Method for preparing micro-foamed pp having both oriented cells and ordered β-type lamellar crystals

Abstract

The present invention belongs to the technical field of new materials. Disclosed is a method for preparing micro-foamed PP having both oriented cells and ordered β-type lamellar crystals. The method comprises the following steps: drying PP pellets and an amide β nucleating agent, then uniformly mixing same, subjecting the mixture to melt blending and extrusion granulation in an extruder, and then subjecting master batches to extrusion or injection, so as to obtain a profile containing β-type oriented crystals; and subjecting the profile to solid-phase thermal stretching, so as to further crush, rearrange and orientate the crystals, cooling same, and then subjecting same to supercritical CO2 foaming, so as to obtain micro-foamed PP having both orientated cells and ordered β-type lamellar crystals. In the present invention, by utilizing a nucleating agent, a processing process and a force field effect, the generation of a β type crystal structure of PP is controlled and a lamellar crystal stacking form is regulated and controlled; and then by utilizing the high strength, high modulus and difficult foamability of the lamellar crystals, the nucleation and growth of cells in an intercrystalline limited space are controlled, thereby achieving the synchronous design of a final cell structure and a crystal aggregation form, reducing the apparent density of PP, and also improving the mechanical properties of PP.

Description

A method for preparing micro-foamed PP with both oriented cells and ordered β-type lamellae Technical Field

[0001] The present invention belongs to the technical field of new materials, and in particular relates to a method for preparing micro-foamed PP having both oriented cells and ordered β-type lamellae. Background Art

[0002] Energy conservation and emission reduction are crucial to achieving green development. The lightweighting of vehicles, such as automobiles and drones, is closely related to this, and reusable, high-performance, lightweight plastics are urgently needed to replace traditional metal materials. Polypropylene (PP) boasts high-temperature and corrosion resistance, is non-toxic and odorless, and can be processed multiple times. Its high production volume surpasses polyethylene and polyvinyl chloride, ranking first among the five major general-purpose plastics. It is widely used in the manufacture of important industrial products such as automotive interior and exterior components and drone bodies. Therefore, achieving lightweight and high-strength PP is of great significance.

[0003] A key approach to producing lightweight, high-strength PP materials is to construct a crystal structure with both oriented pores and a high degree of crystallization. The creation of a pore structure reduces the apparent density of the PP material, while a high degree of crystallization ensures the material's excellent mechanical properties and heat resistance. The orientation of the pores and crystals is key to achieving a high load-to-weight ratio.

[0004] The existing methods for preparing oriented pore structures mainly include 3D printing and ice template method. 3D printing is the simplest and most effective method to achieve oriented pores. Brett et al. [COMPTON BG, LEWIS J A. Advanced Materials, 2014, 26(34): 5930-5.] used 3D printing to prepare a honeycomb epoxy resin composite foam with a density of 0.38 g / cm 3 In addition, the ice template method is also an effective method for preparing oriented pore materials [SAMITSU S, PRUKSAWAN S, YOKOYAMA H, et al. Macromolecules, 2022, 55(9): 3734-46.], Wang [WANG B, TORRES-RENDON JG, YU J, et al. ACS Applied Materials & Interfaces, 2015, 7(8): 4595-607.] and others used the ice template method to orient cellulose nanocrystals, so that the tensile strength of the porous oriented film finally prepared reached 187 MPa, which is twice that of the unoriented one.

[0005] Although 3D printing and ice template method have certain advantages in preparing porous oriented materials, 3D printing has low efficiency and limited precision, and cannot obtain materials with very small pores, while the ice template method is almost not applicable to the preparation of porous PP.

[0006] Therefore, developing new methods to achieve high load-to-weight ratio PP is not only an important requirement for the development of lightweight equipment, but will also help promote the development of the recyclable plastics industry.

[0007] Summary of the Invention

[0008] To address the aforementioned issues in the prior art, the present invention provides a method for preparing micro-foamed PP with both oriented cells and ordered β-type lamellae. Addressing the difficulty of achieving both lightweight and high strength in existing PP foams, the present invention proposes a novel high-load-to-weight ratio micro-foamed PP material preparation technology. This technology is used to produce PP foam with a large number of tiny oriented cells and a highly oriented β-type crystal structure, thereby reducing the apparent density of PP while improving its mechanical properties.

