CN118725462A - Polypropylene foam material, preparation method and application thereof - Google Patents

Abstract

The invention relates to a preparation method suitable for a polypropylene foaming material, and the bead material can be used for preparing a buffering foam part by steam forming and is used for automobiles, packaging, buildings and the like. The foamable material is of a core-shell double-layer structure, and the outer layer is of low-melting-point polypropylene Ding Mogui copolymer and an antioxidant; the inner layer is ethylene propylene random copolymer polypropylene, propylene Ding Mogui copolymer polypropylene, maleic anhydride grafted polypropylene, cell nucleating agent and antioxidant. The foaming beads are prepared into microparticles by adopting multilayer coextrusion granulation, and the microparticles, the dispersing agent, the dispersion reinforcing agent, the surfactant and the water are put into a high-pressure foaming kettle for foaming to obtain the polypropylene foaming material. The invention can improve the production efficiency of the foaming polypropylene, save water resources and energy consumption and ensure the excellent performance of the final product.

Description

Polypropylene foam material, preparation method and application thereof

Technical Field

The invention relates to a polypropylene foam material and a preparation method thereof. The polypropylene foam material can be used to prepare a buffer foam product through steam molding, and the product is used in automobiles, packaging, construction and the like.

Background Art

Foam materials are widely used in transportation, construction, electronics, packaging, sports and medicine, agriculture and other fields due to their light weight, excellent thermal insulation, cushioning and sound insulation. The mainstream foam materials on the market mainly include foam resins such as foamed polystyrene, foamed polypropylene, foamed polyethylene, foamed PET, and foamed polyurethane. Foamed polypropylene beads and their molded products have excellent mechanical properties and environmental protection properties. Compared with traditional foam materials, they are easier to recycle and play a vital role in the field of foamed plastics. Especially in the field of transportation vehicles such as automobiles, foamed polypropylene materials have been used in various parts, such as bumper energy absorption blocks, sun visors, tool boxes, seat internal brackets, etc.

The existing foamed polypropylene bead materials use talcum powder, kaolin, alumina, calcium carbonate and other inorganic dispersants and sodium dodecylbenzene sulfonate surfactants in the autoclave foaming process to improve and avoid the adhesion of polypropylene beads in the high-temperature autoclave foaming process, and ensure that independently dispersed foamed polypropylene beads can be obtained during the pressure release process. The inorganic dispersants and surfactants remaining on the surface of the foamed polypropylene beads need to be sprayed and washed away with pure water, and then the surface of the particles are dehydrated by wind and centrifugal force and then dried. The washing process will result in the generation of wastewater and a decrease in production efficiency. If the inorganic dispersants and surfactants on the surface of the particles are not washed away, the polypropylene beads obtained by direct drying will be affected by the inorganic dispersants and surfactants during the steam molding process, resulting in a significant loss in the tensile strength and compressive strength of the molded parts.

Patent CN112341662 uses kaolin as an inorganic dispersant and sodium dodecylbenzene sulfonate as a surfactant in the polypropylene foaming process. The beads obtained by foaming and unloading pressure are rinsed with clean water and then dried to obtain polypropylene foam beads. The washing step in the patent will produce waste water, and washing will reduce the temperature of the beads themselves, which will prolong the subsequent drying time and reduce production efficiency.

Patent CN104053712B also uses kaolin as a dispersant and sodium alkylbenzene sulfonate as a surfactant in the polypropylene foaming process, and foaming and pressure relief are performed to obtain polypropylene foam beads.

It can be seen that there are still many problems in the preparation of existing polypropylene foam materials.

