Preparation and application of environment-friendly P, N and Si synergistic flame retardant

Abstract

The invention discloses preparation and application of an environment-friendly P, N and Si synergistic flame retardant. According to the flame retardant (KDOPO-LDH (at) GO), a plurality of flame-retardant elements such as P, N and Si are integrated, so that an efficient synergistic flame-retardant system is constructed. Wherein the existence of the N element and the Si element effectively promotes the uniform dispersion of the flame retardant in an epoxy resin matrix; meanwhile, the unique molecular structure of the P-containing component endows the carbon-forming agent with excellent carbon-forming performance under a high-temperature condition. Through the synergistic effect of multiple elements, the flame retardant property of the epoxy resin is remarkably improved, and meanwhile, the good mechanical property of the material is effectively maintained. In addition, halogen-free and low-toxicity substances are mainly generated in the pyrolysis process of the flame retardant, and the green chemical concept is met.

Description

Technical Field

[0001] This invention belongs to the fields of flame retardant technology and polymer material technology, and relates to a method for preparing an environmentally friendly P, N, Si synergistic flame retardant and its application in epoxy resin materials. Background Technology

[0002] Epoxy resin (EP), as a high-performance polymer material, is widely used in aerospace, electronics, and construction. However, its limiting oxygen index (LOI) is only about 19.8%, making it a highly flammable material, and there is an urgent need for efficient flame-retardant modification. Although traditional halogenated flame retardants can improve the flame retardancy of EP, they release toxic gases and corrosive fumes during combustion, which contradicts the trend of green chemistry development.

[0003] Currently, research on environmentally friendly flame retardant systems focuses on two main directions: 1. Halogen-free element-synergistic flame retardants: such as phosphorus-nitrogen (PN) systems (e.g., melamine pyrophosphate) or phosphorus-silicon (P-Si) systems (e.g., the macromolecular flame retardant containing P / Si / N / S described in CN106366670A). These flame retardants achieve expansion flame retardancy through multiphase synergy of acid, char, and gas sources, but have significant drawbacks: Insufficient thermal stability: For example, the initial decomposition temperature of the macromolecular flame retardant CN106366670A is only 260-350℃, and it is prone to premature decomposition during EP high-temperature curing (120-140℃) and processing. Poor compatibility: The linear macromolecular structure is difficult to disperse uniformly in the EP matrix, resulting in a significant decrease in mechanical properties (tensile strength loss >15%).

[0004] 2. Nano-reinforced flame retardants: Two-dimensional materials, such as layered hydrogen hydroxide (LDH) and graphene oxide (GO), can suppress heat / mass transfer through a "physical barrier effect." However: Severe tendency to aggregate: LDH / GO is prone to aggregation due to its high specific surface area and van der Waals forces (as CN117866293A points out, "the inherent poor dispersibility of two-dimensional materials restricts practical applications"). Flame retardant efficiency is limited: pure LDH or GO mainly relies on condensed phase barrier and lacks gas phase free radical quenching ability, resulting in limited improvement in limiting oxygen index (typically LOI < 30%).

[0005] To overcome the above-mentioned shortcomings, existing technologies attempt to combine LDH / GO with phosphorus-based flame retardants, for example: CN117866293A uses a physical coating method to atomize and deposit organosilicon treatment agents onto the surface of phosphorus and nitrogen powders, thereby improving processing fluidity; CN106366670A constructs a P-Si-NS quaternary synergistic structure through intramolecular condensation polymerization.

[0006] However, these solutions still have fundamental limitations: Physically compounded type (e.g. CN117866293A): Organosilicon is only used as a surface coating and does not form a chemical bond with the flame retardant component, making it easy to peel off at the interface in the EP matrix; Single molecular type (such as CN106366670A): lacks carrier support, molecular chains are prone to breakage at high temperatures, and the problem of nano-dispersion cannot be solved.

[0007] In summary, developing a flame retardant specifically for EP that possesses the following properties still faces challenges: (1) Achieving molecular-level synergy of P / N / Si multi-element through chemical bonding; (2) Inhibit LDH / GO aggregation and improve its dispersion stability in EP; (3) Balance high flame retardancy (LOI≥35%) with mechanical property retention rate (bending strength loss<10%). Summary of the Invention

[0008] To address the problems existing in the above-mentioned background technology, the present invention provides a method for preparing an environmentally friendly P, N, Si synergistic flame retardant, achieving: 1. Environmentally friendly characteristics: The pyrolysis products of the composite material are low-toxicity and halogen-free substances, meeting the requirements of green chemistry; 2. Synergistic flame retardancy of gas phase / condensed phase: free radical quenching of chain reaction + decomposition to produce inert gas + catalytic char formation; 3. Flame retardancy-mechanical property balance: Through multi-component synergy, while improving the limiting oxygen index and reducing the heat release rate, the tensile toughness of epoxy resin is maintained.

