A high-porosity retention rate of a polypropylene foamed bead and a molded article thereof

Abstract

The present invention discloses a kind of through-hole polypropylene foamed beads with high through-hole retention rate and its molded parts.The foamed beads contain hollow through-holes throughout the foamed beads.The core layer material of the foamed beads contains a certain amount of through-hole structure retention agent, which can make polypropylene crystals grow rapidly at a higher temperature, accelerate the transition of polypropylene from melt form to solid semi-crystalline state, and thus solidify the structure form of the through-holes inside the filaments; at the same time, the core layer material also contains a synergistic aid, which can absorb a large amount of heat during the extrusion process, further solidifying the through-hole structure of the particles. During the extrusion process of the particles, even if the water temperature for cooling the filaments is slightly higher or the cutting knife for cutting the filaments is slightly dull, the polypropylene particles can still maintain a relatively high through-hole rate, and the corresponding foamed beads have a relatively short production cycle during molding, especially a relatively short water cooling cycle, greatly improving the production efficiency.

Description

Technical Field

[0001] This invention relates to a through-cell polypropylene foamed bead with high pore retention rate and its molded parts, belonging to the field of foamed materials. Background Technology

[0002] Polypropylene foamed beads are widely used in automotive parts, rail transportation, packaging, construction, sports and leisure, and other fields due to their high degree of lightweighting and excellent mechanical properties.

[0003] Generally, the process of steam molding polypropylene foam beads includes the following steps: 1. Before steam molding, the foam beads need to be pre-pressurized for more than ten hours to give them a certain internal pressure; 2. The foam beads with internal pressure are injected into the molding machine for steam molding; 3. Cooling water is sprayed on the surface of the molded part to cool it down and achieve preliminary shaping; 4. The water-cooled molded part is placed in an oven at about 80°C for more than 4 hours to bake it and achieve final shaping.

[0004] CN116102768A discloses a polypropylene foamed bead with tubular micropores, and CN116120623A discloses a polypropylene foamed bead with conical micropores. The high porosity of the foamed bead can effectively shorten the pre-pressing time before molding, the water cooling time during molding, and the baking time after molding, thereby greatly reducing the entire molding cycle and significantly improving molding production efficiency.

[0005] However, maintaining a high porosity in polypropylene microparticles is often difficult during continuous production. The main reasons are: 1. Commonly used gantry pelletizers, when pelletizing polypropylene resin filaments, often only effectively cut the upper half of the filaments due to the small cutting angle α of the cutter, relying on dragging force to break the lower half. When the cutter blade is sharp (blade angle < 35°), effective cutting dominates, and the pores of the microparticles can be effectively maintained; however, when the cutter blade becomes dull (blade angle ≥ 35°), the filaments deform partially due to dragging force, and the pores deform accordingly. The pores at both ends of the microparticles are easily blocked, resulting in a decrease in porosity. 2. Microporous materials are very sensitive to the temperature of the cooling water. Slightly higher water temperatures, such as exceeding 35℃, will severely affect the cooling efficiency of the polypropylene resin filaments. Due to untimely resin cooling and solidification, the pores at both ends of the microparticles are easily blocked during pelletizing. For the reasons mentioned above, to ensure the high porosity of microporous polypropylene, very stringent requirements are placed on the pelletizing and cooling processes during extrusion. The blade angle of the pelletizer cutter must be <35° and the water temperature of the cooling filaments must be <35℃. Otherwise, the porosity of the foamed beads will be directly affected, which will further affect the bead forming cycle, prolong the forming cycle, and reduce the forming efficiency.

[0006] Therefore, it is essential to develop a type of through-hole polypropylene foamed bead and foamed component with high pore retention rate. Summary of the Invention

[0007] To address the problem of low pore retention rate in the production of through-cell polypropylene, this invention discloses a through-cell polypropylene foamed bead with high pore retention rate and its molded parts. The through-cell polypropylene foamed bead contains a pore structure maintaining agent and synergistic additives, which help maintain the pore structure of the polypropylene microparticles. Even with slight dulling of the pelletizer blades and high cooling water temperature for the resin filaments, the through-cell polypropylene microparticles still maintain a high pore rate, resulting in a shorter production cycle and higher production efficiency during the molding process.

