Polypropylene foamed material, preparation method and application thereof

Abstract

The present application relates to a kind of preparation methods suitable for polypropylene foamed material, and the bead material can be prepared by steam forming cushion foam parts, for automobile, packaging, building and other applications.The foamable material is core-shell double-layer structure, outer layer is low melting point propylene-butylene random copolymer polypropylene, antioxidant;Inner layer is ethylene-propylene random copolymer polypropylene, propylene-butylene random copolymer polypropylene, maleic anhydride grafted polypropylene, cell nucleating agent, antioxidant.Said foaming bead is prepared by multilayer co-extrusion granulation microparticles, microparticles, dispersant, dispersion enhancer, surfactant, water are input into high-pressure foaming kettle and foamed to obtain polypropylene foamed material.By the present application, the production efficiency of foamed polypropylene can be improved, water resources and energy consumption can be saved, and the excellent performance of the final product can be guaranteed.

Description

Technical Field

[0001] This invention relates to a polypropylene foam material and its preparation method. The polypropylene foam material can be steam-molded to prepare cushioning foam parts for use in automobiles, packaging, construction and other applications. Background Technology

[0002] Foamed materials, due to their lightweight, excellent thermal insulation, cushioning, and sound insulation properties, have wide applications in transportation, construction, electronics, packaging, sports and medical fields, and agriculture. The mainstream foamed materials on the market include expanded polystyrene, expanded polypropylene, expanded polyethylene, expanded PET, and expanded polyurethane resins. Expanded polypropylene beads and their molded products possess excellent mechanical and environmental properties, and are easier to recycle than traditional foamed materials, playing a crucial role in the foamed plastics industry. Particularly in the automotive and other transportation vehicle sectors, expanded polypropylene materials are used in various components, such as bumper energy absorbers, sun visors, toolboxes, and seat internal supports.

[0003] Existing foamed polypropylene beads utilize inorganic dispersants such as talc, kaolin, alumina, and calcium carbonate, along with sodium dodecylbenzene sulfonate surfactants, during the autoclaving process to improve and prevent bead adhesion during high-temperature autoclaving, ensuring independently dispersed foamed polypropylene beads during depressurization. However, residual inorganic dispersants and surfactants on the surface of the foamed polypropylene beads require washing with pure water, followed by dehydration using airflow and centrifugal force, and then drying. This washing process generates wastewater and reduces production efficiency. If the inorganic dispersants and surfactants are not washed off the particle surface, the resulting dried polypropylene beads will suffer from impaired melt bonding during steam molding due to the influence of these substances, leading to significant losses in the tensile and compressive strength of the molded parts.

[0004] Patent CN112341662 uses kaolin as an inorganic dispersant and sodium dodecylbenzenesulfonate as a surfactant in the polypropylene foaming process. The beads obtained after foaming and depressurization are rinsed with water and then dried to obtain polypropylene foamed beads. The washing step in the patent generates wastewater, and washing lowers the temperature of the beads themselves, which prolongs the subsequent drying time and reduces production efficiency.

[0005] Patent CN104053712B also uses kaolin as a dispersant and sodium alkylbenzene sulfonate as a surfactant in the polypropylene foaming process, and obtains polypropylene foam beads by foaming and depressurization.

[0006] It is evident that there are still many problems in the preparation of existing polypropylene foam materials. Summary of the Invention

[0007] The purpose of this invention is to address the aforementioned shortcomings of existing technologies by providing a method for preparing polypropylene foam materials. This method utilizes dispersants, dispersant reinforcing agents, and specific surfactants to prevent the adhesion of polypropylene beads during high-temperature autoclaving. The foam beads prepared by this method do not require subsequent cleaning; after drying, they are directly steam-molded. During molding, the bead surface melts, and the residual dispersant, dispersant reinforcing agent, and surfactant on the bead surface react with the acid anhydride in the inner layer of the bead, achieving a cross-linking and grafting effect. This allows for the preparation of high-performance molded parts. This invention can improve the production efficiency of foamed polypropylene beads, save water and energy resources, and ensure the excellent performance of the final product.