[0009] To achieve the above object, the present invention provides the following technical solutions:

[0010] The present invention provides a method for preparing micro-foamed PP having a large number of oriented cells and ordered β-type lamellae, comprising the following steps:

[0011] (1) drying PP pellets and an amide β-nucleating agent at 90° C. for 2 hours and then mixing them uniformly; melt-blending the resulting mixture, and extruding and granulating it to obtain mixed pellets;

[0012] (2) extruding or injection molding the mixed pellets obtained in step (1) to obtain a profile;

[0013] (3) hot stretching the profile obtained in step (2), and cooling after stretching;

[0014] (4) The profile after stretching and cooling in step (3) is heated, infiltrated under pressure, and foamed under pressure release in a CO2 atmosphere, and then cooled again to obtain the micro-foamed PP having both oriented bubbles and ordered β-type lamellae.

[0015] As a preferred embodiment of the present invention, in step (1), the amide β-nucleating agent is a nucleating agent with dicyclohexyl terephthalamide as a main component.

[0016] As a preferred embodiment of the present invention, in step (1), the mass of the amide β-nucleating agent is 0.05-2% of the mass of the PP pellets, and the maximum heating temperature of the melt blending is 180-200°C.

[0017] As a preferred embodiment of the present invention, in step (2), the maximum heating temperature of the extrusion is 210-235°C, and the die temperature is 180-200°C.

[0018] As a preferred embodiment of the present invention, in step (3), the heat stretching temperature is 70-110° C., the stretching ratio is 10-80% (i.e., the length is stretched by 10-80% on the basis of the original length), and the cooling is carried out in water at room temperature.

[0019] As a preferred embodiment of the present invention, in step (4), the pressure-maintaining infiltration temperature is 80-120° C., the pressure is 20-25 MPa, and the time is 2-12 h.

[0020] As a preferred embodiment of the present invention, in step (4), the pressure relief foaming process is to relieve pressure from the infiltration pressure to normal pressure, the pressure relief completion time is 15 to 60 seconds, and the re-cooling is to cool along with the foaming kettle.

[0021] The present invention also provides a PP foam prepared according to any of the above methods for preparing micro-foamed PP having both oriented cells and ordered β-type lamellae.

[0022] The present invention also provides an application of the PP foam described above in the preparation of automotive interior and exterior parts, drone bodies, water sports equipment, wind turbine blade fillings, thermal insulation materials and building templates.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] The present invention utilizes the high strength, high modulus and non-foaming properties of PP lamellae, and regulates the stacking form of β-type lamellae through the coupling of crystallization template and external field. The narrow amorphous area between adjacent lamellae is cleverly used as a restricted foaming space to control the nucleation and growth of the foam cells, thereby realizing the design of the final foam cell structure and aggregation form, thereby establishing a simple and efficient high load-to-weight ratio PP directional processing method.

[0025] An amide β-nucleating agent is used, which has a strong interaction with supercritical CO2 (ScCO2) and can self-assemble by relying on its own intermolecular hydrogen bonds. It is controlled to self-assemble into long strip crystal templates with different aspect ratios and surface physical states in the PP melt, thereby reducing the apparent density of PP during foaming and improving its mechanical properties.

[0026] The present invention starts from a deep understanding of the basic principles of ScCO2 foaming and the ordered structure reinforcement effect, simulates the ordered pore morphology of wood, and through the advantageous crystal construction and intercrystalline confined space design, utilizes the synergistic reinforcement effect of the gas-solid two phases under external load, transforms the pore defects into reinforcement phase, and prepares micro-foamed PP with a high load-to-weight ratio, providing a new path and new method for the preparation of high-performance, lightweight, and recyclable plastic products.

[0027] The micro-foamed PP of the present invention having a large number of tiny oriented bubbles and ordered β-type lamellae is obtained by co-extruding a β-nucleating agent with PP, then subjecting it to controllable stretching deformation, and then subjecting it to ScCO2 foaming. The operations involved, such as co-extrusion, stretching, and ScCO2 foaming, are simple, environmentally friendly, and easy to industrialize. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0029] In FIG1 , a to c are appearance pictures of the micro-foamed PP samples prepared in Comparative Example 3, Comparative Example 1 and Example 3, respectively.

