A kind of foamed polypropylene beads capable of rapid microwave molding and molded products thereof

Abstract

The invention discloses a foamed polypropylene bead capable of rapid microwave molding. The foamed polypropylene bead has a core-shell structure. The core layer material comprises high-melting-point high-modulus polypropylene, the shell layer material comprises low-melting-point propylene copolymer and / or polypropylene blend and a certain amount of microwave auxiliary heating agent, and the foamed bead shell layer or surface thereof further comprises a surfactant. The microwave auxiliary heating agent can rapidly heat up under a relatively low microwave heating power, so that the low-melting-point polypropylene material on the surface layer is rapidly melted and melt-sintered with other surrounding foamed polypropylene bead shell layers. The surfactant can further improve the heating efficiency. The foamed polypropylene bead does not contain the microwave auxiliary heating agent and the surfactant, and the low-power microwave will not cause the internal temperature of the foamed bead to rise too much, so that the original independent closed-cell structure can be maintained. The obtained molded product has high maturation degree and good surface sintering, and the entire molding process is short in time, low in energy consumption, and saves a lot of costs, which is green and environmentally friendly.

Description

Technical Field

[0001] The invention relates to a foamed polypropylene bead capable of rapid microwave molding and a molded product thereof, belonging to the field of foaming materials. Background Art

[0002] Foamed polypropylene bead materials are widely used in automotive parts, rail transportation, packaging turnover, construction, sports and leisure and other fields because of their high foaming ratio and lightweight.

[0003] In order to obtain various molded parts with high degree of freedom in shape, expanded polypropylene beads are usually formed by steam compression molding, that is, the expanded polypropylene beads are first filled into the mold, then the mold is closed, and water vapor is injected into the cavity through the pores inside the mold to heat the beads. After the expanded polypropylene beads are sintered, they are cooled and demolded to obtain foamed parts.

[0004] In the above process, in order to make the foamed polypropylene beads heat and sinter more evenly and fully, it is often necessary to preheat the mold with steam; and during the molding process, the mold also needs to be kept at a certain temperature by steam. These two factors lead to a large amount of steam energy consumption actually being consumed in the heating and heat preservation process of the mold, and the heating and sintering of the foamed polypropylene beads only accounts for less than 10% of the total steam energy consumption.

[0005] Microwave-molded foamed polypropylene was born under this background. Compared with steam compression molding, microwave molding does not require heating of the mold. Heat is generated by electromagnetic waves acting on the foaming material. The foaming material can absorb most of the heat, with high energy efficiency and low energy consumption. On the other hand, in order to ensure that the foaming bead material is not overheated and burned during the microwave molding process, a relatively low microwave heating power is often used. However, lower microwave power will lead to longer heating time, which has a greater impact on the molding time cycle and lower work efficiency, which also limits the further promotion of microwave molding in actual production applications.

[0006] Therefore, it is necessary to develop a foamed polypropylene bead material and foamed parts that can be quickly microwaved. Summary of the invention

[0007] In order to solve the problem of long molding time and low molding efficiency of microwave-molded foamed polypropylene beads, the present invention discloses a foamed polypropylene bead that can be quickly microwave-molded and its molded parts. The surface layer of the foamed polypropylene beads contains a certain amount of microwave heating aid and surfactant, which can help the foamed bead material to quickly heat up under low microwave heating power, so that the low-melting-point polypropylene material on the surface layer quickly melts and melts and sinters with other surrounding polypropylene bead surfaces; while the interior of the foamed polypropylene beads does not contain microwave heating aid and surfactant, and the polypropylene material in the core layer has a high melting point, and low-power microwaves will not cause the internal temperature of the foamed beads to rise too much, and the original independent pore structure can be maintained. After the microwave heating is completed and cooled, a foamed polypropylene part with high degree of maturation, good surface sintering, and the original closed-cell structure inside the foamed beads can be obtained; more importantly, the entire molding process is short in time, low in energy consumption, high in energy utilization, saving a lot of costs, and being green and environmentally friendly.