Summary of the invention

The purpose of the present invention is to provide a method for preparing a polypropylene foam material in view of the above-mentioned shortcomings existing in the prior art, using a dispersant, a dispersion enhancer, and a specific surfactant to avoid the adhesion of polypropylene beads during the high-temperature autoclave foaming process. The foamed beads prepared by this method do not need to undergo subsequent cleaning treatment, and can be directly steam-formed after drying. During the forming process, the surface of the beads melts, and the dispersant, dispersion enhancer, and surfactant remaining on the surface of the beads can react with the acid anhydride in the inner layer of the beads to play a cross-linking and grafting role, thereby preparing molded parts with excellent performance. The present invention can improve the production efficiency of foamed polypropylene beads, save water resources and energy consumption, and can ensure the excellent performance of the final product.

In order to achieve the above-mentioned invention object, the present invention adopts the following technical solutions:

On the one hand, the present invention provides a polypropylene foam material, which comprises polypropylene microparticles, a dispersant, an optional dispersion enhancer, a surfactant, and water. The polypropylene microparticles are a core-shell double-layer structure, and the outer layer structural resin is a low-melting point propylene-butyl random copolymer polypropylene and an antioxidant, which accounts for 5 to 20% of the total mass of the beads; the inner layer structural resin is an ethylene-propylene random copolymer polypropylene, a propylene-butyl random copolymer polypropylene, a maleic anhydride grafted polypropylene, a bubble nucleating agent, and an antioxidant, which accounts for 80 to 95% of the total mass of the beads.

In the present invention, the dispersant is selected from Budford RS-608, Dongxin DX450W or a homemade dispersant.

In the present invention, the dispersion enhancer is a dendritic polyamide amine resin, whose terminal group is -NH ₂ , and the number of terminal groups in a single molecule is 4 to 32, such as Weihai Chenyuan CYD-100A, CYD-110A, CYD-120A, CYD-130A.

In the present invention, the surfactant is selected from fatty amine polyoxyethylene ethers, such as dodecylamine polyoxyethylene ether, tetradecylamine polyoxyethylene ether, hexadecylamine polyoxyethylene ether, octadecylamine polyoxyethylene ether, preferably dodecylamine polyoxyethylene ether.

In the present invention, based on 100% weight of polypropylene microparticles, the dispersant accounts for 1-2% weight of polypropylene microparticles, the dispersion enhancer accounts for 0.1-0.3% weight of polypropylene microparticles, the surfactant accounts for 0.2-0.5% weight of polypropylene microparticles, and the mass ratio of polypropylene microparticles to water is 1:2-1:3.

In the present invention, the outer layer structural resin comprises the following raw materials in parts by mass:

99-99.8 parts of random copolymer of propylene and butylene,

0.2-1 part of antioxidant;

In the present invention, the inner layer structural resin comprises the following raw materials in parts by mass:

50-75 parts of ethylene-propylene random copolymer polypropylene,

10-35 parts of random copolymer of propylene and butylene,

Maleic anhydride grafted polypropylene 2-10 parts,

0.1 to 5 parts of cell nucleating agent,

Antioxidant 0.2 to 1 part.

In the present invention, the melting point of the random copolymer of propylene and butylene in the outer layer structural resin is 120-135° C., and its melt index is 5-10 g/10 min (230° C., 2.16 kg).

In the present invention, the antioxidant in the outer layer resin is any one or more of hindered phenol antioxidants, phosphite antioxidants and thioester antioxidants, specifically any one or more of 1010, 168 and DSTDP.

In the present invention, the melting point of the ethylene-propylene random copolymer polypropylene in the inner layer structural resin is 140-155° C., and the melt index thereof is 5-20 g/min (230° C., 2.16 kg).

In the present invention, the melting point of the random copolymer of propylene and butylene in the inner layer structural resin is 120-135° C., and the melt index is 5-10 g/10 min (230° C., 2.16 kg).

In the present invention, the melting point of the maleic anhydride grafted polypropylene in the inner layer structural resin is 162-168° C., and the grafting rate of maleic anhydride is 5-15%.

In the present invention, the cell nucleating agent in the inner layer structural resin is a mixture of one or more proportions of talcum powder, kaolin, montmorillonite, zinc borate, and nano-silicon dioxide; the nucleating agent can adjust the cell size of the foamed beads and improve the uniformity of the cell size; it accounts for 0.1-5% of the total mass of the outer layer of the particles.