[0008] To achieve the above-mentioned objectives, the present invention adopts the following technical solution:

[0009] On one hand, the present invention provides a polypropylene foam material, which comprises polypropylene microparticles, a dispersant, an optional dispersing reinforcing agent, a surfactant, and water. The polypropylene microparticles have a core-shell double-layer structure. The outer layer resin is a low-melting-point propylene-butadiene random copolymer polypropylene and an antioxidant, accounting for 5-20% of the total mass of the beads. The inner layer resin is an ethylene-propylene random copolymer polypropylene, a propylene-butadiene random copolymer polypropylene, a maleic anhydride-grafted polypropylene, a cell nucleating agent, and an antioxidant, accounting for 80-95% of the total mass of the beads.

[0010] In this invention, the dispersant is selected from Badifu RS-608, Dongxin DX450W or a self-made dispersant.

[0011] In this invention, the dispersing reinforcing agent is a dendritic polyamide amine resin with a terminal group of -NH2 and a number of terminal groups of 4 to 32 per molecule, such as Weihai Chenyuan CYD-100A, CYD-110A, CYD-120A, and CYD-130A.

[0012] In this invention, the surfactant is selected from fatty amine polyoxyethylene ethers, such as dodecylamine polyoxyethylene ether, tetradecylamine polyoxyethylene ether, hexadecylamine polyoxyethylene ether, octadecylamine polyoxyethylene ether, and preferably dodecylamine polyoxyethylene ether.

[0013] In this invention, based on the weight of polypropylene microparticles as 100%, the dispersant accounts for 1-2% of the weight of polypropylene microparticles, the dispersing reinforcing agent accounts for 0.1-0.3% of the weight of polypropylene microparticles, the surfactant accounts for 0.2-0.5% of the weight of polypropylene microparticles, and the mass ratio of polypropylene microparticles to water is 1:2 to 1:3.

[0014] In this invention, the outer layer structure resin comprises the following raw materials in parts by weight:

[0015] 99-99.8 parts of propylene-butyl random copolymer polypropylene

[0016] Antioxidant 0.2 to 1 part;

[0017] In this invention, the inner layer structure resin comprises the following raw materials in parts by weight:

[0018] 50-75 parts of ethylene-propylene random copolymer polypropylene

[0019] 10-35 parts of propylene-butyl random copolymer polypropylene

[0020] 2-10 parts of maleic anhydride-grafted polypropylene

[0021] Nucleating agent for bubbles: 0.1–5 parts

[0022] Antioxidant 0.2 to 1 part.

[0023] In this invention, the propylene-butadiene random copolymer polypropylene in the outer structural resin has a melting point of 120-135°C and a melt index of 5-10 g / 10 min (230°C, 2.16 kg).

[0024] In this invention, the antioxidant in the outer resin is any one or more of hindered phenolic antioxidants, phosphite antioxidants, and thioester antioxidants, specifically selected from any one or more of 1010, 168, and DSTDP.

[0025] In this invention, the ethylene-propylene random copolymer polypropylene in the inner layer structure resin has a melting point of 140-155°C and a melt index of 5-20 g / min (230°C, 2.16 kg).

[0026] In this invention, the propylene-butadiene random copolymer polypropylene in the inner layer structure resin has a melting point of 120-135°C and a melt index of 5-10 g / 10 min (230°C, 2.16 kg).

[0027] In this invention, the maleic anhydride-grafted polypropylene in the inner layer structure resin has a melting point of 162-168°C and a grafting rate of maleic anhydride of 5-15%.

[0028] In this invention, the cell nucleating agent in the inner layer resin is a mixture of one or more proportions of talc, kaolin, montmorillonite, zinc borate, and nano-silica; the nucleating agent can adjust the cell size of the foamed beads and improve the uniformity of cell size; it accounts for 0.1-5% of the total mass of the outer layer of the microparticles.

