Flame-retardant polypropylene composite material and preparation method thereof

Abstract

The invention relates to the technical field of polypropylene, in particular to a flame-retardant polypropylene composite material and a preparation method thereof. The composite material is prepared from polypropylene, modified ammonium polyphosphate, dicumyl peroxide, an antioxidant and a lubricant. The modified ammonium polyphosphate is realized through silicon dioxide coating and branched molecular chain grafting, and the specific steps are as follows: firstly, hydrolytic polymerization is performed to form an alkenylated silicon dioxide coating layer, and then a flexible branched structure is introduced through sulfydryl-alkene click reaction and addition reaction. The material realizes collaborative optimization of flame retardance and mechanical properties, and is suitable for the fields of buildings, automobiles, electronics and the like.

Description

Technical Field

[0001] The present invention relates to the technical field of polypropylene, in particular to a flame-retardant polypropylene composite material and a preparation method thereof. Background Art

[0002] With the widespread application of polymer materials in fields such as construction, automobiles, and electronics, the requirements for their flame retardancy and safety are becoming increasingly stringent. Polypropylene, one of the most widely produced general-purpose thermoplastics, is widely used due to its excellent overall performance. However, its flammability limits its application in safety-critical applications. Therefore, the development of highly effective flame-retardant polypropylene materials has become a key research direction in materials science.

[0003] Traditional flame retardant technology primarily relies on additive flame retardants. While halogen-based flame retardants are effective, they produce toxic gases and corrosive fumes during combustion, posing serious risks to the environment and human health. With increasingly stringent environmental regulations, halogen-free flame retardant technology is becoming a trend. Phosphorus-based flame retardants, as the primary halogen-free flame retardant option, exert their flame retardant effect through a combination of gas-phase dilution and condensed-phase carbonization, offering the advantages of low toxicity and low smoke. Ammonium polyphosphate, a key phosphorus-nitrogen synergistic flame retardant, has attracted significant attention due to its highly effective flame retardant mechanism. During combustion, ammonium polyphosphate thermally decomposes to produce phosphoric acid and polyphosphoric acid, which undergo an esterification reaction with the polymer matrix to form a carbonized layer. Simultaneously, the released ammonia dilutes the combustible gas concentration, achieving a dual gas-phase and condensed-phase flame retardant effect. However, the practical application of ammonium polyphosphate faces numerous technical challenges.

[0004] First, ammonium polyphosphate (AMP) is a polar inorganic compound, resulting in significant surface energy differences and chemical structure mismatches with the non-polar polypropylene matrix. This leads to weak interfacial bonding and facilitates agglomeration and phase separation within the matrix. This poor dispersion not only reduces flame retardancy but also introduces stress concentration points within the material, severely degrading mechanical properties. Second, the addition of AMP typically requires a high dosage to achieve the desired flame retardancy rating. However, high loadings significantly increase the material's brittleness, leading to a significant decrease in impact strength and toughness. This trade-off between flame retardancy and mechanical properties has become a key bottleneck restricting the practical application of AMP flame retardant systems. Furthermore, the thermal stability of AMP during processing is a significant concern. Within the typical processing temperature range of polypropylene, AMP may partially decompose. The released phosphoric acid and ammonia gases not only affect processing performance but can also corrode equipment. Furthermore, the volatilization of decomposition products reduces the effective flame retardant content in the final product.

[0005] To address these technical challenges, researchers have attempted a variety of surface modification strategies, including silane coupling agent treatment, polymer coating, and surface grafting. However, existing modification technologies often suffer from limited modification effects, complex processes, and high costs, making it difficult to achieve an ideal balance between flame retardancy and mechanical properties. In particular, in terms of improving interfacial compatibility, while traditional silane coupling agent modification can improve interfacial bonding to a certain extent, its rigid chemical bridging structure often makes the material brittle and significantly reduces impact toughness. While simple physical coating methods can improve dispersibility, they lack effective chemical bonding and are prone to interfacial delamination under stress.

[0006] Therefore, there is an urgent need to develop a new modification technology that can not only effectively improve the dispersibility and compatibility of ammonium polyphosphate in the polypropylene matrix, but also maintain or even improve the comprehensive mechanical properties of the material, so as to meet the actual application needs of high-performance flame-retardant polypropylene materials. Summary of the Invention

[0007] In view of this, the purpose of the present invention is to provide a flame retardant polypropylene composite material and a preparation method thereof, so as to solve the problems of poor dispersibility of ammonium polyphosphate in the polymer matrix, low interfacial compatibility, and difficulty in balancing flame retardancy and mechanical properties.

