Flame-retardant polypropylene composite and method for producing the same

Abstract

The present application relates to the technical field of polypropylene, in particular to a kind of flame-retardant polypropylene composite material and preparation method thereof.The composite material is composed of polypropylene, modified ammonium polyphosphate, dicumyl peroxide, antioxidant and lubricant.Modified ammonium polyphosphate is realized by silica coating and branched molecular linkage grafting, and the specific steps are first hydrolysis polymerization to form alkenyl silica coating layer, then introduce flexible branched structure by thiol-ene click reaction and addition reaction.The present application realizes the synergistic optimization of flame retardant and mechanical property, and is suitable for building, automobile, electronic and other fields.

Description

TECHNICAL FIELD

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

[0002] With the wide application of polymer materials in the fields of construction, automobiles, electronics and electrical appliances, the flame-retardant safety requirements are becoming increasingly stringent. As one of the most commonly used thermoplastics, polypropylene is widely used due to its excellent comprehensive performance, but its flammable characteristics limit its application in high safety requirements. Therefore, the development of high-efficiency flame-retardant polypropylene materials has become an important research direction in the field of material science.

[0003] Traditional flame-retardant technology mainly relies on additive flame retardants. Although halogen-based flame retardants have a significant effect, they produce toxic gases and corrosive smoke during combustion, causing serious harm to the environment and human health. With the increasingly stringent environmental regulations, halogen-free flame-retardant technology has become a development trend. As a major halogen-free flame-retardant scheme, phosphorus-based flame retardants play a flame-retardant role through gas-phase dilution and condensed-phase carbonization mechanisms, and have the advantages of low toxicity and low smoke. Ammonium polyphosphate, as an important phosphorus-nitrogen synergistic flame retardant, has attracted much attention due to its efficient flame-retardant mechanism. During the combustion process, ammonium polyphosphate decomposes to produce phosphoric acid and polyphosphoric acid, which undergo esterification with the polymer matrix to form a carbonized layer, while the released ammonia gas dilutes the concentration of combustible gases, achieving a dual flame-retardant effect of gas phase and condensed phase. However, the practical application of ammonium polyphosphate faces many technical challenges.

[0004] Firstly, ammonium polyphosphate is a polar inorganic compound, which has a significant difference in surface energy and a mismatch in chemical structure with the non-polar polypropylene matrix, resulting in weak interfacial adhesion and easy formation of agglomeration and phase separation in the matrix. This poor dispersion state not only reduces the flame-retardant efficiency, but also introduces stress concentration points in the material, severely deteriorating the mechanical properties. Secondly, a high amount of ammonium polyphosphate is usually required to achieve the desired flame-retardant grade, but high filler content significantly increases the brittleness of the material, resulting in a significant decrease in impact strength and toughness. This trade-off between flame-retardant performance and mechanical properties is a key bottleneck that restricts the practical application of ammonium polyphosphate flame-retardant systems. In addition, the thermal stability of ammonium polyphosphate during processing cannot be ignored. Within the typical processing temperature range of polypropylene, ammonium polyphosphate may partially decompose, releasing phosphoric acid and ammonia gas that not only affects the processing performance but also can cause corrosion to the equipment, and the volatilization of the decomposition products also reduces the effective flame-retardant content in the final product.

[0005] To solve the above technical problems, researchers have tried various surface modification strategies, including silane coupling agent treatment, polymer coating, surface grafting and other methods. However, the existing modification technology often has the problems of limited modification effect, complex process, high cost and other problems, which is difficult to achieve the ideal balance of flame retardation performance and mechanical properties. Especially in the improvement of interfacial compatibility, although the traditional silane coupling agent modification can improve the interfacial bonding force to a certain extent, the rigid chemical bridging structure often makes the material brittle, and the impact toughness decreases significantly. While the simple physical coating method can improve the dispersibility, but it lacks effective chemical bonding, and is prone to interfacial peeling under stress.