[0030] In Figure 2, a1 and e1, a2 and e2, a3 and e3, a4 and e4 are polarized micrographs, scanning electron microscope photographs, two-dimensional wide-angle and small-angle X-ray scattering data of the PP profiles to be foamed prepared in step (4) of Comparative Example 1 and Comparative Example 2, respectively, and b1~d1, b2~d2, b3~d3, b4~b4 are polarized micrographs, scanning electron microscope photographs, two-dimensional wide-angle and small-angle X-ray scattering data of the PP profiles to be foamed prepared in step (4) of Examples 1 to 3, respectively. DETAILED DESCRIPTION

[0031] Various exemplary embodiments of the present invention are now described in detail. This detailed description should not be considered as a limitation of the present invention, but should be understood as a more detailed description of certain aspects, features, and embodiments of the present invention. It should be understood that the terms used in the present invention are only for describing specific embodiments and are not intended to limit the present invention.

[0032] In addition, for numerical ranges in the present invention, it is understood that each intervening value between the upper and lower limits of the range is also specifically disclosed. Each smaller range between any stated value or intervening value in a stated range and any other stated value or intervening value in the stated range is also included in the present invention. The upper and lower limits of these smaller ranges may independently be included or excluded in the range.

[0033] Unless otherwise indicated, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art. Although only preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein may also be used in the practice or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials associated with the documents. In the event of any conflict with any incorporated document, the contents of this specification shall prevail.

[0034] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments described herein without departing from the scope or spirit of the invention. Other embodiments will be apparent to those skilled in the art from the description of the invention. The description and examples are intended to be exemplary only.

[0035] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

[0036] In the following examples, the PP pellets used are isotactic polypropylene; the room temperature water refers to water at 20-35°C; and the raw materials used in the following examples and comparative examples are all commercially available. A detailed description thereof will not be repeated below.

[0037] Example 1

[0038] The preparation of micro-foamed PP with both oriented cells and ordered β-lamellae is as follows:

[0039] (1) Drying PP pellets and 0.1 wt% of the amide β-nucleating agent dicyclohexyl terephthalamide (TMB-5) in an oven at 90° C. for 2 hours, and then mixing them uniformly in a high-speed mixer;

[0040] (2) The maximum heating temperature is set to 200° C., and other temperatures are set according to conventional methods. The PP and nucleating agent blend is melt-blended and extruded into granules in a twin-screw extruder to obtain mixed granules.

[0041] (3) The mixed pellets obtained in step (2) are extruded in a single-screw extruder to obtain a sheet. The maximum heating temperature during the extrusion process is controlled at 210° C., the die temperature is controlled at 200° C., and other temperatures are set as conventionally.

[0042] (4) The sheet obtained in step (3) is subjected to heat stretching at 100° C. with a stretching ratio of 30%, and then rapidly cooled in room temperature water after stretching.

[0043] (5) The profile after stretching and cooling in step (4) is placed in a high-pressure reactor, filled with CO2 and then depressurized and released, and the air is removed by two cycles. The temperature is raised to the infiltration temperature of 80°C, the pressure is maintained at 22 MPa, and the pressure is maintained for 4 hours. After the pressure maintenance is completed, the pressure relief valve is opened and the pressure drops to normal pressure within 40 seconds. The foam is taken out after natural cooling of the reactor to obtain a micro-foamed PP having both oriented bubbles and ordered β-type lamellae.

[0044] Example 2

[0045] The same as Example 1, except that in step (3), the maximum heating temperature of the extrusion process is controlled at 220°C.

[0046] Example 3

[0047] The same as Example 1, except that in step (3), the maximum heating temperature of the extrusion process is controlled at 235°C.

[0048] Example 4

[0049] The preparation of micro-foamed PP with both oriented cells and ordered β-lamellae is as follows:

[0050] (1) Drying PP pellets and an amide β-nucleating agent, dicyclohexyl terephthalamide (TMB-5), at a content of 0.05 wt% of the PP pellets in an oven at 90° C. for 2 hours, and then mixing them uniformly in a high-speed mixer;

[0051] (2) The maximum heating temperature is set to 180° C., and other temperatures are set according to conventional methods. The PP and nucleating agent blend is melt-blended and extruded into granules in a twin-screw extruder to obtain mixed granules.