[0008] Based on the above purpose, the present invention provides a flame retardant polypropylene composite material, which is prepared from the following raw materials, by weight: 400-600 parts of polypropylene, 60-140 parts of modified ammonium polyphosphate, 8-12 parts of dicumyl peroxide, 2-6 parts of antioxidant and 1-4 parts of lubricant.

[0009] Preferably, the polypropylene is copolymerized polypropylene.

[0010] Preferably, the preparation method of the modified ammonium polyphosphate is as follows: (1) Tetraethyl silicate and vinyl triethoxysilane are hydrolyzed and polymerized to encapsulate ammonium polyphosphate to obtain olefinated silica-encapsulated ammonium polyphosphate; (2) Triallyl isocyanurate and dodecanethiol in a molar ratio of 1:1 were reacted through a thiol-ene click reaction to obtain olefinated silica-coated ammonium polyphosphate; (3) Alkylated allyl isocyanurate, N-methylethylenediamine and olefinated silica encapsulate ammonium polyphosphate through the addition reaction of amino groups and alkenes to obtain modified ammonium polyphosphate.

[0011] Preferably, in step (1), the weight ratio of tetraethyl silicate, vinyltriethoxysilane and ammonium polyphosphate is 20-60:3-12:50-150.

[0012] Preferably, the degree of polymerization of the ammonium polyphosphate in step (1) is ≥1000.

[0013] Preferably, the photoinitiator for the thiol-ene click reaction in step (2) is photoinitiator 1173.

[0014] Preferably, in step (3), the weight ratio of alkylated allyl isocyanurate, N-methylethylenediamine and alkylated silica-coated ammonium polyphosphate is 50-150:40-150:3-11.

[0015] Preferably, the antioxidant is one or a mixture of antioxidant 1010 and antioxidant 168.

[0016] Preferably, the lubricant is one or a mixture of calcium stearate, ethylene bisstearamide, stearamide and oleamide.

[0017] Furthermore, the present invention also provides a method for preparing a flame-retardant polypropylene composite material, comprising the following steps: adding polypropylene, modified ammonium polyphosphate, dicumyl peroxide, an antioxidant and a lubricant into a mixer, melt blending, then extruding through a twin-screw extruder, water-cooling and pelletizing, and hot-air drying to obtain a flame-retardant polypropylene composite material.

[0018] Preferably, the melt blending temperature is 190-210° C., and the time is 8-12 minutes.

[0019] Preferably, the screw speed of the twin-screw extruder is 300-500 rpm, the temperature of each zone is 190-230° C., and the melt pressure is 10-15 MPa.

[0020] The present invention achieves synergistic improvement of flame retardancy and mechanical properties by constructing a branched molecular chain-modified ammonium polyphosphate flame retardant system, producing significant technical effects and application value.

[0021] The modification strategy of this invention fundamentally overcomes the technical bottlenecks of traditional ammonium polyphosphate flame-retardant systems through a dual modification mechanism of silica coating and branched molecular grafting. The silica coating acts as a transition layer at the inorganic-organic interface, effectively improving the compatibility between ammonium polyphosphate and the polymer matrix and reducing agglomeration. The introduction of branched molecular chains, through the synergistic effect of flexible alkyl segments and active alkenyl termini, creates a unique "flexible bridging" interface structure.

[0022] In terms of flame retardancy, modified ammonium polyphosphate (AMPP) forms a denser and more stable carbonized layer during combustion. The branched molecular chains, when combined with the phosphoric acid produced by the decomposition of AMPP at high temperatures, form a three-dimensional network framework, significantly enhancing the mechanical strength and thermal stability of the carbon layer. This enhanced carbon layer not only effectively blocks heat transfer and the diffusion of combustible gases, but also suppresses the formation of melt dripping during combustion, improving flame retardancy and safety.

[0023] In terms of optimizing interface performance, the technical solution of this invention achieves an organic combination of enhanced rigidity and flexible cushioning. The chemical bonding formed by the alkenyl end groups and the polypropylene molecular chains provides strong interfacial adhesion, ensuring effective stress transfer; while the flexible alkyl segments act as molecular buffers, absorbing and dispersing localized stress concentrations and preventing premature cracking at the interface. This "rigid and flexible" interface design enables the material to exhibit excellent toughness and impact resistance while maintaining high strength.