[0006] Therefore, it is urgent to develop a new type of modification technology which can effectively improve the dispersibility and compatibility of ammonium polyphosphate in polypropylene matrix, and can maintain or even improve the comprehensive mechanical properties of the material, to meet the practical application requirements of high-performance flame-retardant polypropylene materials. SUMMARY

[0007] Therefore, the purpose of the present application is to provide a flame-retardant polypropylene composite material and a preparation method thereof, to solve the problems of poor dispersibility, low interfacial compatibility, and difficult to balance flame retardation and mechanical properties of ammonium polyphosphate in polymer matrix.

[0008] Based on the above purpose, the present application provides a flame-retardant polypropylene composite material, which 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.

[0009] Preferably, the polypropylene is copolymerized polypropylene.

[0010] Preferably, the preparation method of the modified ammonium polyphosphate is as follows:

[0011] (1) tetraethyl silicate and vinyl triethoxysilane are hydrolyzed and polymerized to coat ammonium polyphosphate to obtain alkenyl silica-coated ammonium polyphosphate;

[0012] (2) triallyl isocyanurate and dodecanethiol are reacted by thiol-ene click reaction at a molar ratio of 1:1 to obtain alkenyl silica-coated ammonium polyphosphate;

[0013] (3) alkylated allyl isocyanurate, N-methylethylenediamine and alkenyl silica-coated ammonium polyphosphate are reacted by amino and alkene addition reaction to obtain modified ammonium polyphosphate.

[0014] Preferably, the weight ratio of tetraethyl silicate, vinyl triethoxysilane and ammonium polyphosphate in step (1) is 20-60:3-12:50-150.

[0015] Preferably, the polyphosphoric acid ammonium in step (1) has a polymerization degree of ≥1000.

[0016] Preferably, the photo initiator of the thiol-ene click reaction in step (2) is photo initiator 1173.

[0017] Preferably, the weight ratio of the alkylated allyl isocyanurate, N-methylethylenediamine and allylated silica-coated polyphosphoric acid ammonium in step (3) is 50-150:40-150:3-11.

[0018] Preferably, the antioxidant is one or a mixture of both of antioxidant 1010 and antioxidant 168.

[0019] Preferably, the lubricant is one or a mixture of several of calcium stearate, ethylene bis-stearamide, stearamide and oleic acid amide.

[0020] Further, the application also provides a preparation method of the flame-retardant polypropylene composite material, comprising the following steps: adding polypropylene, modified polyphosphoric acid ammonium, dicumyl peroxide, antioxidant and lubricant into a mixing machine, melt blending, then extruding through a double screw extruder, water cooling and granulating, hot air drying, to obtain the flame-retardant polypropylene composite material.

[0021] Preferably, the temperature of the melt blending is 190-210℃, and the time is 8-12min.

[0022] Preferably, the screw rotation speed of the double screw extruder is 300-500rpm, the temperature of each zone is 190-230℃, and the melt pressure is 10-15MPa.

[0023] The application realizes the synergistic improvement of the flame-retardant performance and mechanical performance by constructing the branched molecular chain modified polyphosphoric acid ammonium flame-retardant system, and produces significant technical effects and application value.

[0024] The modification strategy of the application fundamentally solves the technical bottleneck of the traditional polyphosphoric acid ammonium flame-retardant system through the dual modification mechanism of silica coating and branched molecular grafting. The silica coating layer acts as a transition layer of inorganic-organic interface, effectively improving the compatibility between the polyphosphoric acid ammonium and the polymer matrix, and reducing the occurrence of agglomeration. The introduction of branched molecular chains constructs a unique "flexible bridging" interface structure through the synergistic effect of flexible alkyl segments and active alkenyl terminals.

[0025] In terms of flame retardation, the modified ammonium polyphosphate can form a more compact and stable carbonized layer structure during combustion. The branched molecular chains form a three-dimensional network framework with phosphoric acid generated by the decomposition of ammonium polyphosphate at high temperatures, significantly improving the mechanical strength and thermal stability of the carbon layer. This enhanced carbon layer not only effectively blocks heat transfer and combustible gas diffusion, but also suppresses the dripping phenomenon of the material during combustion, improving the flame retardant safety.

