High flame-retardant aramid composite materials and their preparation methods

Abstract

This invention discloses a highly flame-retardant aramid composite material and its preparation method, belonging to the field of polymer materials technology. The method includes: activating aramid fibers with a hydroxyl-containing compound to obtain activated aramid fibers; preparing a polymer by copolymerization with ethylene compounds; modifying nano-silica and aluminum hydroxide with a silane coupling agent to obtain a modified inorganic filler; mixing the polymer and the modified inorganic filler to form a dispersion; impregnating the activated aramid fibers with the dispersion and then drying to obtain the highly flame-retardant aramid composite material. This invention employs a halogen-free and phosphorus-free environmentally friendly composite system composed of modified nano-silica and modified aluminum hydroxide. The silane coupling agent improves the compatibility of the organic-inorganic interface, and the physical barrier, cooling effect, and charring effect synergistically retard the flame, resulting in a limiting oxygen index of up to 40.7% and a UL94 flame retardancy rating of V-0, while maintaining good mechanical properties and long-term service stability. This material overcomes the environmental and safety defects of traditional phosphorus-based and halogen-based flame retardants.

Description

Technical Field

[0001] This invention relates to the field of polymer materials technology, specifically to highly flame-retardant aramid composite materials, and also to a method for preparing highly flame-retardant aramid composite materials. Background Technology

[0002] Aramid fiber, as a high-performance polymer material, has broad application prospects in harsh environments such as energy storage and conversion and aerospace due to its excellent mechanical strength and high-temperature stability. It is one of the key materials that cannot be replaced by aramid fiber.

[0003] However, under extreme high-temperature conditions, the internal chemical bonds of aramid fibers are prone to loosening, leading to a significant decrease in the mechanical properties of the material. This severely limits its reliable application in the aforementioned harsh environments and fails to meet the requirements for structural integrity and long-term service stability under extreme working conditions.

[0004] To address the aforementioned issues, the industry primarily employs two technical approaches to enhance the flame-retardant properties of aramid fibers, thereby delaying their thermal decomposition at high temperatures: one approach is physical blending modification. This method involves directly adding nanoscale or molecular-level flame retardants (such as phosphorus-based, nitrogen-based, and silicon-based flame retardants) to the aramid spinning solution, and then co-spinning them to produce inherently flame-retardant fibers. For example, Chinese invention patent application CN120291265A, entitled "An Aramid Flame-Retardant Fabric and Its Preparation Method," utilizes modified carboxylated cellulose nanocrystals compounded with metal oxides and decabromodiphenyl ethane to form a flame retardant, which is then applied to aramid-related substrates. One approach involves compounding the fibers together, using alkaline treatment to enhance the affinity between the fibers and flame retardants, and reinforcing the fiber surface with nanocrystals to improve the flame retardant effect. Another approach is surface chemical modification, which focuses on post-treatment of the formed aramid fibers. This involves introducing active groups or coatings containing flame-retardant elements such as phosphorus, nitrogen, and silicon onto the surface through surface coatings, chemical grafting, etc., to construct a heat-resistant barrier. For example, Chinese invention patent application CN106883587A, entitled "Aramid Fiber and its Modified Form as Flame Retardant Thermoplastic Plastics," uses acid treatment to graft phosphorus onto the surface of aramid fibers to achieve flame-retardant modification. The core of both methods is to promote the formation of a dense and stable char layer on the fibers at high temperatures, thereby isolating them from heat and oxygen.

[0005] However, both of the above-mentioned technical approaches still have obvious drawbacks: CN120291265A uses decabromodiphenyl ethane and antimony trioxide to form a "halogen-antimony" synergistic system. Although this type of flame retardant has high flame retardant efficiency, it produces toxic and corrosive fumes during combustion, posing a threat to the environment and human health. It is currently being gradually restricted and phased out globally. CN106883587A relies on a phosphorus-nitrogen synergistic flame retardant system, with phosphorus and nitrogen as the core flame retardant elements. Although the system is halogen-free, the phosphorus compounds it contains have significant environmental controversies. Its use will harm the natural environment and human health, making it insufficiently environmentally friendly. Summary of the Invention

[0006] The first objective of this invention is to provide a method for preparing a highly flame-retardant aramid composite material, wherein the composite material prepared by this method retains the mechanical properties of the original aramid and produces no harmful gases or toxic residues after combustion.

[0007] A second objective of this invention is to provide a highly flame-retardant aramid composite material.

[0008] The technical solution adopted in this invention is a method for preparing highly flame-retardant aramid composite materials, comprising the following steps: Step 1: Immerse the aramid fiber in a solution containing hydroxyl groups for activation, and then dry it to obtain activated aramid fiber; Step 2: Copolymerize the ethylene compounds to obtain the polymer; Step 3: Modify nano-silica and aluminum hydroxide respectively using silane coupling agents to obtain modified nano-silica and modified aluminum hydroxide; Step 4: Mix the polymer with modified nano-silica and modified aluminum hydroxide to obtain a dispersion. Then, dip the activated aramid fiber into the dispersion and dry it to obtain a high flame retardant aramid composite material.