[0009] This invention constructs a synergistic effect of N, P, and Si multi-flame retardant elements by covalently linking KH550-modified DOPO (9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide) with LDH@GO. Specifically, it includes: (1) Preparation of layered double hydroxide LDH: Dissolve NaOH and Na2CO3 in deionized water at a molar ratio of 0.8:1 to 1:1 to form solution A; Ni(NO3)2·6H2O, Mg(NO3)2·6H2O and Al(NO3)3·9H2O are dissolved in deionized water in a molar ratio of Ni:Mg:Al=1:2:1 to form solution B; Under pH 9-10 conditions, solutions A and B were co-precipitated, crystallized at 60-70℃ for 15-20 h, and then washed and dried to obtain LDH. (2) Preparation of graphene oxide (GO): In an ice bath at 2-6℃, H3PO4 and concentrated H2SO4 were mixed at a volume ratio of 1:8 to 1:10. Graphite sheets were added, and KMnO4 with a mass of 7-8 times that of the graphite sheets was added in batches. The mixture was reacted at 45-55℃ for 8-12 hours and then cooled. H2O2 was added until the solution turned golden yellow and no more bubbles were produced. The solution was washed with hydrochloric acid and deionized water until pH=7. The solution was then freeze-dried to obtain GO. (3) Preparation of LDH@GO composite carrier: Solution A is obtained by dispersing GO and citric acid in deionized water at a mass ratio of 3-5:1 and then sonicating. Dissolve LDH in water to obtain solution B; Solution B was added dropwise to solution A, and the mixture was stirred for 1 hour while maintaining pH = 10 ± 0.2. The mixture was then transferred to a reaction vessel and reacted at 80°C for 20-25 hours. After washing with water until neutral, the mixture was freeze-dried to obtain LDH@GO. (4) Preparation of silane-modified flame retardant monomer KDOPO: Disperse KH550 and terephthalaldehyde in ethanol at a molar ratio of 1:2 to 2:1 and stir at 75 to 85°C for 4 to 8 hours to obtain solution A; Disperse DOPO in ethanol to obtain solution B; Solution B was added dropwise to solution A at a molar ratio of KH550:DOPO = 1:1.5~1.7, and the reaction was carried out at 100℃ for 10~12 h. After washing and drying, KDOPO was obtained. (5) Preparation of KDOPO-LDH@GO flame retardant: LDH@GO was dispersed in a 1:1 volume ratio ethanol / water mixture, KDOPO was added, and the mixture was reacted at 70-80℃ for 8-10 h under N2 atmosphere. After washing until neutral, the mixture was dried to obtain KDOPO-LDH@GO. The mass ratio of KDOPO to LDH@GO was 0.8:1 to 1.2:1.

[0010] Another object of the present invention is to provide the application of the flame retardant in epoxy resins.

[0011] The flame retardant is dispersed in ethanol at a total weight of 10-15 wt% of the mixture; Add the fluid epoxy resin and mix at 65~75℃. After vacuum solvent removal, the product is injected into the mold and pre-cured at 75~85℃ for 0.5~1h, and then finally cured at 120~140℃ for 6~8h.

[0012] Compared with existing technologies, this invention utilizes the hydrolysis of ethoxy groups in KDOPO molecules to form Si-O covalent bonds with hydroxyl groups on the LDH@GO surface (Si-O-LDH@GO), constructing a synergistic flame-retardant system integrating nitrogen (N), phosphorus (P), and silicon (Si). This system not only significantly improves the flame-retardant properties of the material but also effectively addresses the problems of poor compatibility and easy aggregation of two-dimensional materials in the matrix through covalent bonding. Furthermore, the combustion decomposition products of the composite material are environmentally friendly substances, meeting the core requirements of green chemistry. Attached Figure Description

[0013] Figure 1 These are the infrared spectra of DOPO and KDOPO, KDOPO-LDH, and KDOPO-LDH@GO prepared according to the embodiments of the present invention.

[0014] Figure 2 These are XRD patterns of DOPO and KDOPO, KDOPO-LDH, and KDOPO-LDH@GO prepared according to embodiments of the present invention.

[0015] Figure 3 This is the XPS image of KDOPO-LDH@GO prepared according to an embodiment of the present invention. Where a is the full spectrum, b is Si 2p, c is P 2p, and d is N 1s.

[0016] Figure 4 These are SEM and EDS images of DOPO and KDOPO, KDOPO-LDH, and KDOPO-LDH@GO prepared according to the embodiments of the present invention.

[0017] Figure 5 The thermogravimetric (TG) test curves of pure EP and DOPO / EP, KDOPO / EP, KDOPO-LDH / EP and KDOPO-LDH@GO / EP prepared in the embodiments of the present invention are shown in air atmosphere.