[0026] In terms of interface performance optimization, the technical solution of the present application realizes the organic combination of rigidity enhancement and flexibility buffering. The chemical bond formed by the alkenyl end and the polypropylene molecular chain provides strong interfacial bonding force, ensuring effective stress transfer; and the flexible alkyl segment acts as a molecular buffer pad, which can absorb and disperse local stress concentration to prevent premature interface cracking. This "rigidity and flexibility" interface design enables the material to maintain high strength while exhibiting excellent toughness and impact resistance.

[0027] From the perspective of processing performance, the good dispersibility of the modified ammonium polyphosphate significantly improves the processing fluidity and molding quality of the composite material. Uniform dispersion reduces local stress concentration and hot spot formation, reducing the risk of decomposition during processing, while improving the surface quality and dimensional stability of the product.

[0028] In summary, through innovative molecular design concepts and modification strategies, the present application successfully solves the technical problems of traditional ammonium polyphosphate flame retardant systems, providing 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

[0029] To make the purpose, technical scheme and advantages of the present application clearer, the present application is further described in detail below with specific examples.

[0030] In the specific embodiment of the present application, the polypropylene is a copolymerized polypropylene with a brand name of EP548R from CNOOC Shell, and a melt index of 25 g / 10 min; the ammonium polyphosphate is a high-polymer ammonium polyphosphate with a degree of polymerization of ≥1000 and a model number of XS-APPII, purchased from Zhejiang Xusen Flame Retardant Co., Ltd. Example 1

[0031] (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℃, adjust the pH to 9.3 with ammonia water, then add 20 g of tetraethyl silicate and 3 g of vinyl triethoxysilane dropwise, stir for 5 h, centrifuge, wash with deionized water and anhydrous ethanol for 3 times each, and vacuum dry to obtain alkenyl silica-coated ammonium polyphosphate;

[0032] (2) 24.9 g of triallyl isocyanurate and 20.2 g of dodecanethiol were mixed and stirred for 20 min, 0.4 g of a photoinitiator 1173 was added in a dark environment, stirred for 20 min, and reacted at room temperature under ultraviolet light (365 nm) irradiation for 5 h to obtain an alkylated allyl isocyanurate;

[0033] (3) 50 g of allylated silica-coated ammonium polyphosphate and 40 g of alkylated allyl isocyanurate were added to 300 g of methanol and 100 g of deionized water, ultrasonicated for 20 min, 3 g of N-methylethylenediamine was added, the temperature was raised to 48°C, and the reaction was stirred for 40 h, centrifuged, washed with methanol for 3 times, and vacuum dried to obtain modified ammonium polyphosphate;

[0034] (4) 400 g of polypropylene, 60 g of modified ammonium polyphosphate, 8 g of dicumyl 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, then extruded through a twin-screw extruder, the screw rotation speed was 300 rpm, the temperature of each zone was 190-210°C, the melt pressure was 10 MPa, water-cooled pelletizing was performed, and hot air drying was performed at 75°C for 3 h to obtain a flame-retardant polypropylene composite material. Example 2:

[0035] (1) 100 g of ammonium polyphosphate and 1 g of emulsifier OP-10 were added to 75 g of deionized water and 225 g of anhydrous ethanol, the temperature was raised to 45°C, the pH was adjusted to 9.5 with ammonia water, then 41.7 g of tetraethyl silicate and 7.6 g of vinyl triethoxysilane were added dropwise, and the reaction was stirred for 6 h, centrifuged, washed with deionized water and anhydrous ethanol for 3 times, and vacuum dried to obtain allylated silica-coated ammonium polyphosphate;

[0036] (2) 49.9 g of triallyl isocyanurate and 40.5 g of dodecanethiol were mixed and stirred for 30 min, 0.9 g of a photoinitiator 1173 was added in a dark environment, stirred for 30 min, and reacted at room temperature under ultraviolet light (365 nm) irradiation for 6 h to obtain an alkylated allyl isocyanurate;

[0037] (3) 100 g of allylated 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, ultrasonicated for 30 min, 7.5 g of N-methylethylenediamine was added, the temperature was raised to 50°C, and the reaction was stirred for 48 h, centrifuged, washed with methanol for 3 times, and vacuum dried to obtain modified ammonium polyphosphate;