[0009] The invention is further characterized by: The compound containing a hydroxyl group is at least one of hydrogen peroxide, ethylene glycol, and sodium hydroxide, and the concentration of its solution is 0.5-3.5 mol / L.

[0010] The copolymerization method is as follows: ethylene compounds and initiators are added to an ether solvent and mixed thoroughly, and then reacted at 40-60℃ for 6-8 hours to obtain the polymer; The ethylene compounds are at least one of hydroxyethyl acrylate, hydroxyethyl methacrylate, styrene, methacrylic acid, or ethylene-acrylic acid copolymer.

[0011] The initiator is at least one of benzoyl peroxide and azobisisobutyronitrile.

[0012] The mass ratio of ethylene compounds to initiators is (10-40):(0.1-1.0).

[0013] The modification method is as follows: add nano-silica or aluminum hydroxide to a solvent and mix evenly, then add a silane coupling agent to react, and finally separate the precipitate and dry it. The mass ratio of silane coupling agent to nano silica is (0.5-2.5):(1-5); the mass ratio of silane coupling agent to aluminum hydroxide is (1-5):(2-6).

[0014] The silane coupling agent is at least one of aminosilane coupling agents, epoxysilane coupling agents, and methacryloxysilane coupling agents; the solvent is at least one of alcohol solvents, ketone solvents, and deionized water.

[0015] The mass ratio of the polymer to the modified nano-silica and the modified aluminum hydroxide is (1-2):(0.5-3):(0.5-3).

[0016] The dipping time is 10-30 minutes.

[0017] The second technical solution adopted in this invention is a highly flame-retardant aramid composite material prepared by the above method.

[0018] The beneficial effects of this invention are: The highly flame-retardant aramid composite material prepared in this invention uses nano-silica as a key flame-retardant component, which forms a dense siliceous heat-insulating and oxygen-barrier layer on the material surface. This siliceous layer is tightly attached to the surface of the aramid fiber substrate, constructing a physical barrier. On the one hand, it effectively blocks the diffusion and transmission of oxygen into the substrate, and on the other hand, it inhibits the outward escape of flammable gases generated by the pyrolysis of the substrate. It suppresses the spread of combustion from the two core aspects of the combustion reaction, namely "oxygen supply" and "fuel release," while significantly reducing smoke emissions. Aluminum hydroxide undergoes an endothermic decomposition reaction under combustion conditions. In addition to reducing the system temperature through heat absorption, its decomposition products further transform into a dense inorganic alumina coating. This inorganic coating and the carbon layer formed by nano-silica constitute a dual barrier structure, further sealing the pores and cracks on the substrate surface, enhancing the barrier effect on heat conduction and gas diffusion, and synergistically improving the overall flame-retardant efficiency. At the same time, the polymer is compounded with inorganic-organic flame retardants, utilizing the synergistic effect between components to improve the quality of the char layer and broaden the flame-retardant temperature range, thereby achieving better results than a single flame-retardant system. To address the common problems of poor interfacial compatibility and insufficient bonding between inorganic fillers and the organic polymer matrix, leading to easy detachment in traditional organic-inorganic composite flame-retardant systems, this invention employs a silane coupling agent to modify the surface of two inorganic flame-retardant fillers: nano-silica and aluminum hydroxide. The trimethoxysilyl group at one end of the coupling agent molecule hydrolyzes to generate silanol groups, which condense with the hydroxyl groups on the filler surface to form covalent bonds. The methacryloyloxy group at the other end is compatible with olefins and can also form chemical bonds with olefins through copolymerization, thereby establishing a strong chemical bond at the interface. This effectively transfers stress, prevents interfacial delamination, and significantly improves the interfacial bonding between the inorganic and organic phases. This ensures uniform and stable coating and dispersion of the flame-retardant filler on the aramid fiber surface, avoiding flame-retardant performance degradation due to filler detachment during use. In addition, the carbon layer and alumina produced after the combustion reaction of nano-silica and aluminum hydroxide are all green and non-toxic inorganic substances, without the generation of harmful gases or toxic residues. This not only meets the safety performance requirements of the material during use, but also complies with the technical standards of "low toxicity and no secondary pollution" for flame retardant materials in the environmental protection field, thus achieving a balance between flame retardant performance, use stability and environmental safety. Attached Figure Description

[0019] Figure 1(a) is a microstructure diagram of the highly flame-retardant aramid composite material prepared in Example 1 of the present invention before combustion; Figure 1(b) is a microstructure diagram of the highly flame-retardant aramid composite material prepared in Example 1 of the present invention after combustion; Figure 2 This is a comparison chart of thermogravimetric analysis of the high flame-retardant aramid composite material prepared in Example 1 of this invention and the original aramid fiber; Figure 3 This is a comparison chart of the mass loss rate of the high flame-retardant aramid composite material prepared in Example 1 of the present invention and the original aramid fiber before and after 2 minutes of combustion. Detailed Implementation