[0008] In order to achieve the above object, the present invention adopts the following technical solution:

[0009] A foamed polypropylene bead capable of rapid microwave molding has a core-shell structure, wherein the core layer material comprises 80-99.97wt%, preferably 90-99.97wt% of high-melting-point high-modulus polypropylene, the shell layer material comprises 80-99.96wt%, preferably 90-99.96wt% of low-melting-point propylene copolymer and / or polypropylene blend and 0.01-5wt% of microwave auxiliary heating agent, the high-melting-point high-modulus polypropylene has a melting point of 130-160°C and a bending modulus of 800-1100M Pa, a melt index of 5 to 10 g / 10 min, the melting point of the low-melting-point propylene copolymer and / or polypropylene blend is 105 to 125 ° C, the microwave heating agent is one or more of metal powder, metal oxide, nitride, glass fiber, bamboo fiber, carbon fiber, graphene, ferrite, ceramic, etc., the size of the microwave heating agent is 10 to 1000 nm, and the mass ratio of the core layer material to the shell layer material is 80:20 to 99:1, and more preferably 90:10 to 99:1.

[0010] Furthermore, the shell material of the foamed polypropylene beads further includes 0.01 to 5 wt% of a surfactant, and the surfactant is at least one of an anionic surfactant, a cationic surfactant, a zwitterionic surfactant, and a nonionic surfactant. Under the action of the microwave electromagnetic field, the ionic surfactant is ionized, which can promote the absorption of microwaves by the foamed beads; the nonionic surfactant contains a hydrophilic group, which promotes the absorption and wetting of water attached to the surface of the beads. The polarity of water is relatively high, and it can heat up quickly under the action of microwaves. These factors can make the shell material heat up and melt quickly, and quickly increase the degree of sintering between the beads.

[0011] Furthermore, the anionic surfactant is at least one of stearic acid, oleic acid, lauric acid, sulfates, sulfonates, etc., the cationic surfactant is at least one of ammonium salts, quaternary ammonium salts, heterocyclic surfactants, etc., the zwitterionic surfactant is at least one of lecithin, amino acid, betaine surfactants, etc., and the nonionic surfactant is at least one of fatty acid glycerides, polyols, fatty acid sorbitan, polysorbate, polyoxyethylene, polyoxyethylene-polyoxypropylene copolymers, etc.

[0012] Furthermore, the cell structure of the core layer is an independent closed-cell cell structure with a pore size of 50-200 microns.

[0013] Furthermore, the high melting point high modulus polypropylene is homopolymer polypropylene and / or copolymer polypropylene, and the copolymer polypropylene includes ethylene-propylene binary copolymer polypropylene and / or ethylene-propylene-butylene ternary copolymer polypropylene. Selecting such high melting point high modulus polypropylene as the core layer material can provide the foamed beads with better heat resistance and rigidity.

[0014] Furthermore, the propylene copolymer with low melting point is an ethylene-propylene binary copolymer or an ethylene-propylene-butylene ternary copolymer, and the polypropylene blend with low melting point is a polyethylene / polypropylene blend.

[0015] Furthermore, the metal powder is iron powder, copper powder, copper-tin alloy powder, gold powder and / or silver powder, etc., the metal oxide is magnesium oxide, iron oxide, aluminum oxide and / or copper oxide, etc., the nitride is silicon nitride and / or boron nitride, etc., the ferrite is a sintered composite of ferric oxide and nickel oxide, zinc oxide, manganese oxide, etc. The microwave heating aid has a high dielectric constant and dielectric loss, is conducive to absorbing microwaves and quickly converting them into heat energy, heating the skin layer of the foamed polypropylene beads and fully sintering them.

[0016] Compared with traditional steam heating molding, most of the heat is lost in the mold. Microwave heating can increase the temperature of the beads themselves, and very little heat is consumed in the mold. Under the action of the external alternating electromagnetic field generated by microwaves, the polar molecules in the foamed polypropylene beads are polarized, forming dipoles or the existing dipoles are rearranged, and swing at a speed of hundreds of millions of times per second with the high-frequency alternating electromagnetic field. In order for the molecules to rearrange with the changing direction of the high-frequency magnetic field, they need to overcome the obstacles and interference of the original molecular thermal motion and intermolecular interactions, generate "frictional heat", and the electromagnetic field energy is directly converted into thermal energy, which greatly increases the temperature of the foamed polypropylene beads. Compared with polypropylene materials, microwave-assisted heating agent materials with higher dielectric constants and dielectric losses have higher and faster heat generation capabilities during the "friction" process, which results in a higher and faster temperature speed of the foamed polypropylene shell containing microwave-assisted heating agents under the same microwave power; in addition, the shell layer of the foamed polypropylene itself uses polypropylene with a lower melting point as the base material, which allows the shell layer material to melt quickly under lower microwave power, and the beads are quickly sintered, but the core layer of the foamed beads is kept at a relatively low temperature to ensure that the independent bubble structure in the core layer is not destroyed by melting.