In the present invention, the antioxidant in the inner layer structural resin is any one or more of hindered phenol antioxidants, phosphite antioxidants and thioester antioxidants, specifically any one or more of 1010, 168 and DSTDP.

On the other hand, in the present invention, multi-layer co-extrusion granulation is used to prepare polypropylene microparticles, and polypropylene microparticles, dispersant, dispersion enhancer, surfactant, and water are put into a high-pressure foaming kettle for foaming to obtain a polypropylene foam material.

In the present invention, the method for preparing polypropylene microparticles comprises the following steps:

(1) placing the raw materials of the outer layer in a twin-screw extruder for high-temperature melt dispersion extrusion granulation to obtain the outer layer resin;

(2) placing the raw materials of the inner layer in a twin-screw extruder for high-temperature melt dispersion extrusion granulation to obtain the inner layer resin;

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

In the present invention, the screw speed of the high-temperature melt dispersion extrusion granulation in steps (1) to (3) is 100-500 r/min, and the extrusion temperature is 190-210°C.

In a specific embodiment, the self-made dispersant used in the autoclave foaming is internally micro-crosslinked polymer microspheres, and the preparation method thereof comprises the following steps:

(1) preparing a mixed monomer: mixing 70 to 92 parts of styrene, 2 to 10 parts of divinylbenzene, 2 to 5 parts of acrylamide, and 5 to 15 parts of hydroxybutyl acrylate to form a mixed monomer;

(2) Preparing a pre-emulsion: 90 to 100 parts of a mixed monomer, 30 to 40 parts of water, and 1 to 2 parts of an emulsifier, dodecyl ammonium chloride, are mixed, stirred, and dispersed to prepare a pre-emulsion;

(3) preparing a seed emulsion: adding 200 to 250 parts of water into a reaction kettle, adding 1 to 10 parts by weight of a pre-emulsified liquid thereto, heating to 80 to 85° C., adding 0.5 to 1 part of an initiator, ammonium persulfate, dissolved in 10 to 15 parts of water, and keeping the temperature to prepare a seed emulsion;

(4) adding the remaining amount of the pre-emulsion and 0.25-0.5 parts of initiator ammonium persulfate dissolved in 10-15 parts of water to the seed emulsion, and keeping the temperature for 1 hour to 1.5 hours after the addition is completed; cooling to room temperature, spray drying to obtain internal micro-crosslinked polymer microspheres with hydroxyl functional groups, and the particle size of the polymer microspheres is 100-500 nm.

On the other hand, the present invention also provides a foaming method for polypropylene foam material, which comprises the following steps: uniformly dispersing polypropylene microparticles, a dispersant, an optional dispersion enhancer, and a surfactant in an aqueous dispersion medium, introducing a foaming agent into an autoclave to displace air, and then increasing the temperature and pressure to allow the foaming agent to penetrate into the interior of the polypropylene microparticles. After maintaining the temperature and pressure for a certain period of time, the polypropylene microparticles together with the aqueous dispersion medium are released into a normal temperature and pressure environment. Under the action of an instantaneous pressure difference, the high pressure inside the polypropylene microparticles causes them to foam and expand instantly, thereby preparing polypropylene foam beads.

In the present invention, the foaming agent is a mixture of one or more of carbon dioxide, nitrogen, butane, pentane and heptane in any proportion. To meet environmental protection requirements, the foaming agent is more preferably carbon dioxide.

In the present invention, the foaming temperature is 120-165° C., and the foaming pressure is 2-6 MPa.

The technical solution of the present invention has the following advantages over the prior art solution:

1. The polypropylene foam bead material of the present invention adopts a core-shell structure, and the outer layer is a low-melting-point propylene-butane random copolymer polypropylene, which does not foam during the autoclave foaming process, and can be sintered and formed at a lower steam pressure during the steam molding process, thereby obtaining excellent surface quality, and the inner layer pore structure will not be destroyed by high temperature; the inner layer is a high-melting-point polypropylene resin, which is conducive to the molded product having high rigidity mechanical properties and better dimensional stability.

