lubricant PE wax, and 0.5% of molecular weight regulator di-tert-butyl peroxide through an extruder; the functional additive masterbatch B is prepared by mixing 70 parts of polypropylene resin, 10 parts of nucleating agent zinc borate, 5 parts of BASF antioxidant of type 1010, and 1 part of dispersing lubricant PE wax through an extruder.

[0073] Comparative Example 1

[0074] A polypropylene foam particle microwave absorbing material, the preparation method of which includes the following steps:

[0075] Step S1: By weight, 48 parts of polypropylene resin, 50 parts of red phosphorus coating, and 2 parts of PE wax dispersing lubricant are mixed and then extruded to form flame retardant masterbatch.

[0076] Step S2: By weight, 14 parts of polypropylene resin, 70 parts of conductive masterbatch, 15 parts of flame retardant masterbatch obtained in step S1, and 1 part of nucleating agent zinc borate are mixed and extruded through a twin-screw extruder to obtain 0.2-3.2 mg of columnar polypropylene resin microwave absorbing microparticles.

[0077] Step S3: Add the columnar polypropylene resin microwave absorbing microparticles obtained in step S2 to the reactor. Add water and sodium dodecylbenzenesulfonate (3.5% by weight of water) as a dispersant to the high-pressure reactor. Start stirring at a speed of 300 r / min. Heat the reactor and gradually introduce carbon dioxide. Continue heating until the melting point of the columnar polypropylene resin microwave absorbing microparticles reaches 145°C. Release the material and wash it to obtain polypropylene foam microwave absorbing particles.

[0078] Step S4: The polypropylene foam absorbing particles obtained in step S3 are molded to obtain polypropylene foam absorbing material.

[0079] Comparative Example 2

[0080] A polypropylene foam particle microwave absorbing material, the preparation method of which includes the following steps:

[0081] Step S1: By weight, 48 parts of polypropylene resin, 50 parts of red phosphorus coating, and 2 parts of PE wax dispersing lubricant are mixed and then extruded to form flame retardant masterbatch.

[0082] Step S2: By weight, 14 parts of polypropylene resin, 70 parts of conductive masterbatch, 15 parts of flame retardant masterbatch obtained in step S1, and 1 part of nucleating agent zinc borate are mixed and extruded through a twin-screw extruder to obtain 0.2-3.2 mg of columnar polypropylene resin microwave absorbing microparticles.

[0083] Step S3: Add the columnar polypropylene resin microwave absorbing microparticles obtained in step S2 to the reactor. Add water and sodium dodecylbenzenesulfonate (3.5% by weight of water) as a dispersant to the high-pressure reactor. Start stirring at a speed of 300 r / min. Heat the reactor and gradually introduce carbon dioxide. Continue heating until the melting point of the columnar polypropylene resin microwave absorbing microparticles reaches 145°C. Release the material and wash it to obtain polypropylene foam microwave absorbing particles.

[0084] Step S4: By weight, add 50 parts of 50% acrylic resin, 5 parts of carbon black, 80 parts of expanded graphite flame retardant, and 100 parts of ammonia solvent to a dispersion mixer and mix for 20 minutes until uniform to obtain flame retardant coating.

[0085] Step S5: By weight, add 30 parts of flame retardant coating obtained in step S4 and 60 parts of polypropylene foam absorbing particles obtained in step S3 into a mixing tank, mix for 20 minutes until uniform, and then put into a drying device to dry at 50-80℃ for 20 minutes to obtain polypropylene foam absorbing particles coated with flame retardant coating.

[0086] Step S6: The polypropylene foam absorbing particles coated with flame retardant coating obtained in step S5 are molded into polypropylene foam absorbing material.

[0087] The microwave absorbing materials prepared in Examples 1 to 3 and Comparative Examples 1 to 2 were tested. The test sample dimensions were 600*600*50mm. The test results are shown in Table 1 and Figure 3 below.

[0088] Table 1

[0089] in conclusion:

[0090] As shown in Table 1, the microwave absorbing material prepared in Comparative Example 1 cannot meet the flame retardant requirements for microwave absorbing materials used in microwave anechoic chambers. The microwave absorbing material in Comparative Example 2 is coated with a flame retardant coating based on Comparative Example 1. However, due to the conductivity of expanded graphite as a flame retardant, the surface resistance of the microwave absorbing material prepared in Comparative Example 2 is lower, and it cannot meet the resistance requirements of some microwave absorbing materials.