[0020] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0021] The method for preparing a highly flame-retardant aramid composite material provided by this invention includes the following steps: Step 1: Activate aramid fibers with a compound containing hydroxyl groups, and obtain activated aramid fibers after drying; Specifically, aramid fibers are activated by immersing them in a 0.5-3.5 mol / L solution of a hydroxyl-containing compound. This compound includes one of hydrogen peroxide, ethylene glycol, or sodium hydroxide. The activation reaction is carried out at 40-60°C for 3-6 hours, followed by drying at 45-65°C for 10-12 hours. The activated aramid fibers obtained through this treatment have a large number of hydrophilic hydroxyl groups attached to their surface. These hydrophilic hydroxyl groups improve the compatibility of the aramid fiber surface with inorganic and organic materials.

[0022] Step 2: Copolymerize the ethylene compounds to obtain the polymer; Specifically, the copolymerization process involves thoroughly mixing an ethylene compound and an initiator in an ether solvent, purging with nitrogen for deoxygenation, and then conducting a polymerization reaction to obtain the polymer. The mass ratio of the ethylene compound to the initiator is (10-40):(0.1-1.0); the reaction temperature is 40°C. 60℃, reaction time is 6 8h; preferably, the mass ratio of ethylene compound to solvent is 10:(10-40).

[0023] The ethylene compound is at least one of hydroxyethyl acrylate, hydroxyethyl methacrylate, styrene, methacrylic acid, or ethylene-acrylic acid copolymer; the initiator is at least one of benzoyl peroxide or azobisisobutyronitrile; and the solvent is an ether solvent, such as methyl ether, diethyl ether, or tetrahydrofuran.

[0024] Step 3: Modify nano-silica and aluminum hydroxide respectively using silane coupling agents to obtain modified nano-silica and modified aluminum hydroxide; The specific modification method is as follows: Nano-silica or aluminum hydroxide is added to a solvent and mixed evenly, then a silane coupling agent is added and reacted at 80-100℃ for 9-12 hours. After reaction, the precipitate is separated and dried. The silane coupling agent includes one of aminosilane coupling agents, epoxysilane coupling agents, or methacryloxysilane coupling agents. The solvent includes at least one of alcohol solvents, ketone solvents, or deionized water. The drying temperature is 60℃. 80℃, drying time is 10 12h.

[0025] The mass ratio of the silane coupling agent to nano silica is (0.5-2.5):(1-5), and the preferred mass ratio of solvent to silane coupling agent is (20-50):(0.5-2.5); the mass ratio of silane coupling agent to aluminum hydroxide is (1-5):(2-6), and the preferred mass ratio of solvent to silane coupling agent is (10-50):(1-5).

[0026] The interfacial compatibility between nano-silica and aluminum hydroxide modified with silane coupling agent is improved, which can improve the situation where they cannot be uniformly dispersed in organic polymers.

[0027] Step 4: Mix the polymer with modified nano-silica and modified aluminum hydroxide to obtain a dispersion, then dip the activated aramid fiber into the dispersion and dry it to obtain a high flame retardant aramid composite material. Specifically, the polymer prepared in step 2 is uniformly mixed with the modified nano-silica and modified aluminum hydroxide prepared in step 3 at a mass ratio of (1-2):(0.5-3):(0.5-3) to obtain a dispersion. The dispersion is then diluted with a solvent, and activated aramid fibers are immersed in the dispersion for 10-30 minutes. After completion, the fibers are removed and dried to obtain a highly flame-retardant aramid composite material. The solvent used is either an alcohol or an ether solvent; the drying temperature is controlled at 60°C. Dry at 80℃ for 3-5 hours.

[0028] Example 1 The preparation method of the high flame-retardant aramid composite material provided in this embodiment includes the following steps: Step 1: Immerse 10g of para-aramid fiber in a 1mol / L sodium hydroxide solution, react at 40℃ for 3h, then remove and dry at 60℃ for 10h to obtain activated aramid fiber; Step 2: Add 10g styrene and 0.2g benzoyl peroxide to 10g tetrahydrofuran while stirring and mix thoroughly. Then, purge with nitrogen for 20min to remove oxygen and react at 40℃ for 6h. Use diethyl ether as a solvent to precipitate the polymer generated in the reaction, and finally obtain the polymer.

[0029] Step 3: Add 3g of nano-silica to a mixed solution of 25g of anhydrous ethanol and 10g of deionized water and mix well. Then, add 1g of γ-methacryloyloxypropyltrimethoxysilane (KH-570 silane coupling agent) while stirring. After refluxing at 80℃ for 9h, centrifuge three times, discard the supernatant, and dry the precipitate at 60℃ for 10h to obtain modified nano-silica.