[0017] Furthermore, the core layer material further includes 0.01-2wt% of a nucleating agent, which is one or more of calcium carbonate, talcum powder, zinc borate, sodium chloride, silicon dioxide, etc., and has a particle size of 1-20 μm. The nucleating agent usually has a certain incompatibility with the polypropylene substrate, and promotes the growth of pores at the interface between the nucleating agent and the polypropylene during the foaming process of the polypropylene beads, playing a role of heterogeneous nucleation. At the same time, the nucleating agent also has the effect of reducing the foaming pressure and uniform pores.

[0018] Furthermore, the foamed polypropylene beads may also be added with masterbatch, lubricant, antioxidant, anti-ultraviolet agent, etc. according to requirements.

[0019] The preparation method of the above-mentioned foamed polypropylene beads capable of rapid microwave forming is as follows:

[0020] (1) The core layer material of the foamed polypropylene beads is mixed evenly and then put into a twin-screw extruder A, and the shell layer material of the foamed polypropylene beads is mixed evenly and then put into a twin-screw extruder B;

[0021] (2) Co-extrusion is achieved through a double-layer die, with the material in extruder A being the core layer and the material in extruder B being the shell layer;

[0022] (3) The extruded filaments are water-cooled and pelletized using a pelletizer to prepare expandable polypropylene particles that can be quickly microwaved;

[0023] (4) The expandable polypropylene microparticles are placed in a high-pressure foaming kettle for foaming to obtain expanded polypropylene beads.

[0024] When the shell layer of the foamed beads does not contain a surfactant, when preparing a foamed polypropylene molded product, it is necessary to spray a surfactant solution on the surface of the foamed polypropylene beads before microwave molding, the mass ratio of the surfactant to water in the surfactant solution is 1:1 to 1:10, the mass ratio of the spraying amount of the surfactant solution to the foamed polypropylene beads is 1:2 to 1:20, the microwave heating power is 1 to 100 kW, more preferably 10 to 60 kW, further preferably 20 to 50 kW, and the time is 1 to 500 s, more preferably 10 to 100 s, further preferably 15 to 60 s.

[0025] When the shell layer of the foamed beads contains a surfactant, a small amount of water needs to be sprayed onto the surface of the foamed beads before microwave molding. The mass ratio of water to foamed polypropylene beads is 1:10 to 1:50. The foamed polypropylene beads are then subjected to microwave molding to prepare foamed polypropylene molded parts. The microwave heating power is 1 to 100 kW, more preferably 10 to 60 kW, and further preferably 20 to 50 kW. The time is 1 to 500 s, more preferably 10 to 100 s, and further preferably 15 to 60 s.

[0026] The present invention has the following beneficial effects:

[0027] 1. The shell layer of the above-mentioned expanded polypropylene beads contains a microwave heating aid, which has a high dielectric constant and dielectric loss. It can quickly generate high heat through polarization and oscillation of polar molecules at a low microwave power, so that the temperature of the shell layer of the expanded polypropylene beads rises rapidly. On the other hand, since the core layer of the expanded polypropylene beads does not contain a microwave heating aid with a high dielectric constant and dielectric loss, less heat is generated at the same low microwave power, resulting in a lower temperature of the core layer of the expanded polypropylene beads. The low-melting-point polypropylene substrate used in the shell layer of the expanded polypropylene itself can make the shell layer material melt quickly under the action of a low microwave power, and the beads are quickly sintered, but the independent pore structure in the core layer is kept from being destroyed by melting.

[0028] 2. The surfactant in the shell layer of the foamed polypropylene beads or on the surface of the beads is ionized under the action of the microwave electromagnetic field, or contains hydrophilic groups to promote the absorption of highly polar water, thereby improving the heating efficiency and heat conduction efficiency, further causing the shell material to heat up and melt rapidly, and improving the degree of sintering between the beads.

[0029] 3. In ordinary microwave molding, in order to ensure that the foamed bead material is not overheated and burned during the microwave molding process, a relatively low microwave heating power is often used. However, a low microwave power will lead to a long heating time, which has a great impact on the molding time cycle and low working efficiency. The foamed polypropylene bead material of the present invention can be quickly molded under a low microwave power to obtain a foamed molded part with a good sintering degree. The entire molding production process has low energy consumption, a short cycle, and is green, energy-saving and environmentally friendly. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is the polypropylene foam molding prepared in Example 1. DETAILED DESCRIPTION

[0031] The present invention will now be further described in detail with reference to embodiments.

