A polypropylene-based multi-component synergistic flame-retardant foam material, its preparation method, and cables.

Abstract

This invention relates to the field of polymer materials technology, and discloses a polypropylene-based multi-element synergistic flame-retardant foam material, its preparation method, and cables. The preparation method of the polypropylene-based multi-element synergistic flame-retardant foam material includes the following steps: Step 1, modifying bromine-terminated polypropylene with nitrogen and phosphorus compounds to obtain modified polypropylene; Step 2, modifying a surface-modified composite inorganic filler with a silicon-containing compound to obtain a functionalized composite inorganic filler; Step 3, mixing polypropylene, modified polypropylene, and the functionalized composite inorganic filler, melt-blending, foaming, and cooling to obtain the polypropylene-based multi-element synergistic flame-retardant foam material. The polypropylene-based multi-element synergistic flame-retardant foam material of this invention has excellent mechanical properties, flame-retardant properties, and UV aging resistance. When used in cables, it can improve the flame-retardant performance of the cables and extend their service life.

Description

Technical Field

[0001] This invention relates to the field of polymer materials technology, specifically to a polypropylene-based multi-element synergistic flame-retardant foam material, its preparation method, and cables. Background Technology

[0002] The flammability of polymer materials poses significant safety hazards, severely limiting their application. Flame-retardant modification of polymer materials is therefore crucial for fire safety. Polypropylene (PP) foam has gained considerable attention in recent years. Compared to pure PP, foamed PP maintains good impact resistance, high operating temperature, and good chemical resistance while addressing the shrinkage issue of PP and exhibiting lower density, effectively saving costs. This has led to its widespread application in numerous fields, particularly in wires and cables. However, PP foam is flammable, with a low ignition point and high combustion temperature, generating significant heat and easily allowing fire to spread. Therefore, improving flame retardancy is one of the effective methods to expand the application range of PP foam.

[0003] For example, Chinese patent application CN109942957A provides a high flame-retardant foamed polypropylene material, and Chinese patent application CN107603027A discloses a flame-retardant foamed polypropylene board. Both of these patent applications provide methods to solve the flammability problem of foamed polypropylene. However, some cables are used in homes and are prone to aging after being exposed to wind and sun. If foamed polypropylene material is applied to cables, it is also necessary to improve the UV resistance of the foamed polypropylene material to extend the service life of the cables. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a method for preparing a polypropylene-based multi-element synergistic flame-retardant foam material, comprising the following steps:

[0005] Step 1: Modify bromine-terminated polypropylene with nitrogen and phosphorus compounds to obtain modified polypropylene;

[0006] Step 2: Modify the surface-modified composite inorganic filler with a silicon-containing compound to obtain a functionalized composite inorganic filler;

[0007] Step 3: Mix polypropylene, modified polypropylene and functionalized composite inorganic filler, melt blend, foam, and cool to obtain polypropylene-based multi-element synergistic flame-retardant foam material.

[0008] Preferably, in step one, the method for preparing the nitrogen and phosphorus compound is as follows:

[0009] Acryloyl chloride was added to chloroform and stirred. At 0-5°C, a 1,3-propanediamine / chloroform mixture was added and stirred at 23-27°C for 100-150 min. Then, deionized water and sodium hydroxide were added and stirred for another 40-80 min. The mixture was purified to obtain a nitrogen-containing compound. The mass ratio of acryloyl chloride, chloroform, 1,3-propanediamine / chloroform mixture, deionized water, and sodium hydroxide was (9.4-18.8):(150-250):(21.4-38.4):(100-150):(4-8).

[0010] In a nitrogen atmosphere, a nitrogen-containing compound, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, and ethanol were mixed in a mass ratio of (9.1-18.2):(11.3-21.6):(300-500), stirred until homogeneous, heated to 55-65℃, and stirred for 5-7 hours. The mixture was then purified to obtain a nitrogen-phosphorus compound.

[0011] In the above process, the acyl chloride of acryloyl chloride reacts with the amino group of 1,3-propanediamine to form an amide bond, yielding a nitrogen-containing compound with two carbon-carbon double bonds. Next, the double bond at one end of the nitrogen-containing compound undergoes an addition reaction with 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide to generate a nitrogen-phosphorus compound.

[0012] Preferably, in step one, the method for preparing the modified polypropylene is as follows:

[0013] In a nitrogen atmosphere, toluene, cuprous chloride, nitrogen and phosphorus compounds, and brominated end-capped polypropylene are mixed and stirred at 88-92℃ for 40-80 min. Then, 1,1,4,7,10,10-hexamethyltriethylenetetramine is added, and the reaction is carried out for 10-16 h. The reaction is then terminated by adding ethanol and purified. The mass ratio of toluene, cuprous chloride, nitrogen and phosphorus compounds, brominated end-capped polypropylene, and 1,1,4,7,10,10-hexamethyltriethylenetetramine is (300-500):(0.08-0.1):(19.1-30.1):(2.5-3):(0.2-0.4).

[0014] In the above process, nitrogen and phosphorus compounds are polymerized with bromine-terminated polypropylene under the initiation of 1,1,4,7,10,10-hexamethyltriethylenetetramine to obtain modified polypropylene. The modified polypropylene contains flame-retardant nitrogen and phosphorus elements and has a good flame-retardant effect. The modified polypropylene has the same non-polar polymer chain as polypropylene, which can penetrate deep into the polypropylene resin and entangle with the polypropylene molecular chain, increasing the crosslinking density of the polypropylene system. In addition, the modified polypropylene also contains polar groups such as amide bonds and PO bonds, which can form hydrogen bonds with functionalized composite inorganic fillers. Therefore, the modified polypropylene of the present invention can be used as a flame retardant and as a compatibilizer for polypropylene matrix and functionalized composite inorganic fillers, improving the dispersion effect of functionalized composite inorganic fillers.