[0024] From a processing performance perspective, the good dispersibility of modified ammonium polyphosphate significantly improves the processing fluidity and molding quality of the composite material. The uniform dispersion reduces local stress concentration and hot spot formation, lowering the risk of decomposition during processing while improving the surface quality and dimensional stability of the product.

[0025] In summary, the present invention successfully solves the technical difficulties of traditional ammonium polyphosphate flame retardant systems through innovative molecular design concepts and modification strategies, provides a new technical path and theoretical support for the development of high-performance flame-retardant polymer materials, and has important theoretical significance and broad application prospects. DETAILED DESCRIPTION

[0026] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to specific embodiments.

[0027] In the specific embodiment of the present invention, the polypropylene is copolymerized polypropylene, brand EP548R, CNOOC Shell, and has a melt index of 25 g / 10 min; the ammonium polyphosphate is high-polymerized ammonium polyphosphate, with a degree of polymerization ≥1000, model XS-APPII, purchased from Zhejiang Xusen Flame Retardant Co., Ltd. Example 1:

[0028] (1) Add 50 g of ammonium polyphosphate and 0.5 g of emulsifier OP-10 to 45 g of deionized water and 175 g of anhydrous ethanol, heat to 40 ° C, adjust the pH to 9.3 with ammonia water, then dropwise add 20 g of tetraethyl silicate and 3 g of vinyltriethoxysilane, stir and react for 5 h, centrifuge, wash with deionized water and anhydrous ethanol three times each, and vacuum dry to obtain olefinated silica-coated ammonium polyphosphate; (2) 24.9 g of triallyl isocyanurate and 20.2 g of dodecanethiol were mixed and stirred for 20 min. 0.4 g of photoinitiator 1173 was added in a dark environment and stirred for 20 min. The mixture was reacted at room temperature under ultraviolet light (365 nm) for 5 h to obtain alkylated allyl isocyanurate. (3) 50 g of alkylated silica-coated ammonium polyphosphate and 40 g of alkylated allyl isocyanurate were added to 300 g of methanol and 100 g of deionized water, and ultrasonicated for 20 min. Then, 3 g of N-methylethylenediamine was added, and the temperature was raised to 48 ° C. The mixture was stirred and reacted for 40 h. The mixture was centrifuged, washed with methanol three times, and vacuum dried to obtain modified ammonium polyphosphate. (4) 400 g of polypropylene, 60 g of modified ammonium polyphosphate, 8 g of diisopropylbenzene peroxide, 2 g of antioxidant 1010, and 1 g of calcium stearate were added to a mixer, melt-blended at 190 °C for 8 min, and then extruded through a twin-screw extruder with a screw speed of 300 rpm, a temperature of 190-210 °C in each zone, a melt pressure of 10 MPa, water-cooled pelletizing, and hot air dried at 75 °C for 3 h to obtain a flame-retardant polypropylene composite material. Example 2:

[0029] (1) Add 100 g of ammonium polyphosphate and 1 g of emulsifier OP-10 to 75 g of deionized water and 225 g of anhydrous ethanol, heat to 45 ° C, adjust the pH to 9.5 with ammonia water, then dropwise add 41.7 g of tetraethyl silicate and 7.6 g of vinyltriethoxysilane, stir and react for 6 h, centrifuge, wash with deionized water and anhydrous ethanol three times each, and vacuum dry to obtain olefinated silica-coated ammonium polyphosphate; (2) 49.9 g of triallyl isocyanurate and 40.5 g of dodecanethiol were mixed and stirred for 30 min. 0.9 g of photoinitiator 1173 was added in a dark environment and stirred for 30 min. The mixture was reacted at room temperature under ultraviolet light (365 nm) for 6 h to obtain alkylated allyl isocyanurate. (3) 100 g of alkylated silica-coated ammonium polyphosphate and 90.3 g of alkylated allyl isocyanurate were added to 600 g of methanol and 200 g of deionized water, and ultrasonicated for 30 min. Then, 7.5 g of N-methylethylenediamine was added, and the temperature was raised to 50 ° C. and stirred for 48 h. The mixture was centrifuged, washed with methanol three times, and vacuum dried to obtain modified ammonium polyphosphate. (4) 500 g of polypropylene, 100 g of modified ammonium polyphosphate, 10 g of diisopropylbenzene peroxide, 4 g of antioxidant 1010, and 2.5 g of calcium stearate were added to a mixer, melt-blended at 200 °C for 10 min, and then extruded through a twin-screw extruder with a screw speed of 400 rpm, a temperature of 200-220 °C in each zone, a melt pressure of 12 MPa, water-cooled pelletizing, and hot air dried at 80 °C for 4 h to obtain a flame-retardant polypropylene composite material. Example 3:

[0030] (1) 150 g of ammonium polyphosphate and 2 g of emulsifier OP-10 were added to 105 g of deionized water and 275 g of anhydrous ethanol, and the temperature was raised to 50 ° C. The pH was adjusted to 9.8 with aqueous ammonia. Then 60 g of tetraethyl silicate and 12 g of vinyltriethoxysilane were added dropwise. The mixture was stirred for 7 h, centrifuged, washed with deionized water and anhydrous ethanol 3 times each, and vacuum dried to obtain olefinated silica-coated ammonium polyphosphate; (2) 74.7 g of triallyl isocyanurate and 40.7 g of dodecanethiol were mixed and stirred for 40 min. 1.2 g of photoinitiator 1173 was added in a dark environment and stirred for 40 min. The mixture was reacted at room temperature under ultraviolet light (365 nm) for 7 h to obtain alkylated allyl isocyanurate. (3) 150 g of alkylated silica-coated ammonium polyphosphate and 150 g of alkylated allyl isocyanurate were added to 1000 g of methanol and 300 g of deionized water, and ultrasonicated for 40 min. Then, 11 g of N-methylethylenediamine was added, and the temperature was raised to 52 °C. The mixture was stirred for 56 h, centrifuged, washed with methanol three times, and vacuum dried to obtain modified ammonium polyphosphate. (4) 600 g of polypropylene, 140 g of modified ammonium polyphosphate, 12 g of diisopropylbenzene peroxide, 6 g of antioxidant 1010, and 4 g of calcium stearate were added to a mixer, melt-blended at 210 °C for 12 min, and then extruded through a twin-screw extruder with a screw speed of 500 rpm, a temperature of 210-230 °C in each zone, a melt pressure of 15 MPa, water-cooled pelletizing, and hot air dried at 85 °C for 5 h to obtain a flame-retardant polypropylene composite material.

[0031] Comparative Example 1: The difference between Comparative Example 1 and Example 2 is that the alkylated allyl isocyanurate in step (3) is replaced by an equimolar amount of triallyl isocyanurate; The specific steps are as follows: (1) Add 100 g of ammonium polyphosphate and 1 g of emulsifier OP-10 to 75 g of deionized water and 225 g of anhydrous ethanol, heat to 45 ° C, adjust the pH to 9.5 with ammonia water, then dropwise add 41.7 g of tetraethyl silicate and 7.6 g of vinyltriethoxysilane, stir and react for 6 h, centrifuge, wash with deionized water and anhydrous ethanol three times each, and vacuum dry to obtain olefinated silica-coated ammonium polyphosphate; (2) 100 g of olefinated silica-coated ammonium polyphosphate and 49.9 g of triallyl isocyanurate were added to 600 g of methanol and 200 g of deionized water, ultrasonicated for 30 min, and then 7.5 g of N-methylethylenediamine was added. The temperature was raised to 50 ° C. and stirred for 48 h. The mixture was centrifuged, washed with methanol three times, and vacuum dried to obtain modified ammonium polyphosphate. (3) 500 g of polypropylene, 100 g of modified ammonium polyphosphate, 10 g of diisopropylbenzene peroxide, 4 g of antioxidant 1010, and 2.5 g of calcium stearate were added to a mixer, melt-blended at 200 °C for 10 min, and then extruded through a twin-screw extruder with a screw speed of 400 rpm, a temperature of 200-220 °C in each zone, a melt pressure of 12 MPa, water-cooled pelletizing, and hot air dried at 80 °C for 4 h to obtain a flame-retardant polypropylene composite material.