[0030] Similarly, 3g of aluminum hydroxide was added to a mixed solution of 20g anhydrous ethanol and 20g deionized water and mixed thoroughly. Then, 1.5g of γ-methacryloyloxypropyltrimethoxysilane was added while stirring. After refluxing at 60°C for 10h, the mixture was centrifuged three times. The supernatant was discarded, and the precipitate was dried at 60°C for 10h to obtain modified aluminum hydroxide.

[0031] Step 4, The polymer prepared in step 2 was uniformly mixed with the modified nano-silica and modified aluminum hydroxide prepared in step 3 at a mass ratio of 1:3:3 to obtain a dispersion. The dispersion was diluted 1:1 with tetrahydrofuran, and then the activated aramid fiber prepared in step 1 was immersed in the diluted dispersion for 10 min. After that, it was dried at 60℃ for 3 h to obtain a high flame retardant aramid composite material.

[0032] Example 2 The preparation method of the high flame-retardant aramid composite material provided in this embodiment includes the following steps: Step 1: Immerse 10g of para-aramid fiber in 0.5mol / L sodium hydroxide solution, react at 45℃ for 4h, and then dry at 50℃ for 9h to obtain activated aramid fiber. Step 2: Add 10g styrene and 0.15g benzoyl peroxide to 12.5g tetrahydrofuran while stirring until well mixed. Then, purge with nitrogen for 20 minutes to remove oxygen and react at 45°C for 7 hours. Use diethyl ether as a solvent to precipitate the polymer generated in the reaction, and finally obtain the polymer.

[0033] Step 3: Add 3g of nano-silica to a mixture of 20g of anhydrous ethanol and 10g of deionized water and mix well. Then, add 0.8g of γ-methacryloyloxypropyltrimethoxysilane (KH-570 silane coupling agent) while stirring. After refluxing at 85°C for 9.5h, centrifuge three times, discard the supernatant, and dry the precipitate at 55°C for 9h to obtain modified nano-silica.

[0034] Similarly, 3g of aluminum hydroxide was added to a mixture of 25g of anhydrous ethanol and 20g of deionized water and mixed thoroughly. Then, 1.2g of γ-methacryloyloxypropyltrimethoxysilane was added while stirring. After refluxing at 85°C for 11 hours, the mixture was centrifuged three times. The supernatant was discarded, and the precipitate was dried at 55°C for 10.5 hours to obtain modified aluminum hydroxide.

[0035] Step 4: The polymer prepared in Step 2 is mixed with the modified nano-silica and modified aluminum hydroxide prepared in Step 3 at a mass ratio of 1:2:1 to obtain a dispersion. The dispersion is diluted 1:1 with tetrahydrofuran, and then the activated aramid fiber prepared in Step 1 is immersed in the diluted dispersion for 20 min. After immersion, it is dried at 65°C for 4 h to obtain the high flame retardant aramid composite material of the present invention.

[0036] Example 3 The preparation method of the high flame-retardant aramid composite material provided in this embodiment includes the following steps: Step 1: Immerse 10g of para-aramid fiber in a 1.5mol / L sodium hydroxide solution, react at 48℃ for 3.5h, then remove and dry at 55℃ for 9.5h to obtain activated aramid fiber; Step 2: Add 10g styrene and 0.18g benzoyl peroxide to 13g tetrahydrofuran while stirring and mix thoroughly. Then, purge with nitrogen for 20min to remove oxygen and react at 50℃ for 6.5h. Use diethyl ether as a solvent to precipitate the polymer generated in the reaction, and finally obtain the polymer.

[0037] Step 3: Add 3g of nano-silica to a mixture of 30g of anhydrous ethanol and 12g of deionized water and mix well. Then, add 0.9g of γ-methacryloyloxypropyltrimethoxysilane (KH-570 silane coupling agent) while stirring. After refluxing at 88℃ for 10h, centrifuge three times, discard the supernatant, and dry the precipitate at 56℃ for 9.5h to obtain modified nano-silica.

[0038] Similarly, 3g of aluminum hydroxide was added to a mixture of 30g of anhydrous ethanol and 27g of deionized water and mixed thoroughly. Then, 1.8g of γ-methacryloyloxypropyltrimethoxysilane was added while stirring. After refluxing at 90℃ for 10.5h, the mixture was centrifuged three times. The supernatant was discarded, and the precipitate was dried at 54℃ for 11h to obtain modified aluminum hydroxide.

[0039] Step 4: The polymer prepared in Step 2 is mixed with the modified nano-silica and modified aluminum hydroxide prepared in Step 3 at a mass ratio of 1:1:2 to obtain a dispersion. The dispersion is diluted 1:1 with tetrahydrofuran. The activated aramid fiber prepared in Step 1 is then immersed in the diluted dispersion for 30 min. After immersion, it is dried at 70°C for 3.5 h to obtain the high flame retardant aramid composite material of the present invention.