[0015] Preferably, in step one, the method for preparing the bromine-terminated polypropylene is as follows:

[0016] Step A1: In a hydrogen atmosphere, toluene, a mixed solution of methylaluminoxane / toluene, and a mixed solution of divinylbenzene / toluene are mixed and stirred at 600-700 rpm for 4-8 minutes at 28-32°C. Then, a mixed solution of dimethylsilylbis(2-methyl-4-phenylindenyl)zirconia dichloride / toluene is added, followed by the introduction of propylene at 3 bar. The propylene is continuously introduced to maintain a constant pressure of 4 bar in the reaction system. The mixture is kept at 28-32°C for 10-20 minutes. The pressure is then released, acidified ethanol is added, and the mixture is purified to obtain styrene-terminated polypropylene. The volume ratio of toluene, the mixed solution of methylaluminoxane / toluene, the mixed solution of divinylbenzene / toluene, and the mixed solution of dimethylsilylbis(2-methyl-4-phenylindenyl)zirconia dichloride / toluene is (50-100):(4.3-8.6):(6-12):(10-20).

[0017] Step A2: At 58-62℃, styrene-terminated polypropylene and 1,1,2,2-tetrachloroethane are mixed at a mass ratio of (2-3):(120-150) to obtain a suspension; HBr is bubbled into the above suspension, and after 5-7 hours, ethanol is added to purify the solution to obtain bromine-terminated polypropylene.

[0018] In the above process, dimethylsilylbis(2-methyl-4-phenylindenyl)zirconia is used as a catalyst to copolymerize divinylbenzene and propylene to obtain styrene-terminated polypropylene; the styrene-terminated polypropylene is then reacted with HBr to obtain bromine-terminated polypropylene.

[0019] Preferably, in step two, the preparation method of the functionalized composite inorganic filler is as follows:

[0020] In a nitrogen atmosphere, eugenol, triethylamine, dichloromethane, and a mixture of dichlorodiphenylsilane / dichloromethane were mixed in a mass ratio of (3.6-7.2):(2-4):(100-150):(18.8-31.6), heated to 38-42℃, and reacted at a constant temperature for 2.5-3.5 h. After purification, a silicon-containing compound was obtained.

[0021] In a nitrogen atmosphere, surface-modified composite inorganic filler, ethanol, and silicon-containing compound are mixed in a mass ratio of (3-5):(100-150):(5.4-8.6), stirred and reacted at 70-76℃ for 7-9 hours, and purified to obtain functionalized composite inorganic filler.

[0022] In the above process, eugenol and dichlorodiphenylsilane combine through a Schiff base reaction to obtain a silicon-containing compound. The phenoxy and aromatic functional groups in the silicon-containing compound have strong ultraviolet absorption capabilities, and silicon can have a flame-retardant effect. Furthermore, the presence of flexible siloxanes and rigid structures can enhance the strength and toughness of the matrix. Next, the aldehyde group of the silicon-containing compound reacts with the amino group of the surface-modified composite inorganic filler to graft the silicon-containing compound onto the surface-modified composite inorganic filler, thereby obtaining a functionalized composite inorganic filler.

[0023] Preferably, in step two, the preparation method of the surface-modified composite inorganic filler is as follows:

[0024] Step B1: Mix montmorillonite / zinc oxide composite material, aluminum chloride hexahydrate, magnesium chloride hexahydrate, and deionized water in a mass ratio of (2.4-4.8):(3.9-7.8):(3.3-6.6):(300-500), and stir until transparent. Then, add sodium hydroxide aqueous solution and sodium carbonate aqueous solution to the mixture simultaneously until the pH of the mixture is 7. Keep the pH constant and stir at 110-130℃ for 15-17 hours. Purify to obtain composite inorganic filler.

[0025] Step B2: Mix silane coupling agent KH-550, deionized water, and ethanol, then add acetic acid to adjust the pH to 3.5-4.5, stir at 55-65℃ for 3-5 hours, then add the composite inorganic filler, continue stirring and reacting for 1-2 hours, purify, and obtain the surface-modified composite inorganic filler; wherein, the mass ratio of silane coupling agent KH-550, deionized water, ethanol, and composite inorganic filler is (4.7-9.4):(45-90):(40-80):(2.5-4.5).

[0026] Furthermore, the preparation method of the montmorillonite / zinc oxide composite material is as follows:

[0027] Montmorillonite, sodium alginate, and deionized water were mixed in a mass ratio of (2-4):(20-40):(1000-2000) and stirred to form a montmorillonite composite sol. Zinc nitrate hexahydrate and deionized water were mixed in a mass ratio of (6-12):(200-400) to obtain a zinc nitrate aqueous solution. The montmorillonite composite sol and the zinc nitrate aqueous solution were mixed and stirred for 20-30 hours to obtain a gel. The gel was calcined at 480-520℃ for 2.5-3.5 hours to obtain a montmorillonite / zinc oxide composite material.

[0028] In the above process, montmorillonite has a nanosheet structure, resulting in a very large specific surface area after dissociation. It possesses abundant adsorbed water and structural water. During combustion, MMT can form a protective char layer on the polymer surface, providing the necessary barrier properties for flame retardancy, thereby reducing the release of combustion degradation products and the transfer of heat from the material surface. Hydrotalcite (LDH) also has a sheet structure and exhibits flame retardant and smoke-suppressing effects. For example, the metal oxides (such as magnesium oxide and aluminum oxide) generated after the decomposition of magnesium aluminum hydrotalcite have high melting points and can form a heat-insulating layer on the matrix surface, thus providing protection. Therefore, both montmorillonite and LDH can be used as flame-retardant fillers and reinforcing fillers in polymer materials. However, due to the van der Waals forces between LDH nanosheets, LDH easily aggregates in solution or solid phase. Therefore, this invention uses zinc nitrate as a zinc precursor in the montmorillonite sheet... Structurally, in-situ growth of nano-zinc oxide was achieved. Then, using aluminum chloride hexahydrate and magnesium chloride hexahydrate as magnesium and aluminum precursors, ZnMgAl-LDH was generated in-situ on montmorillonite sheets, solving the problem of easy aggregation of hydrotalcite. Most importantly, the two-dimensional montmorillonite and ZnMgAl-LDH have excellent physical barrier effects, forming a network structure that inhibits the escape of combustible gases and the transfer of heat. The metal oxides generated by ZnMgAl-LDH have excellent catalytic effects, providing a stable coating layer on the surface of polypropylene materials during combustion, which can inhibit the release of toxic gases. At the same time, when ZnMgAl-LDH is thermally decomposed, it can absorb heat and dilute combustible gases by generating water vapor during the decomposition process. In addition, ZnMgAl-LDH hydrotalcite has a good ultraviolet shielding effect.