[0032] Comparative Example 2: The difference between Comparative Example 2 and Example 2 is that the N-methylethylenediamine in step (3) is replaced by an equal molar amount of ethylenediamine; The specific steps are as follows: (1) Add 100 g of ammonium polyphosphate and 1 g of emulsifier OP-10 to 75 g of deionized water and 225 g of anhydrous ethanol, heat to 45 ° C, adjust the pH to 9.5 with ammonia water, then dropwise add 41.7 g of tetraethyl silicate and 7.6 g of vinyltriethoxysilane, stir and react for 6 h, centrifuge, wash with deionized water and anhydrous ethanol three times each, and vacuum dry to obtain olefinated silica-coated ammonium polyphosphate; (2) 49.9 g of triallyl isocyanurate and 40.5 g of dodecanethiol were mixed and stirred for 30 min. 0.9 g of photoinitiator 1173 was added in a dark environment and stirred for 30 min. The mixture was reacted at room temperature under ultraviolet light (365 nm) for 6 h to obtain alkylated allyl isocyanurate. (3) 100 g of alkylated silica-coated ammonium polyphosphate and 90.3 g of alkylated allyl isocyanurate were added to 600 g of methanol and 200 g of deionized water, and ultrasonicated for 30 min. Then, 6.1 g of ethylenediamine was added, and the temperature was raised to 50 ° C. and stirred for 48 h. The mixture was centrifuged, washed with methanol three times, and vacuum dried to obtain modified ammonium polyphosphate; (4) 500 g of polypropylene, 100 g of modified ammonium polyphosphate, 10 g of diisopropylbenzene peroxide, 4 g of antioxidant 1010, and 2.5 g of calcium stearate were added to a mixer, melt-blended at 200 °C for 10 min, and then extruded through a twin-screw extruder with a screw speed of 400 rpm, a temperature of 200-220 °C in each zone, a melt pressure of 12 MPa, water-cooled pelletizing, and hot air dried at 80 °C for 4 h to obtain a flame-retardant polypropylene composite material.

[0033] Comparative Example 3: The difference between Comparative Example 3 and Example 2 is that the modified ammonium polyphosphate in step (4) is replaced by olefinated silica-coated ammonium polyphosphate; The specific steps are as follows: (1) Add 100 g of ammonium polyphosphate and 1 g of emulsifier OP-10 to 75 g of deionized water and 225 g of anhydrous ethanol, heat to 45 ° C, adjust the pH to 9.5 with ammonia water, then dropwise add 41.7 g of tetraethyl silicate and 7.6 g of vinyltriethoxysilane, stir and react for 6 h, centrifuge, wash with deionized water and anhydrous ethanol three times each, and vacuum dry to obtain olefinated silica-coated ammonium polyphosphate; (2) 500 g of polypropylene, 100 g of olefinated silica-coated ammonium polyphosphate, 10 g of diisopropylbenzene peroxide, 4 g of antioxidant 1010, and 2.5 g of calcium stearate were added to a mixer, melt-blended at 200 °C for 10 min, and then extruded through a twin-screw extruder with a screw speed of 400 rpm, a temperature of 200-220 °C in each zone, a melt pressure of 12 MPa, water-cooled pelletizing, and hot air dried at 80 °C for 4 h to obtain a flame-retardant polypropylene composite material.

[0034] Comparative Example 4: The difference between Comparative Example 4 and Example 2 is that the amount of dodecanethiol added in step (2) is adjusted to 81 g; The specific steps are as follows: (1) Add 100 g of ammonium polyphosphate and 1 g of emulsifier OP-10 to 75 g of deionized water and 225 g of anhydrous ethanol, heat to 45 ° C, adjust the pH to 9.5 with ammonia water, then dropwise add 41.7 g of tetraethyl silicate and 7.6 g of vinyltriethoxysilane, stir and react for 6 h, centrifuge, wash with deionized water and anhydrous ethanol three times each, and vacuum dry to obtain olefinated silica-coated ammonium polyphosphate; (2) 49.9 g of triallyl isocyanurate and 81 g of dodecanethiol were mixed and stirred for 30 min. 0.9 g of photoinitiator 1173 was added in a dark environment and stirred for 30 min. The mixture was reacted at room temperature under ultraviolet light (365 nm) for 6 h to obtain alkylated allyl isocyanurate. (3) 100 g of alkylated silica-coated ammonium polyphosphate and 90.3 g of alkylated allyl isocyanurate were added to 600 g of methanol and 200 g of deionized water, and ultrasonicated for 30 min. Then, 7.5 g of N-methylethylenediamine was added, and the temperature was raised to 50 ° C. and stirred for 48 h. The mixture was centrifuged, washed with methanol three times, and vacuum dried to obtain modified ammonium polyphosphate. (4) 500 g of polypropylene, 100 g of modified ammonium polyphosphate, 10 g of diisopropylbenzene peroxide, 4 g of antioxidant 1010, and 2.5 g of calcium stearate were added to a mixer, melt-blended at 200 °C for 10 min, and then extruded through a twin-screw extruder with a screw speed of 400 rpm, a temperature of 200-220 °C in each zone, a melt pressure of 12 MPa, water-cooled pelletizing, and hot air dried at 80 °C for 4 h to obtain a flame-retardant polypropylene composite material.