[0018] Figure 6 These are vertical combustion test diagrams of pure EP and 10DOPO / EP, 10KDOPO / EP, 10KDOPO-LDH / EP, 10KDOPO-LDH@GO / EP, and 15KDOPO-LDH@GO / EP prepared according to embodiments of the present invention.

[0019] Figure 7 These are cone calorimetric test data for pure EP and 10DOPO / EP, 10KDOPO / EP, 10KDOPO-LDH / EP, 10KDOPO-LDH@GO / EP, and 15KDOPO-LDH@GO / EP prepared according to embodiments of the present invention.

[0020] Figure 8These are digital photographs and microscopic morphology images of carbon layers of pure EP and 10DOPO / EP, 10KDOPO / EP, 10KDOPO-LDH / EP, 10KDOPO-LDH@GO / EP, and 15KDOPO-LDH@GO / EP prepared according to embodiments of the present invention.

[0021] Figure 9 These are mechanical property diagrams of pure EP and 10DOPO / EP, 10KDOPO / EP, 10KDOPO-LDH / EP, 10KDOPO-LDH@GO / EP, and 15KDOPO-LDH@GO / EP prepared according to examples of this invention. Detailed Implementation

[0022] The present invention will be further explained and described below with reference to specific embodiments.

[0023] Example 1 EP was placed in an oven and heated at 80°C to obtain fluidized EP. 4g of DOPO was dispersed in 15mL of anhydrous ethanol and placed in a water bath at 70°C. The mixture was stirred, and then 30mL of fluidized EP was added. The mixture was stirred thoroughly to form a homogeneous solution. The anhydrous ethanol in the homogeneous solution was removed by a vacuum filter until no obvious bubbles appeared. The resulting mixture was poured into a preheated mold, transferred to an oven, and cured at 80°C for 30 min. The temperature was then raised to 130°C and held for 8 h to obtain the intrinsic flame retardant material 10DOPO / EP.

[0024] Example 2 11.68 mL of KH550 and 6.7 g of terephthalaldehyde were dispersed in 100 mL of ethanol and reacted at 78 °C for 6 h to obtain solution A; 17.28 g of DOPO was dispersed in 10 mL of ethanol to obtain solution B; solution B was slowly added dropwise to solution A, and the temperature was raised to 100 °C and reacted for 12 h to obtain the target product KDOPO, which was then washed with deionized water.

[0025] EP was placed in an oven and heated at 80°C to obtain fluidized EP. 4g of KDOPO was dispersed in 15mL of anhydrous ethanol and placed in a water bath at 70°C. The mixture was stirred, and then 30mL of fluidized EP was added. The mixture was stirred thoroughly to form a homogeneous solution. The anhydrous ethanol in the homogeneous solution was removed by a vacuum filter until no obvious bubbles appeared. The resulting mixture was poured into a preheated mold, transferred to an oven, and cured at 80°C for 30 min. The temperature was then raised to 130°C and held for 8 h to obtain the intrinsic flame retardant material 10KDOPO / EP.

[0026] Example 3 11.68 mL of KH550 and 6.7 g of terephthalaldehyde were dispersed in 100 mL of ethanol and reacted at 78 °C for 6 h to obtain solution A; 17.28 g of DOPO was dispersed in 10 mL of ethanol to obtain solution B; solution B was slowly added dropwise to solution A, and the temperature was raised to 100 °C and reacted for 12 h to obtain the target product KDOPO, which was then washed with deionized water.

[0027] Weigh 0.2 mol NaOH and 0.025 mol Na2CO3, add 100 mL of deionized water to obtain solution A. Weigh Ni(NO3)2·6H2O, Mg(NO3)2·6H2O, and Al(NO3)3·9H2O in a molar ratio (Ni:Mg:Al=1:2:1), add 100 mL of deionized water to obtain solution B. Add solutions A and B dropwise to a beaker simultaneously, stir vigorously, and maintain pH=9-10. After stirring for 1 h, transfer to a reaction vessel and crystallize at 65℃ for 18 h. After crystallization, wash and dry, collect the LDH and store it dry.

[0028] 5 g of LDH was dispersed in a 1:1 mixture of ethanol and water. The mixture was kept at 70°C with continuous stirring for 1 h to ensure uniform dispersion of LDH in the solvent. Then, 5 g of KDOPO was added, and the mixture was reacted at 70°C for 10 h under a nitrogen atmosphere to promote the chemical reaction between KDOPO and LDH. Subsequently, the mixture was filtered and washed repeatedly with deionized water until the pH reached neutral (pH = 7). Finally, the product was dried in an oven at 60°C to remove residual moisture and solvent. The dried solid was the desired KDOPO-LDH.

