A composite polypropylene foamed beads, its preparation method and molding body

Abstract

This invention relates to the field of polypropylene foam bead preparation technology, and particularly to a method for preparing composite polypropylene foam beads, comprising the following steps: S1: mixing expanded polystyrene virgin particles with conductive carbon black, pre-stirring at 55-75°C, then adding an organic solvent and continuing stirring to obtain a coating slurry; S2: adding polypropylene foam beads to the coating slurry for surface coating, and performing pre-pressing treatment to obtain composite polypropylene foam beads. By coating the surface of the polypropylene foam beads with microwave-absorbing components, making them concentrated in the outer layer where they actually exert their microwave-absorbing effect, the amount of nano-absorbing components used is reduced while meeting the required resistance value for microwave absorption performance, thus lowering raw material costs; simultaneously, using highly flame-retardant polypropylene foam beads as a substrate, the resulting composite polypropylene foam beads and their molded bodies can still achieve a flame-retardant oxygen index of over 32% while meeting microwave absorption performance requirements.

Description

Technical Field

[0001] This invention relates to the field of polypropylene foamed beads preparation technology, and in particular to a composite polypropylene foamed beads, its preparation method and molding body. Background Technology

[0002] With the continuous growth in demand for wireless communication, artificial intelligence terminals, and electromagnetic compatibility testing, the performance requirements for absorbing materials in microwave anechoic chambers and other electromagnetic testing environments are constantly increasing. Existing absorbing materials are typically made of lightweight expanded polyurethane foam (EPU), expanded polystyrene foam (EPS), or expanded polypropylene foam (EPP) to convert incident electromagnetic waves into heat energy and dissipate it internally. Among these, EPU is prone to sagging deformation at its pointed tip under high temperatures, exhibiting insufficient dimensional stability; while EPS, although lower in cost, is brittle and has poor heat resistance, typically requiring temperatures to be controlled below 80°C to prevent deformation, thus limiting its application in high-frequency, high-heat-load testing environments; in contrast, EPP combines better heat resistance, resilience, and toughness, making it more suitable as a substrate for molded absorbing bodies.

[0003] However, existing microwave-absorbing EPP molded bodies typically achieve lower surface resistivity by increasing the amount of conductive carbon black added during the modification process to meet microwave absorption requirements. However, with the increase in the amount of conductive carbon black added, the flame retardant performance of the EPP molded body is difficult to improve simultaneously, and the oxygen index in the industry is usually only maintained at around 28%. Summary of the Invention

[0004] To address the problem that existing EPP molded bodies cannot simultaneously meet microwave absorption performance and improve flame retardancy, this invention provides a method for preparing composite EPP foam beads. By coating the surface of EPP foam beads with microwave-absorbing components, these components are concentrated in the outermost layer where microwave absorption actually occurs. This method reduces the amount of nano-absorbing components used, thereby lowering raw material costs, while still meeting the required resistance value for microwave absorption performance. Furthermore, by using highly flame-retardant EPP foam beads as the substrate, the resulting composite EPP foam beads and their molded bodies can achieve a flame-retardant oxygen index of over 32% while meeting microwave absorption performance requirements.

[0005] The technical solution adopted by this invention to solve its technical problem is: A method for preparing composite EPP foamed beads includes the following steps: S1: Mix expanded polystyrene virgin particles with conductive carbon black, pre-stir at 55-75℃, then add organic solvent and continue stirring to obtain the coating slurry; S2: EPP foam beads are added to the coating slurry for surface coating and pre-pressed to obtain composite EPP foam beads.

[0006] Optionally, the conductive carbon black is nanoscale conductive carbon black with a particle size of 20-50 nm and a bulk density of 120-150 g / L.

[0007] Optionally, the mass ratio of the conductive carbon black to the expanded polystyrene virgin particles is (1-1.6):1.

[0008] Optionally, the organic solvent is selected from at least one of ethyl acetate or acetone.