[0032] Embodiment 1:

[0033] Preparation of rapidly microwaveable expanded polypropylene beads:

[0034] (1) According to the mass ratio of Example 1 in Table 1, the high melting point and high modulus polypropylene (melting point 145°C, flexural modulus 1000 MPa, melt index 7 g / 10 min, purchased from China Petrochemical Corporation), the nucleating agent (calcium carbonate, purchased from Jiangxi Guangyuan Chemical Co., Ltd.), the lubricant (oleic acid amide, purchased from Zhengzhou Zhuochuang Chemical Products Co., Ltd.) and the antioxidant (1010, purchased from BASF AG, Germany) of the core layer were mixed uniformly and put into a twin-screw extruder A, and the ethylene-propylene copolymer (melting point 120℃, melt index 8g / 10min, purchased from Sinopec), microwave heating agent (silicon nitride, CW-Si3N4-002, purchased from Shanghai Chaowei Nanotechnology Co., Ltd.), surfactant (tridecanol polyether-10 phosphate, i.e. PEG-10 ester, purchased from Nantong Chenrun Chemical Co., Ltd.), lubricant (oleic acid amide, purchased from Zhengzhou Zhuochuang Chemical Products Co., Ltd.) and antioxidant (1010, purchased from BASF AG, Germany) were mixed evenly and put into twin-screw extruder B;

[0035] (2) Co-extrusion is achieved through a double-layer die, with the material in extruder A being the core layer and the material in extruder B being the shell layer, with the mass ratio of the core layer to the shell layer being 95:5;

[0036] (3) Cooling the extruded filaments with water and pelletizing them to prepare expandable polypropylene microparticles with a length of 1.2 to 2.5 mm and a unit weight of 0.5 to 1.8 mg;

[0037] (4) Add the above-mentioned expandable polypropylene microparticles and water into a foaming kettle, add dispersant butter and surfactant sodium dodecyl sulfate at the same time, heat the foaming kettle and introduce carbon dioxide physical foaming agent to make the foaming agent penetrate into the interior of the polypropylene microparticles to form a homogeneous system. When the reactor reaches the set foaming temperature of 145°C and the foaming pressure of 2.0 MPa, maintain it for 10 minutes, then instantly release the expandable polypropylene microparticles into a foaming pipe with an internal air pressure of less than 0.1 MPa and an atmosphere temperature of 80-100°C for foaming and expansion. The expandable polypropylene microparticles spend 4-15 seconds in the foaming pipe, and finally obtain foamed polypropylene beads that can be quickly microwaved.

[0038] Preparation of foamed polypropylene molded parts:

[0039] (1) The above-mentioned expanded polypropylene beads were subjected to an air pressure of 0.3 MPa for 10 hours.

[0040] (2) The surface of the foamed polypropylene beads after air pressure was sprayed with water for wetting treatment, the mass ratio of water to foamed polypropylene beads was 1:20, and then the wet foamed polypropylene beads were subjected to microwave molding, the microwave frequency was 2450MHz, and the microwave power was 20kw. The molded parts were baked in a drying room at 80℃ to finally obtain the finished foamed parts. The process parameters during the molding process and the degree of maturation of the foamed parts are shown in Table 1.

[0041] Embodiment 2:

[0042] Except that the microwave auxiliary heating agent in the shell layer was replaced by alumina (CW-AL203-002, purchased from Shanghai Chaowei Nanotechnology Co., Ltd.), other components and preparation conditions were consistent with those in Example 1.

[0043] Embodiment 3:

[0044] Except that the microwave heating aid in the shell layer was replaced by copper-tin alloy powder (CW-Sn-Cu, purchased from Shanghai Chaowei Nanotechnology Co., Ltd.), other components and preparation conditions were consistent with those in Example 1.

[0045] Embodiment 4:

[0046] The difference from Example 1 is that when preparing the foamed polypropylene beads that can be quickly microwaved, no surfactant is added to the extruder B; when preparing the foamed polypropylene molded parts, the water spraying wetting treatment in step (2) is replaced by: a surfactant (PEG-10 ester, purchased from Nantong Chenrun Chemical Co., Ltd.) and water are mixed in a mass ratio of 1:3 and stirred to prepare a surfactant solution, and the above-mentioned surfactant solution is evenly sprayed on the surface of the foamed polypropylene beads after pressure loading, and the ratio of the surfactant solution spraying amount to the foamed beads mass is 1:9, and a foamed polypropylene bead material with the surfactant solution attached is obtained and microwaved. The other components and preparation conditions are the same as those in Example 1.