[0029] In this invention, the antioxidant in the inner layer structure resin is any one or more of hindered phenolic antioxidants, phosphite antioxidants and thioester antioxidants, specifically selected from any one or more of 1010, 168 and DSTDP.

[0030] On the other hand, in this invention, polypropylene microparticles are prepared by multi-layer co-extrusion granulation. Polypropylene microparticles, dispersant, dispersant reinforcing agent, surfactant, and water are added to a high-pressure foaming kettle and foamed to obtain polypropylene foam material.

[0031] In this invention, the method for preparing polypropylene microparticles includes the following steps:

[0032] (1) The outer layer raw material is placed in a twin-screw extruder for high-temperature melt dispersion extrusion granulation to obtain the outer layer resin;

[0033] (2) The inner layer raw material is placed in a twin-screw extruder for high-temperature melt dispersion extrusion granulation to obtain the inner layer resin;

[0034] (3) The outer layer resin and the inner layer resin are respectively placed in two single-screw extruders for plasticization, and core-shell composite polypropylene microparticles are obtained by extrusion pelletizing through co-extrusion die. The mass ratio of the outer layer resin is 5-20%, and the mass ratio of the inner layer resin is preferably 80-95%. The length of the microparticles prepared by extrusion pelletizing is 1-2 mm, and the weight is controlled at 1-2 mg.

[0035] In this invention, the screw speed for high-temperature melt dispersion extrusion granulation in steps (1) to (3) is 100-500 r / min, and the extrusion temperature is 190-210℃.

[0036] In one specific embodiment, the self-made dispersant used in the autoclaving foaming is an internally micro-crosslinked polymer microsphere, and its preparation method includes the following steps:

[0037] (1) Preparation of mixed monomers: 70-92 parts of styrene, 2-10 parts of divinylbenzene, 2-5 parts of acrylamide, and 5-15 parts of hydroxybutyl acrylate are mixed to form mixed monomers;

[0038] (2) Preparation of preemulsion: Mix and disperse 90-100 parts of mixed monomers, 30-40 parts of water and 1-2 parts of emulsifier dodecyl ammonium chloride to obtain preemulsion;

[0039] (3) Preparation of seed emulsion: Add 200-250 parts of water to the reaction vessel, add 1-10 parts of pre-emulsion by mass, heat to 80-85℃, add 0.5-1 parts of initiator ammonium persulfate dissolved in 10-15 parts of water, and keep warm to prepare seed emulsion;

[0040] (4) Add the remaining pre-emulsion and 0.25 to 0.5 parts of initiator ammonium persulfate dissolved in 10 to 15 parts of water dropwise to the seed emulsion. After the dropwise addition is complete, keep warm for 1 to 1.5 hours. After cooling to room temperature, spray dry to obtain polymer microspheres with internal micro-crosslinks containing hydroxyl functional groups. The particle size of the polymer microspheres is 100 to 500 nm.

[0041] On the other hand, the present invention also provides a method for foaming polypropylene foam material, the method comprising the following steps: uniformly dispersing polypropylene microparticles, dispersant, optional dispersing reinforcing agent, and surfactant in an aqueous dispersion medium; introducing a foaming agent into an autoclave to replace air; raising the temperature and pressure to allow the foaming agent to penetrate into the interior of the polypropylene microparticles; maintaining the temperature and pressure for a certain period of time; and releasing the polypropylene microparticles together with the aqueous dispersion medium into a normal temperature and pressure environment; under the action of instantaneous pressure difference, the high pressure inside the polypropylene microparticles causes them to foam and expand instantaneously, thereby preparing polypropylene foam beads.

[0042] In this invention, the foaming agent is one or more of carbon dioxide, nitrogen, butane, pentane, and heptane in any proportion. To meet environmental protection requirements, carbon dioxide is more preferred as the foaming agent.

[0043] In this invention, the foaming temperature is 120–165°C and the foaming pressure is 2–6 MPa.