[0009] Optionally, the mass ratio of the organic solvent to the expanded polystyrene virgin particles is (0.8-1.6):1.

[0010] Optionally, in step S2, the volume ratio of the coating slurry to the EPP foamed beads is 1:(15-40).

[0011] Optionally, the oxygen index of the EPP foam beads is ≥32%.

[0012] Another object of the present invention is to provide a composite EPP foamed bead, which is prepared by the method described above for preparing composite EPP foamed beads.

[0013] Another object of the present invention is to provide a composite EPP foamed molded body, which is obtained by steam molding of the composite EPP foamed beads as described above.

[0014] Optionally, the steam forming is performed using a double-sided steam pressure of 1.0-1.6 bar.

[0015] The beneficial effects of this invention are: The method for preparing composite EPP foam beads provided by this invention distributes the microwave absorbing components mainly on the surface of the EPP foam beads in a coating manner, rather than dispersing them entirely inside the beads. This concentrates the microwave absorbing components in the outer surface layer region where electromagnetic wave absorption actually occurs. Therefore, while meeting the required resistance value for microwave absorption performance, it can effectively reduce the amount of nano-absorbing components used, significantly reducing raw material costs. Furthermore, this application uses highly flame-retardant EPP foam beads as the substrate, avoiding the problem of decreased flame retardancy caused by increasing the overall amount of conductive carbon black in existing technologies. This allows the resulting composite EPP foam beads and their molded bodies to meet microwave absorption performance while maintaining a flame-retardant oxygen index of over 32%. In addition, because the composite EPP foam beads obtained in this application have a good surface coating structure and molding adaptability, the energy consumption required in the subsequent steam molding process is significantly reduced, and the required molding pressure is only 1 / 3 of that of traditional microwave-absorbing EPP molded bodies, thereby further reducing production costs and increasing industrial application value. Attached Figure Description

[0016] Figure 1This is a schematic diagram of the structure of the composite EPP foam beads in this invention; Figure 2 This is a schematic diagram illustrating the molding process of the composite EPP foamed beads in this invention. In the diagram: 1-EPP foamed beads; 2-coating slurry. Detailed Implementation

[0017] The present invention will now be described in further detail. The embodiments described below are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0018] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below.

[0019] Existing microwave-absorbing EPP foam beads typically employ a method of integrally incorporating microwave-absorbing additives such as conductive carbon black and flame retardants during the EPP modification process to simultaneously impart microwave absorption and flame-retardant properties to the product. This approach suffers from at least the following problems: (1) In order to obtain a lower surface resistivity and meet the requirements of microwave absorption, it is often necessary to increase the amount of conductive carbon black added. However, after the amount of conductive carbon black added increases, the flame retardant performance of EPP products is difficult to improve simultaneously. The oxygen index in the industry is usually only maintained at around 28%. (2) In the actual electromagnetic wave absorption process, the outer surface layer of the absorbing cone mainly plays the role of absorbing waves. However, the existing overall doping method will cause the conductive absorbing components to be distributed throughout the entire bead or even the entire product, resulting in a large number of absorbing components being in the ineffective area, causing waste of raw materials and increased costs. (3) Conventional high flame retardant EPP materials have high molding steam pressure, usually 3.5-3.9 bar, and high steam consumption, resulting in long molding cycle and high production energy consumption, which is not conducive to large-scale industrial low-cost production. (4) If an unsuitable conductive paste carrier or an unreasonable coating amount is used, problems such as unstable surface resistivity, poor coating adhesion, excessively hard particle surface, and reduced product toughness may occur, making it difficult to balance the microwave absorption performance, flame retardant performance and mechanical properties.

[0020] Therefore, in order to solve the above problems, the present invention provides a method for preparing composite EPP foamed beads, comprising the following steps: S1: Mix expanded polystyrene virgin particles with conductive carbon black, pre-stir at 55-75℃, then add organic solvent and continue stirring to obtain the coating slurry; S2: EPP foam beads are added to the coating slurry for surface coating and pre-pressed to obtain composite EPP foam beads.