[0038] (4) Put 500 g of polypropylene, 100 g of modified ammonium polyphosphate, 10 g of dicumyl peroxide, 4 g of antioxidant 1010, and 2.5 g of calcium stearate into a mixer, melt blend at 200°C for 10 min, then extrude through a twin-screw extruder with a screw speed of 400 rpm, zone temperatures of 200-220°C, and a melt pressure of 12 MPa. Water-cool and cut into particles, then dry at 80°C with hot air for 4 h to obtain a flame-retardant polypropylene composite material. Example 3:

[0039] (1) Put 150 g of ammonium polyphosphate and 2 g of emulsifier OP-10 into 105 g of deionized water and 275 g of anhydrous ethanol, heat to 50°C, adjust the pH to 9.8 with ammonia water, then add 60 g of tetraethyl silicate and 12 g of vinyl triethoxysilane dropwise, stir for 7 h, centrifuge, wash with deionized water and anhydrous ethanol 3 times each, and vacuum dry to obtain ammonium polyphosphate coated with alkenyl silica;

[0040] (2) Mix 74.7 g of triallyl isocyanurate and 40.7 g of dodecanethiol, stir for 40 min, add 1.2 g of photoinitiator 1173 in a light-proof environment, stir for 40 min, and react at room temperature under ultraviolet light (365 nm) for 7 h to obtain alkylated allyl isocyanurate;

[0041] (3) Put 150 g of ammonium polyphosphate coated with alkenyl silica and 150 g of alkylated allyl isocyanurate into 1000 g of methanol and 300 g of deionized water, ultrasonic for 40 min, then add 11 g of N-methylethylenediamine, heat to 52°C, stir for 56 h, centrifuge, wash with methanol 3 times, and vacuum dry to obtain modified ammonium polyphosphate;

[0042] (4) Put 600 g of polypropylene, 140 g of modified ammonium polyphosphate, 12 g of dicumyl peroxide, 6 g of antioxidant 1010, and 4 g of calcium stearate into a mixer, melt blend at 210°C for 12 min, then extrude through a twin-screw extruder with a screw speed of 500 rpm, zone temperatures of 210-230°C, and a melt pressure of 15 MPa. Water-cool and cut into particles, then dry at 85°C with hot air for 5 h to obtain a flame-retardant polypropylene composite material.

[0043] Comparative Example 1:

[0044] The difference between Comparative Example 1 and Example 2 is that the alkylated allyl isocyanurate in step (3) is replaced with an equal molar amount of triallyl isocyanurate;

[0045] The specific steps are as follows:

[0046] (1) 100 g ammonium polyphosphate and 1 g emulsifier OP-10 were added into 75 g deionized water and 225 g anhydrous ethanol, heated to 45℃, adjusted to pH 9.5 with ammonia water, then 41.7 g tetraethyl silicate and 7.6 g vinyl triethoxysilane were added dropwise, stirred for 6 h, centrifuged, washed with deionized water and anhydrous ethanol for 3 times, vacuum dried to obtain allyl silicane-coated ammonium polyphosphate;

[0047] (2) 100 g allyl silicane-coated ammonium polyphosphate and 49.9 g triallyl isocyanurate were added into 600 g methanol and 200 g deionized water, ultrasonic for 30 min, then 7.5 g N-methylethylenediamine was added, heated to 50℃, stirred for 48 h, centrifuged, washed with methanol for 3 times, vacuum dried to obtain modified ammonium polyphosphate;

[0048] (3) 500 g polypropylene, 100 g modified ammonium polyphosphate, 10 g dicumyl peroxide, 4 g antioxidant 1010 and 2.5 g calcium stearate were added into a mixer, melt blended at 200℃ for 10 min, then extruded through a twin-screw extruder, screw speed was 400 rpm, temperature of each zone was 200-220℃, melt pressure was 12 MPa, water-cooled and pelletized, dried at 80℃ for 4 h by hot air to obtain a flame-retardant polypropylene composite material.