[0029] EP was placed in an oven and heated at 80°C to obtain a fluidized EP. 4g of KDOPO-LDH was dispersed in 15mL of anhydrous ethanol and placed in a water bath at 70°C. The mixture was stirred, and then 30mL of fluidized EP was added. The mixture was stirred thoroughly to form a homogeneous solution. The anhydrous ethanol in the homogeneous solution was removed by a vacuum filter until no obvious bubbles appeared. The resulting mixture was poured into a preheated mold, transferred to an oven, and cured at 80°C for 30 min. The temperature was then raised to 130°C and held for 8 h to obtain the intrinsic flame retardant material 10KDOPO-LDH / EP.

[0030] Example 4 11.68 mL of KH550 and 6.7 g of terephthalaldehyde were dispersed in 100 mL of ethanol and reacted at 78 °C for 6 h to obtain solution A; 17.28 g of DOPO was dispersed in 10 mL of ethanol to obtain solution B; solution B was slowly added dropwise to solution A, and the temperature was raised to 100 °C and reacted for 12 h to obtain the target product KDOPO, which was then washed with deionized water.

[0031] Under low temperature conditions, 15 mL of H₃PO₄ was added to a 250 mL round-bottom flask. Then, 135 mL of concentrated H₂SO₄ was slowly poured into the flask, and ice was added to maintain the temperature of the ice-water bath at approximately 4 °C. The mixture was stirred for 10 minutes until a homogeneous acidic solution was formed. Next, 3 g of graphite sheets were slowly added to the prepared acidic solution, and stirring was continued for 10 minutes to ensure thorough mixing of the graphite sheets and the acidic solution. Then, 21 g of KMnO₄ was added to the mixture in portions. The mixture was transferred to a 50 °C oil bath and stirred continuously for 10 hours to form a viscous substance. The resulting substance was cooled to room temperature, and deionized water was added to a total volume of 1000 mL. H₂O₂ (30%) was then slowly added, stopping when the solution turned golden yellow and no more bubbles were produced. The mixture was filtered and washed with 5% hydrochloric acid solution, and then washed with deionized water until the pH reached 7. The precipitate was freeze-dried to obtain graphene oxide (GO) product.

[0032] Dissolve 25 mg of GO in 50 mL of deionized water, sonicate for 20 min, then add 100 mg of citric acid and sonicate for another 10 min to obtain a GO solution containing citric acid. Prepare 50 mL of a mixed alkaline solution of NaOH and Na₂CO₃, where [OH⁻] / [CO₃²⁻] = 1 / 2. 2- =1. Prepare 50 mL of a mixed metal salt solution containing Ni(NO3)2·6H2O, Mg(NO3)2·6H2O, and Al(NO3)3·9H2O (Ni:Mg:Al = 1:2:1). Under a N2 atmosphere, the mixed alkaline solution and the mixed metal salt solution are simultaneously and slowly added dropwise to a GO solution containing citric acid, maintaining the pH at 10 ± 0.2, and stirring continuously for 1 h. After stirring, the resulting product is placed in a stainless steel reactor and kept at 80 °C for 24 h. Then, it is washed with deionized water until neutral, and freeze-dried to obtain LDH@GO.

[0033] 5 g of LDH@GO was dispersed in a 1:1 mixture of ethanol and water. The mixture was kept at 70°C with continuous stirring for 1 h to ensure uniform dispersion of the LDH@GO complex in the solvent. Then, 5 g of KDOPO was added, and the reaction was carried out at 70°C for 10 h under a nitrogen atmosphere to promote the chemical reaction between KDOPO and LDH@GO. Subsequently, the mixture was filtered and washed repeatedly with deionized water until the pH reached neutral (pH = 7). Finally, the product was dried in an oven at 60°C to remove residual moisture and solvent. The dried solid was the desired KDOPO-LDH@GO.

[0034] The EP was placed in an oven and heated at 80°C to obtain a fluidized EP. 4g of KDOPO-LDH@GO was dispersed in 15mL of anhydrous ethanol and placed in a water bath at 70°C. The mixture was stirred, and then 30mL of fluidized EP was added. The mixture was stirred thoroughly to form a homogeneous solution. The anhydrous ethanol in the homogeneous solution was removed by a vacuum filter until no obvious bubbles appeared. The resulting mixture was poured into a preheated mold, transferred to an oven, and cured at 80°C for 30 min. The temperature was then raised to 130°C and held for 8 h to obtain the intrinsic flame retardant material 10KDOPO-LDH@GO / EP.

[0035] Example 5 11.68 mL of KH550 and 6.7 g of terephthalaldehyde were dispersed in 100 mL of ethanol and reacted at 78 °C for 6 h to obtain solution A; 17.28 g of DOPO was dispersed in 10 mL of ethanol to obtain solution B; solution B was slowly added dropwise to solution A, and the temperature was raised to 100 °C and reacted for 12 h to obtain the target product KDOPO, which was then washed with deionized water.