[0040] Example 4 The preparation method of the high flame-retardant aramid composite material provided in this embodiment includes the following steps: Step 1: Immerse 10g of para-aramid fiber in a 3mol / L sodium hydroxide solution, react at 42℃ for 5h, and then dry at 50℃ for 11.5h to obtain activated aramid fiber; Step 2: Add 10g styrene and 0.23g benzoyl peroxide to 11g tetrahydrofuran while stirring and mix thoroughly. Then, purge with nitrogen for 20min to remove oxygen and react at 55℃ for 7.5h. Finally, use diethyl ether as a solvent to precipitate the polymer generated in the reaction to obtain the final polymer.

[0041] Step 3: Add 3g of nano-silica to a mixture of 27g of anhydrous ethanol and 20g of deionized water and mix well. Then, add 1.2g of γ-methacryloyloxypropyltrimethoxysilane (KH-570 silane coupling agent) while stirring. After refluxing at 95℃ for 10h, centrifuge three times, discard the supernatant, and dry the precipitate at 60℃ for 10.5h to obtain modified nano-silica.

[0042] Similarly, 3g of aluminum hydroxide was added to a mixed solution of 12g of anhydrous ethanol and 34g of deionized water and mixed evenly. Then, 2.2g of γ-methacryloyloxypropyltrimethoxysilane was added while stirring. After refluxing at 95°C for 11 hours, the mixture was centrifuged three times. The supernatant was discarded, and the precipitate was dried at 75°C for 10 hours to obtain modified aluminum hydroxide.

[0043] Step 4: The polymer prepared in Step 2 is mixed with the modified nano-silica and modified aluminum hydroxide prepared in Step 3 at a mass ratio of 1:2:2 to obtain a dispersion. The dispersion is diluted with tetrahydrofuran at a ratio of 1:1. The activated aramid fiber prepared in Step 1 is then immersed in the diluted dispersion for 15 minutes. After immersion, it is dried at 58°C for 5 hours to obtain the high flame retardant aramid composite material of the present invention.

[0044] Example 5 Step 1: Immerse 10g of para-aramid fiber in a 3.5mol / L hydrogen peroxide solution, react at 60℃ for 6h, and then dry at 60℃ for 10h to obtain activated aramid fiber. Step 2: 10g of hydroxyethyl acrylate and 1.0g of azobisisobutyronitrile are added to 40g of dimethyl ether while stirring and mixed evenly. After purging with nitrogen for 10min to remove oxygen, the mixture is reacted at 60℃ for 8h. Then, diethyl ether is used as a solvent to precipitate the polymer generated in the reaction, and finally the polymer is obtained.

[0045] Step 3: Add 1g of nano-silica to 20g of acetone and mix well. Then add 0.5g of aminosilane coupling agent while stirring. After refluxing at 100℃ for 12h, centrifuge three times, discard the supernatant, and dry the precipitate at 60℃ for 10h to obtain modified nano-silica.

[0046] Similarly, 2g of aluminum hydroxide was added to 20g of acetone and mixed evenly. Then, 3g of aminosilane coupling agent was added while stirring. After refluxing at 60℃ for 10h, the mixture was centrifuged three times. The supernatant was discarded, and the precipitate was dried at 80℃ for 12h to obtain modified aluminum hydroxide.

[0047] Step 4: The polymer prepared in Step 2 is mixed with the modified nano-silica and modified aluminum hydroxide prepared in Step 3 at a mass ratio of 2:0.5:0.5 to obtain a dispersion. The dispersion is diluted 1:1 with tetrahydrofuran. The activated aramid fiber prepared in Step 1 is then immersed in the diluted dispersion for 20 min. After immersion, it is dried at 80℃ for 3 h to obtain a high flame retardant aramid composite material.

[0048] Example 6 Step 1: 10g of para-aramid fiber was immersed in a 2mol / L ethylene glycol solution and reacted at 50℃ for 6h. After that, it was taken out and dried at 60℃ for 10h to obtain activated aramid fiber. Step 2: Add 4g of methacrylic acid and 0.5g of azobisisobutyronitrile to 10g of diethyl ether while stirring and mix well. Then, purge with nitrogen for 30 minutes to remove oxygen and react at 60°C for 8 hours. Use diethyl ether as a solvent to precipitate the polymer generated in the reaction, and finally obtain the polymer.

[0049] Step 3: Add 5g of nano-silica to 50g of deionized water and mix well. Then, add 2.5g of epoxy silane coupling agent (KH-560) while stirring. After refluxing at 100℃ for 12h, centrifuge three times, discard the supernatant, and dry the precipitate at 60℃ for 10h to obtain modified nano-silica.

[0050] Similarly, 6g of aluminum hydroxide was added to 50g of deionized water and mixed evenly. Then, 5g of KH-560 was added while stirring. After refluxing at 60℃ for 10h, the mixture was centrifuged three times. The supernatant was discarded, and the precipitate was dried at 80℃ for 12h to obtain modified aluminum hydroxide.