[0044] The technical solution of this invention has the following advantages over existing technical solutions:

[0045] 1. The polypropylene foam beads material in this invention adopts a core-shell structure. The outer layer is a low-melting-point propylene-butadiene random copolymer polypropylene, which does not foam during autoclaving and can be fused and molded at a lower steam pressure during steam molding, resulting in excellent appearance quality. The inner layer cell structure is not destroyed by high temperature. The inner layer is a high-melting-point polypropylene resin, which is beneficial for the molded products to have high rigidity mechanical properties and better dimensional stability.

[0046] 2. The dendritic polyamide resin used as a dispersant and reinforcing agent in the high-temperature autoclaving foaming of this invention carries a positive charge in the aqueous phase environment of high-pressure CO2, and has excellent aqueous phase dispersion ability. This can reduce the amount of dispersant and emulsifier used, and prevent the adhesion between polypropylene beads during autoclaving foaming.

[0047] 3. The dispersant used in the high-temperature autoclaving foaming process of this invention has a smaller particle size compared to traditional inorganic dispersants, and has better dispersibility in the aqueous phase, which can effectively prevent the polypropylene beads from sticking together during autoclaving foaming.

[0048] 4. The dispersant and reinforcing agent, dendritic polyamide amine resin, dispersant, and emulsifier, fatty amine polyoxyethylene ether, used in the high-temperature autoclaving foaming of this invention, all have functional groups on their surfaces that can undergo grafting reactions with acid anhydride groups. During the steam molding process, the shell layer of the core-shell polypropylene foam beads melts, and the dispersant, dispersing reinforcing agent, and surfactant remaining on the bead surface can react with the acid anhydride groups in the inner layer of the bead, playing a cross-linking grafting role and enhancing the interfacial forces between the beads, thus producing molded parts with excellent performance. This preparation method can improve the production efficiency of foamed polypropylene beads, save water resources and energy consumption, and ensure the excellent performance of the final product. Detailed Implementation

[0049] To better understand the technical solution of the present invention, the following embodiments further illustrate the content of the present invention, but the content of the present invention is not limited to the following embodiments. Unless otherwise specified, the raw materials used in the embodiments or comparative examples are commercially available raw materials.

[0050] The apparatus and main raw material sources used in the embodiments and comparative examples of this invention are as follows:

[0051] Twin-screw extruder: Model SK-26, screw length-to-diameter ratio 50:1; Nanjing Keya Chemical Complete Equipment Co., Ltd.

[0052] Single-screw co-extrusion equipment: Model HRJSJ-35, screw length-to-diameter ratio 28:1, Foshan Hairuijia Precision Extrusion Machinery Co., Ltd.

[0053] Nucleating agent: Talc powder AH51205L, Liaoning Aihai Talc Co., Ltd.

[0054] Lubricant: Zinc stearate, FARKS GmbH, Italy;

[0055] Fiber-Butyl Random Copolymer Polypropylene: FL7632L, Singapore Polyolefins Ltd.

[0056] EPDM random copolymer polypropylene: RP344P-K, North China Huajin Chemical Industry Co., Ltd.

[0057] Maleic anhydride-grafted polypropylene: UMEX 1001, Sanyo Chemicals

[0058] Main antioxidant: 1010, a rising star in chemistry.

[0059] Co-antioxidant: 168, New Star Chemistry

[0060] Inorganic dispersant: Kaolin DB-1, China Kaolin Co., Ltd.

[0061] Dispersant Reinforcing Agent: CYD-130A, Weihai Chenyuan Molecular New Materials Co., Ltd.

[0062] All other raw materials were commercially available and of analytical grade.