[0029] Preferably, in step three, the mass ratio of polypropylene, modified polypropylene, and functionalized composite inorganic filler is 100:(15-30):(9-15); the melt blending conditions are: melt blending for 8-15 minutes at a screw speed of 30-40 r / min and a temperature of 170-220℃.

[0030] Preferably, in step three, the foaming agent used is supercritical carbon dioxide, and the amount of foaming agent is 2-3% of the mass of polypropylene.

[0031] The polypropylene-based multi-element synergistic flame-retardant foam material is prepared using the aforementioned method.

[0032] The present invention also discloses a cable made using the above-mentioned polypropylene-based multi-element synergistic flame-retardant foam material; the polypropylene-based multi-element synergistic flame-retardant foam material is extruded and shaped using a physical foaming wire extruder, and the nitrogen injection pressure is controlled at 150 bar to obtain a cable (outer diameter 1.35 mm) that wraps the conductor (diameter 0.48 mm).

[0033] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0034] 1. This invention uses functionalized composite inorganic fillers as functional fillers in polypropylene-based multi-element synergistic flame-retardant foam materials. The functionalized composite inorganic fillers have good dispersibility in polypropylene-based multi-element synergistic flame-retardant foam materials, and the addition of functionalized composite inorganic fillers can improve the flame-retardant performance, UV aging resistance and mechanical properties of polypropylene-based multi-element synergistic flame-retardant foam materials.

[0035] 2. The modified polypropylene of the present invention has multiple functions: On the one hand, the modified polypropylene has the same non-polar polymer chain as polypropylene, which can penetrate deep into the polypropylene resin and entangle with the polypropylene molecular chain, enhance the crosslinking density of the polymer system, improve the mechanical properties of polypropylene-based multi-component synergistic flame-retardant foaming materials, and the molecular chain entanglement can increase the resistance to movement of polypropylene molecular chain segments, enhance the melt strength of polypropylene, and thus improve the foaming performance of polypropylene; on the other hand, the flame-retardant nitrogen and phosphorus elements in the modified polypropylene, together with the montmorillonite, hydrotalcite and flame-retardant silicon elements in the functionalized composite inorganic filler, exert excellent flame-retardant effects; furthermore, the modified polypropylene also contains amide bonds and PO bonds, which are polar groups, and can form hydrogen bonds with the functionalized composite inorganic filler. Therefore, the modified polypropylene of the present invention can be used as a flame retardant and as a compatibilizer for polypropylene matrix and functionalized composite inorganic filler, improving the dispersion effect of functionalized composite inorganic filler.

[0036] In summary, the polypropylene-based multi-element synergistic flame-retardant foam material of the present invention has excellent mechanical properties, flame-retardant properties, and UV aging resistance. When used in cables, it can improve the flame-retardant properties of the cables and extend their service life. Attached Figure Description

[0037] Figure 1 This is a comparison chart of the limiting oxygen index tests of flame-retardant foamed polypropylene samples prepared using the polypropylene-based multi-component synergistic flame-retardant foaming materials of Examples 3-5 and Comparative Examples 2-5 of the present invention.

[0038] Figure 2 This is a schematic diagram illustrating the synthesis of the nitrogen-containing compound of the present invention;

[0039] Figure 3 This is a schematic diagram illustrating the synthesis of the nitrogen and phosphorus compounds of the present invention;

[0040] Figure 4 This is a schematic diagram illustrating the synthesis of the silicon-containing compound of the present invention. Detailed Implementation

[0041] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0042] Example 1

[0043] This embodiment discloses a method for preparing bromine-terminated polypropylene, including the following steps:

[0044] Step A1: Hydrogen gas (1.0 bar) is introduced into the reactor, followed by the addition of 75 mL toluene, 6.5 mL of a 1.4 mol / L methylaluminoxane / toluene mixed solution, and 9 mL of a 1.75 mol / L divinylbenzene / toluene mixed solution. The mixture is stirred at 650 rpm for 6 min at 30 °C. Then, 15 mL of a 0.2 mol / L dimethylsilylbis(2-methyl-4-phenylindenyl)zirconium dichloride / toluene mixed solution is added. Propylene is then introduced at 3 bar to initiate the polymerization reaction. During the polymerization process, propylene is continuously introduced while maintaining a constant pressure of 4 bar in the reaction system. The mixture is kept at 30 °C for 15 min. The pressure inside the reactor is then released, and the reaction mixture is discharged from the reactor and quenched with acidified ethanol. The resulting polymer is filtered and washed with tetrahydrofuran to remove excess divinylbenzene. Finally, the polymer is vacuum dried at 50 °C for 12 h to obtain styrene-terminated polypropylene.

[0045] Step A2: At 60°C, 2.5g of styrene-terminated polypropylene was added to 135g of 1,1,2,2-tetrachloroethane to obtain a suspension; dried HBr was bubbled into the above suspension, and after 6 hours, 270g of ethanol was added, filtered, and the resulting polymer powder was vacuum dried at 50°C for 12 hours to obtain bromine-terminated polypropylene.

[0046] Example 2

[0047] This embodiment discloses a method for preparing surface-modified composite inorganic fillers, including the following steps:

[0048] Step B1: Add 3g of montmorillonite and 30g of sodium alginate to 1500g of deionized water and stir to form a montmorillonite composite sol; add 9g of zinc nitrate hexahydrate to 300g of deionized water to obtain a zinc nitrate aqueous solution; mix the montmorillonite composite sol with the zinc nitrate aqueous solution and stir for 25h to obtain a gel; calcine the gel at 500℃ for 3h to obtain a montmorillonite / zinc oxide composite material.

[0049] 3.6 g of montmorillonite / zinc oxide composite material, 5.9 g of aluminum chloride hexahydrate, and 5 g of magnesium chloride hexahydrate were added to 400 g of deionized water and stirred until transparent. Then, 2.4 mol / L sodium hydroxide aqueous solution and 0.4 mol / L sodium carbonate aqueous solution were added to the mixture at a flow rate of 10 mL / min until the pH of the mixture reached 7. The pH was kept constant and the mixture was stirred at 120 °C for 16 h. After centrifugation, the resulting solid product was washed with ethanol and then vacuum dried at 60 °C to obtain the composite inorganic filler.