[0052] (3) The mixed pellets obtained in step (2) are injection molded to obtain a sheet. The maximum heating temperature of the injection molding process is controlled at 210° C., the nozzle temperature is controlled at 180° C., and other temperatures can be set according to conventional methods.

[0053] (4) The sheet obtained in step (3) was heat-stretched at 70° C. with a stretching ratio of 10%, and then rapidly cooled in room temperature water after stretching.

[0054] (5) The profile after stretching and cooling in step (4) is placed in a high-pressure reactor, filled with CO2 and then depressurized and released, and the air is removed by two cycles. The temperature is raised to the infiltration temperature of 100°C, the pressure is maintained at 20 MPa, and the pressure is maintained for 2 hours. After the pressure maintenance is completed, the pressure relief valve is opened and the pressure is reduced to normal pressure within 60 seconds. The foam is taken out after natural cooling of the reactor to obtain a micro-foamed PP having both oriented bubbles and ordered β-type lamellae.

[0055] Example 5

[0056] The preparation of micro-foamed PP with both oriented cells and ordered β-lamellae is as follows:

[0057] (1) Drying PP pellets and an amide β-nucleating agent, dicyclohexyl terephthalamide (TMB-5), at a content of 2 wt% of the PP pellets in an oven at 90° C. for 2 hours, and then mixing them uniformly in a high-speed mixer;

[0058] (2) The maximum heating temperature is set to 190° C., and other temperatures are set according to conventional methods. The PP and nucleating agent blend is melt-blended and extruded into granules in a twin-screw extruder to obtain mixed granules.

[0059] (3) The mixed pellets obtained in step (2) are extruded into a sheet in a single screw extruder. The maximum heating temperature during the extrusion process is controlled at 210° C., the die temperature is controlled at 190° C., and other temperatures are set as conventionally.

[0060] (4) The sheet obtained in step (3) was subjected to heat stretching at 85° C. with a stretching ratio of 80%, and then rapidly cooled in ice water after stretching.

[0061] (5) The profile after stretching and cooling in step (4) is placed in a high-pressure reactor, filled with CO2 and then depressurized and released, and the air is removed by two cycles. The temperature is raised to the infiltration temperature of 120°C, the pressure is maintained at 25 MPa, and the pressure is maintained for 12 hours. After the pressure maintenance is completed, the pressure relief valve is opened and the pressure drops to normal pressure within 15 seconds. The foam is taken out after natural cooling of the reactor to obtain a micro-foamed PP having both oriented bubbles and ordered β-type lamellae.

[0062] Figure 1 shows PP foamed by supercritical CO2 under different conditions, a to c are the appearance pictures of the micro-foamed PP samples prepared in comparative example 3, comparative example 1 and embodiment 3, i.e., the samples of PP extruded and solid-phase foamed without adding β-nucleating agent; the solid-phase foamed PP sample with β-nucleating agent added and extruded but without the regulation of lamellae arrangement; and the appearance of the micro-foamed PP sample prepared in embodiment 3 with a large number of oriented bubbles and ordered β-type lamellae.

[0063] It can be seen that PP without a β-nucleating agent, that is, basically without β-type lamellae, can hardly be foamed, while PP containing β-type lamellae can be foamed, and the foaming ratio can become higher after stretching and orientation.

[0064] Comparative Examples 1-2

[0065] The same as Example 1, except that in step (3), the maximum heating temperature of the extrusion process is controlled at 200° C. and 255° C. respectively.

[0066] In Figure 2, a1 and e1, a2 and e2, a3 and e3, and a4 and e4 are polarized light micrographs, scanning electron micrographs, and two-dimensional wide-angle and small-angle X-ray scattering data of the PP profiles to be foamed prepared in step (4) of Comparative Examples 1 and 2, respectively. b1-d1, b2-d2, b3-d3, and b4-b4 are polarized light micrographs, scanning electron micrographs, and two-dimensional wide-angle and small-angle X-ray scattering data of the PP profiles to be foamed prepared in step (4) of Examples 1-3, respectively. As can be seen from Figure 2, during the extrusion process to obtain the profiles, the aspect ratio of the crystallization template, the structure and arrangement of the PP lamellae, and the PP lamellae were regulated by varying the maximum processing temperature. When the maximum processing temperature is low (Comparative Example 1), the nucleating agent cannot self-assemble into a distinct fibrous structure, and cannot induce PP to form a distinct ordered β-type lamellae, resulting in a decrease in the orderliness of the intercrystalline foamable space; when the maximum processing temperature is appropriate (Examples 1 to 3), the nucleating agent can self-assemble into fibrous structures with different aspect ratios, which can induce PP to form a distinct ordered β-type lamellae structure, and the lamellae stacking method is adjustable, so that a large number of ordered intercrystalline restricted foaming spaces can be formed in PP, and the spatial ordered arrangement form is adjustable; when the maximum processing temperature is too high (Comparative Example 2), the nucleating agent can only self-assemble into a dendritic form, and can only induce PP to form a randomly arranged β-type lamellae structure, resulting in an inability to construct an ordered intercrystalline foaming space in PP.