[0091] Table 1 and Figure 3 also show the changes in the 30M-2000M reflectance resonance peaks of the absorbing materials prepared in the examples and comparative examples. The materials prepared in Comparative Example 1 and Comparative Example 2 contain the same absorber, but Comparative Example 2 is coated with a layer of flame retardant containing expanded graphite, which reduces the resistance of the absorbing material, causing the resonance peak to shift to lower frequencies and the high-frequency reflectance to decrease. Furthermore, due to the poor compatibility between polypropylene and the adhesive acrylic resin, the resulting polypropylene foam absorbing material, after being encapsulated in polypropylene foam particles and then pressed into a product using a foam molding machine, has poor mechanical properties, is very brittle, and is easily damaged, resulting in a high defect rate and hindering finished product transportation and subsequent installation operations.

[0092] The absorbent materials prepared in Examples 1 to 3 of this invention meet or exceed national standards in terms of flame retardancy, volume resistivity, and electromagnetic parameters. Specifically, Figure 1 shows the microstructure of the inner layer of flame-retardant polypropylene at the center of the whole foam particle prepared in Example 1, with a large number of expanded graphite sheets composited inside the flame-retardant polypropylene. Figure 2 shows the stacking state of the foam particles prepared in Example 1, as well as a cut image of some microwave-absorbing foam particles. It can be seen that the inner flame-retardant polypropylene layer is filled with a large amount of expanded graphite flame retardant. Due to the low proportion of conductive carbon black added, the inner layer appears gray; the outer layer is a conductive polypropylene layer, which is clearly black. In addition, since the melting point of the flame-retardant polypropylene layer is higher than that of the conductive polypropylene layer, it can be clearly seen from the complete single foam particle in the lower right of Figure 2 that the inner flame-retardant polypropylene layer has a small expansion and shrinkage pores. Therefore, the flame-retardant polypropylene layer does not occupy a large volume of the whole microwave-absorbing foam particle and has no impact on the mechanical properties and microwave absorption performance of the final foam microwave-absorbing material.

[0093] On the other hand, Example 1 directly prepared flame-retardant polypropylene microwave-absorbing foam particles using an extruder and reactor, eliminating the need for a coating process. Calculations show that a 2.5 cubic meter reactor can produce 6 tons of flame-retardant polypropylene microwave-absorbing foam particles per day. However, the method in Comparative Example 2 requires foaming the particles obtained in Comparative Example 1 and then coating them with an adhesive to produce flame-retardant polypropylene foam particles. Calculations show that Comparative Example 2, using a 2.5 cubic meter mixing tank, can produce 60 kg of foam particles per hour, and only 1440 kg of flame-retardant polypropylene microwave-absorbing foam particles per day. Calculations also show that the solvent accounts for approximately 30% of the total mass of the microwave-absorbing foam particles; after drying, the solvent evaporates, representing a wasted material, resulting in a material cost that is more than 10% higher than that of Example 1.

[0094] This invention boasts high production efficiency. One extruder can produce 6 tons of polyolefin resin microwave absorbing particles per day in 24 hours, and one 2.5 cubic meter high-pressure reactor can foam 6 tons of polyolefin microwave absorbing foam particles per day in 24 hours. The production of 150 tons of microwave absorbing material required for a 10,000 square meter large anechoic chamber can be completed in just 25 days, significantly shortening the project duration. Compared to Comparative Example 2, the production efficiency is increased by 416%, demonstrating the excellent technical effects achieved by this invention.

[0095] 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 multilayer co-extruded flame-retardant polyolefin foam particle microwave absorbing material, characterized in that: Each foam particle has a layered structure, including a flame-retardant polyolefin layer and a conductive polyolefin layer, wherein the flame-retardant polyolefin layer is the inner layer and the conductive polyolefin layer is the outer layer; or the flame-retardant polyolefin layer is the middle layer and the conductive polyolefin layer is both the inner and outer layers; or the flame-retardant polyolefin layer is the outer layer and the conductive polyolefin layer is the inner layer.

2. The multilayer co-extruded flame-retardant polyolefin foam particle microwave absorbing material according to claim 1, characterized in that, By weight, the flame-retardant polyolefin comprises: 20-80 parts of polyolefin resin A, 20-80 parts of flame retardant A, 0-15 parts of conductive masterbatch, and 0-20 parts of functional additive masterbatch A, which are mixed and then extruded into flame-retardant polyolefin masterbatch A particles.