[0021] For step S1, the present invention preferably uses nano-sized conductive carbon black with a particle size of 20-50 nm and a bulk density of 120-150 g / L to improve the dispersibility of conductive carbon black in the subsequent coating slurry and the efficiency of its wave absorption function.

[0022] In this application, expanded polystyrene virgin particles are selected as an important component of the surface coating system, namely, virgin particles obtained by introducing foaming gas during the polymerization of styrene. These virgin particles can gradually release internal gas and generate slight foaming during subsequent stirring, thereby forming a porous or hilly micro-surface morphology on the particle surface, which is beneficial for the adsorption and adhesion of conductive carbon black on its surface. Simultaneously, the density of the slightly foamed expanded polystyrene virgin particles is closer to the bulk density of the conductive carbon black, improving the uniformity of their mixing. Furthermore, polystyrene itself is a non-conductive material and will not produce ineffective dispersion inside the beads like conductive carbon black. Therefore, it can serve as a carrier for the microwave absorbing component, allowing the conductive carbon black to be mainly concentrated on the surface of the beads. Polystyrene softens more easily when heated. Although it is not a typical crystalline resin and does not have a definite melting point, its glass transition allows it to enter a softening flow state, which is conducive to forming a continuous coating layer on the surface of the beads and improving the interfacial welding behavior during subsequent steam molding. Through such a surface coating structure, the microwave absorbing component can be mainly concentrated in the outer surface area of ​​the beads. At the same time, the surface layer is easier to soften and participate in welding, thereby reducing the steam pressure required for molding, reducing energy consumption, and lowering production costs.

[0023] Furthermore, the preferred mass ratio of conductive carbon black to expanded polystyrene virgin particles is (1-1.6):1. When the mass ratio of conductive carbon black to expanded polystyrene virgin particles is less than 1:1, the content of the effective conductive component in the resulting coating slurry is too low, which is not conducive to achieving the target surface resistivity after subsequent surface coating. When the mass ratio is greater than 1.6:1, the solid content in the coating slurry is too high, which can easily affect the uniformity of the slurry and the adhesion effect during subsequent surface coating.

[0024] The preferred pre-stirring temperature of this invention is 55-75℃, the stirring speed is 1500-2000 r / min, and the stirring time is 3-8 min, so that the conductive carbon black is initially and uniformly distributed on the surface of the expanded polystyrene virgin particles. During the pre-stirring process, the microporous structure on the surface of the expanded polystyrene virgin particles is conducive to the adhesion of conductive carbon black, thus providing a foundation for the subsequent formation of a uniform and stable coating slurry.

[0025] After pre-stirring, add organic solvent to the above mixture and continue stirring to obtain the coating slurry; preferably, the stirring speed is between 2000-2500 r / min and the stirring time is 5-8 min.

[0026] The preferred organic solvents of this invention preferably meet the following conditions: firstly, their solubility parameters are similar to those of polystyrene, so as to facilitate moderate swelling or dissolution of the expanded polystyrene virgin particles during stirring; secondly, they have low toxicity, biodegradability, or environmental friendliness; and thirdly, they have low boiling points to facilitate subsequent volatilization and discharge. Further, the preferred organic solvents are selected from at least one of ethyl acetate or acetone. It should be noted that the above-mentioned low toxicity, biodegradability, or environmental friendliness are preferred characteristics, not absolute limitations on the organic solvents. Other organic solvents that can achieve the swelling or dissolution of expanded polystyrene virgin particles to form a coating slurry, thereby achieving the technical effects of this invention, may also be used in this invention.

[0027] The preferred mass ratio of organic solvent to expanded polystyrene virgin particles in this invention is (0.8-1.6):1. When the amount of organic solvent added is too low, it is not conducive to the full swelling or dissolution of the expanded polystyrene virgin particles, thus affecting the formation of a uniform slurry with the conductive carbon black; when the amount of organic solvent added is too high, it is not conducive to subsequent volatilization and removal, and may affect the stability of the subsequent surface coating process.