[0008] To achieve the above objectives, the present invention provides the following technical solution:

[0009] A type of through-cell polypropylene foamed bead with high pore retention rate, wherein the foamed bead has hollow through-cells penetrating the entire bead, and the average diameter ϕ of the hollow through-cells is 1-4 mm, preferably 1-3 mm, more preferably 1-2 mm, and even more preferably 1-1.5 mm; the ratio of the average diameter of the foamed bead to the average diameter of the hollow through-cell micropores, ϕ bead / ϕ pore, is 1.2-4.5, preferably 1.6-4; the foamed bead has a core-shell structure, and its core layer material comprises 80-99.99 wt%, preferably 90-99 wt%. The pore structure retainer comprises 0.01-5 wt%, preferably 0.01-3 wt%, more preferably 0.01-1 wt%, of a pore structure retainer and 0.01-10 wt%, preferably 0.1-3 wt%, of a synergistic agent. The pore structure retainer is one or more of lithium adipate, sodium benzoate, aluminum p-tert-butylbenzoate, bicycloheptandicarboxylate, sodium bis(4-tert-butylphenyl)phosphate, sodium 2,2'-methylenebis(4,6-di-tert-butylphenyl)phosphate, N,N'-dicyclohexyl-2,6-naphthalenediamide, polyvinylcyclohexane, and poly3-methyl-1-butene. The synergistic agent is a polyhydroxy compound.

[0010] When polypropylene resin is extruded through the extrusion orifice of an extruder, the pore structure retainer enables the polypropylene crystals to grow rapidly at higher temperatures, accelerating the transformation of polypropylene from a melt state to a solid semi-crystalline state. This solidifies the pore structure within the filament, ensuring that the pore structure is well maintained even with slightly higher cooling water temperatures. Furthermore, the pore structure retainer also improves the mechanical rigidity of polypropylene. When cut by a slightly duller pelletizer blade, more brittle fracture occurs at the cutting point, and the pores at both ends of the particles are not blocked due to excessive deformation, thus ensuring a higher porosity of the particles.

[0011] The synergist is a polyhydroxy compound that can form strong intramolecular and intermolecular hydrogen bonds, exhibiting high cohesive energy. When the synergist is mixed with a porosity maintainer, the hydrogen bonds in the synergist are broken, forming new interactions with polar substances or polar groups in the porosity maintainer. Since the energy required to break hydrogen bonds is much higher than the energy released by the newly formed interactions, this process requires the absorption of a large amount of energy and heat. The extruded resin filaments cool rapidly, solidify, and retain a high porosity. It is important to note that the synergist needs to be used simultaneously with the porosity maintainer for better endothermic curing ability, resulting in a higher microparticle porosity. If the synergist is used alone, because polypropylene is a non-polar material with weaker interaction with hydroxyl groups, the endothermic effect is relatively weaker, and the improvement in microparticle porosity is not as significant as with the former.

[0012] In the foamed beads, if the ϕ pore size is less than 1 mm, the particles are prone to clogging during production, resulting in a decrease in the open-cell rate. A lower open-cell rate cannot help the foamed beads establish sufficient internal pressure within a short pre-compression time, and the water cooling time during molding and the baking time of the molded foamed part cannot be significantly shortened. If the ϕ pore size is greater than 4 mm, defects such as pinholes and pits are easily generated on the surface of the foamed part. If the ϕ bead / ϕ pore size ratio is greater than 4.5, the diameter of the hollow open-cell ϕ pore is relatively small, and the reduction in pre-compression time before molding, water cooling time during molding, and baking time after molding is not significant. If the ϕ bead / ϕ pore size ratio is less than 1.2, the diameter of the hollow open-cell ϕ pore is relatively large, and the pore walls of the foamed beads are thin. The surface of the molded foamed part has many defects such as pinholes and pits, seriously affecting the aesthetics and mechanical properties of the part, and may even cause local shrinkage and severe dimensional deformation.

[0013] Furthermore, the polypropylene is a high-melting-point, high-modulus polypropylene, and the shell material of the foamed beads includes 90-100 wt%, preferably 95-100 wt%, of a low-melting-point polyethylene / polypropylene blend. The high-melting-point, high-modulus polypropylene has a melting point of 140-160°C and a flexural modulus of 800-1180 MPa. The polyethylene / polypropylene blend has a melting point of 105-125°C. The mass ratio of the core material to the shell material is 80:20-99:1, more preferably 90:10-99:1.

[0014] Furthermore, the synergistic adjuvant is one or more of glycerol, pentaerythritol, sorbitol, glucose, sucrose, and lactose.