[0036] Under low temperature conditions, 15 mL of H₃PO₄ was added to a 250 mL round-bottom flask. Then, 135 mL of concentrated H₂SO₄ was slowly poured into the flask, and ice was added to maintain the temperature of the ice-water bath at approximately 4 °C. The mixture was stirred for 10 minutes until a homogeneous acidic solution was formed. Next, 3 g of graphite sheets were slowly added to the prepared acidic solution, and stirring was continued for 10 minutes to ensure thorough mixing of the graphite sheets and the acidic solution. Then, 21 g of KMnO₄ was added to the mixture in portions. The mixture was transferred to a 50 °C oil bath and stirred continuously for 10 hours to form a viscous substance. The resulting substance was cooled to room temperature, and deionized water was added to a total volume of 1000 mL. H₂O₂ (30%) was then slowly added, stopping when the solution turned golden yellow and no more bubbles were produced. The mixture was filtered and washed with 5% hydrochloric acid solution, and then washed with deionized water until the pH reached 7. The precipitate was freeze-dried to obtain graphene oxide (GO) product.

[0037] Dissolve 25 mg of GO in 50 mL of deionized water, sonicate for 20 min, then add 100 mg of citric acid and sonicate for another 10 min to obtain a GO solution containing citric acid. Prepare 50 mL of a mixed alkaline solution of NaOH and Na₂CO₃, where [OH⁻] / [CO₃²⁻] = 1 / 2. 2-=1. Prepare 50 mL of a mixed metal salt solution containing Ni(NO3)2·6H2O, Mg(NO3)2·6H2O, and Al(NO3)3·9H2O (Ni:Mg:Al = 1:2:1). Under a N2 atmosphere, the mixed alkaline solution and the mixed metal salt solution are simultaneously and slowly added dropwise to a GO solution containing citric acid, maintaining the pH at 10 ± 0.2, and stirring continuously for 1 h. After stirring, the resulting product is placed in a stainless steel reactor and kept at 80 °C for 24 h. Then, it is washed with deionized water until neutral, and freeze-dried to obtain GO@LDH.

[0038] 5 g of LDH@GO was dispersed in a 1:1 mixture of ethanol and water. The mixture was kept at 70°C with continuous stirring for 1 h to ensure uniform dispersion of the LDH@GO complex in the solvent. Then, 5 g of KDOPO was added, and the reaction was carried out at 70°C for 10 h under a nitrogen atmosphere to promote the chemical reaction between KDOPO and LDH@GO. Subsequently, the mixture was filtered and washed repeatedly with deionized water until the pH reached neutral (pH = 7). Finally, the product was dried in an oven at 60°C to remove residual moisture and solvent. The dried solid was the desired KDOPO-LDH@GO.

[0039] The EP was placed in an oven and heated at 80°C to obtain a fluidized EP. 6g of KDOPO-LDH@GO was dispersed in 15mL of anhydrous ethanol and placed in a water bath at 70°C. The mixture was stirred, and then 30mL of fluidized EP was added. The mixture was stirred thoroughly to form a homogeneous solution. The anhydrous ethanol in the homogeneous solution was removed by a vacuum filter until no obvious bubbles appeared. The resulting mixture was poured into a preheated mold, transferred to an oven, and cured at 80°C for 30 min. The temperature was then raised to 130°C and held for 8 h to obtain the intrinsic flame retardant material 15KDOPO-LDH@GO / EP.

[0040] The structural characterization and performance evaluation of the products from Examples 1-5 are as follows: Figure 1 These are the infrared spectra of DOPO, KDOPO, KDOPO-LDH, and KDOPO-LDH@GO. The DOPO curve shows that it is located at 1591 cm⁻¹. −1 1477 cm −1 and 1433 cm −1 Related to the characteristic absorption of the benzene ring, 2425 cm⁻¹ −1 The absorption peak at this location corresponds to the P-H bond stretching vibration. In the KH550-DOPO spectrum, the absorption peak is located at 3057 cm⁻¹. −1 It was significantly weakened. Meanwhile, at 2922 cm... −1The newly added C-H bond peak and 1638 cm⁻¹ −1 The presence of an amide bond (C═O) peak confirms the amidation reaction between DOPO and KH550. Specifically, the peak at 2425 cm⁻¹... −1 The P-H bond disappears at 1073 cm⁻¹. −1 The increased absorption peak of the Si-O bond stretching vibration at the site provides evidence for the successful synthesis of KH550-DOPO.

[0041] Figure 2 These are the XRD patterns of DOPO, KDOPO, KDOPO-LDH, and KDOPO-LDH@GO. The characteristic diffraction peak of KDOPO is located at 2θ = 21.6°, while the DOPO curve shows many sharp crystalline peaks at 2θ = 8.5, 12.5, 22.7, and 25.7°. In KDOPO, these peaks become broad (12.1°, 22.3°). This phenomenon is due to the addition of KH550, which lengthens the polymer molecular chains, making the arrangement of DOPO irregular and thus reducing crystallinity. KDOPO-LDH@GO shows the (003) crystal plane diffraction peak of LDH at 11.3°, proving that the layered structure is fully preserved.