[0035] Performance testing: Spline injection molding: The flame-retardant polypropylene composite material pellets of Examples 1-3 and Comparative Examples 1-4 were used to prepare standard test specimens using an injection molding machine at a melt temperature of 220° C. and a holding pressure of 60 MPa.

[0036] Vertical burning test: Refer to GB / T 2408-2006. Fix the specimen vertically on the fixture. Aim the flame of an alcohol burner (20±1 mm height) at the center of the lower end of the specimen and ignite for 10 seconds. Record the afterflame time after the flame is removed.

[0037] Limiting Oxygen Index Test: Referring to GB / T 2406.2-2009, the specimen was vertically placed in the test tube of the oxygen index instrument. A nitrogen and oxygen mixed flow (23±2°C) was introduced from the bottom. The initial oxygen concentration was set to ±2% of the estimated value. The oxygen concentration gradient was adjusted by ±0.2%. The minimum oxygen concentration (LOI value) required for the specimen to burn continuously for 3 minutes or a burning length of 50 mm was recorded. The results are shown in Table 1.

[0038] Tensile strength and elongation at break test: Referring to GB / T 1040.2-2006, the specimens were stretched at a speed of 50 mm / min using a universal testing machine to test the tensile strength and elongation at break. The results are shown in Table 1.

[0039] Impact strength test: Referring to GB / T 1843-2008, a V-notched (2 mm depth) specimen was fixed horizontally on an impact testing machine. The pendulum energy was 7.5 J and the impact velocity was 3.5 m / s. The impact strength was tested. The results are shown in Table 1.

[0040] Table 1 Performance test results

[0041] Data Analysis: The data from Example 2 show that the flame-retardant polypropylene composite material prepared by the present invention exhibits a significant improvement in comprehensive performance. The flame-retardant function achieves a self-extinguishing effect, which may be due to the three-dimensional cross-linked network formed by the branched molecular chains grafted onto the surface of ammonium polyphosphate. This structure promotes the formation of a dense carbon layer under the action of heat. The olefin groups at the ends of the branched molecular chains may participate in the cross-linking reaction when the matrix melts, thereby enhancing the interfacial bonding strength; at the same time, the long alkyl chain structure in the branched chains improves the dispersion uniformity of the flame retardant in the polypropylene. This synergistic effect enables the material to have significantly better flame retardant performance than the basic polymer system while maintaining mechanical properties.

[0042] Compared to Comparative Example 1, Example 2 exhibits superior flame retardancy and impact resistance. This difference may stem from the regulatory effect of the dodecyl chains on molecular chain conformation: the flexible alkyl chains in the branched molecular chains reduce the resistance to polymer segment motion, while the interfacial crosslinking involving the terminal alkenyl groups enhances stress transfer efficiency. The steric hindrance of the alkyl chains may optimize the pore structure during carbon layer expansion, forming a more effective heat-oxygen isolation barrier. This structural property suppresses melt droplets during combustion and prolongs the integrity of the carbon layer.

[0043] Compared with Comparative Example 2, Example 2 shows a more stable balance between material strength and toughness. This technical effect may be attributed to the steric hindrance effect of N-methyl-ethylenediamine. The methyl substituent limits the excessive cross-linking density of the molecular chain, avoiding the sudden change of the rigidity of the cross-linked network. The moderate distribution of cross-linking points may form a carbon layer skeleton with both toughness and thermal stability. The alkenyl group at the end of the molecular chain delays interfacial debonding through bonding with the matrix, jointly ensuring the structural stability of the material under thermal-mechanical coupling environment.