[0049] Comparative Example 2:

[0050] The difference between Comparative Example 2 and Example 2 is that N-methylethylenediamine in step (3) is replaced by an equimolar amount of ethylenediamine;

[0051] The specific steps are as follows:

[0052] (1) 100 g ammonium polyphosphate and 1 g emulsifier OP-10 were added into 75 g deionized water and 225 g anhydrous ethanol, heated to 45℃, adjusted to pH 9.5 with ammonia water, then 41.7 g tetraethyl silicate and 7.6 g vinyl triethoxysilane were added dropwise, stirred for 6 h, centrifuged, washed with deionized water and anhydrous ethanol for 3 times, vacuum dried to obtain allyl silicane-coated ammonium polyphosphate;

[0053] (2) 49.9 g triallyl isocyanurate and 40.5 g dodecanethiol were mixed and stirred for 30 min, 0.9 g photoinitiator 1173 was added in a dark environment and stirred for 30 min, then reacted at room temperature for 6 h under ultraviolet light (365 nm) irradiation to obtain alkylated allyl isocyanurate;

[0054] (3) 100 g of allyl silicate-coated ammonium polyphosphate and 90.3 g of alkylated allyl isocyanurate were added into 600 g of methanol and 200 g of deionized water, and ultrasonic treatment was performed for 30 min. Then, 6.1 g of ethylenediamine was added, and the temperature was raised to 50°C. The reaction was stirred for 48 h, centrifuged, washed with methanol for 3 times, and vacuum dried to obtain modified ammonium polyphosphate;

[0055] (4) 500 g of polypropylene, 100 g of modified ammonium polyphosphate, 10 g of dicumyl peroxide, 4 g of antioxidant 1010, and 2.5 g of calcium stearate were added into a mixer, and melt blended at 200°C for 10 min. Then, the mixture was extruded through a twin-screw extruder, with a screw rotation speed of 400 rpm, a temperature of each zone of 200-220°C, and a melt pressure of 12 MPa. The extrudate was water-cooled, cut into particles, and dried by hot air at 80°C for 4 h to obtain a flame-retardant polypropylene composite material.

[0056] Comparative Example 3:

[0057] The difference between Comparative Example 3 and Example 2 is that the modified ammonium polyphosphate in step (4) is replaced by allyl silicate-coated ammonium polyphosphate.

[0058] The specific steps are as follows:

[0059] (1) 100 g of ammonium polyphosphate and 1 g of emulsifier OP-10 were added into 75 g of deionized water and 225 g of anhydrous ethanol, and the temperature was raised to 45°C. The pH was adjusted to 9.5 with ammonia water, and then 41.7 g of tetraethyl silicate and 7.6 g of vinyl triethoxysilane were added dropwise. The reaction was stirred for 6 h, centrifuged, washed with deionized water and anhydrous ethanol for 3 times, and vacuum dried to obtain allyl silicate-coated ammonium polyphosphate.

[0060] (2) 500 g of polypropylene, 100 g of allyl silicate-coated ammonium polyphosphate, 10 g of dicumyl peroxide, 4 g of antioxidant 1010, and 2.5 g of calcium stearate were added into a mixer, and melt blended at 200°C for 10 min. Then, the mixture was extruded through a twin-screw extruder, with a screw rotation speed of 400 rpm, a temperature of each zone of 200-220°C, and a melt pressure of 12 MPa. The extrudate was water-cooled, cut into particles, and dried by hot air at 80°C for 4 h to obtain a flame-retardant polypropylene composite material.

[0061] Comparative Example 4:

[0062] The difference between Comparative Example 4 and Example 2 is that the amount of dodecanethiol in step (2) is adjusted to 81 g.

[0063] The specific steps are as follows:

[0064] (1) 100 g ammonium polyphosphate and 1 g emulsifier OP-10 were added into 75 g deionized water and 225 g anhydrous ethanol, and the temperature was raised to 45 °C. The pH was adjusted to 9.5 with ammonia water, and then 41.7 g tetraethyl silicate and 7.6 g vinyl triethoxysilane were added dropwise. The reaction was stirred for 6 h, centrifuged, washed with deionized water and anhydrous ethanol for 3 times, and vacuum dried to obtain allyl silicane-coated ammonium polyphosphate;