[0051] Step 4: The polymer prepared in Step 2 is mixed with the modified nano-silica and modified aluminum hydroxide prepared in Step 3 at a mass ratio of 1.5:1.5:2.5 to obtain a dispersion. The dispersion is diluted 1:1 with tetrahydrofuran. The activated aramid fiber prepared in Step 1 is then immersed in the diluted dispersion for 20 min. After immersion, it is dried at 80℃ for 3 h to obtain a high flame retardant aramid composite material.

[0052] Comparative Example 1 The preparation method of the high flame-retardant aramid composite material provided in this comparative example includes the following steps: Step 1: Immerse 10g of para-aramid fiber in a 1mol / L sodium hydroxide solution, react at 40℃ for 3h, then remove and dry at 60℃ for 10h to obtain activated aramid fiber; Step 2: Add 10g styrene and 0.2g benzoyl peroxide to 10g tetrahydrofuran while stirring and mix thoroughly. Then, purge with nitrogen for 20min to remove oxygen and react at 40℃ for 6h. Use diethyl ether as a solvent to precipitate the polymer generated in the reaction, and finally obtain the polymer.

[0053] Step 3: Add 3g of nano-silica to a mixed solution of 25g of anhydrous ethanol and 10g of deionized water and mix well. Then, add 1g of γ-methacryloyloxypropyltrimethoxysilane (KH-570 silane coupling agent) while stirring. After refluxing at 80℃ for 9h, centrifuge three times, discard the supernatant, and dry the precipitate at 60℃ for 10h to obtain modified nano-silica.

[0054] Step 4: The polymer prepared in Step 2 and the modified nano-silica prepared in Step 3 are uniformly mixed at a mass ratio of 1:3 to obtain a dispersion. The dispersion is diluted 1:1 with tetrahydrofuran, and then the activated aramid fiber prepared in Step 1 is immersed in the diluted dispersion for 10 min. After immersion, it is dried at 60°C for 3 h to obtain the high flame retardant aramid composite material of the present invention.

[0055] Comparative Example 2 The preparation method of the high flame-retardant aramid composite material provided in this comparative example includes the following steps: Step 1: Immerse 10g of para-aramid fiber in a 1mol / L sodium hydroxide solution, react at 40℃ for 3h, and then dry at 60℃ for 10h to obtain activated aramid fiber. Step 2: Add 10g styrene and 0.2g benzoyl peroxide to 10g tetrahydrofuran while stirring and mix thoroughly. Then, purge with nitrogen for 20min to remove oxygen and react at 40℃ for 6h. Use diethyl ether as a solvent to precipitate the polymer generated in the reaction, and finally obtain the polymer.

[0056] Step 3: Add 3g of aluminum hydroxide to a mixed solution of 20g of anhydrous ethanol and 20g of deionized water and mix well. Then, while stirring, add 1.5g of γ-methacryloyloxypropyltrimethoxysilane (KH-570 silane coupling agent). After refluxing at 60°C for 10h, centrifuge three times, discard the supernatant, and dry the precipitate at 60°C for 10h to obtain modified aluminum hydroxide.

[0057] Step 4: The polymer prepared in Step 2 and the modified aluminum hydroxide prepared in Step 3 are mixed uniformly at a mass ratio of 1:3 to obtain a dispersion. The dispersion is diluted 1:1 with tetrahydrofuran, and then the activated aramid fiber prepared in Step 1 is immersed in the diluted dispersion for 10 min. After immersion, it is dried at 60°C for 3 h to obtain the high flame retardant aramid composite material of the present invention.

[0058] Comparative Example 3 The preparation method of the high flame-retardant aramid composite material provided in this comparative example includes the following steps: Step 1: Immerse 10g of para-aramid fiber in a 1mol / L sodium hydroxide solution, react at 40℃ for 3h, and then dry at 60℃ for 10h to obtain activated aramid fiber. Step 2: Add 10g styrene and 0.2g benzoyl peroxide to 10g tetrahydrofuran while stirring and mix thoroughly. Then, purge with nitrogen for 20min to remove oxygen and react at 40℃ for 6h. Use diethyl ether as a solvent to precipitate the polymer generated in the reaction, and finally obtain the polymer.

[0059] Step 3: Dilute the polymer prepared in Step 2 with tetrahydrofuran at a ratio of 1:1, then dip the activated aramid fiber prepared in Step 1 into the diluted polymer for 10 min, remove it and dry it at 60°C for 3 h to obtain the high flame retardant aramid composite material of the present invention.

[0060] Comparative Example 4 The preparation method of the high flame-retardant aramid composite material provided in this comparative example includes the following steps: Step 1: Immerse 10g of para-aramid fiber in a 1mol / L sodium hydroxide solution, react at 40℃ for 3h, and then dry at 60℃ for 10h to obtain activated aramid fiber. Step 2: Add 10g styrene and 0.2g benzoyl peroxide to 10g tetrahydrofuran while stirring and mix thoroughly. Then, purge with nitrogen for 20min to remove oxygen and react at 40℃ for 6h. Use diethyl ether as a solvent to precipitate the polymer generated in the reaction, and finally obtain the polymer.