[0050] Step B2: Mix 7.1g of silane coupling agent KH-550, 67.5g of deionized water, and 60g of ethanol. Then add acetic acid to adjust the pH to 4. Stir at 60℃ for 4h. Then add 3.5g of composite inorganic filler. Keep the temperature constant and continue stirring for 1.5h. After the reaction is complete, centrifuge. The obtained solid product is washed with ethanol and then vacuum dried at 60℃ to obtain the surface-modified composite inorganic filler.

[0051] Example 3

[0052] This embodiment discloses a method for preparing a polypropylene-based multi-element synergistic flame-retardant foam material, including the following steps:

[0053] Step 1: Add 9.4g of acryloyl chloride to 150g of chloroform and stir for 20min. Then, at 0℃, while stirring, add 21.4g of 13wt% 1,3-propanediamine / chloroform mixture dropwise over 30min. Stir the reaction mixture at 23℃ for 150min. Then add 100g of deionized water and 4g of sodium hydroxide and continue stirring for 40min. Filter the mixture. Wash the resulting solid with deionized water and dry it under vacuum at 80℃ to obtain a nitrogen-containing compound.

[0054] In a nitrogen atmosphere, 9.1 g of a nitrogen-containing compound and 11.3 g of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide were added to 300 g of ethanol, stirred until homogeneous, heated to 55 °C, and stirred for 7 h. After the reaction was completed, the solvent was evaporated to obtain the nitrogen-phosphorus compound.

[0055] In a nitrogen atmosphere, 300 g of toluene, 0.08 g of cuprous chloride, 19.1 g of nitrogen and phosphorus compounds, and 2.5 g of bromine-terminated polypropylene were mixed and stirred at 88 °C for 80 min. Then, 0.2 g of 1,1,4,7,10,10-hexamethyltriethylenetetramine was added to initiate the reaction. After reacting for 16 h, ethanol was added to terminate the reaction. The mixture was filtered, and the resulting mixture was swollen in dichloromethane and precipitated in hexane. The swelling and precipitation operations were repeated three times. Finally, the product was dried under vacuum at 50 °C to obtain modified polypropylene.

[0056] Step 2: Under a nitrogen atmosphere, 3.6 g of eugenol, 2 g of triethylamine and 100 g of dichloromethane were mixed and stirred. 18.8 g of a 16 wt% dichlorodiphenylsilane / dichloromethane mixture was added, and the mixture was heated to 38 °C and reacted at a constant temperature for 3.5 h. After the reaction was completed, the mixture was cooled to room temperature, filtered to remove the triethylamine hydrochloride byproduct, and the resulting filtrate was washed three times with deionized water, extracted, separated into layers, and dried with anhydrous sodium sulfate. After drying, the liquid product mixture was rotary evaporated at 50 °C to remove the solvent, and then dried under vacuum at 60 °C for 12 h to obtain the silicon-containing compound.

[0057] In a nitrogen atmosphere, 3g of surface-modified composite inorganic filler was added to 100g of ethanol and stirred for 20min. Then, 5.4g of silicon-containing compound was added and stirred at 70℃ for 9h. After the reaction was completed, the product was centrifuged and washed with ethanol. The product was then vacuum dried at 60℃ to obtain the functionalized composite inorganic filler.

[0058] Step 3: Add polypropylene, modified polypropylene, and functionalized composite inorganic filler to a foaming extruder. Melt-blend for 15 minutes at a screw speed of 30 r / min and a temperature of 170℃, and then foam. During the foaming process, extrusion is carried out by the screw to induce foaming. After foaming, cool to obtain polypropylene-based multi-component synergistic flame-retardant foam material. The mass ratio of polypropylene, modified polypropylene, and functionalized composite inorganic filler is 100:15:9. The foaming agent used is supercritical carbon dioxide, and the amount of foaming agent is 2% of the mass of polypropylene.

[0059] Example 4

[0060] This embodiment discloses a method for preparing a polypropylene-based multi-element synergistic flame-retardant foam material, including the following steps:

[0061] Step 1: Add 18.8g of acryloyl chloride to 250g of chloroform and stir for 40min. Then, at 5℃, while stirring, add 38.4g of 13wt% 1,3-propanediamine / chloroform mixture dropwise over 60min. Stir the reaction mixture at 27℃ for 100min, then add 150g of deionized water and 8g of sodium hydroxide and continue stirring for 80min. Filter the mixture, wash the resulting solid with deionized water, and then vacuum dry it at 80℃ to obtain a nitrogen-containing compound.

[0062] In a nitrogen atmosphere, 18.2 g of a nitrogen-containing compound and 21.6 g of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide were added to 500 g of ethanol, stirred until homogeneous, heated to 65 °C, and stirred for 5 h. After the reaction was completed, the solvent was evaporated to obtain the nitrogen-phosphorus compound.

[0063] In a nitrogen atmosphere, 500g of toluene, 0.1g of cuprous chloride, 30.1g of nitrogen and phosphorus compounds, and 3g of bromine-terminated polypropylene were mixed and stirred at 92°C for 40 min. Then, 0.4g of 1,1,4,7,10,10-hexamethyltriethylenetetramine was added to initiate the reaction. After reacting for 10 h, ethanol was added to terminate the reaction. The mixture was filtered, and the resulting mixture was swollen in dichloromethane and precipitated in hexane. The swelling and precipitation operations were repeated 3-5 times. Finally, the product was dried under vacuum at 50°C to obtain modified polypropylene.

[0064] Step 2: Under a nitrogen atmosphere, 7.2 g of eugenol, 4 g of triethylamine and 150 g of dichloromethane were mixed and stirred. Then, 31.6 g of a 16 wt% dichlorodiphenylsilane / dichloromethane mixture was added, and the mixture was heated to 42 °C and reacted at a constant temperature for 2.5 h. After the reaction was completed, the mixture was cooled to room temperature, filtered to remove the triethylamine hydrochloride byproduct, and the resulting filtrate was washed three times with deionized water, extracted, separated into layers, and dried with anhydrous sodium sulfate. After drying, the liquid product mixture was rotary evaporated at 50 °C to remove the solvent, and then dried in a vacuum at 60 °C for 12 h to obtain a silicon-containing compound.