[0015] Furthermore, the high-melting-point, high-modulus polypropylene can be homopolymer polypropylene, copolymer polypropylene, or a mixture of homopolymer polypropylene and copolymer polypropylene, wherein the copolymer polypropylene can be ethylene-propylene binary copolymer polypropylene, ethylene-propylene-butadiene ternary copolymer polypropylene, etc., and the melt index of the high-melting-point, high-modulus polypropylene is 5-10 g / 10 min. Using this type of high-melting-point, high-modulus polypropylene as the core material can provide better heat resistance and rigidity for the foamed beads, ensuring that the independent cells after foaming are not "burned down" by steam, and also inhibiting the shrinkage and deformation of the molded parts.

[0016] Furthermore, the core layer material further comprises 0.01–2 wt% of an inorganic nucleating agent, wherein the inorganic nucleating agent is one or more selected from calcium carbonate, talc, zinc borate, barium sulfate, sodium chloride, and silicon dioxide, with a particle size of 1–20 μm. The inorganic nucleating agent exhibits a certain degree of incompatibility with the polypropylene substrate, promoting cell growth at the interface between the inorganic nucleating agent and the polypropylene, thus playing a role in heterogeneous nucleation. Simultaneously, the inorganic nucleating agent also reduces foaming pressure and promotes uniform cell size.

[0017] Furthermore, the polyethylene / polypropylene blend is a blend of high-density polyethylene, low-density polyethylene, or linear low-density polyethylene with polypropylene, and its melt index is 4–15 g / 10 min. During the molding process, when steam pressure acts on the foamed polypropylene beads, the lower-melting-point polyethylene / polypropylene blend shell melts rapidly, and the internal molecular chains begin to entangle. However, the core layer of high-melting-point polypropylene foamed beads maintains its complete closed-cell structure. In the subsequent cooling process, the shell layer of polyethylene / polypropylene blend is rapidly cooled, and the entangled molecular chains are instantly "frozen," resulting in a tight and firm fusion between the beads. The resulting molded part has a high degree of curing and a fuller, more aesthetically pleasing surface.

[0018] Furthermore, the polypropylene microparticles with high porosity retention are heated from 25°C to 220°C at a heating rate of 10°C / min, held at that temperature for 3 minutes, and then cooled from 220°C to 25°C at a cooling rate of 10°C / min. This process is repeated twice, heating back to 220°C at the same rate. The resulting DSC (Differential Scanning Calorimeter) curve shows an enthalpy value of over 40 J / g for the melting peak during the second heating. When the enthalpy value of the melting peak during the second heating reaches over 40 J / g, the polypropylene exhibits rapid crystallization at high temperatures and high rigidity, retaining a relatively complete porous structure and possessing a high porosity. Consequently, the foamed beads have a shorter molding cycle, especially the water cooling cycle of the molded parts.

[0019] The preparation method of the above-mentioned through-cell polypropylene foam beads with high pore retention rate is as follows:

[0020] (1) The core material of the foamed beads is mixed evenly and then fed into twin-screw extruder A; the shell material of the foamed beads is mixed evenly and then fed into twin-screw extruder B.

[0021] (2) Co-extrusion is achieved through a special double-layer die and through-hole design of the filament outlet, with the material in extruder A as the core layer and the material in extruder B as the shell layer, and through-holes are formed inside the filament.

[0022] (3) The extruded filaments are cooled with water and pelletized using a pelletizer to obtain expandable polypropylene microparticles with a core-shell structure containing through-pores.

[0023] (4) The expandable polypropylene microparticles with a core-shell structure and through holes are fed into a high-pressure foaming kettle for foaming to obtain the through-hole polypropylene foam beads.

[0024] A foamed polypropylene molded part is obtained by sintering the above-mentioned through-pore polypropylene foamed beads with steam.