[0042] Figure 3 This is the XPS image of KDOPO-LDH@GO prepared in this embodiment of the invention. The binding energy of Al 2p is 78.08 eV, and the binding energy of Mg 1s is 1308.08 eV. The binding energy of Ni 2p3 is 870.08 eV, constituting LDH. In addition, high-resolution XPS of Si, P, N, and Si 2P were further analyzed. Figure 3 As can be seen from b, the spectral peaks at 100.98 eV, 101.71 eV, 102.36 eV, and 106.32 eV correspond, respectively, to Si-C bonds, Si-O-C bonds, Si-O-M bonds (where M represents Mg / Al), and Si-O-Si bonds. This result indicates that the coupling agent was successfully grafted onto the LDH surface, forming Si-O-M bonds. The appearance of Si-O-Si bonds may be due to the self-condensation of KH550. High-resolution XPS spectra of P2P Figure 3 The peaks at 131.58 eV, 132.58 eV, and 133.38 eV are attributed to P-C, P-ph, and P(═O)-O-C, respectively. This is consistent with the reaction products of KH550 and DOPO, as these compounds contain phosphorus and nitrogen, which can form chemical bonds with carbon. The XPS spectrum of N 1s... Figure 3The values ​​of d at 399.00 eV and 201.40 eV correspond to C-N and C-NH-C bonds, respectively, because DOPO contains nitrogen, which may form chemical bonds with carbon after the reaction. These results demonstrate the successful synthesis of KDOPO-LDH@GO through the reaction of organic DOPO with a silane coupling agent.

[0043] Figure 4 These are SEM and EDS images of DOPO, KDOPO, KDOPO-LDH, and KDOPO-LDH@GO. Figure 4 a shows the surface morphology of the unmodified DOPO flame retardant, which exhibits a relatively smooth and flat surface. Figure 4 b shows a SEM image of the DOPO flame retardant modified with KH550 silane coupling agent. The surface showed significant roughening, indicating that KH550 was successfully grafted, improving the interfacial adhesion between the flame retardant and the substrate. Figure 4 c is a SEM image of DOPO (KDOPO) modified with KH550 and LDH after being combined. Its layered structure can be clearly observed, which indicates that KDOPO and LDH have been successfully combined. Figure 4 Figure d shows the SEM image of the KDOPO-LDH@GO composite flame retardant, combined with the elemental distribution map of energy dispersive spectroscopy (EDS) (Figure d). 1-6 The results confirmed that N, P, and Si were uniformly distributed, indicating that the multi-component flame retardant was successfully prepared.

[0044] Figure 5 These are thermogravimetric (TG) test curves of pure EP, 10DOPO / EP, 10KDOPO / EP, 10KDOPO-LDH / EP, 10KDOPO-LDH@GO / EP, and 15KDOPO-LDH@GO / EP in air atmosphere. Figure 5 As shown in Figure a, at 800 °C, the final char content of pure EP is only 0.5 wt%, while that of 15% KDOPO-LDH@GO / EP increases to 22.6 wt%, indicating an improved carbonization capacity. The weight loss temperature range of pure EP resin is relatively large, with 10% KDOPO / EP exhibiting a higher char content. D max The most likely reason is that the addition of DOPO promotes char formation. The char layer effectively isolates heat and oxygen transfer, slowing down further polymer degradation. However, as the temperature rises, once a certain critical temperature is reached, the polymer molecular chains rapidly break and decompose, causing the material to lose weight rapidly within a certain temperature range. D maxThe addition of KH550 enhances the compatibility between the filler and the polymer matrix, improving the filler's dispersibility in the EP matrix. Simultaneously, it avoids rapid decomposition caused by localized agglomeration, resulting in a more stable decomposition process and thus reducing the maximum weight loss rate. D max .from Figure 5 As can be seen from b, among all materials, pure EP has the highest EP content. T g At the lowest level, after adding flame retardants T g Varying degrees of increase were observed. This phenomenon clearly shows that once LDH@GO flame retardant is added, it hinders the free movement of polymer molecular chains. Hydrogen bonds and other forces form between the surface active groups and the molecular chains. Simultaneously, the addition of KH550 improves the dispersibility of the filler, enhancing the interaction forces between molecular chains, leading to… T g Enhance Figure 6 These are vertical burning test images of pure EP, 10DOPO / EP, 10KDOPO / EP, 10KDOPO-LDH / EP, 10KDOPO-LDH@GO / EP, and 15KDOPO-LDH@GO / EP. As can be seen from the images, pure EP exhibits a longer burning time and more burning droplets. 10% KDOPO / EP achieves a V-1 flame retardancy rating with a shorter burning time. The addition of LDH further improves the flame retardancy of 10% KDOPO-LDH / EP, with secondary ignition extinguishing within ten seconds. With the further addition of GO, 15% KDOPO-LDH@GO / EP shows even better flame retardancy compared to the 10% sample, with faster initial flame extinguishing and inability to ignite directly on the second attempt. Therefore, 15% KDOPO-LDH@GO has a better flame retardant effect on EP.