[0065] In a nitrogen atmosphere, 5g of surface-modified composite inorganic filler was added to 150g of ethanol and stirred for 40min. Then, 8.6g of silicon-containing compound was added and stirred at 76℃ for 7h. After the reaction was completed, the product was centrifuged and washed with ethanol. The product was then vacuum dried at 60℃ to obtain the functionalized composite inorganic filler.

[0066] Step 3: Add polypropylene, modified polypropylene, and functionalized composite inorganic filler to a foaming extruder. Melt-blend for 8 minutes at a screw speed of 40 r / min and a temperature of 220℃, and then foam. During the foaming process, extrusion is carried out by pushing through the screw to achieve foaming. After foaming is completed, cool to obtain polypropylene-based multi-element synergistic flame-retardant foam material. The mass ratio of polypropylene, modified polypropylene, and functionalized composite inorganic filler is 100:30:15. The foaming agent used is supercritical carbon dioxide, and the amount of foaming agent is 3% of the mass of polypropylene.

[0067] Example 5

[0068] This embodiment discloses a method for preparing a polypropylene-based multi-element synergistic flame-retardant foam material, including the following steps:

[0069] Step 1: Add 14.1g of acryloyl chloride to 200g of chloroform and stir for 30min. Then, at 3℃, while stirring, add 29.9g of 13wt% 1,3-propanediamine / chloroform mixture dropwise over 45min. Stir the reaction mixture at 25℃ for 125min. Then add 125g of deionized water and 6g of sodium hydroxide and continue stirring for 60min. Filter the mixture. Wash the resulting solid with deionized water and dry it under vacuum at 80℃ to obtain a nitrogen-containing compound.

[0070] In a nitrogen atmosphere, 13.7 g of a nitrogen-containing compound and 16.5 g of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide were added to 400 g of ethanol, stirred until homogeneous, heated to 60 °C, and stirred for 6 h. After the reaction was completed, the solvent was evaporated to obtain the nitrogen-phosphorus compound.

[0071] In a nitrogen atmosphere, 400 g of toluene, 0.09 g of cuprous chloride, 24.6 g of nitrogen-phosphorus compound, and 2.8 g of bromine-terminated polypropylene were mixed and stirred at 90 °C for 60 min. Then, 0.3 g of 1,1,4,7,10,10-hexamethyltriethylenetetramine was added to initiate the reaction. After reacting for 13 h, ethanol was added to terminate the reaction. The mixture was filtered, and the resulting mixture was swollen in dichloromethane and precipitated in hexane. The swelling and precipitation operations were repeated 4 times. Finally, the product was dried under vacuum at 50 °C to obtain modified polypropylene.

[0072] Step 2: Under a nitrogen atmosphere, 5.4 g of eugenol, 3 g of triethylamine and 125 g of dichloromethane were mixed and stirred. Then, 25.2 g of 16 wt% dichlorodiphenylsilane / dichloromethane mixture was added, heated to 40 °C, and reacted at a constant temperature for 3 h. After the reaction was completed, the mixture was cooled to room temperature, filtered to remove the triethylamine hydrochloride byproduct, and the resulting filtrate was washed three times with deionized water, extracted, separated into layers, and dried with anhydrous sodium sulfate. After drying, the liquid product mixture was rotary evaporated at 50 °C to remove the solvent, and then dried in a vacuum at 60 °C for 12 h to obtain the silicon-containing compound.

[0073] In a nitrogen atmosphere, 4g of surface-modified composite inorganic filler was added to 125g of ethanol and stirred for 30min. Then, 7g of silicon-containing compound was added and stirred at 73℃ for 8h. After the reaction was completed, the product was centrifuged and washed with ethanol and then vacuum dried at 60℃ to obtain the functionalized composite inorganic filler.

[0074] Step 3: Add polypropylene, modified polypropylene, and functionalized composite inorganic filler to a foaming extruder. Melt-blend for 11 minutes at a screw speed of 35 r / min and a temperature of 195℃, and then foam. During the foaming process, extrusion is carried out by the screw to induce foaming. After foaming, cool to obtain polypropylene-based multi-component synergistic flame-retardant foam material. The mass ratio of polypropylene, modified polypropylene, and functionalized composite inorganic filler is 100:22.5:12. The foaming agent used is supercritical carbon dioxide, and the amount of foaming agent is 2.5% of the mass of polypropylene.

[0075] The bromine-terminated polypropylene and surface-modified composite inorganic filler in Examples 3-5 above are the bromine-terminated polypropylene prepared in Example 1 and the surface-modified composite inorganic filler prepared in Example 2.

[0076] Comparative Example 1

[0077] This comparative example discloses a method for preparing a surface-modified inorganic filler, comprising the following steps:

[0078] 7.1g of silane coupling agent KH-550, 67.5g of deionized water, and 60g of ethanol were mixed, and then acetic acid was added to adjust the pH to 4. The mixture was stirred at 60℃ for 4h, and then 3.5g of montmorillonite was added. The temperature was kept constant, and the reaction was continued to be stirred for 1.5h. After the reaction was completed, the mixture was centrifuged, and the resulting solid product was washed with ethanol and then vacuum dried at 60℃ to obtain the surface-modified composite inorganic filler.

[0079] Comparative Example 2

[0080] This comparative example discloses a method for preparing a polypropylene-based multi-element synergistic flame-retardant foam material, including the following steps:

[0081] Step 1: Add 14.1g of acryloyl chloride to 200g of chloroform and stir for 30min. Then, at 3℃, while stirring, add 29.9g of 13wt% 1,3-propanediamine / chloroform mixture dropwise over 45min. Stir the reaction mixture at 25℃ for 125min. Then add 125g of deionized water and 6g of sodium hydroxide and continue stirring for 60min. Filter the mixture. Wash the resulting solid with deionized water and dry it under vacuum at 80℃ to obtain a nitrogen-containing compound.

[0082] In a nitrogen atmosphere, 13.7 g of a nitrogen-containing compound and 16.5 g of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide were added to 400 g of ethanol, stirred until homogeneous, heated to 60 °C, and stirred for 6 h. After the reaction was completed, the solvent was evaporated to obtain the nitrogen-phosphorus compound.