2. The dendritic polyamide resin, a dispersion enhancer used in the high-temperature autoclave foaming of the present invention, carries a positive charge in the aqueous phase environment of high-pressure _CO2 and has excellent aqueous phase dispersion ability, which can reduce the amount of dispersant and emulsifier used and prevent adhesion between polypropylene beads during autoclave foaming.

3. Compared with the traditional inorganic dispersants, the dispersant used in the high-temperature autoclave foaming of the present invention has a smaller particle size and better dispersibility in the water phase, which can effectively prevent the adhesion of polypropylene beads during the autoclave foaming process.

4. The dendritic polyamide amine resin, dispersant, and emulsifier fatty amine polyoxyethylene ether used in the high-temperature autoclave foaming of the present invention all have functional groups on their surfaces that can undergo grafting reactions with anhydride groups. During the steam molding process, the shell layer of the core-shell polypropylene foamed beads melts, and the dispersant, dispersion enhancer, and surfactant remaining on the surface of the beads can react with the anhydride groups in the inner layer of the beads to play a cross-linking and grafting role, thereby enhancing the interfacial force between the beads and preparing molded parts with excellent performance. Such a preparation method can improve the production efficiency of foamed polypropylene beads, save water resources and energy consumption, and ensure the excellent performance of the final product.

DETAILED DESCRIPTION

In order to better understand the technical solution of the present invention, the content of the present invention is further described below in conjunction with the examples, but the content of the present invention is not limited to the following examples. The raw materials used in the examples or comparative examples are all commercially available raw materials unless otherwise specified.

The devices and main raw material sources used in the embodiments and comparative examples of the present invention are as follows:

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

Single screw co-extrusion equipment: model HRJSJ-35, screw length-diameter ratio is 28:1, Foshan Hairuijia Precision Extrusion Machinery Co., Ltd.

Nucleating agent: Talc AH51205L, Liaoning Aihai Talc Co., Ltd.

Lubricant: zinc stearate, Italy Faji Co., Ltd.;

Propylene butadiene random copolymer polypropylene: FL7632L, Singapore Polyolefins Pte. Ltd.

Ethylene-propylene random copolymer polypropylene: RP344P-K, North Huajin Chemical Industry Co., Ltd.

Maleic anhydride grafted polypropylene: UMEX 1001, Sanyo Chemical

Primary antioxidant: 1010, Xinxiu Chemical

Secondary antioxidant: 168, Xinxiu Chemical

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

Dispersion enhancer: CYD-130A, Weihai Chenyuan Molecular New Materials Co., Ltd.

Other raw materials were commercially available and of analytical grade.

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

Tensile strength: ISO 1798:2008

Compression strength: ISO 844:2014;

Bulk density: ISO 844;

Preparation Example 1 is the preparation of a homemade dispersant;

Preparation Example 1

(1) preparing a mixed monomer: mixing 83 parts of styrene, 5 parts of divinylbenzene, 4 parts of acrylamide, and 8 parts of hydroxybutyl acrylate to form a mixed monomer;

(2) Preparing a pre-emulsion: 100 parts of the mixed monomer, 40 parts of water, and 1.5 parts of an emulsifier, dodecyl ammonium chloride, are mixed, stirred, and dispersed to prepare a pre-emulsion;

(3) preparing seed emulsion: adding 200 parts of water into a reaction kettle, adding 5 parts of pre-emulsion by weight, heating to 85° C., adding 0.5 parts of initiator ammonium persulfate dissolved in 10 parts of water, and keeping the temperature to prepare seed emulsion;

(4) The remaining amount of pre-emulsion and 0.25 parts of initiator ammonium persulfate dissolved in 10 parts of water were added dropwise to the seed emulsion, and the mixture was kept warm for 1 hour. After cooling to room temperature, the emulsion was spray-dried to obtain internally micro-crosslinked polymer microspheres with hydroxyl functional groups, which were recorded as dispersant 1.