[0063] The performance characterization method of the polypropylene foam material of the present invention is as follows:

[0064] Tensile strength: ISO 1798:2008

[0065] Compressive strength: ISO 844:2014;

[0066] Bulk density: ISO 844;

[0067] Preparation Example 1 is the preparation of a self-made dispersant;

[0068] Preparation Example 1

[0069] (1) Preparation of mixed monomers: 83 parts styrene, 5 parts divinylbenzene, 4 parts acrylamide and 8 parts hydroxybutyl acrylate are mixed to form mixed monomers;

[0070] (2) Preparation of preemulsion: Mix and disperse 100 parts of mixed monomers, 40 parts of water and 1.5 parts of emulsifier dodecyl ammonium chloride to obtain preemulsion;

[0071] (3) Preparation of seed emulsion: Add 200 parts of water to the reaction vessel, add 5 parts of pre-emulsion by mass, heat to 85°C, add 0.5 parts of initiator ammonium persulfate dissolved in 10 parts of water, and keep warm to prepare seed emulsion;

[0072] (4) Add the remaining pre-emulsion and 0.25 parts of initiator ammonium persulfate dissolved in 10 parts of water dropwise to the seed emulsion. After the addition is complete, keep it warm for 1 hour. After cooling to room temperature, spray dry the emulsion to obtain polymer microspheres with internal micro-crosslinks containing hydroxyl functional groups, which is designated as dispersant 1.

[0073] Preparation Examples 2-3

[0074] Dispersant 2 was obtained by spray drying of Badifu RS-608 emulsion, and dispersant 3 was obtained by spray drying of Dongxin DX450W emulsion.

[0075] Examples and Comparative Examples

[0076]

[0077]

[0078] The preparation method of the outer layer resin is as follows: weigh each raw material according to the weight ratio, then put it into a high-speed mixer and mix for 3 minutes. Then feed it from the main feed port of the twin-screw extruder. The screw speed is 500 r / min and the extrusion temperature is 200℃. After high-temperature melting and dispersion, the outer layer resin is obtained by extrusion granulation.

[0079] The preparation method of the inner layer resin is as follows: weigh each raw material according to the weight ratio, then put them into a high-speed mixer and mix for 3 minutes. Then feed them from the main feed port of the twin-screw extruder. The screw speed is 500 r / min and the extrusion temperature is 200℃. After high-temperature melting and dispersion, the inner layer resin is obtained by extrusion granulation.

[0080] The preparation method of polypropylene microparticles is as follows: the outer layer resin and the inner layer resin are respectively placed in two single-screw extruders for plasticization, the extrusion temperature is 200℃, and the core-shell composite polypropylene microparticles are obtained by extrusion pelletizing through a co-extrusion die. The weight of the microparticles prepared by extrusion pelletizing is controlled at 1.5mg.

[0081] Example 5

[0082] 500g of polypropylene microparticles S1, 0.5g of dispersant CYD-130A, 1g of surfactant dodecylamine polyoxyethylene ether, and 5g of dispersant 1 were uniformly dispersed in 1kg of aqueous dispersion medium. Carbon dioxide was introduced into an autoclave to replace the air, and the temperature and pressure were increased to allow the carbon dioxide to penetrate into the interior of the raw polypropylene microparticles. After maintaining the temperature and pressure for 45 minutes, the polypropylene microparticles, along with the aqueous dispersion medium, were released into a normal temperature and pressure environment. Under the action of the instantaneous pressure difference, the high pressure inside the polypropylene microparticles caused them to foam and expand instantaneously, thereby preparing polypropylene foamed beads. The foaming temperature was 140℃, and the foaming pressure was 4.0MPa.

[0083] Examples 6-8

[0084] Polypropylene foamed beads were prepared according to the amounts of each raw material in Table 3. For specific preparation methods, please refer to Example 5.

[0085] Comparative Examples 5-10

[0086] Polypropylene foamed beads were prepared according to the amounts of each raw material in Table 3. For specific preparation methods, please refer to Example 5.

[0087] Preparation of polypropylene foamed products: The obtained polypropylene foamed beads are injected into a flat mold with a length of 300mm, a width of 200mm, and a thickness of 30mm. After being heated by steam and cooled to set, rectangular foamed products are obtained. The foamed products are cut into samples and tested for tensile strength and compressive strength.