[0067] Comparative Example 3

[0068] The same as Example 1, except that the amount of amide β-nucleating agent added in step (1) is 0.

[0069] Comparative Example 4

[0070] Same as Example 1, except that the soaking temperature in step (5) is 70°C.

[0071] Comparative Example 5

[0072] Same as Example 1, except that the soaking temperature in step (5) is 160°C.

[0073] Comparative Example 6

[0074] The same as Example 1, except that the β-nucleating agent used in step (1) is a non-amide β-nucleating agent, specifically a polypropylene β-crystal nucleating agent WBG-Ⅱ purchased from Guangdong Weilinna New Materials Technology Co., Ltd.

[0075] Comparative Example 7

[0076] The same as Example 1, except that the holding time in step (5) is 30 minutes.

[0077] Comparative Example 8

[0078] The same as Example 1, except that the pressure relief time in step (5) is 180 seconds.

[0079] Comparative Example 9

[0080] (1) Drying PP pellets and 0.1 wt% of the amide β-nucleating agent dicyclohexyl terephthalamide (TMB-5) in an oven at 90° C. for 2 hours, and then mixing them uniformly in a high-speed mixer;

[0081] (2) The maximum heating temperature was set to 200° C., and the other temperature settings were the same as in Example 1. The PP and nucleating agent blend was melt-blended and extruded into granules in a twin-screw extruder to obtain mixed granules.

[0082] (3) The mixed pellets obtained in step (2) were extruded in a single-screw extruder to obtain a sheet. The maximum heating temperature during the extrusion process was controlled at 210° C., the die temperature was controlled at 200° C., and other temperature settings were the same as in Example 1.

[0083] (4) The sheet obtained in step (3) is placed in a high-pressure reactor, filled with CO2 and then depressurized and released, and the air is removed by two cycles. The temperature is raised to the infiltration temperature of 80°C, the pressure is maintained at 22 MPa, and the pressure is maintained for 4 hours. After the pressure maintenance is completed, the pressure relief valve is opened and the pressure drops to normal pressure within 40 seconds. The foam is taken out after the reactor is naturally cooled to obtain micro-foamed PP.

[0084] Performance testing

[0085] The apparent density, tensile modulus in the extrusion direction, tensile strength, and elongation at break of the PP foams prepared in Examples 1 to 5 and Comparative Examples 1 to 9 are as follows, based on the test standards and the results obtained.

[0086] Table 1 Performance test results of PP foams prepared in Examples 1 to 5 and Comparative Examples 1 to 9

[0087] The apparent density was calculated by dividing the mass by the volume, and the tensile properties were measured on a universal testing machine according to ASTM D882.