[0061] Step 3: The polymer prepared in Step 2 is uniformly mixed with unmodified nano-silica and unmodified aluminum hydroxide at a mass ratio of 1:3:3 to obtain a dispersion. The dispersion is diluted 1:1 with tetrahydrofuran, and then the activated aramid fiber prepared in Step 1 is immersed in the diluted dispersion for 10 min. After that, it is dried at 60°C for 3 h to obtain the high flame retardant aramid composite material of the present invention.

[0062] Flame retardant properties of the high flame retardant aramid composite materials prepared in Examples 1-4 and Comparative Examples 1-4 were tested using 100mm × 10mm (length × width) specimens. Vertical burning tests were performed according to GB / T 17591-2006, limiting oxygen index tests were performed according to GB / T5454-1997, tensile strength tests were performed according to ASTM D3039 / D3039M, and interfacial shear strength tests were performed according to ASTM D7334. Specific data are shown in Table 1.

[0063] Table 1 Flame Retardant Performance Table

[0064] The original para-aramid fiber had a UL94 flame retardancy rating of V-0, a limiting oxygen index (LOI) of 30.3%, a tensile strength of 3.15 GPa, and an interfacial shear strength of 28.8 MPa. According to Table 1 and the performance data of the original aramid fiber, the high flame retardant aramid composite material provided by this invention achieves a significant improvement in flame retardant performance: the original aramid fiber had a limiting oxygen index (LOI) of 30.3%, while the composite materials prepared in Examples 1-4 all had LOIs between 36.8% and 40.7%, reaching a maximum of 40.7%. Furthermore, all examples maintained a UL94 V-0 flame retardancy rating, far superior to Comparative Examples 1 and 2 containing only a single inorganic flame retardant filler, and Comparative Example 3 without inorganic filler. This fully demonstrates that the composite system of "polymer + modified nano-silica + modified aluminum hydroxide" used in this invention has excellent flame retardant effects. Meanwhile, the compatibility of the organic-inorganic system was significantly improved after modification: Examples 1-4 modified with silane coupling agents showed interfacial shear strengths of 33.6 MPa-50.4 MPa, far exceeding the 28.8 MPa of the original aramid fiber, and significantly better than Comparative Example 4 using unmodified inorganic fillers. This indicates that the modification treatment not only solved the problem of easy detachment of inorganic fillers, but also further optimized the flame retardant performance. In addition, the high flame retardant aramid composite material prepared by this invention also showed good mechanical properties: the tensile strength of Examples 1-4 was 2.51 GPa-2.79 GPa, slightly lower than the 3.15 GPa of the original aramid fiber, but significantly higher than Comparative Example 4, and the interfacial shear strength was comprehensively superior to that of the original aramid fiber. This shows that while achieving high flame retardancy and environmental friendliness, this invention maximized the preservation and optimization of the material's mechanical properties, achieving a unity of flame retardant performance, service stability, and mechanical properties.

[0065] Furthermore, a comparison of the microstructures in Figures 1(a) and 1(b) reveals that the nano-silica and aluminum hydroxide modified with the silane coupling agent KH570 achieve a uniform and dense coating dispersion on the surface of the aramid fiber. This is because, firstly, the silane coupling agent modifies the surface of the inorganic filler, improving its compatibility with organic polymers; secondly, the activation treatment of the aramid fiber with hydroxyl compounds forms a large number of hydrophilic hydroxyl groups on the fiber surface, promoting the grafting bonding between the coating and the fiber, effectively avoiding the problems of inorganic filler agglomeration and uneven dispersion in traditional systems. Simultaneously, after combustion, the fiber skeleton of the high flame-retardant aramid composite material retains its complete strip or filamentous morphology, with a large amount of particulate and flocculent combustion residues adhering to the surface. These residues are inorganic compounds (such as aluminum oxide) and incompletely carbonized substances generated during combustion, proving that the composite flame-retardant system can form a continuous and stable protective layer during combustion, blocking the transfer of heat and oxygen while supporting the fiber structure, preventing material disintegration due to combustion, further confirming the effectiveness of the flame-retardant mechanism.

[0066] Figure 2 By comparing the thermogravimetric (TG) curves of the highly flame-retardant aramid fiber prepared in Example 1 of this invention with those of the original aramid fiber, it was found that the highly flame-retardant aramid fiber exhibits significantly improved thermal stability and char-forming properties compared to the original aramid fiber. The original aramid fiber begins to enter the main decomposition stage at approximately 500℃, while the highly flame-retardant aramid fiber has a higher initial decomposition temperature for the aramid segment, around 550℃, and a higher mass retention rate in the high-temperature range of 600-800℃ compared to the original aramid fiber. This indicates that the synergistic flame-retardant system of modified aluminum hydroxide and modified nano-silica effectively promotes the formation of a dense and stable char layer, inhibits the thermal decomposition of polymer chains and the release of combustible volatiles, thereby significantly improving the flame retardancy and thermo-oxidative stability of the material.