[0088] Table 3. Raw material usage and performance of foamed products in Examples 5-8 and Comparative Examples 5-10

[0089]

[0090]

[0091] Note: Foamed bead adhesion status: from 0 to 5, where the higher the number, the more severe the adhesion; 0 represents no adhesion.

[0092] Table 3 shows that Examples 5-8, using polymer microsphere dispersant, dodecylamine polyoxyethylene ether, and CYD-130A as the dispersion system, produced foamed beads without adhesion, indicating that the dispersion system has excellent dispersion effect and can avoid the adhesion of polypropylene microparticles under high temperature and pressure. This results in polypropylene foamed beads with lower density and higher foaming ratio. Compared with Comparative Examples 5 and 6, Example 5 exhibits higher tensile and compressive strength. This is because the shell of the polypropylene foamed beads melts during steam molding, and the polymer microsphere dispersant, dodecylamine polyoxyethylene ether, and CYD-130A in Example 5 can react with the anhydride groups in the inner layer of the polypropylene microparticles, playing a cross-linking and grafting role, thus enhancing the interfacial forces between the beads.

[0093] Comparative Example 5 used polymer microsphere dispersant and dodecylamine polyoxyethylene ether as the dispersion system. Compared with Example S5, it did not add dispersant enhancer CYD-130A, but added a higher content of dodecylamine polyoxyethylene ether. However, the foamed beads obtained in the end still had some adhesion.

[0094] Comparative Example 6 uses kaolin dispersant, dodecylamine polyoxyethylene ether, and CYD-130A as the dispersion system. Compared with Example 6, it does not add polymer microsphere dispersant, so the dispersion effect during the foaming process is slightly worse, and some foam beads still stick together.

[0095] The polypropylene microparticles used in Comparative Example 7 do not have anhydride groups in their inner layer. During the steam molding process, the polymer microsphere dispersant, dodecylamine polyoxyethylene ether, and CYD-130A cannot undergo a grafting reaction with the polypropylene foam beads, thus affecting the interfacial forces. Therefore, compared with Example 5, its tensile strength and compressive strength are lower.

[0096] In Comparative Examples 8-10, a traditional dispersion system using kaolin dispersant and sodium dodecylbenzenesulfonate was used. With increasing content of sodium dodecylbenzenesulfonate and kaolin, the dispersion effect during the foaming process increased. The amount of dispersion system added up to Comparative Example 10 resulted in foamed beads without adhesion, but the amount of dispersion system added was excessive. However, the traditional dispersion system used in Comparative Examples 8-10 could not undergo a grafting reaction with the polypropylene foamed beads during steam molding, which affected the melt bonding between the beads. Consequently, the tensile strength and compressive strength gradually decreased with increasing amount of the traditional dispersion system.

[0097] The above description of the embodiments is intended to enable those skilled in the art to understand and apply the present invention. Those skilled in the art can readily make various modifications to the embodiments and apply the general principles described herein to other embodiments without inventive effort. Therefore, the present invention is not limited to the embodiments described herein, and any improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the invention should be within the protection scope of the present invention.

Claims

1. A polypropylene foam material, comprising polypropylene microparticles, a dispersant, a dispersing enhancer, a surfactant, and water, wherein the polypropylene microparticles have a core-shell double-layer structure, the outer layer resin being a low-melting-point propylene-butyl random copolymer polypropylene and an antioxidant, accounting for 5-20% of the total mass of the beads; the inner layer resin being an ethylene-propylene random copolymer polypropylene, a propylene-butyl random copolymer polypropylene, maleic anhydride-grafted polypropylene, a cell nucleating agent, and an antioxidant, accounting for 80-95% of the total mass of the beads, wherein the dispersant is selected from Badifu RS-6.

08. Dongxin DX450W, wherein the dispersing reinforcing agent is dendritic polyamide amine resin with -NH2 terminal groups, and the number of terminal groups per molecule is 4 to 32. The surfactant is selected from fatty amine polyoxyethylene ether. Based on the weight of 100% polypropylene microparticles, the dispersing agent accounts for 1 to 2% of the weight of polypropylene microparticles, the dispersing reinforcing agent accounts for 0.1 to 0.3% of the weight of polypropylene microparticles, the surfactant accounts for 0.2 to 0.5% of the weight of polypropylene microparticles, and the mass ratio of polypropylene microparticles to water is 1:2 to 1:

3.