[0047] Embodiment 5:

[0048] Except that the sprayed surfactant was polypropylene glycol (15) stearyl alcohol ether, i.e., PPG-15 (purchased from Nantong Chenrun Chemical Co., Ltd.), other components and preparation conditions were the same as those in Example 4.

[0049] Embodiment 6:

[0050] Except that the sprayed surfactant was propylene glycol polyoxyethylene ether, APEG-800 (purchased from Nantong Chenrun Chemical Co., Ltd.), other components and preparation conditions were the same as those in Example 4.

[0051] Embodiment 7:

[0052] When preparing the foamed polypropylene molded product, the water spraying wetting treatment in step (2) is replaced by: a surfactant (PEG-10 ester, purchased from Nantong Chenrun Chemical Co., Ltd.) and water are mixed in a mass ratio of 1:3 and stirred to prepare a surfactant solution, and the surfactant solution is evenly sprayed onto the surface of the foamed polypropylene beads after pressure loading, and the ratio of the surfactant solution spraying amount to the foamed beads mass is 1:9, to obtain a foamed polypropylene bead material with the surfactant solution attached. Other components and preparation conditions are the same as those in Example 1.

[0053] Comparative Example 1:

[0054] Except that silicon nitride, a microwave auxiliary heating agent, is not added to the shell layer of the foamed polypropylene beads, other components and preparation conditions are consistent with those of Example 1.

[0055] Comparative Example 2:

[0056] Except that the surfactant PEG-10 is not added to the shell layer of the foamed polypropylene beads, other components and preparation conditions are the same as those in Example 1.

[0057] Comparative Example 3:

[0058] Except that silicon nitride, a microwave auxiliary heating agent, is not added to the shell layer of the foamed polypropylene beads, the other components and preparation conditions are the same as those in Example 4.

[0059] Comparative Example 4:

[0060] Except that the microwave heating aid silicon nitride and the surfactant PEG-10 are not added to the shell layer of the foamed polypropylene beads, the rest is the same as Example 1.

[0061]

[0062] The ripening degree of the foamed parts in Table 1 is from good to bad, and is divided into 5 very good, 4 good, 3 average, 2 poor, and 1 very poor. 5: When the number of beads with destroyed cells on the broken surface of the foamed part accounts for 90% or more of the total number of beads; 4: The number of beads with destroyed cells on the broken surface of the foamed part accounts for 70%-90% of the total number of beads; 3: The number of beads with destroyed cells on the broken surface of the foamed part accounts for 50%-70% of the total number of beads; 2: The number of beads with destroyed cells on the broken surface of the foamed part accounts for 30%-50% of the total number of beads; 1: The number of beads with destroyed cells on the broken surface of the foamed part accounts for less than 30% of the total number of beads.

[0063] Combining Examples 1-3 and Comparative Example 1, as well as Example 4 and Comparative Example 3, it can be seen that the addition of microwave heating aids significantly reduces the microwave heating time during molding, and the obtained foamed parts have a good degree of maturation. Different microwave heating aids have different efficiencies in converting heat energy in a high-frequency alternating electric field, and silicon nitride reduces the microwave heating time most significantly.

[0064] Combining Examples 1 and 4 with Comparative Example 2, it can be seen that the surfactant blended in the shell layer or sprayed on the surface of the foamed beads can also shorten the microwave heating time during molding and achieve a better degree of maturation. Moreover, spraying the surfactant on the surface of the foamed beads has the characteristics of simple processing technology and more significant shortening of microwave heating time.

[0065] It can be seen from Examples 4-6 that spraying different surfactants on the surface of the foamed beads can reduce the microwave heating time during molding to varying degrees.

[0066] From Examples 1, 4 and 7, it can be seen that, compared with only spraying the surfactant on the surface of the foamed beads or blending it in the shell of the foamed beads, the effect of simultaneously blending and coating the surfactant on shortening the microwave heating time is not obvious. Therefore, it is sufficient to choose one of the surfactant application methods.

[0067] Combining Examples 1 and 4 with Comparative Examples 1-4, it can be seen that the simultaneous addition of microwave heating aid and surfactant (blended or coated) has a better synergistic effect than the addition of microwave heating aid or surfactant (blended or coated) alone, and can significantly reduce the microwave heating time. In Comparative Example 4, when neither microwave heating aid nor surfactant is added, the microwave heating time of the foamed product is long, the efficiency is low, and the degree of maturation of the product is also poor.