[0025] Compared with the prior art, the beneficial effects of the present invention are:

[0026] The expandable, porous polypropylene microparticles contain a pore-retaining agent, which allows polypropylene crystals to grow rapidly at higher temperatures, accelerating the transformation of polypropylene from a melt state to a solid semi-crystalline state, thereby solidifying the pore structure inside the filament. Simultaneously, the addition of synergistic additives absorbs a large amount of heat during extrusion, further solidifying the pore structure of the microparticles. Even if the water temperature for cooling the filament is slightly high or the cutting blade for cutting the filament is slightly dull during microparticle extrusion, the polypropylene microparticles can still maintain a high porosity. Correspondingly, the foamed beads have a shorter production cycle during molding, especially a shorter water cooling cycle, significantly improving production efficiency. Attached Figure Description

[0027] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used in conjunction with embodiments of the invention to explain the invention and do not constitute a limitation thereof. In the drawings:

[0028] Figure 1 These are the through-pore polypropylene foam beads prepared in Example 1;

[0029] Figure 2 The porous polypropylene foam beads prepared in Comparative Example 3 are shown. Detailed Implementation

[0030] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0031] Example 1:

[0032] Preparation of through-pore polypropylene foam beads:

[0033] (1) According to the component ratio (mass ratio) of Example 1 in Table 1, the high melting point and high modulus polypropylene (melting point 142℃, melt index 7g / 10min, flexural modulus 1000MPa, purchased from China Petrochemical Corporation), the through-pore structure retainer (sodium 2,2'-methylene bis(4,6-di-tert-butylphenyl) phosphate, NA-11, purchased from Adico Corporation of Japan), the synergistic agent (glycerol, purchased from Zibo Lishuo Chemical Co., Ltd.), and the nucleating agent (calcium carbonate, purchased from Jiangxi Guangyuan New Materials Co., Ltd.) were mixed. The polyethylene / polypropylene blend (melting point 120℃, melt index 8g / 10min, purchased from Sinopec Corporation), lubricant (oleamide, purchased from Zhengzhou Zhuochuang Chemical Products Co., Ltd.), and antioxidant (1010, purchased from BASF AG, Germany) were mixed evenly and then fed into twin-screw extruder A. The polyethylene / polypropylene blend (melting point 120℃, melt index 8g / 10min, purchased from Sinopec Corporation), lubricant (oleamide, purchased from Zhengzhou Zhuochuang Chemical Products Co., Ltd.), and antioxidant (1010, purchased from BASF AG, Germany) were mixed evenly and then fed into twin-screw extruder B.

[0034] (2) Co-extrusion is achieved through a double-layer die, with the material in extruder A as the core layer and the material in extruder B as the shell layer, and through holes are formed inside the filament. The mass ratio of the core layer to the shell layer is 95:5.

[0035] (3) The extruded filaments are cooled by water and granulated. The cooling water temperature is 35-40℃ and the blade angle is 35° to prepare expandable polypropylene microparticles with a core-shell structure and tubular through-holes inside. The microparticles are 1.2-2.5 mm long and weigh 0.5-1.8 mg.

[0036] (4) Add expandable polypropylene microparticles with a core-shell structure and water to the foaming kettle, and add dispersant butter and surfactant sodium dodecyl sulfate at the same time. Heat the foaming kettle and introduce carbon dioxide physical foaming agent to allow the foaming agent to penetrate into the polypropylene microparticles and form a homogeneous system. When the set foaming temperature and foaming pressure are reached in the reactor, maintain for 10 minutes. Then, release the expandable polypropylene microparticles instantly into the foaming pipe with an internal air pressure of less than 0.1 MPa and an atmosphere temperature of 80 to 100°C for foaming expansion. The expandable polypropylene microparticles spend 4 to 15 seconds in the foaming pipe, and finally obtain microporous polypropylene foam beads.

[0037] Preparation of foamed polypropylene molded parts:

[0038] After the prepared through-pore polypropylene foam beads were subjected to an air pressure of 0.3 MPa for several hours, they were steam molded and baked in an 80℃ oven to obtain the foamed parts. The process parameters and performance of the foamed parts are shown in Table 1.

[0039] Example 2: Except for the addition of 0.1 wt% of the through-pore structure retainer bicycloheptane dicarboxylate (HPN-20E, purchased from Milliken, USA) to the core layer, through-pore polypropylene foam beads and foamed polypropylene molded parts were prepared using the same method as in Example 1.

[0040] Example 3: Except for the addition of 3 wt% of glycerol (purchased from Zibo Lishuo Chemical Co., Ltd.) as a synergist for through-hole structure in the core layer, through-hole polypropylene foam beads and foamed polypropylene molded parts were prepared using the same method as in Example 1.

[0041] Example 4: Except for the addition of 1 wt% of sorbitol (purchased from Shandong Tianli Pharmaceutical Co., Ltd.) as a synergistic agent for through-pore structure in the core layer, through-pore polypropylene foam beads and foamed polypropylene molded parts were prepared using the same method as in Example 1.