Preparation Example 2-3

The Badford RS-608 emulsion was spray dried to obtain dispersant 2, and the Eastcom DX450W emulsion was spray dried to obtain dispersant 3.

Examples and Comparative Examples

The preparation method of the outer layer resin is as follows: weigh each raw material according to the weight ratio, put it into a high-speed mixer and mix it for 3 minutes, then feed it from the main feeding port of a twin-screw extruder, the screw speed is 500r/min, the extrusion temperature is 200°C, and after high-temperature melting and dispersion, the outer layer resin is extruded and granulated.

The preparation method of the inner layer resin is as follows: weigh each raw material according to the weight ratio, then put it into a high-speed mixer and mix it for 3 minutes, then feed it from the main feeding port of the twin-screw extruder, the screw speed is 500r/min, the extrusion temperature is 200℃, and after high-temperature melting and dispersion, the inner layer resin is extruded and granulated.

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

Example 5

500g of polypropylene microparticles S1, 0.5g of dispersion enhancer CYD-130A, 1g of surfactant dodecylamine polyoxyethylene ether, and 5g of dispersant 1 are uniformly dispersed in 1kg of aqueous dispersion medium, carbon dioxide is introduced into the autoclave to replace the air, and the temperature and pressure are increased so that the carbon dioxide can penetrate into the interior of the raw polypropylene microparticles. After maintaining the temperature and pressure for 45 minutes, the polypropylene microparticles together with the aqueous dispersion medium are released into a normal temperature and pressure environment. Under the action of the instantaneous pressure difference, the high pressure inside the polypropylene microparticles causes them to foam and expand instantly, thereby preparing polypropylene foam beads. The foaming temperature is 140°C and the foaming pressure is 4.0MPa.

Embodiments 6 to 8

The polypropylene foam beads were prepared according to the amounts of the raw materials in Table 3. The specific preparation method is shown in Example 5.

Comparative Examples 5 to 10

The polypropylene foam beads were prepared according to the amounts of the raw materials in Table 3. The specific preparation method is shown in Example 5.

Preparation of polypropylene foam products: The prepared polypropylene foam beads are injected into a flat plate mold of 300mm long, 200mm wide and 30mm thick. After heating with water vapor and cooling and shaping, a rectangular foam product is obtained. The foam product is cut into samples and the tensile strength and compressive strength are tested.

Table 3: Raw material dosage of Examples 5 to 8 and Comparative Examples 5 to 10 and properties of their foamed products

Note: Foam beads adhesion: from 0 to 5, the larger the number, the more serious the adhesion, 0 means no adhesion

It can be found from Table 3 that Examples 5 to 8 use polymer microsphere dispersant, dodecylamine polyoxyethylene ether, and CYD-130A as the dispersion system, and the foam beads finally obtained have no adhesion, indicating that the dispersion system has a good dispersion effect and can avoid the adhesion of polypropylene microparticles under high temperature and high pressure, so that the prepared polypropylene foam beads have a lower density and a higher foaming ratio. Compared with the tensile strength and compressive strength of Comparative Examples 5 and 6 and Example 5, Example 5 has higher tensile strength and compressive strength, because the shell layer of the polypropylene foam beads melts during the steam molding process, and the polymer microsphere dispersant, dodecylamine polyoxyethylene ether, and CYD-130A in Example 5 can react with the anhydride group of the inner layer of the polypropylene microparticles to play a cross-linking and grafting role, thereby enhancing the interfacial force between the beads.

Comparative Example 5 used polymer microsphere dispersant and dodecylamine polyoxyethylene ether as the dispersion system. Compared with Example S5, it did not add the dispersion enhancer CYD-130A, but added a higher content of dodecylamine polyoxyethylene ether. However, the foamed beads finally obtained were still partially adhered.