2. The polypropylene foam material as described in claim 1, characterized in that, The dispersion enhancer is selected from Weihai Chenyuan CYD-100A, CYD-110A, CYD-120A, and CYD-130A; and / or the surfactant is selected from dodecylamine polyoxyethylene ether, tetradecylamine polyoxyethylene ether, hexadecylamine polyoxyethylene ether, and octadecylamine polyoxyethylene ether.

3. The polypropylene foam material as described in claim 1, characterized in that, The outer structural resin comprises the following raw materials in parts by weight: 99-99.8 parts of propylene-butyl random copolymer polypropylene, 0.2-1 part of antioxidant; The inner layer structure resin comprises the following parts by weight of raw materials: 50-75 parts of ethylene-propylene random copolymer polypropylene, 10-35 parts of propylene-butyl random copolymer polypropylene, 2-10 parts of maleic anhydride grafted polypropylene, 0.1-5 parts of cell nucleating agent, and 0.2-1 parts of antioxidant.

4. The polypropylene foam material as described in claim 3, characterized in that, The outer layer resin, a propylene-butadiene random copolymer polypropylene, has a melting point of 120-135℃ and a melt index of 5-10 g / 10 min at 230℃ and 2.16 kg; and / or, the inner layer resin, an ethylene-propylene random copolymer polypropylene, has a melting point of 140-155℃ and a melt index of 5-20 g / min at 230℃ and 2.16 kg; and / or, the inner layer resin, a propylene-butadiene random copolymer polypropylene, has a melting point of 120-135℃ and a melt index of 5-10 g / 10 min at 230℃ and 2.16 kg; and / or, the inner layer resin, a maleic anhydride-grafted polypropylene, has a melting point of 162-168℃ and a maleic anhydride grafting rate of 5-15%.

5. The polypropylene foam material according to any one of claims 1-4, characterized in that, The method for preparing polypropylene microparticles includes the following steps: (1) The raw material of the outer layer structure resin is placed in a twin-screw extruder for high-temperature melt dispersion extrusion granulation to obtain the outer layer resin; (2) The raw material of the inner layer structure resin is placed in a twin-screw extruder for high-temperature melt dispersion extrusion granulation to obtain the inner layer resin; (3) The outer layer resin and the inner layer resin are respectively placed in two single-screw extruders for plasticization, and core-shell composite polypropylene microparticles are obtained by extrusion pelletizing through co-extrusion die. The outer layer resin accounts for 5~20% of the mass and the inner layer resin accounts for 80~95% of the mass. The length of the microparticles prepared by extrusion pelletizing is 1~2mm and the weight is controlled at 1~2mg. In steps (1) to (3), the screw speed for high-temperature melt dispersion extrusion granulation is 100-500 r / min, and the extrusion temperature is 190-210℃.

6. The foaming method of polypropylene foam material according to any one of claims 1-5, the method comprising the following steps: uniformly dispersing polypropylene microparticles, dispersant, dispersion reinforcing agent and surfactant in an aqueous dispersion medium; introducing a foaming agent into an autoclave to replace air; raising the temperature and pressure to allow the foaming agent to penetrate into the interior of the polypropylene microparticles; maintaining the temperature and pressure for a certain period of time; and releasing the polypropylene microparticles together with the aqueous dispersion medium into a normal temperature and pressure environment; under the action of instantaneous pressure difference, the high pressure inside the polypropylene microparticles causes them to foam and expand instantaneously, thereby preparing polypropylene foam beads.

7. The foaming method as described in claim 6, characterized in that, The foaming agent is one or more of carbon dioxide, nitrogen, butane, pentane, and heptane in any proportion; and / or the foaming temperature is 120~165℃ and the foaming pressure is 2~6MPa.

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