Claims

1. A foamed polypropylene bead that can be quickly microwaved, characterized in that: The invention has a core-shell structure, wherein the core layer material comprises 80-99.97wt% of high-melting-point high-modulus polypropylene and 0.01-2wt% of a nucleating agent, the shell layer material comprises 80-99.96wt% of a low-melting-point propylene copolymer and / or a polypropylene blend and 0.01-5wt% of a microwave auxiliary heating agent, the high-melting-point high-modulus polypropylene has a melting point of 130-160°C, a bending modulus of 800-1100MPa, and a melt index of 5-10g / 10min, the low-melting-point propylene copolymer and / or the polypropylene blend has a melting point of 105-125°C, the microwave auxiliary heating agent is one or more of metal powder, metal oxide, nitride, glass fiber, bamboo fiber, carbon fiber, graphene, ferrite, and ceramic, the size of the microwave auxiliary heating agent is 10-1000nm, and the mass ratio of the core layer material to the shell layer material is 80:20-99:

1.

2. The rapidly microwaveable foamed polypropylene beads according to claim 1, characterized in that: The shell material further comprises 0.01 to 5 wt % of a surfactant, wherein the surfactant is at least one of an anionic surfactant, a cationic surfactant, a zwitterionic surfactant, and a nonionic surfactant.

3. The rapidly microwaveable foamed polypropylene beads according to claim 2, characterized in that: The anionic surfactant is at least one of stearic acid, oleic acid, lauric acid, sulfates, and sulfonates; the cationic surfactant is at least one of ammonium salts, quaternary ammonium salts, and heterocyclic surfactants; the zwitterionic surfactant is at least one of lecithin, amino acid, and betaine surfactants; and the nonionic surfactant is at least one of fatty acid glycerides, polyols, fatty acid sorbitan, polysorbate, polyoxyethylene, and polyoxyethylene-polyoxypropylene copolymers.

4. The rapidly microwaveable foamed polypropylene beads according to claim 1 or 2, characterized in that: The cell structure of the core layer is an independent closed-cell cell structure with a pore diameter of 50-200 microns.

5. The rapidly microwaveable foamed polypropylene beads according to claim 1 or 2, characterized in that: The high melting point and high modulus polypropylene is homopolymer polypropylene and / or copolymer polypropylene, and the copolymer polypropylene includes ethylene-propylene binary copolymer polypropylene and / or ethylene-propylene-butylene ternary copolymer polypropylene.

6. The rapidly microwaveable expanded polypropylene beads according to claim 1 or 2, characterized in that: The low melting point propylene copolymer is an ethylene-propylene binary copolymer and / or an ethylene-propylene-butylene ternary copolymer, and the low melting point polypropylene blend is a polyethylene / polypropylene blend.

7. The rapidly microwaveable expanded polypropylene beads according to claim 1 or 2, characterized in that: The metal powder is iron powder, copper powder, copper-tin alloy powder, gold powder and / or silver powder, the metal oxide is magnesium oxide, iron oxide, aluminum oxide and / or copper oxide, the nitride is silicon nitride and / or boron nitride, and the ferrite is a sintered composite of ferric oxide and nickel oxide, zinc oxide and manganese oxide.

8. The rapidly microwaveable expanded polypropylene beads according to claim 1 or 2, characterized in that: The nucleating agent is one or more of calcium carbonate, talcum powder, zinc borate, sodium chloride and silicon dioxide, and the particle size is 1 to 20 μm.

9. A foamed polypropylene molded part, characterized in that: The foamed polypropylene beads according to claim 1 are prepared by a microwave heating molding process, wherein a surfactant solution is sprayed on the surface of the foamed polypropylene beads before microwave molding, the mass ratio of the surfactant to water in the surfactant solution is 1:1 to 1:10, the mass ratio of the spraying amount of the surfactant solution to the foamed polypropylene beads is 1:2 to 1:20, the microwave heating power is 1 to 100 kW, and the time is 1 to 500 s.

10. A foamed polypropylene molded part, characterized in that: The foamed polypropylene beads according to any one of claims 2 to 8 are prepared by a microwave heating molding process, water is sprayed on the surface of the foamed polypropylene beads before microwave molding, the mass ratio of water to foamed polypropylene beads is 1:10 to 1:50, the microwave heating power is 1 to 100 kW, and the time is 1 to 500 s.

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