[0044] Example 2 demonstrates a significant performance improvement compared to Comparative Example 3, demonstrating that the grafting of branched molecules onto the ammonium polyphosphate is a core technical feature. This branched molecular structure likely enhances functionality through two mechanisms: first, the flexible alkyl segments enhance interfacial compatibility through molecular chain entanglement, forming a microscopic continuous phase; second, the terminal olefin groups chemically bond with the polypropylene molecular chains, creating "rivet-like" reinforcement points within the matrix. This composite interface structure not only inhibits the aggregation and migration of the flame retardant but also induces an energy dissipation mechanism through molecular chain slippage under impact loads.

[0045] Comparing the data from Example 2 with that from Comparative Example 4, excessive thiol modification maintains flame retardancy but sacrifices impact strength. This phenomenon suggests that the concentration of terminal alkenyl groups has a threshold effect on interfacial properties: a moderate alkyl chain / alkenyl ratio can create a molecular structure that is both rigid and flexible. Excessive alkylation may weaken the alkenyl crosslink density, increasing the spacing between nodes in the crosslinked network and reducing stress transfer efficiency. Simultaneously, the plasticizing effect of long-chain alkyl groups may reduce the entanglement strength of the molecular chains, ultimately exhibiting a characteristic response under impact loading, resulting in an earlier shift in the brittle-to-ductile transition point.

[0046] Those skilled in the art should understand that the discussion of any of the above embodiments is merely illustrative and is not intended to imply that the scope of the present invention is limited to these examples. Within the scope of the present invention, the technical features in the above embodiments or different embodiments may be combined, the steps may be implemented in any order, and there are many other variations of the different aspects of the present invention as described above, which are not provided in detail for the sake of simplicity.

Claims

1. A flame retardant polypropylene composite material, characterized in that: The invention is prepared from the following raw materials in parts by weight: 400-600 parts of polypropylene, 60-140 parts of modified ammonium polyphosphate, 8-12 parts of dicumyl peroxide, 2-6 parts of antioxidant and 1-4 parts of lubricant; The preparation method of the modified ammonium polyphosphate is as follows: (1) Tetraethyl silicate and vinyl triethoxysilane are hydrolyzed and polymerized to encapsulate ammonium polyphosphate to obtain olefinated silica-encapsulated ammonium polyphosphate; (2) Triallyl isocyanurate and dodecanethiol in a molar ratio of 1:1 were reacted through a thiol-ene click reaction to obtain olefinated silica-coated ammonium polyphosphate; (3) Alkylated allyl isocyanurate, N-methylethylenediamine and olefinated silica encapsulate ammonium polyphosphate through the addition reaction of amino groups and alkenes to obtain modified ammonium polyphosphate.

2. The flame retardant polypropylene composite material according to claim 1, characterized in that: The polypropylene is copolymerized polypropylene.

3. The flame retardant polypropylene composite material according to claim 1, characterized in that: In the step (1), the weight ratio of tetraethyl silicate, vinyl triethoxysilane and ammonium polyphosphate is 20-60:3-12:50-150.

4. The flame retardant polypropylene composite material according to claim 1, characterized in that: The degree of polymerization of ammonium polyphosphate in step (1) is ≥1000.

5. The flame retardant polypropylene composite material according to claim 1, characterized in that: The photoinitiator for the thiol-ene click reaction in step (2) is photoinitiator 1173.

6. The flame retardant polypropylene composite material according to claim 1, characterized in that: In the step (3), the weight ratio of alkylated allyl isocyanurate, N-methylethylenediamine and alkylated silica-coated ammonium polyphosphate is 50-150:40-150:3-11.

7. The flame retardant polypropylene composite material according to claim 1, characterized in that: The antioxidant is one or a mixture of antioxidant 1010 and antioxidant 168.

8. The flame retardant polypropylene composite material according to claim 1, characterized in that: The lubricant is one or a mixture of calcium stearate, ethylene bisstearamide, stearamide and oleamide.

9. A method for preparing the flame-retardant polypropylene composite material according to any one of claims 1 to 8, characterized in that: The method comprises the following steps: adding polypropylene, modified ammonium polyphosphate, dicumyl peroxide, antioxidant and lubricant into a mixer, melt-blending, extruding through a twin-screw extruder, water-cooling and pelletizing, and hot-air drying to obtain a flame-retardant polypropylene composite material.

10. The method for preparing a flame-retardant polypropylene composite material according to claim 9, characterized in that: The temperature of the melt blending is 190-210° C., and the time is 8-12 minutes.

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