[0028] During continued stirring, the surface pores and internal cell structure of the expanded polystyrene (EPS) virgin particles facilitate the penetration of organic solvents into the particles, causing further swelling or partial dissolution of the EPS virgin particles. This results in the formation of a coating slurry with certain adhesion and flowability together with the conductive carbon black. In the resulting coating slurry, the EPS virgin particles primarily serve as a carrier for the conductive carbon black, supporting and fixing it, and providing an adhesion medium for subsequent coating onto the surface of EPP foam beads; the conductive carbon black serves as the main effective component for achieving conductivity and microwave absorption functions.

[0029] Through the above step S1, a coating slurry with relatively uniform dispersion and good coating adaptability can be obtained, which provides a basis for subsequently coating the coating slurry onto the surface of EPP foam beads and preparing composite EPP foam beads.

[0030] Regarding step S2, the present invention preferably uses high flame-retardant EPP foam beads with an oxygen index ≥32%. By using EPP foam beads with a high oxygen index as the substrate, on the one hand, it can provide a better flame-retardant foundation for the obtained composite EPP foam beads, and on the other hand, it can avoid the problem of reduced flame-retardant performance caused by increasing the overall amount of conductive carbon black in the prior art.

[0031] The preferred grade of this invention is ATMIS-C82(V0) from Huitong Lightweight Materials Co., Ltd., with a density of 20-90 g / L; further, a density of 45-50 g / L is preferred, because if the density is too high, it will increase the pressure required for molding, and if the density is too low, it may lead to a decrease in the stability of the resistance value.

[0032] In the initial stage of surface coating, the system temperature is controlled between 55-75℃, and the stirring speed is controlled between 400-800 r / min, maintained for 20-30 min. This stage is mainly used to ensure that the coating slurry obtained in step S1 is uniformly coated on the surface of the EPP foamed beads to form a conductive coating layer on the outer surface of the beads. At the same time, the organic solvent can fully evaporate under these conditions to avoid solvent residue from adversely affecting the subsequent molding process. If the solvent does not evaporate sufficiently, it will weaken the adhesion between particles, which is not conducive to the control of molding stability and will further affect the stability of the final microwave absorption function.

[0033] After completing the initial coating, the system temperature is preferably gradually reduced to 25-30°C at a cooling rate of 3-5°C / min. After cooling, the stirring speed is increased to 1200-1500 r / min and maintained for 3-5 minutes. The main purpose of this stage is to break up some of the EPP foam beads that are stuck together after the system reaches room temperature, thereby improving the dispersion of the beads, reducing agglomeration, and providing conditions for subsequent pre-pressing and steam molding.

[0034] During the surface coating process, EPP foam beads are added to the coating slurry, allowing the slurry to adhere to the surface of the EPP foam beads. The preferred volume ratio of coating slurry to EPP foam beads is 1:(15-40). This volume ratio is primarily used to control the amount of coating slurry on the surface of the EPP foam beads. If the volume ratio is too large, less coating slurry will adhere to the surface of each unit volume of EPP foam beads, resulting in an excessively thin surface coating layer. This leads to unstable surface conductivity and is detrimental to subsequent microwave absorption performance. Conversely, if the volume ratio is too small, more coating slurry will adhere to the surface of each unit volume of EPP foam beads, resulting in an excessively thick coating layer. This can cause the bead surface to harden, which is detrimental to maintaining the toughness of the EPP foam beads and their subsequent molding performance.

[0035] Through the aforementioned surface coating treatment, the expanded polystyrene (EPS) virgin particles in the coating slurry can act as a carrier to fix and adhere conductive carbon black to the surface of the EPP foam beads. This results in the conductive carbon black being primarily distributed in the outer surface region of the EPP foam beads, rather than being predominantly distributed within the beads. Since the microwave absorption effect mainly occurs in the surface region of the material, this surface coating method helps improve the utilization efficiency of the conductive components and reduces the ineffective distribution of conductive carbon black within the beads, thus ensuring both conductivity and microwave absorption performance while also maintaining flame retardant properties.