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

The inner layer of the polypropylene microparticles used in Comparative Example 7 has no anhydride groups. During the steam molding process of the polypropylene foam beads, the polymer microsphere dispersant, dodecylamine polyoxyethylene ether, and CYD-130A cannot undergo a grafting reaction with them, thereby affecting the interfacial force. Therefore, compared with Example 5, its tensile strength and compressive strength are lower.

Comparative Examples 8 to 10 use a traditional dispersion system of kaolin dispersant and sodium dodecylbenzene sulfonate. As the content of sodium dodecylbenzene sulfonate and kaolin increases, the dispersion effect of the foaming process increases. The amount of dispersion system added is up to Comparative Example 10, and the foamed beads finally obtained have no adhesion. However, the amount of dispersion system added is too high. However, the traditional dispersion system used in Comparative Examples 8 to 10 cannot undergo a grafting reaction with the polypropylene foamed beads during the steam molding process, which affects the melt bonding between the beads, resulting in a gradual decrease in its tensile strength and compressive strength as the amount of the traditional dispersion system increases.

The above description of the embodiments is to facilitate the understanding and application of the present invention by those skilled in the art. Those familiar with the art can easily make various modifications to the embodiments and apply the general principles described herein to other embodiments without creative work. Therefore, the present invention is not limited to the implementation cases here, and improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the present invention should be within the scope of protection of the present invention.

Claims (10)

1. The polypropylene foaming material comprises polypropylene microparticles, a dispersing agent, an optional dispersion reinforcing agent, a surfactant and water, wherein the polypropylene microparticles are of a core-shell double-layer structure, and the outer-layer structural resin is low-melting-point polypropylene Ding Mogui-copolymer and an antioxidant, and accounts for 5-20% of the total mass of the beads; the inner layer structure resin is ethylene propylene random copolymer polypropylene, propylene Ding Mogui copolymer polypropylene, maleic anhydride grafted polypropylene, cell nucleating agent and antioxidant, which account for 80-95% of the total mass of the beads.

2. The polypropylene foam of claim 1, wherein the dispersant is selected from the group consisting of barnforc RS-608, east DX450W, and homemade dispersants.

3. The polypropylene foam material according to claim 1 or 2, wherein the dispersion enhancer is a dendritic polyamidoamine resin, the terminal groups of which are-NH ₂, and the number of terminal groups of a single molecule is 4 to 32, such as the wedney source CYD-100A, CYD-110A, CYD-120A, CYD-130A; and/or the surfactant is selected from fatty amine polyoxyethylene ether, such as laurylamine polyoxyethylene ether, tetradecylamine polyoxyethylene ether, hexadecylamine polyoxyethylene ether, octadecylamine polyoxyethylene ether, preferably laurylamine polyoxyethylene ether.

4. A polypropylene foam material according to any one of claims 1 to 3, wherein the dispersant is 1 to 2% by weight of the polypropylene particles, the dispersion enhancer is 0.1 to 0.3% by weight of the polypropylene particles, the surfactant is 0.2 to 0.5% by weight of the polypropylene particles, and the mass ratio of the polypropylene particles to water is 1: 2-1: 3.

5. The polypropylene foam material according to any one of claims 1 to 4, wherein the outer layer structural resin comprises the following raw materials in parts by mass:

99 to 99.8 parts of polypropylene copolymer of propylene Ding Mogui and 0.2 to 1 part of antioxidant;

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

50-75 parts of ethylene propylene random copolymer polypropylene, 10-35 parts of propylene Ding Mogui copolymer polypropylene, 2-10 parts of maleic anhydride grafted polypropylene, 0.1-5 parts of cell nucleating agent and 0.2-1 part of antioxidant.