[0042] Comparative Example 1: Through-pore polypropylene foam beads and foamed polypropylene molded parts were prepared using the same method as in Example 1, except that no through-pore structure retainer (sodium 2,2'-methylene bis(4,6-di-tert-butylphenyl) phosphate) was added.

[0043] Comparative Example 2: Except for the absence of the synergistic additive glycerol, porous polypropylene foam beads and foamed polypropylene molded parts were prepared using the same method as in Example 1.

[0044] Comparative Example 3: Except for the absence of the through-pore structure retainer (sodium 2,2'-methylene bis(4,6-di-tert-butylphenyl) phosphate) and the absence of the synergistic additive glycerol, through-pore polypropylene foam beads and foamed polypropylene molded parts were prepared using the same method as in Example 1.

[0045] Comparative Example 4: Except that the cooling water temperature of the filaments was 50°C during the extrusion process, through-pore polypropylene foam beads and foamed polypropylene molded parts were prepared using the same method as in Example 1.

[0046] Comparative Example 5: Except that the blade angle of the pelletizer was 45° (the blade was relatively blunt) during the extrusion process, through-cell polypropylene foam beads and foamed polypropylene molded parts were prepared using the same method as in Example 1.

[0047]

[0048] In Table 1, the diameter of the foamed beads and the diameter of the through holes are the average values ​​measured with vernier calipers from 100 randomly selected foamed beads. The through hole retention rate is the percentage of beads with through holes that are visible to the naked eye out of 100 randomly selected foamed beads.

[0049] As can be seen from Examples 1 and 2 and Comparative Example 1, the through-cell structure retainer has a good effect on maintaining the through-cell structure of foamed polypropylene beads. Adding small amounts of NA-11 and HPN-20E can increase the through-cell retention rate from 43% to 74% and 68% respectively, thereby reducing the water cooling time during molding from 165s to 120s and 140s.

[0050] As can be seen from Examples 1 and 3 and Comparative Example 2, when adding a through-hole structure retainer, adding the synergist glycerol can further improve the diameter and retention rate of the through-hole. Moreover, as the amount of glycerol added increases from 1 wt% to 3 wt%, the diameter and retention rate of the through-hole further increase, and the water cooling time of molding is further shortened.

[0051] As can be seen from Examples 1 and 4, different through-hole structure synergists have different abilities to maintain through-hole retention rate and shorten water cooling time.

[0052] As can be seen from Example 1 and Comparative Example 3, if neither the through-hole structure retainer nor the synergistic additive is added, the diameter of the through-hole will become smaller, some of the bead surfaces will be blocked, the through-hole retention rate will decrease significantly, and the water cooling time during molding will be longer.

[0053] As can be seen from Example 1 and Comparative Example 4, when the water temperature of the cooling filament is significantly higher during the extrusion process, even with the presence of a through-hole structure retainer and synergistic additives, the through-hole retention rate of the foamed beads is low, and the water cooling time of the molded parts is longer.

[0054] As can be seen from Example 1 and Comparative Example 5, when the blade of the pelletizer becomes severely dull during the extrusion process, even with the presence of a through-hole structure retainer and synergistic additives, the through-hole retention rate of the foamed beads is low, and the water cooling time of the molded parts is long.

[0055] Finally, it should be noted that the above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A type of through-cell polypropylene foamed bead with high pore retention rate, characterized in that, The foamed beads have hollow through-holes that extend throughout the entire foamed bead, and the average diameter ϕ of the hollow through-holes is... 孔 The average diameter of the foamed beads is 1-4 mm, and the ratio ϕ of the average diameter of the hollow pores is... 珠粒 / ϕ 孔 The content is 1.2 to 4.5; the foamed beads have a core-shell structure, and the core material includes 80 to 99.99 wt% polypropylene, 0.01 to 5 wt% pore structure retainer and 0.01 to 10 wt% synergistic agent. The pore structure retainer is one or more of lithium adipate, sodium benzoate, aluminum p-tert-butylbenzoate, bicycloheptanide dicarboxylate, sodium bis(4-tert-butylphenyl)phosphate, N,N'-dicyclohexyl-2,6-naphthalenedicarboxamide, polyvinylcyclohexane, and poly3-methyl-1-butene. The synergistic agent is one or more of glycerol, pentaerythritol, glucose, sucrose, and lactose.