[0036] After surface coating, the resulting beads undergo pre-compression treatment to obtain composite EPP foamed beads. The preferred pre-compression process involves gradually increasing the air pressure from atmospheric pressure to 0.4-0.5 MPa over 6-8 hours, and then gradually decreasing the pressure to 0.2-0.35 MPa over 1.5-2 hours. This pre-compression treatment adjusts the internal and external pressure states of the surface-coated EPP foamed beads, making them more suitable for subsequent steam molding; it also helps improve the stability of the beads during the subsequent molding process.

[0037] Another object of the present invention is to provide a composite EPP foamed bead, which is prepared by the method described above for preparing composite EPP foamed beads.

[0038] Another object of the present invention is to provide a composite EPP foam molded body, which is obtained by steam molding of composite EPP foam beads as described above. In the present invention, the molded body is preferably a microwave absorbing cone.

[0039] The present invention preferably uses steam forming with a double-sided steam pressure of 1.0-1.6 bar.

[0040] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below.

[0041] Unless otherwise specified, the carbon black in the embodiments and comparative examples of this invention has a particle size of 30 nm and a bulk density of 130 g / L; the EPP foam beads have an oxygen index higher than 32%, are grade ATMIS-C82(V0) from Huitong Lightweight Materials Co., Ltd., and have a density of 45-50 g / L. Example 1

[0042] This embodiment provides a method for preparing composite EPP foamed beads, including the following steps: S1: Add 3 kg of expanded polystyrene virgin particles and 3 kg of conductive carbon black into a temperature-controlled stirring vessel and mix them. Pre-stir at 70°C (stirring speed 1500 r / min, stirring time 5 min); after pre-stirring, add 3.3 kg of ethyl acetate to the stirring vessel and continue stirring (stirring speed 2200 r / min, stirring time 8 min) to obtain the coating slurry; S2: Take 46.5L of the above coating slurry, add 930L of EPP foam beads to the coating slurry for surface coating, and after pre-pressing, obtain composite EPP foam beads. In the initial stage of surface coating, the system temperature was controlled at 75℃, the stirring speed was controlled at 600 r / min, and maintained for 20 min; after coating was completed, the system temperature was gradually reduced to 25℃ at a cooling rate of 3℃ / min. After cooling was completed, the stirring speed was increased to 1500 r / min and maintained for 3 min. The pre-compression process is as follows: the air pressure is gradually increased from atmospheric pressure to 0.5 MPa over 8 hours, and then gradually decreased to 0.3 MPa over 1.5 hours. Example 2

[0043] This embodiment provides a method for preparing composite EPP foamed beads, including the following steps: S1: Add 3 kg of expanded polystyrene virgin particles and 4.5 kg of conductive carbon black into a temperature-controlled stirring vessel and mix them. Pre-stir at 55°C (stirring speed 2000 r / min, stirring time 3 min); after pre-stirring, add 4.2 kg of ethyl acetate to the stirring vessel and continue stirring (stirring speed 2000 r / min, stirring time 5 min) to obtain the coating slurry; S2: Take 46.5L of the above coating slurry, add 1860L of EPP foam beads to the coating slurry for surface coating, and after pre-pressing, obtain composite EPP foam beads. In the initial stage of surface coating, the system temperature was controlled at 55℃, the stirring speed was controlled at 400 r / min, and maintained for 30 min; after coating was completed, the system temperature was gradually reduced to 30℃ at a cooling rate of 5℃ / min. After cooling was completed, the stirring speed was increased to 1200 r / min and maintained for 3 min. The pre-compression process is as follows: the air pressure is gradually increased from atmospheric pressure to 0.4 MPa over 6 hours, and then gradually decreased to 0.2 MPa over 2 hours. Example 3