6. The polypropylene foam material according to claim 5, wherein the polypropylene Ding Mogui copolymer of the outer layer structural resin has a melting point of 120 to 135 ℃ and a melt index of 5 to 10g/10min (230 ℃,2.16 kg); and/or the melting point of the ethylene propylene random copolymer polypropylene of the inner layer structure resin is 140-155 ℃, and the melt index of the ethylene propylene random copolymer polypropylene is 5-20 g/min (230 ℃,2.16 kg); and/or the polypropylene copolymer of the polypropylene Ding Mogui-135 deg.c and with the melt index of 5-10 g/10min (230 deg.c, 2.16 kg); and/or the melting point of the maleic anhydride grafted polypropylene of the inner layer structure resin is 162-168 ℃, and the grafting rate of the maleic anhydride is 5-15%.

7. The polypropylene foam according to any one of claims 1 to 6, wherein the method for producing polypropylene microparticles comprises the steps of:

(1) Placing the raw materials of the outer layer structure resin into a double-screw extruder for high-temperature melting, dispersing, extruding and granulating to obtain the outer layer resin;

(2) Placing the raw materials of the inner layer structure resin into a double-screw extruder for high-temperature melting, dispersing, extruding and granulating to obtain the inner layer resin;

(3) Respectively plasticizing the outer layer resin and the inner layer resin in two single screw extruders, and extruding and granulating through a co-extrusion die head to obtain core-shell composite polypropylene particles, wherein the mass ratio of the outer layer resin is 5-20%, and the mass ratio of the inner layer resin is preferably 80-95%; the length of the microparticles prepared by extrusion granulation is 1-2 mm, and the weight is controlled to be 1-2 mg;

Preferably, the screw speed of the high-temperature melt-dispersing extrusion granulation in the steps (1) - (3) is 100-500r/min, and the extrusion temperature is 190-210 ℃.

8. The polypropylene foam material according to any one of claims 1 to 7, wherein the self-made dispersing agent used for the autoclave foaming is an internal micro-crosslinked polymer microsphere, and the preparation method comprises the following steps:

(1) Preparing a mixed monomer: mixing 70-92 parts of styrene, 2-10 parts of divinylbenzene, 2-5 parts of acrylamide and 5-15 parts of hydroxybutyl acrylate to form a mixed monomer;

(2) Preparing a pre-emulsion: mixing 90-100 parts of mixed monomer, 30-40 parts of water and 1-2 parts of emulsifier dodecyl ammonium chloride, stirring and dispersing to prepare a pre-emulsion;

(3) Preparing seed emulsion: adding 200-250 parts of water into a reaction kettle, adding 1-10 parts of pre-emulsion by mass into the reaction kettle, heating to 80-85 ℃, adding 0.5-1 part of initiator ammonium persulfate dissolved in 10-15 parts of water, and carrying out heat preservation to obtain seed emulsion;

(4) Dripping the rest pre-emulsion and 0.25-0.5 part of initiator ammonium persulfate dissolved in 10-15 parts of water into the seed emulsion, and preserving heat for 1-1.5 h after dripping; and cooling to room temperature, and spray drying to obtain the internal micro-crosslinked polymer microsphere with hydroxyl functional groups, wherein the particle size of the polymer microsphere is 100-500 nm.

9. A method of foaming a polypropylene foam material according to any one of claims 1 to 8, said method comprising the steps of: uniformly dispersing polypropylene particles, a dispersing agent, an optional dispersion enhancer and a surfactant in an aqueous dispersion medium, introducing a foaming agent into an autoclave to replace air, heating and boosting to enable the foaming agent to permeate into the polypropylene particles, and releasing the polypropylene particles together with the aqueous dispersion medium into a normal-temperature and normal-pressure environment after heat preservation and pressure maintaining are carried out for a certain time, wherein under the action of instant pressure difference, the high pressure in the polypropylene particles causes the polypropylene particles to be instantly foamed and expanded, so that the polypropylene foam beads are prepared.

10. The foaming method according to claim 9, wherein the foaming agent is a mixed foaming agent of one or more of carbon dioxide, nitrogen, butane, pentane and heptane according to any proportion, preferably carbon dioxide; and/or the foaming temperature is 120-165 ℃ and the foaming pressure is 2-6 MPa.