2. The through-cell polypropylene foam beads with high pore retention rate according to claim 1, characterized in that, The average diameter ϕ of the hollow through hole 孔 It is 1-3mm.

3. The through-cell polypropylene foam beads with high pore retention rate according to claim 2, characterized in that, The average diameter ϕ of the hollow through hole 孔 It is 1-2 mm.

4. The through-cell polypropylene foam beads with high pore retention rate according to claim 3, characterized in that, The average diameter ϕ of the hollow through hole 孔 It is 1 to 1.5 mm.

5. The through-cell polypropylene foam beads with high pore retention rate according to claim 1, characterized in that, The ratio ϕ of the average diameter of the foamed beads to the average diameter of the hollow pores 珠粒 / ϕ 孔 1.6~4.

6. The through-cell polypropylene foam beads with high pore retention rate according to claim 1, characterized in that, The core layer material of the foamed beads comprises 90-99 wt% polypropylene.

7. The through-cell polypropylene foam beads with high pore retention rate according to claim 6, characterized in that, The core layer material of the foamed beads comprises 93-98 wt% polypropylene.

8. The through-cell polypropylene foam beads with high pore retention rate according to claim 1, characterized in that, The core layer material of the foamed beads includes 0.01 to 3 wt% of a through-pore structure retainer.

9. The through-cell polypropylene foam beads with high pore retention rate according to claim 8, characterized in that, The core layer material of the foamed beads includes 0.01 to 1 wt% of a through-pore structure retainer.

10. The through-cell polypropylene foam beads with high pore retention rate according to claim 1, characterized in that, The core layer material of the foamed beads includes 0.1 to 3 wt% of synergistic additives.

11. The through-cell polypropylene foam beads with high pore retention rate according to claim 1, characterized in that, The polypropylene is a high-melting-point, high-modulus polypropylene. The shell material of the foamed beads includes 90-100 wt% of a low-melting-point polyethylene / polypropylene blend. The high-melting-point, high-modulus polypropylene has a melting point of 140-160°C and a flexural modulus of 800-1180 MPa. The polyethylene / polypropylene blend has a melting point of 105-125°C. The mass ratio of the core material to the shell material is 80:20-99:

1.

12. The through-cell polypropylene foam beads with high pore retention rate according to claim 11, characterized in that, The shell material of the foamed beads comprises 95-100 wt% of a low-melting-point polyethylene / polypropylene blend.

13. The through-cell polypropylene foam beads with high pore retention rate according to claim 11, characterized in that, The mass ratio of the core material to the shell material is 90:10 to 99:

1.

14. The through-cell polypropylene foam beads with high pore retention rate according to claim 11, characterized in that, The high-melting-point, high-modulus polypropylene is homopolymer polypropylene, copolymer polypropylene, or a mixture of homopolymer polypropylene and copolymer polypropylene. The copolymer polypropylene is ethylene-propylene binary copolymer polypropylene or ethylene-propylene-butadiene ternary copolymer polypropylene. The melt index of the high-melting-point, high-modulus polypropylene is 5-10 g / 10 min.

15. The through-cell polypropylene foam beads with high pore retention rate according to claim 1, characterized in that, The core layer material further includes 0.01 to 2 wt% of a nucleating agent, wherein the nucleating agent is one or more of calcium carbonate, talc, zinc borate, barium sulfate, sodium chloride, and silicon dioxide, and the particle size of the nucleating agent is 1 to 20 μm.

16. The through-cell polypropylene foam beads with high pore retention rate according to claim 11, characterized in that, The polyethylene / polypropylene blend is a blend of high-density polyethylene, low-density polyethylene, or linear low-density polyethylene with polypropylene, and its melt index is 4-15 g / 10 min.

17. The through-cell polypropylene foam beads with high pore retention rate according to claim 1, characterized in that, The polypropylene microparticles were heated from 25°C to 220°C at a heating rate of 10°C / min, held at that temperature for 3 minutes, and then cooled from 220°C to 25°C at a cooling rate of 10°C / min. The same heating and cooling rates were then applied to raise the temperature to 220°C a second time. The enthalpy value obtained from integrating the melting peak during the second heating in the DSC curve was 40 J / g or higher.

18. A foamed polypropylene molded part, characterized in that, The through-cell polypropylene foam beads with high pore retention rate as described in any one of claims 1 to 17 are obtained by steam sintering molding process.

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