[0067] Figure 3 By comparing the mass changes of the highly flame-retardant aramid fiber prepared in Example 1 of this invention with those of the original aramid fiber before and after 2 minutes of combustion, it was found that the highly flame-retardant aramid fiber exhibited significantly enhanced thermal stability and flame-retardant properties. Data showed that the mass loss rate of the original aramid fiber after combustion was 83.33%, while the mass loss rate of the highly flame-retardant aramid fiber was only 47.98%. These results indicate that the modified aluminum hydroxide and modified nano-silica in the highly flame-retardant system effectively promoted the formation of a dense char layer during combustion. Simultaneously, the polymer, in synergy with the flame-retardant filler, transformed from a flammable component into an effective char precursor, which, together with the former two, promoted the formation of a dense and stable char layer, thereby strengthening the condensed-phase flame-retardant mechanism and significantly improving the flame-retardant properties and thermo-oxidative stability of the material.

[0068] This invention creates a halogen-free and phosphorus-free flame-retardant system by coating and dispersing nano-silica and aluminum hydroxide onto raw aramid fibers. This method minimizes damage to the aramid fiber itself and eliminates phosphorus- and halogen-based flame retardants. The nano-silica and aluminum hydroxide, upon combustion, produce green and non-toxic inorganic products such as a carbon layer and alumina, without generating harmful gases or toxic residues. This meets both material safety requirements and environmental standards for "low toxicity and no secondary pollution." By modifying the nano-silica and aluminum hydroxide with a silane coupling agent, the two inorganic flame-retardant fillers are uniformly and stably coated onto the aramid fiber surface. One end of the silane coupling agent molecule hydrolyzes to generate silanol groups, which condense with the hydroxyl groups on the inorganic filler surface to form covalent bonds. The other end forms chemical bonds with the polymer through a copolymerization reaction, establishing a strong chemical connection at the interface. This effectively prevents interfacial delamination and avoids flame-retardant performance degradation due to filler detachment during use, significantly improving the long-term service stability of the material.

Claims

1. A method for preparing a highly flame-retardant aramid composite material, characterized in that, Includes the following steps: Step 1: Immerse the aramid fiber in a solution containing hydroxyl groups for activation, and then dry it to obtain activated aramid fiber; Step 2: Copolymerize the ethylene compounds to obtain the polymer; Step 3: Modify nano-silica and aluminum hydroxide respectively using silane coupling agents to obtain modified nano-silica and modified aluminum hydroxide; Step 4: Mix the polymer with modified nano-silica and modified aluminum hydroxide to obtain a dispersion. Then, dip the activated aramid fiber into the dispersion and dry it to obtain a high flame retardant aramid composite material.

2. The method for preparing the high flame-retardant aramid composite material according to claim 1, characterized in that, The hydroxyl-containing compound is at least one of hydrogen peroxide, ethylene glycol, and sodium hydroxide, and the concentration of its solution is 0.5-3.5 mol / L.

3. The method for preparing the high flame-retardant aramid composite material according to claim 1, characterized in that, The copolymerization method is as follows: ethylene compounds and initiators are added to an ether solvent and mixed thoroughly, and then reacted at 40-60℃ for 6-8 hours to obtain the polymer; The ethylene compounds are at least one of hydroxyethyl acrylate, hydroxyethyl methacrylate, styrene, methacrylic acid, or ethylene-acrylic acid copolymer.

4. The method for preparing the high flame-retardant aramid composite material according to claim 3, characterized in that, The initiator is at least one of benzoyl peroxide and azobisisobutyronitrile.

5. The method for preparing the high flame-retardant aramid composite material according to claim 3, characterized in that, The mass ratio of the ethylene compound to the initiator is (10-40):(0.1-1.0).

6. The method for preparing the high flame-retardant aramid composite material according to claim 1, characterized in that, The modification method is as follows: nano-silica or aluminum hydroxide is added to a solvent and mixed evenly, then a silane coupling agent is added to carry out the reaction, and the precipitate is separated and dried after the reaction is completed. The mass ratio of silane coupling agent to nano silica is (0.5-2.5):(1-5); the mass ratio of silane coupling agent to aluminum hydroxide is (1-5):(2-6).

7. The method for preparing the high flame-retardant aramid composite material according to claim 6, characterized in that, The silane coupling agent is at least one of aminosilane coupling agents, epoxysilane coupling agents, and methacryloxysilane coupling agents; the solvent is at least one of alcohol solvents, ketone solvents, and deionized water.

8. The method for preparing the high flame-retardant aramid composite material according to claim 1, characterized in that, The mass ratio of the polymer to the modified nano-silica and the modified aluminum hydroxide is (1-2):(0.5-3):(0.5-3).

9. The method for preparing the high flame-retardant aramid composite material according to claim 1, characterized in that, The dipping time is 10-30 minutes.

10. A highly flame-retardant aramid composite material prepared using the preparation method of any one of the highly flame-retardant aramid composite materials according to claims 1-9.

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