[0045] Table 1. Flame retardant performance test data of pure EP and samples prepared in Examples 1-5 Furthermore, based on the flame retardant performance test data in Table 1, it can be seen that the KDOPO-LDH@GO composite flame retardant system achieves LOI>25% and UL-94 V-0 rating with a low addition amount of 10%~15% through the synergistic effect of chemical (phosphorus-based flame retardancy) and physical (nano-carbon layer barrier) methods. Its performance is significantly better than that of traditional DOPO and LDH systems alone.

[0046] Figure 7 These are cone calorimetric test data for pure EP, 10DOPO / EP, 10KDOPO / EP, 10KDOPO-LDH / EP, 10KDOPO-LDH@GO / EP, and 15KDOPO-LDH@GO / EP. Figure 7 It is evident from curve a that the HRR of 15% KDOPO-LDH@GO / EP is significantly lower than that of unmodified EP, decreasing by 56.30%. However, the HRR curve of 10% KDOPO-LDH@GO / EP shows a trend of first decreasing and then increasing. This may be because 10% KDOPO-LDH@GO / EP can effectively prevent heat conduction and diffusion during the initial combustion phase, resulting in a lower HRR. However, as combustion progresses, if heat accumulation leads to localized overheating, it may trigger a more intense combustion reaction, thus causing the HRR to increase. Therefore... Figure 7 b shows that its THR value is also higher than that of the 15% KDOPO-LDH@GO / EP sample, with a THR of 72.59 MJ / m³ for the 10% KDOPO-LDH@GO / EP sample. 2 Compared to unmodified pure EP, it reduced by 24.71%, while the addition of 15% reduced it by 35.71%, resulting in better flame retardant effect for EP. Figure 7 cd represents the smoke production rate (SPR) per unit time and the total smoke output (TSP) generated during the combustion of EP and its composites. Unmodified EP released a large amount of smoke during simulated combustion, with a PSPR and TSP as high as 0.352 m. 2 / s and 25.91 m 2 The PSPR (Power Spread Rate) of 10% KDOPO-LDH / EP, 10% KDOPO-LDH@GO / EP, and 15% KDOPO-LDH@GO / EP all decreased significantly, by 52.27%, 60.23%, and 60.80%, respectively. This is mainly attributed to the fact that the metal ions in LDH can catalyze cross-linking reactions during combustion, promoting the formation of more char layers. Furthermore, LDH has a large specific surface area, enabling it to adsorb small organic molecules generated during combustion. 15% KDOPO-LDH@GO is uniformly dispersed in EP and does not produce smoke during combustion. Its presence in the material effectively dilutes the combustible components that could produce smoke. The generation rates of CO and CO2 also reflect the intensity of gas production by the material during a fire. Figure 7 As shown in ef, both PCO and PCO2 of KDOPO-LDH@GO / EP material are reduced. This is due to the char-promoting and adsorption effects of LDH and the good dispersibility of flame retardant in EP matrix. This proves that KDOPO-LDH@GO can effectively improve the flame retardant performance of EP and exert a good flame retardant and smoke-suppressing effect.

[0047] Figure 8These are digital photographs and microscopic images of the carbon layers of pure EP, 10DOPO / EP, 10KDOPO / EP, 10KDOPO-LDH / EP, 10KDOPO-LDH@GO / EP, and 15KDOPO-LDH@GO / EP. The pure EP was almost completely burned off, and the remaining residue, as observed by scanning electron microscopy, is full of pores and voids. I D / I G The value is also the highest, and the degree of graphitization is low. After modification with DOPO and KDOPO, the char residue of the EP material after combustion exhibits an irregular shape and contains internal cracks. Observation of its microstructure shows that the surface of the char residue still has porosity with varying pore sizes, indicating an increased degree of graphitization. The char layer of 10% KDOPO-LDH / EP and 10% KDOPO-LDH@GO / EP materials has fewer pores, indicating a better barrier effect. The char layer of 15% KDOPO-LDH@GO / EP material is denser after combustion. Observation of its microstructure shows that its surface is relatively rough and the char layer is solid. At the same time, I D / I G A value of 2.19 indicates that the char residue possesses a high degree of graphitization and few structural defects, implying good thermal stability and structural integrity at high temperatures. Combining the physical and chemical properties of the char residue allows for a comprehensive evaluation of the material's flame-retardant performance and the protective effect of the char layer on the matrix at high temperatures. The 15KDOPO-LDH@GO / EP material exhibits superior performance, consistent with results obtained from other tests.