[0065] (2) 49.9 g triallyl isocyanurate and 81 g dodecanethiol were mixed and stirred for 30 min. 0.9 g photoinitiator 1173 was added in a dark environment and stirred for 30 min. The reaction was carried out at room temperature under ultraviolet light (365 nm) for 6 h to obtain alkylated allyl isocyanurate;

[0066] (3) 100 g allyl silicane-coated ammonium polyphosphate and 90.3 g alkylated allyl isocyanurate were added into 600 g methanol and 200 g deionized water, and ultrasonic was performed for 30 min. 7.5 g N-methylethylenediamine was added, and the temperature was raised to 50 °C. The reaction was stirred for 48 h, centrifuged, washed with methanol for 3 times, and vacuum dried to obtain modified ammonium polyphosphate;

[0067] (4) 500 g polypropylene, 100 g modified ammonium polyphosphate, 10 g dicumyl peroxide, 4 g antioxidant 1010, and 2.5 g calcium stearate were added into a mixer, and melt blended at 200 °C for 10 min. Then, the mixture was extruded through a twin-screw extruder at a screw speed of 400 rpm, and the temperature of each zone was 200-220 °C. The melt pressure was 12 MPa. The extrudate was water-cooled and cut into pellets. The pellets were dried by hot air at 80 °C for 4 h to obtain a flame-retardant polypropylene composite material.

[0068] Performance test:

[0069] Sample injection molding: The flame-retardant polypropylene composite materials of Examples 1-3 and Comparative Examples 1-4 were prepared into standard test samples using an injection molding machine at a melt temperature of 220 °C and a holding pressure of 60 MPa.

[0070] Vertical burning test: According to GB / T 2408-2006, the sample was vertically fixed on a clamp, and the lower end of the sample was ignited for 10 seconds by an alcohol lamp flame (20±1 mm in height) aligned with the center. The afterflame time after the flame was removed was recorded.

[0071] Limiting oxygen index test: According to GB / T 2406.2-2009, the sample was vertically loaded into an oxygen index tester cylinder, and a nitrogen-oxygen mixed gas stream (23±2 °C) was introduced from the lower end. The initial oxygen concentration was set to be ±2% of the estimated value, and the oxygen concentration gradient was adjusted to be ±0.2%. The lowest oxygen concentration (LOI value) required for the sample to continue burning for 3 minutes or for the burning length to reach 50 mm was recorded. The results are shown in Table 1.

[0072] Tensile strength and elongation at break test: refer to GB / T 1040.2-2006, the universal testing machine stretched the sample at a speed of 50 mm / min, and the tensile strength and elongation at break were tested, and the results are shown in Table 1.

[0073] Impact strength test: refer to GB / T 1843-2008, the V-shaped notch (depth 2 mm) sample was fixed horizontally on the impact testing machine, the pendulum energy was 7.5 J, the impact speed was 3.5 m / s, the impact strength was tested, and the results are shown in Table 1.

[0074] Table 1 Performance test results

[0075]

[0076] Data analysis:

[0077] According to the data of Example 2, it is shown that the flame-retardant polypropylene composite prepared by the application exhibits a significant improvement in comprehensive performance. The self-extinguishing effect of the flame-retardant function may be due to the three-dimensional cross-linked network formed by the branched molecular chains grafted on the surface of ammonium polyphosphate. This structure promotes the formation of a dense carbon layer under the action of heat. The alkenyl group at the end of the branched molecular chain may participate in the cross-linking reaction when the matrix is melted, enhancing the interfacial bonding force; at the same time, the long alkyl chain structure in the branched chain improves the uniformity of the dispersion of the flame retardant in the polypropylene. This synergistic effect makes the material maintain the mechanical properties while the flame-retardant efficiency is significantly better than the base polymer system.

[0078] Example 2 shows more excellent flame-retardant performance and impact resistance compared to Comparative Example 1. The difference may be due to the regulation of the dodecyl chain on the molecular chain conformation: the flexible alkyl chain in the branched molecular chain reduces the resistance of polymer segment motion, and the interfacial cross-linking of the terminal alkenyl group enhances the stress transfer efficiency. The steric hindrance effect of the alkyl chain may optimize the pore structure of the carbon layer during the expansion process, forming a more effective thermal-oxygen isolation barrier, which can inhibit the dripping and prolong the time of maintaining the integrity of the carbon layer during combustion.