[0044] This embodiment provides a method for preparing composite EPP foamed beads, including the following steps: S1: Add 3 kg of expanded polystyrene virgin particles and 3.6 kg of conductive carbon black into a temperature-controlled stirring vessel and mix them. Pre-stir at 75°C (stirring speed 1800 r / min, stirring time 8 min); after pre-stirring, add 2.4 kg of acetone to the stirring vessel and continue stirring (stirring speed 2500 r / min, stirring time 6 min) to obtain the coating slurry; S2: Take 46.5L of the above coating slurry, add 697.5L of EPP foam beads to the coating slurry for surface coating, and after pre-pressing, obtain composite EPP foam beads. In the initial stage of surface coating, the system temperature was controlled at 65℃, the stirring speed was controlled at 800 r / min, and maintained for 25 min; after coating was completed, the system temperature was gradually reduced to 28℃ at a cooling rate of 4℃ / min. After cooling was completed, the stirring speed was increased to 1300 r / min and maintained for 5 min. The pre-compression process is as follows: the air pressure is gradually increased from atmospheric pressure to 0.5 MPa over 7 hours, and then gradually decreased to 0.35 MPa over 1.5 hours. Example 4

[0045] This embodiment provides a method for preparing composite EPP foamed beads, including the following steps: S1: Add 3 kg of expanded polystyrene virgin particles and 3.9 kg of conductive carbon black into a temperature-controlled stirring vessel and mix them. Pre-stir at 70°C (stirring speed 1600 r / min, stirring time 6 min); after pre-stirring, add 4.5 kg of xylene to the stirring vessel and continue stirring (stirring speed 2400 r / min, stirring time 7 min) to obtain the coating slurry; S2: Take 46.5L of the above coating slurry, add 1395L of EPP foam beads to the coating slurry for surface coating, and after pre-pressing, obtain composite EPP foam beads. In the initial stage of surface coating, the system temperature was controlled at 60℃, the stirring speed was controlled at 500 r / min, and maintained for 25 min; after coating was completed, the system temperature was gradually reduced to 30℃ at a cooling rate of 3℃ / min. After cooling was completed, the stirring speed was increased to 1250 r / min and maintained for 4 min. The pre-compression process is as follows: the air pressure is gradually increased from atmospheric pressure to 0.45 MPa over 8 hours, and then gradually decreased to 0.2 MPa over 2 hours. Example 5

[0046] This embodiment provides a method for preparing composite EPP foamed beads, including the following steps: S1: Add 3 kg of expanded polystyrene virgin particles and 4.8 kg of conductive carbon black into a temperature-controlled stirring vessel and mix them. Pre-stir at 60°C (stirring speed 1900 r / min, stirring time 7 min); after pre-stirring, add 4.8 kg of ethyl acetate to the stirring vessel and continue stirring (stirring speed 2200 r / min, stirring time 6 min) to obtain the coating slurry; S2: Take 46.5L of the above coating slurry, add 1627.5L of EPP foam beads to the coating slurry for surface coating, and after pre-pressing, obtain composite EPP foam beads. In the initial stage of surface coating, the system temperature was controlled at 65℃, the stirring speed was controlled at 800 r / min, and maintained for 20 min; after coating was completed, the system temperature was gradually reduced to 25℃ at a cooling rate of 5℃ / min. After cooling was completed, the stirring speed was increased to 1500 r / min and maintained for 3 min. The pre-compression process is as follows: the air pressure is gradually increased from atmospheric pressure to 0.5 MPa over 6 hours, and then gradually decreased to 0.3 MPa over 1.5 hours.