[0083] In a nitrogen atmosphere, 400 g of toluene, 0.09 g of cuprous chloride, 24.6 g of nitrogen and phosphorus compounds, and 2.8 g of bromine-terminated polypropylene prepared in Example 1 were mixed and stirred at 90 °C for 60 min. Then, 0.3 g of 1,1,4,7,10,10-hexamethyltriethylenetetramine was added to initiate the reaction. After reacting for 13 h, ethanol was added to terminate the reaction. The mixture was filtered, and the resulting mixture was swollen in dichloromethane and precipitated in hexane. The above swelling and precipitation operations were repeated 4 times. Finally, the product was dried under vacuum at 50 °C to obtain modified polypropylene.

[0084] Step 2: Under a nitrogen atmosphere, 5.4 g of eugenol, 3 g of triethylamine and 125 g of dichloromethane were mixed and stirred. Then, 25.2 g of 16 wt% dichlorodiphenylsilane / dichloromethane mixture was added, heated to 40 °C, and reacted at a constant temperature for 3 h. After the reaction was completed, the mixture was cooled to room temperature, filtered to remove the triethylamine hydrochloride byproduct, and the resulting filtrate was washed three times with deionized water, extracted, separated into layers, and dried with anhydrous sodium sulfate. After drying, the liquid product mixture was rotary evaporated at 50 °C to remove the solvent, and then dried in a vacuum at 60 °C for 12 h to obtain the silicon-containing compound.

[0085] In a nitrogen atmosphere, 4g of the surface-modified inorganic filler prepared in Comparative Example 1 was added to 125g of ethanol and stirred for 30min. Then, 7g of silicon-containing compound was added and stirred at 73℃ for 8h. After the reaction was completed, the product was centrifuged and washed with ethanol and then vacuum dried at 60℃ to obtain the functionalized composite inorganic filler.

[0086] Step 3: Add polypropylene, modified polypropylene, and functionalized composite inorganic filler to a foaming extruder. Melt-blend for 11 minutes at a screw speed of 35 r / min and a temperature of 195℃, then foam. During foaming, extrusion is performed by pushing the material through the screw. After foaming, cool to obtain a polypropylene-based multi-element synergistic flame-retardant foamed material. The mass ratio of polypropylene, modified polypropylene, and functionalized composite inorganic filler is 100:22.5:12. The foaming agent used is supercritical carbon dioxide, and the amount of foaming agent is 2.5% of the mass of polypropylene.

[0087] Comparative Example 3

[0088] This comparative example discloses a method for preparing a polypropylene-based multi-element synergistic flame-retardant foam material, including the following steps:

[0089] Step 1: Under a nitrogen atmosphere, 5.4 g of eugenol, 3 g of triethylamine and 125 g of dichloromethane were mixed and stirred. Then, 25.2 g of 16 wt% dichlorodiphenylsilane / dichloromethane mixture was added, and the mixture was heated to 40 °C and reacted at a constant temperature for 3 h. After the reaction was completed, the mixture was cooled to room temperature, filtered to remove the triethylamine hydrochloride byproduct, and the resulting filtrate was washed three times with deionized water, extracted, separated into layers, and dried with anhydrous sodium sulfate. After drying, the liquid product mixture was rotary evaporated at 50 °C to remove the solvent, and then dried in a vacuum at 60 °C for 12 h to obtain a silicon-containing compound.

[0090] In a nitrogen atmosphere, 4g of the surface-modified composite inorganic filler prepared in Example 2 was added to 125g of ethanol and stirred for 30min. Then, 7g of silicon-containing compound was added and stirred at 73℃ for 8h. After the reaction was completed, the product was centrifuged and washed with ethanol and then vacuum dried at 60℃ to obtain the functionalized composite inorganic filler.

[0091] Step 2: Polypropylene, the bromine-terminated polypropylene prepared in Example 1, and the functionalized composite inorganic filler are added to a foaming extruder. The mixture is melt-blended for 11 minutes at a screw speed of 35 r / min and a temperature of 195°C, and then foamed. During foaming, the material is extruded through the screw. After foaming, it is cooled to obtain a polypropylene-based multi-element synergistic flame-retardant foamed material. The mass ratio of polypropylene, bromine-terminated polypropylene, and functionalized composite inorganic filler is 100:22.5:12. The foaming agent used is supercritical carbon dioxide, and the amount of foaming agent is 2.5% of the mass of polypropylene.

[0092] Comparative Example 4

[0093] This comparative example discloses a method for preparing a polypropylene-based multi-element synergistic flame-retardant foam material, including the following steps:

[0094] Step 1: Add 14.1g of acryloyl chloride to 200g of chloroform and stir for 30min. Then, at 3℃, while stirring, add 29.9g of 13wt% 1,3-propanediamine / chloroform mixture dropwise over 45min. Stir the reaction mixture at 25℃ for 125min. Then add 125g of deionized water and 6g of sodium hydroxide and continue stirring for 60min. Filter the mixture. Wash the resulting solid with deionized water and dry it under vacuum at 80℃ to obtain a nitrogen-containing compound.

[0095] In a nitrogen atmosphere, 400 g of toluene, 0.09 g of cuprous chloride, 13.7 g of nitrogen-containing compound, and 2.8 g of bromine-terminated polypropylene prepared in Example 1 were mixed and stirred at 90 °C for 60 min. Then, 0.3 g of 1,1,4,7,10,10-hexamethyltriethylenetetramine was added to initiate the reaction. After reacting for 13 h, ethanol was added to terminate the reaction. The mixture was filtered, and the resulting mixture was swollen in dichloromethane and precipitated in hexane. The above swelling and precipitation operations were repeated 4 times. Finally, the product was dried under vacuum at 50 °C to obtain modified polypropylene.

[0096] Step 2: Under a nitrogen atmosphere, 5.4 g of eugenol, 3 g of triethylamine and 125 g of dichloromethane were mixed and stirred. Then, 25.2 g of 16 wt% dichlorodiphenylsilane / dichloromethane mixture was added, heated to 40 °C, and reacted at a constant temperature for 3 h. After the reaction was completed, the mixture was cooled to room temperature, filtered to remove the triethylamine hydrochloride byproduct, and the resulting filtrate was washed three times with deionized water, extracted, separated into layers, and dried with anhydrous sodium sulfate. After drying, the liquid product mixture was rotary evaporated at 50 °C to remove the solvent, and then dried in a vacuum at 60 °C for 12 h to obtain the silicon-containing compound.