[0079] Example 2 shows a more stable balance of material strength and toughness compared to Comparative Example 2. The technical effect may be attributed to the steric hindrance effect of N-methyl-ethylenediamine, which limits the excessive cross-linking density of the molecular chain, avoiding the sudden change of the cross-linked network rigidity, and the moderate distribution of cross-linking points may form a carbon layer skeleton with toughness and thermal stability, while the alkenyl group at the end of the molecular chain delays the interfacial peeling through the bonding effect with the matrix, which together ensures the structural stability of the material in the thermal-mechanical coupling environment.

[0080] Example 2 exhibits a cross-sectional performance leap compared to Comparative Example 3, which indicates that the branched molecule grafting outside of ammonium polyphosphate is the core technical feature. The branched molecule structure can strengthen the function through two mechanisms: first, the flexible alkyl segment enhances the interface compatibility through molecular chain entanglement, forming a micro-continuous phase; second, the terminal alkenyl group chemically bonds with the polypropylene molecular chain, building "rivet" reinforcing points in the matrix. This composite interface structure not only inhibits the aggregation and migration of the flame retardant, but also induces energy dissipation mechanism through molecular chain slip under impact load.

[0081] Comparing the data of Comparative Example 2 and Comparative Example 4, excessive thiol modification maintains the flame retardancy but sacrifices the impact strength. This phenomenon indicates that the terminal alkenyl group concentration has a threshold effect on the interface performance: a moderate alkyl chain / alkenyl group ratio can form a "rigid-flexible" molecular structure. Excessive alkylation can weaken the crosslinking density of alkenyl groups, resulting in an increase in the spacing between crosslinking network nodes and a decrease in stress transfer efficiency; at the same time, the plasticizing effect of long-chain alkyl groups can reduce the strength of molecular chain entanglement, eventually presenting a characteristic response of the brittle-ductile transition point moving forward under impact load.

[0082] It should be understood by those of ordinary skill in the art that the above discussion of any of the embodiments is merely exemplary and is not intended to suggest that the scope of the present application is limited to these examples; under the idea of the present application, the technical features in the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other changes of different aspects of the present application as described above. In order to be brief, they are not provided in details.

Claims

1. A flame-retardant polypropylene composite, characterized in that, Prepared by the following raw materials: 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 coat ammonium polyphosphate, to obtain allylized silicon dioxide coated ammonium polyphosphate; (2) Triallyl isocyanurate and dodecanethiol are reacted by thiol-ene click reaction at a molar ratio of 1:1, to obtain alkylated allyl isocyanurate; (3) Alkylated allyl isocyanurate, N-methylethylenediamine and allylized silicon dioxide coated ammonium polyphosphate are reacted by addition of amino and olefin, to obtain modified ammonium polyphosphate.

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

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

4. The flame retardant polypropylene composite of claim 1, wherein, The degree of polymerization of ammonium polyphosphate in step (1) is ≥1000.

5. The flame retardant polypropylene composite of claim 1, wherein, The photoinitiator for thiol-ene click reaction in step (2) is photoinitiator 1173.

6. The flame retardant polypropylene composite of claim 1, wherein, The weight ratio of alkylated allyl isocyanurate, N-methylethylenediamine and allylized silicon dioxide coated ammonium polyphosphate in step (3) is 50-150:40-150:3-11.

7. The flame retardant polypropylene composite of claim 1, wherein, The antioxidant is one of antioxidant 1010 and antioxidant 168 or a mixture of the two.

8. The flame retardant polypropylene composite of claim 1, wherein, The lubricant is one of calcium stearate, ethylene bis-stearamide, stearamide and oleic amide or a mixture of several thereof.

9. A process for the preparation of a flame-retardant polypropylene composite 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 mixing machine, melt blending, then extruding through a double screw extruder, water cooling and granulating, hot air drying, to obtain a flame-retardant polypropylene composite material.

10. The process for the preparation of a flame retardant polypropylene composite according to claim 9, characterized in that, The temperature for melt blending is 190-210℃, and the time is 8-12 min.

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