[0047] The difference between Comparative Example 1 and Example 1 is that the expanded polystyrene virgin particles in Example 1 were replaced with polyethylene wax (melting point 68°C), and no solvent was added for dissolution. The specific steps are as follows: S1: Add 3 kg of polyethylene wax and 3 kg of conductive carbon black into a temperature-controlled mixing vessel and mix them. Stir at 70°C (stirring speed is 1500 r / min, stirring time is 13 min); after stirring, the coating slurry is obtained. S2: Take 46.5L of the above coating slurry, add 930L of EPP foam beads to the coating slurry for surface coating, and after pre-pressing, obtain composite EPP foam beads. In the initial stage of surface coating, the system temperature was controlled at 75℃, the stirring speed was controlled at 600 r / min, and maintained for 20 min; after coating was completed, the system temperature was gradually reduced to 25℃ at a cooling rate of 3℃ / min. After cooling was completed, the stirring speed was increased to 1500 r / min and maintained for 3 min. The pre-compression process is as follows: the air pressure is gradually increased from atmospheric pressure to 0.5 MPa over 8 hours, and then gradually decreased to 0.3 MPa over 1.5 hours.

[0048] The difference between Comparative Example 2 and Example 1 is that the EPP foam beads from Example 1 were directly pre-compressed. The specific steps are as follows: The 930L EPP foam beads were pre-compressed. The pre-compressing process was as follows: the air pressure was gradually increased from normal pressure to 0.5 MPa over 8 hours, and then gradually decreased to 0.3 MPa over 1.5 hours.

[0049] The difference between Comparative Example 3 and Example 1 is that the EPP foam beads in Example 1 were replaced with automotive-grade EPP foam beads, purchased from Huitong Lightweight Materials Co., Ltd., brand name ZH. The preparation process of other coating slurries is the same as in Example 1, and the specific steps are as follows: S1: Add 3 kg of expanded polystyrene virgin particles and 3 kg of conductive carbon black into a temperature-controlled stirring vessel and mix them. Pre-stir at 65°C (stirring speed 1500 r / min, stirring time 5 min); after pre-stirring, add 3.3 kg of ethyl acetate to the stirring vessel and continue stirring (stirring speed 2200 r / min, stirring time 8 min) to obtain the coating slurry; S2: Take 46.5L of the above coating slurry, add 930L of automotive grade EPP foam beads to the coating slurry for surface coating, and after pre-pressing, obtain composite EPP foam beads. In the initial stage of surface coating, the system temperature was controlled at 75℃, the stirring speed was controlled at 600 r / min, and maintained for 20 min; after coating was completed, the system temperature was gradually reduced to 25℃ at a cooling rate of 3℃ / min. After cooling was completed, the stirring speed was increased to 1500 r / min and maintained for 3 min. The pre-compression process is as follows: the air pressure is gradually increased from atmospheric pressure to 0.5 MPa over 8 hours, and then gradually decreased to 0.3 MPa over 1.5 hours.

[0050] The difference between Comparative Example 4 and Example 4 is that the expanded polystyrene virgin particles were replaced with commercially available linear low-density polyethylene (LLDPE) resin, brand name SABIC® LLDPE 118NE.

[0051] The difference between Comparative Example 5 and Example 1 is that the volume ratio of the coating slurry to the EPP foamed beads was modified to 1:45.

[0052] The difference between Comparative Example 6 and Example 1 is that the volume ratio of the coating slurry to the EPP foamed beads is modified to 1:10.

[0053] The difference between Comparative Example 7 and Example 1 is that the mass ratio of conductive carbon black, ethyl acetate and expanded polystyrene virgin particles was modified to 1:1.8:1.

[0054] The difference between Comparative Example 8 and Example 1 is that the mass ratio of conductive carbon black, ethyl acetate and expanded polystyrene virgin particles was modified to 1:0.7:1.

[0055] The difference between Comparative Example 9 and Example 1 is that the mass ratio of conductive carbon black, ethyl acetate and expanded polystyrene virgin particles was modified to 2:1.1:1.

[0056] The difference between Comparative Example 10 and Example 1 is that the mass ratio of conductive carbon black, ethyl acetate and expanded polystyrene virgin particles was modified to 0.8:1.1:1.