[0097] In a nitrogen atmosphere, 4g of the surface-modified composite inorganic filler prepared in Example 2 was added to 125g of ethanol and stirred for 30min. Then, 7g of silicon-containing compound was added and stirred at 73℃ for 8h. After the reaction was completed, the product was centrifuged and washed with ethanol and then vacuum dried at 60℃ to obtain the functionalized composite inorganic filler.

[0098] Step 3: Add polypropylene, modified polypropylene, and functionalized composite inorganic filler to a foaming extruder. Melt-blend for 11 minutes at a screw speed of 35 r / min and a temperature of 195℃, and then foam. During the foaming process, extrusion is carried out by the screw to induce foaming. After foaming, cool to obtain polypropylene-based multi-component synergistic flame-retardant foam material. The mass ratio of polypropylene, modified polypropylene, and functionalized composite inorganic filler is 100:22.5:12. The foaming agent used is supercritical carbon dioxide, and the amount of foaming agent is 2.5% of the mass of polypropylene.

[0099] Comparative Example 5

[0100] This comparative example discloses a method for preparing a polypropylene-based multi-element synergistic flame-retardant foam material, including the following steps:

[0101] Step 1: Add 14.1g of acryloyl chloride to 200g of chloroform and stir for 30min. Then, at 3℃, while stirring, add 29.9g of 13wt% 1,3-propanediamine / chloroform mixture dropwise over 45min. Stir the reaction mixture at 25℃ for 125min. Then add 125g of deionized water and 6g of sodium hydroxide and continue stirring for 60min. Filter the mixture. Wash the resulting solid with deionized water and dry it under vacuum at 80℃ to obtain a nitrogen-containing compound.

[0102] In a nitrogen atmosphere, 13.7 g of a nitrogen-containing compound and 16.5 g of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide were added to 400 g of ethanol, stirred until homogeneous, heated to 60 °C, and stirred for 6 h. After the reaction was completed, the solvent was evaporated to obtain the nitrogen-phosphorus compound.

[0103] In a nitrogen atmosphere, 400 g of toluene, 0.09 g of cuprous chloride, 24.6 g of nitrogen-phosphorus compound, and 2.8 g of bromine-terminated polypropylene were mixed and stirred at 90 °C for 60 min. Then, 0.3 g of 1,1,4,7,10,10-hexamethyltriethylenetetramine was added to initiate the reaction. After reacting for 13 h, ethanol was added to terminate the reaction. The mixture was filtered, and the resulting mixture was swollen in dichloromethane and precipitated in hexane. The swelling and precipitation operations were repeated 4 times. Finally, the product was dried under vacuum at 50 °C to obtain modified polypropylene.

[0104] Step 2: Polypropylene, modified polypropylene, and the composite inorganic filler prepared in Example 2 are added to a foaming extruder and melt-blended for 11 minutes at a screw speed of 35 r / min and a temperature of 195°C. Foaming is then carried out by extruding through the screw. After foaming, the material is cooled to obtain a polypropylene-based multi-component synergistic flame-retardant foam material. The mass ratio of polypropylene, modified polypropylene, and composite inorganic filler is 100:22.5:12. The foaming agent used is supercritical carbon dioxide, and the amount of foaming agent is 2.5% of the mass of polypropylene.

[0105] Experimental Example

[0106] The polypropylene-based multi-element synergistic flame-retardant foam material samples prepared in Examples 3-5 and Comparative Examples 2-5 were numbered sequentially as Sample 3, Sample 4, Sample 5, Comparative Sample 2, Comparative Sample 3, Comparative Sample 4, and Comparative Sample 5. Performance tests were performed on Sample 3, Sample 4, Sample 5, Comparative Sample 2, Comparative Sample 3, Comparative Sample 4, and Comparative Sample 5.

[0107] I. Limiting Oxygen Index Test: The limiting oxygen index of each group of samples was tested using an oxygen index meter in accordance with standard GB / T 2406.2-2009.

[0108] II. Tensile Strength Test: Tensile strength was determined using a universal testing machine at room temperature according to standard GB / T 1040.2-2022, with a tensile rate of 10 mm / min. Five specimens were tested in each group, and the average value of the test results was taken.

[0109] III. Yellowing Resistance Test: The UV aging resistance of each group of samples was evaluated using a gray scale according to standard ISO105 / A02.

[0110] The test results are shown in Table 1:

[0111] Table 1

[0112]

[0113] As shown in Table 1, the polypropylene-based multi-element synergistic flame-retardant foamed materials prepared in Examples 3-5 of this invention exhibit excellent flame-retardant properties, UV aging resistance, and mechanical properties. A comparison between sample 2 and sample 5 shows that the composite inorganic filler of this invention has a more significant impact on the mechanical properties of foamed polypropylene compared to montmorillonite. Furthermore, the presence of ZnMgAl-LDH in the composite inorganic filler further improves the flame-retardant properties and UV aging resistance of the foamed polypropylene. A comparison between sample 3 and sample 5 shows that the flame-retardant nitrogen and phosphorus elements in the modified polypropylene, synergistically with the montmorillonite, hydrotalcite, and flame-retardant silicon elements in the functionalized composite inorganic filler, exert excellent flame-retardant effects. Moreover, the introduction of polar groups such as amide and PO bonds into the modified polypropylene by nitrogen and phosphorus compounds can form hydrogen bonds with the functionalized composite inorganic filler, improving the functionalization... The dispersion effect of the composite inorganic filler improves the overall performance of foamed polypropylene. A comparison between sample 4 and sample 5 shows that the introduction of flame-retardant phosphorus in the modified polypropylene improves the flame-retardant properties of the foamed polypropylene. A comparison between sample 5 and sample 6 shows that the silane coupling agent modification and grafting of silicon-containing compounds to the composite inorganic filler, with its strong UV absorption capacity from phenoxy and aromatic functional groups, and the flame-retardant effect of silicon, along with the enhanced strength of the matrix due to the rigid structure, further improves the compatibility between the modified composite inorganic filler and the polypropylene matrix. Therefore, the overall performance of the foamed polypropylene is improved.