3. The multilayer co-extruded flame-retardant polyolefin foam particle microwave absorbing material according to claim 2, characterized in that: The flame retardant A is selected from one or more of the following: organic flame retardants, inorganic flame retardants, ammonium salt flame retardants, organosilicon flame retardants, and halogenated flame retardants.

4. The multilayer co-extruded flame-retardant polyolefin foam particle microwave absorbing material according to claim 1, characterized in that, By weight, the conductive polyolefin comprises: 2-98 parts of polyolefin resin, 1-20 parts of flame retardant masterbatch, 0.1-95 parts of conductive masterbatch, and 1-20 parts of functional additive masterbatch.

5. The multilayer co-extruded flame-retardant polyolefin foam particle microwave absorbing material according to claim 4, characterized in that: The flame retardant masterbatch B comprises a polyolefin resin and a flame retardant B. The polyolefin resin is selected from one or more of polypropylene, polyethylene, ethylene-acrylic acid copolymer, and chlorinated polyolefin resin. The flame retardant B is selected from one or more of organic flame retardants, inorganic flame retardants, ammonium salt flame retardants, organosilicon flame retardants, and halogenated flame retardants. After mixing, the flame retardant masterbatch B particles are obtained by extrusion.

6. The multilayer co-extruded flame-retardant polyolefin foam particle microwave absorbing material according to claim 5, characterized in that: The flame retardant B accounts for 1-20% of the total mass of the conductive polyolefin by weight percentage.

7. The multilayer co-extruded flame-retardant polyolefin foam particle microwave absorbing material according to claim 2 or 4, characterized in that: The polyolefin resin A and polyolefin resin B are respectively selected from one or more of polypropylene resin, polyethylene resin, maleic anhydride-grafted polypropylene, ethylene-acrylic acid copolymer, and chlorinated polyolefin resin.

8. The multilayer co-extruded flame-retardant polyolefin foam particle microwave absorbing material according to claim 2 or 4, characterized in that: The conductive masterbatch includes polypropylene resin or polyethylene resin, microwave absorbing powder and / or dispersing lubricant, wherein the microwave absorbing powder is selected from one or more of carbon black, graphite, carbon nanotubes, graphene, and carbon fiber.

9. The multilayer co-extruded flame-retardant polyolefin foam particle microwave absorbing material according to claim 2 or 4, characterized in that: The functional additive masterbatch A comprises a polyolefin resin and a functional additive A. The polyolefin resin is selected from one or more of polypropylene, polyethylene, maleic anhydride-grafted polypropylene, ethylene-acrylic acid copolymer, and chlorinated polyolefin resin. The functional additive A is selected from one or more of antioxidants, ultraviolet absorbers, antibacterial agents, dispersants, lubricants, and molecular weight regulators. The functional additive masterbatch B comprises a polyolefin resin and a functional additive B. The polyolefin resin is selected from one or more of polypropylene, polyethylene, maleic anhydride-grafted polypropylene, ethylene-acrylic acid copolymer, and chlorinated polyolefin resin. The functional additive B is selected from one or more of nucleating agents, antioxidants, ultraviolet absorbers, antibacterial agents, and dispersants.

10. A method for preparing a multilayer co-extruded flame-retardant polyolefin foam particle microwave absorbing material, characterized in that: Includes the following steps: Step S1: Mix polyolefin resin A, flame retardant A, conductive masterbatch, and functional additive masterbatch A, and then extrude them into flame retardant masterbatch A granules. Step S2: Add the flame-retardant masterbatch A particles obtained in step S1 to extruder A. Simultaneously, add polyolefin resin B, flame-retardant masterbatch B, conductive masterbatch, and functional additive masterbatch B to extruder B. Extrude through a two-layer co-extrusion die to obtain columnar or ring-shaped polyolefin resin microwave-absorbing particles with an inner layer of flame-retardant polyolefin and an outer layer of conductive polyolefin, or vice versa. By replacing the extruder die with a 3-layer co-extrusion die, columnar or ring-shaped polyolefin resin microwave absorbing particles with flame-retardant polyolefin as the middle layer and conductive polyolefin as the inner and outer layers are obtained. Step S3: Add the polyolefin resin microwave absorbing particles obtained in step S2 to the reactor, add water and dispersant to the high-pressure reactor, stir, heat and pressurize by introducing carbon dioxide, continue heating until the melting point of the conductive polyolefin layer resin is reached, and release the material to obtain polyolefin microwave absorbing foam particles with flame retardant polyolefin layer that does not foam or micro-foams while conductive polyolefin layer foams. Step S4: The polyolefin absorbing foam particles obtained in step S3 are molded into absorbing materials.

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