[0057] The composite EPP foamed beads prepared in the various embodiments and comparative examples of the present invention were steam-molded to obtain EPP foamed cones, and then their performance was tested. The specific test process is as follows: 1. Surface resistivity: Measured according to the method specified in ASTM D257. Three different locations of each sample were randomly selected for testing. Five parallel measurements were taken at each location and the average value was recorded as surface resistivity A, surface resistivity B and surface resistivity C, respectively, to examine the stability and uniformity of surface resistivity at different locations of the sample. 2. Flame retardant performance: The flame retardant performance was determined according to the method specified in GB 2406.2-2009. Three different locations were randomly selected for each sample and tested separately. Five parallel measurements were taken at each location and the average value was recorded as oxygen index A, oxygen index B and oxygen index C, respectively, to examine the flame retardant stability of the sample at different locations. 3. Elongation at break: Tested according to ISO1798. Five test strips are used in each zone of each sample. The test is conducted using a universal tensile testing device to examine the toughness of the sample. The higher the toughness, the stronger the reusability of the microwave absorbing cone and the longer the life of the microwave absorbing anechoic chamber. 4. Molding pressure: The EPP-specific K1214 molding equipment is used for molding. The operability of molding is determined by the magnitude of the double-sided steam molding pressure. The lower the molding pressure, the lower the energy consumption.

[0058] The test results are shown in Table 1: Table 1 As can be seen from the data in the table above, the EPP foamed molded body prepared by this invention distributes the microwave absorbing components mainly on the surface of the EPP foam beads in a coating manner, rather than dispersing them entirely inside the beads. This makes the microwave absorbing components more concentrated in the outer surface layer area where they actually play a role. Thus, while meeting the microwave absorption performance, it avoids the problem of reduced flame retardant performance caused by increasing the overall amount of conductive carbon black in the prior art. The resulting composite EPP foamed molded body can still achieve a flame retardant oxygen index of over 32%. At the same time, due to the good surface coating structure and molding adaptability of the beads, the subsequent steam molding energy consumption is significantly reduced, and the required molding pressure is only 1 / 3 of that of traditional microwave absorbing EPP molded bodies. Therefore, it is beneficial to reduce production costs and improve the value of industrial applications.

[0059] Based on the above-described preferred embodiments of the present invention, and through the foregoing description, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

1. A method for preparing composite polypropylene foamed beads, characterized in that, Includes the following steps: S1: Mix expanded polystyrene virgin particles with conductive carbon black, pre-stir at 55-75℃, then add organic solvent and continue stirring to obtain the coating slurry; S2: Polypropylene foam beads are added to the coating slurry for surface coating and pre-pressed to obtain composite polypropylene foam beads.

2. The method for preparing composite polypropylene foamed beads as described in claim 1, characterized in that, The conductive carbon black is nanoscale conductive carbon black with a particle size of 20-50 nm and a bulk density of 120-150 g / L.

3. The method for preparing composite polypropylene foamed beads as described in claim 1, characterized in that, The mass ratio of the conductive carbon black to the expanded polystyrene virgin particles is (1-1.6):

1.

4. The method for preparing composite polypropylene foamed beads as described in claim 1, characterized in that, The organic solvent is selected from at least one of ethyl acetate or acetone.

5. The method for preparing composite polypropylene foamed beads as described in claim 1, characterized in that, The mass ratio of the organic solvent to the expanded polystyrene virgin particles is (0.8-1.6):

1.

6. The method for preparing composite polypropylene foamed beads as described in claim 1, characterized in that, In step S2, the volume ratio of the coating slurry to the polypropylene foam beads is 1:(15-40).

7. The method for preparing composite polypropylene foamed beads as described in claim 1, characterized in that, The oxygen index of the polypropylene foam beads is ≥32%.

8. A composite polypropylene foamed bead, characterized in that, The composite polypropylene foamed beads are prepared using the preparation method described in any one of claims 1-7.

9. A composite polypropylene foam molded body, characterized in that, The composite polypropylene foam beads as described in claim 8 are obtained by steam molding.

10. The composite polypropylene foamed molded article as described in claim 9, characterized in that, The steam forming process is performed using a double-sided steam pressure of 1.0-1.6 bar.

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