[0048] Figure 9 These are mechanical property diagrams of pure EP and 10DOPO / EP, 10KDOPO / EP, 10KDOPO-LDH / EP, 10KDOPO-LDH@GO / EP, and 15KDOPO-LDH@GO / EP prepared according to examples of this invention. From... Figure 9As shown in (a), the addition of KH550 to EP significantly improves the flexural modulus of the material. This is because KH550 can react with the hydroxyl groups in the epoxy resin through its silane groups to form chemical bonds, thereby enhancing intermolecular interactions and crosslinking density. This enhanced crosslinking network improves the rigidity of the material, thus increasing the flexural modulus. The addition of GO significantly improves the flexural strength of the material, mainly because GO has excellent mechanical properties and a high specific surface area. Its addition can form additional physical crosslinking points, enhancing the mechanical properties of the material, while also improving the dispersibility and interfacial compatibility of the material, reducing stress concentration within the material, and thus improving flexural strength. When the content of KDOPO-LDH@GO increases to 15%, the increase in tensile strength and flexural strength decreases. This may be due to the reduced dispersibility within the material caused by excessively high filler content, or the weakened reinforcing effect of the interactions between fillers.

Claims

1. A method for preparing an environmentally friendly P, N, Si synergistic flame retardant, characterized in that, Includes the following steps: (1) Preparation of layered double hydroxide LDH: Dissolve NaOH and Na2CO3 in deionized water at a molar ratio of 0.8:1 to 1:1 to form solution A; Ni(NO3)2·6H2O, Mg(NO3)2·6H2O and Al(NO3)3·9H2O are dissolved in deionized water in a molar ratio of Ni:Mg:Al=1:2:1 to form solution B; Under pH 9-10 conditions, solutions A and B were co-precipitated, crystallized at 60-70℃ for 15-20 h, and then washed and dried to obtain LDH. (2) Preparation of graphene oxide (GO): In an ice bath at 2-6℃, H3PO4 and concentrated H2SO4 were mixed at a volume ratio of 1:8 to 1:

10. Graphite sheets were added, and KMnO4 with a mass of 7-8 times that of the graphite sheets was added in batches. The mixture was reacted at 45-55℃ for 8-12 hours and then cooled. H2O2 was added until the solution turned golden yellow and no more bubbles were produced. The solution was washed with hydrochloric acid and deionized water until pH=7. The solution was then freeze-dried to obtain GO. (3) Preparation of LDH@GO composite carrier: Solution A was obtained by dispersing GO and citric acid in deionized water and sonicating. Dissolve LDH in water to obtain solution B; Solution B was added dropwise to solution A, and the mixture was stirred for 1 hour while maintaining pH = 10 ± 0.

2. The mixture was then transferred to a reaction vessel and reacted at 80°C for 20-25 hours. After washing with water until neutral, the mixture was freeze-dried to obtain LDH@GO. (4) Preparation of silane-modified flame retardant monomer KDOPO: Disperse KH550 and terephthalaldehyde in ethanol at a molar ratio of 1:2 to 2:1 and stir at 75 to 85°C for 4 to 8 hours to obtain solution A; Disperse DOPO in ethanol to obtain solution B; Solution B was added dropwise to solution A at a molar ratio of KH550:DOPO = 1:1.5~1.7, and the reaction was carried out at 100℃ for 10~12 h. After washing and drying, KDOPO was obtained. (5) Preparation of KDOPO-LDH@GO flame retardant: LDH@GO was dispersed in a 1:1 volume ratio ethanol / water mixture, KDOPO was added, and the mixture was reacted at 70-80℃ for 8-10 h under N2 atmosphere. After washing until neutral, the mixture was dried to obtain KDOPO-LDH@GO.

2. The preparation method according to claim 1, characterized in that, The mass ratio of citric acid to GO in step (3) is 3~5:

1.

3. The preparation method according to claim 1, characterized in that, The mass ratio of KDOPO to LDH@GO in step (5) is 0.8:1 to 1.2:

1.

4. An environmentally friendly P, N, Si synergistic flame retardant prepared by any one of claims 1-3, characterized in that, Its structure consists of KDOPO anchored to the surface of the LDH@GO carrier via Si-O covalent bonds.

5. The application of the flame retardant according to claim 4 in epoxy resin, characterized in that, include: The flame retardant is dispersed in ethanol at a total weight of 10-15 wt% of the mixture; Add the fluid epoxy resin and mix at 65~75℃. After vacuum solvent removal, the product is injected into the mold and pre-cured at 75~85℃ for 0.5~1h, and then finally cured at 120~140℃ for 6~8h.

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