[0114] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A process for the preparation of a polypropylene based synergistic flame retardant foamed material, characterized in that, It comprises the following steps: Step one, the nitrogen phosphorus compound is used for modifying the bromine end-capped polypropylene, and the modified polypropylene is obtained, and the specific method is as follows: The acryloyl chloride is added into chloroform, stirred, and 1,3-propanediamine / chloroform mixed solution is added at 0-5℃, stirred and reacted at 23-27℃ for 100-150min, then deionized water and sodium hydroxide are added, and stirred for 40-80min, purified to obtain the nitrogen-containing compound; wherein the mass ratio of acryloyl chloride, chloroform, 1,3-propanediamine / chloroform mixed solution, deionized water and sodium hydroxide is (9.4-18.8):(150-250):(21.4-38.4):(100-150):(4-8); In a nitrogen atmosphere, the nitrogen-containing compound, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide and ethanol are mixed in a mass ratio of (9.1-18.2):(11.3-21.6):(300-500), stirred uniformly, heated to 55-65℃, stirred and reacted for 5-7h, and purified to obtain the nitrogen phosphorus compound; In a nitrogen atmosphere, the toluene, cuprous chloride, nitrogen phosphorus compound and bromine end-capped polypropylene are mixed, stirred at 88-92℃ for 40-80min, then 1,1,4,7,10,10-hexamethyltrivinyltetramine is added, reacted for 10-16h, then ethanol is added to terminate the reaction, and purified; wherein the mass ratio of toluene, cuprous chloride, nitrogen phosphorus compound, bromine end-capped polypropylene and 1,1,4,7,10,10-hexamethyltrivinyltetramine is (300-500):(0.08-0.1):(19.1-30.1):(2.5-3):(0.2-0.4); Step two, the surface modified composite inorganic filler is modified by using the silicon-containing compound to obtain the functionalized composite inorganic filler, and the specific method is as follows: In a nitrogen atmosphere, the eugenol, triethylamine, dichloromethane and dichlorodiphenylsilane / dichloromethane mixed solution are mixed in a mass ratio of (3.6-7.2):(2-4):(100-150):(18.8-31.6), heated to 38-42℃, kept constant temperature for 2.5-3.5h, and purified to obtain the silicon-containing compound; In a nitrogen atmosphere, the surface modified composite inorganic filler, ethanol and silicon-containing compound are mixed in a mass ratio of (3-5):(100-150):(5.4-8.6), stirred and reacted at 70-76℃ for 7-9h, and purified to obtain the functionalized composite inorganic filler; The preparation method of the surface modified composite inorganic filler is as follows: Step B1, the montmorillonite / zinc oxide composite material, aluminum chloride hexahydrate, magnesium chloride hexahydrate and deionized water are mixed in a mass ratio of (2.4-4.8):(3.9-7.8):(3.3-6.6):(300-500), stirred until transparent, then sodium hydroxide aqueous solution and sodium carbonate aqueous solution are added into the mixed system at the same time, until the pH of the mixed system is 7, the pH is kept constant, stirred at 110-130℃ for 15-17h, and purified to obtain the composite inorganic filler; Step B2, mixing silane coupling agent KH-550, deionized water, ethanol, then adding acetic acid to adjust pH to 3.5-4.5, stirring at 55-65℃ for 3-5h, then adding composite inorganic filler again, continuing to stir for 1-2h, purifying to obtain surface modified composite inorganic filler; wherein the mass ratio of silane coupling agent KH-550, deionized water, ethanol, composite inorganic filler is (4.7-9.4):(45-90):(40-80):(2.5-4.5); Step three, mixing polypropylene, modified polypropylene and functionalized composite inorganic filler at a mass ratio of 100:(15-30):(9-15), melt blending, foaming, cooling to obtain polypropylene-based multi-component synergistic flame-retardant foaming material.

2. The preparation method of the polypropylene-based multi-element synergistic flame-retardant foam material according to claim 1, characterized in that, In the step one, the preparation method of the bromine-terminated polypropylene: Step A1, mixing toluene, methylaluminoxane / toluene mixed solution, p-divinylbenzene / toluene mixed solution in a hydrogen atmosphere, stirring at 28-32℃ at a speed of 600-700rpm for 4-8min, then adding dimethylsilyl bis(2-methyl-4-phenyl indenyl) zirconium dichloride / toluene mixed solution, then introducing 3bar propylene, continuously introducing propylene to keep the pressure of the reaction system constant at 4bar, keeping at 28-32℃ for 10-20min, releasing pressure, adding acidified ethanol, purifying to obtain styrene-terminated polypropylene; wherein the volume ratio of toluene, methylaluminoxane / toluene mixed solution, p-divinylbenzene / toluene mixed solution, dimethylsilyl bis(2-methyl-4-phenyl indenyl) zirconium dichloride / toluene mixed solution is (50-100):(4.3-8.6):(6-12):(10-20); Step A2, mixing styrene-terminated polypropylene, 1,1,2,2-tetrachloroethane at a mass ratio of (2-3):(120-150) at 58-62℃ to obtain a suspension; bubbling HBr into the above suspension, after 5-7h, adding ethanol again, purifying to obtain bromine-terminated polypropylene.

3. The preparation method of the polypropylene-based multi-element synergistic flame-retardant foam material according to claim 1, characterized in that, The preparation method of the montmorillonite / zinc oxide composite material: Mixing montmorillonite, sodium alginate, deionized water at a mass ratio of (2-4):(20-40):(1000-2000), stirring to form a montmorillonite composite sol; mixing zinc nitrate hexahydrate, deionized water at a mass ratio of (6-12):(200-400) to obtain a zinc nitrate aqueous solution; mixing the montmorillonite composite sol with the zinc nitrate aqueous solution, stirring for 20-30h to obtain a gel, calcining the gel at 480-520℃ for 2.5-3.5h to obtain the montmorillonite / zinc oxide composite material.

4. The preparation method of the polypropylene-based multi-element synergistic flame-retardant foam material according to claim 1, characterized in that, In the step three, the melt blending conditions are: melt blending at a screw speed of 30-40r / min and a temperature of 170-220℃ for 8-15min; the foaming agent used for foaming is supercritical carbon dioxide, and the amount of foaming agent is 2-3% of the mass of polypropylene.

5. A polypropylene-based multi-effect synergistic flame-retardant foamed material prepared by the method according to any one of claims 1-4.

6. A cable, characterized by including the use of a polypropylene-based multi-effect synergistic flame-retardant foamed material according to claim 5.

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