[0088] As can be seen from Table 1: Adding an amide-type self-assembling β-nucleating agent to PP, while ensuring the appropriate processing temperature, appropriate hot stretching, and appropriate ScCO2 solid-phase foaming infiltration temperature, infiltration pressure, pressure relief and cooling rate, can ensure that the apparent density of the obtained PP foam is significantly reduced and the mechanical properties are significantly improved. However, both too high and too low extrusion temperatures (Comparative Examples 1-2) will result in the inability to construct a directional and orderly arranged β-type lamellar structure in PP, reducing the consistency and synergy of the oriented bubbles and the arrangement of the lamellae, thereby causing a significant reduction in mechanical properties. A short infiltration time (Comparative Example 7) will result in a low foaming ratio, weakening the secondary crystallization behavior brought about by the movement of the molecular chains, and not significantly improving the mechanical properties. An increased pressure relief time (Comparative Example 8) will also lead to slow CO2 overflow, fewer bubbles generated, a low foaming ratio, weak synergistic reinforcement between the oriented bubbles and the directional arranged lamellae, and not significantly improving the mechanical strength. Failure to add a nucleating agent during preparation (Comparative Example 3) results in a significant reduction in the β-crystal content, and the PP hardly foams. If a non-amide β-nucleating agent is added (Comparative Example 6), the effect of inducing PP to form ordered β-type lamellae is weakened, the effect of forming small oriented bubbles on a large scale is not increased, and the mechanical enhancement effect of the micro-foamed sample in the processing direction is not obvious. When the infiltration temperature during ScCO2 foaming is reduced when all other steps and parameters are the same during the processing (Comparative Example 4), the foaming ratio is reduced, the density is increased, the oriented bubbles are difficult to form and function, and the mechanical strength is not significantly improved. When the infiltration temperature during ScCO2 foaming is increased when all other steps and parameters are the same during the processing (Comparative Example 5), the resulting oriented crystal structure and especially the controlled ordered intercrystalline confined space are destroyed, and some β crystals are transformed into α crystals, making the foaming process difficult to control, the foaming ratio is too large, and the mechanical strength is seriously deteriorated. During the preparation process, direct ScCO2 solid-phase foaming without hot stretching can also result in a material with a high apparent density and poor mechanical properties.

[0089] The above description is only a preferred specific embodiment of the present invention, and the scope of protection of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes based on the technical solutions and inventive concepts of the present invention within the technical scope disclosed in the present invention, and these changes should be covered by the scope of protection of the present invention.

Claims

1. A method for preparing micro-foamed PP having both oriented cells and ordered β-type lamellae, characterized in that: The following steps are involved: (1) mixing PP pellets with an amide β-nucleating agent, melt-blending the resulting mixture, and extruding and granulating to obtain mixed pellets; (2) extruding or injection molding the mixed pellets obtained in step (1) to obtain a profile; (3) hot stretching the profile obtained in step (2), and cooling after stretching; (4) The profile stretched and cooled in step (3) is heated, infiltrated under pressure, foamed under pressure release in a CO2 atmosphere, and cooled again to obtain the micro-foamed PP having both oriented cells and ordered β-type lamellae.

2. The method for preparing a micro-foamed PP having both oriented cells and ordered β-type lamellae according to claim 1, characterized in that: In step (1), the amide β-nucleating agent is a nucleating agent with dicyclohexyl terephthalamide as the main component.

3. The method for preparing a micro-foamed PP having both oriented cells and ordered β-type lamellae according to claim 1, characterized in that: The mass of the amide β-nucleating agent is 0.05-2% of the mass of the PP pellets, and the maximum heating temperature of the melt blending is 180-200°C.

4. The method for preparing a micro-foamed PP having both oriented cells and ordered β-type lamellae according to claim 1, characterized in that: In step (2), the maximum heating temperature of the extrusion is 210-235°C, and the die temperature is 180-200°C.

5. The method for preparing micro-foamed PP having both oriented cells and ordered β-type lamellae according to claim 1, characterized in that: In step (3), the heat stretching temperature is 70-110° C., the stretching ratio is 10-80%, and the cooling is carried out in water at room temperature.

6. The method for preparing micro-foamed PP having both oriented cells and ordered β-type lamellae according to claim 1, characterized in that: In step (4), the pressure-maintaining infiltration temperature is 80 to 120° C., the pressure is 20 to 25 MPa, and the pressure-maintaining infiltration time is 2 to 12 hours.

7. The method for preparing micro-foamed PP having both oriented cells and ordered β-type lamellae according to claim 1, characterized in that: In step (4), the pressure relief foaming process is to relieve pressure from the infiltration pressure to normal pressure, and the pressure relief completion time is 15 to 60 seconds. The re-cooling is to cool along with the foaming kettle.

8. A PP foam prepared by the method for preparing a micro-foamed PP having both oriented cells and ordered β-type lamellae according to any one of claims 1 to 7.

9. Use of the PP foam according to claim 8 in the preparation of automotive interior and exterior parts, drone bodies, water sports equipment, wind turbine blade fillings, thermal insulation materials and building templates.

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