Vanillin-based DOPS derivative flame retardant and preparation method thereof

Abstract

The invention relates to the technical field of flame-retardant materials, in particular to a vanillin-based DOPS derivative flame retardant and a preparation method thereof. The flame retardant VD is prepared by carrying out addition reaction on DOPS and vanillin in an organic solvent, carrying out rotary evaporation concentration, dissolving, precipitating and drying. The preparation method comprises the following steps: uniformly mixing VD and epoxy resin at 170-180 DEG C, cooling, adding the curing agent, and carrying out segmented curing. According to the invention, the limit oxygen index of the epoxy resin composite material can be remarkably improved by utilizing a phosphorus-sulfur synergistic flame-retardant effect, and the epoxy resin composite material reaches the UL-94 V-0 level and has no molten drops when the addition amount is 14 wt%; meanwhile, by utilizing a rigid aromatic structure of VD molecules, the bending strength and the tensile strength of the composite material are greatly improved compared with those of pure epoxy resin, the flame-retardant safety and the mechanical reliability of the material are effectively considered, and the composite material has a wide application prospect.

Description

Technical Field

[0001] This invention belongs to the field of flame retardant materials technology, and relates to a polymeric flame retardant and its preparation method. Background Technology

[0002] Epoxy resin, as a class of high-performance thermosetting resins, is widely used in aerospace, industrial coatings, electronic packaging materials, and adhesives due to its excellent mechanical properties, chemical stability, and adhesive capabilities. It is one of the most representative basic materials in modern industrial production. However, the inherent flammability of epoxy resin severely limits its further promotion in high-end fields. During combustion, epoxy resin not only releases a large amount of heat but also produces a large amount of toxic fumes, which can easily cause serious fire accidents. Therefore, how to improve the flame retardant properties of epoxy resin through effective modification methods while maintaining its original excellent physical properties has become a key problem that urgently needs to be solved in the field of materials science.

[0003] Among existing epoxy resin flame retardant modification technologies, additive flame retardant technology has become the most commonly used technical route due to its advantages such as simple process, low cost, and wide availability of raw materials. Among numerous flame retardant systems, phosphaphenanthrene flame retardants have attracted widespread attention from researchers due to their environmental friendliness, high carbon content, good compatibility with the matrix, and long-lasting flame retardant effect. As an important phosphaphenanthrene intermediate, the phosphorus-oxygen double bond in the DOPO molecular structure can generate phosphorus-containing free radicals during thermal decomposition, thereby exerting a flame retardant effect in the gas phase. To further improve the flame retardant efficiency, the phosphorus-oxygen double bond in DOPO is converted into a phosphorus-sulfur double bond, thus preparing 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-sulfide (DOPS). The introduced sulfur element effectively enhances the flame retardant effect of the condensed phase, forming a phosphorus-sulfur synergistic flame retardant mechanism.

[0004] Furthermore, with the deepening of green chemistry and sustainable development concepts, the development of novel flame retardants using bio-based raw materials has become an industry trend. Vanillin, as a bio-based raw material containing highly reactive aldehyde groups, provides an excellent platform for the molecular design of flame retardants. Although there is existing research on phosphorus-based flame retardants, how to efficiently combine the bio-based raw material vanillin (VA) with the DOPS structure, which has a synergistic effect of phosphorus and sulfur, through molecular design to prepare a novel flame retardant that can significantly improve the limiting oxygen index of epoxy resin and achieve a high-level vertical burning standard, while also taking into account or even improving the mechanical properties of the matrix material, such as flexural strength and tensile strength, still faces technical challenges. Therefore, developing a vanillin-based DOPS derivative flame retardant with both high-efficiency flame retardancy and reinforcing effects, and its preparation method, has important theoretical significance and engineering application prospects for expanding the application of epoxy resins in fields such as electronics, aerospace, and transportation. Summary of the Invention

[0005] This invention addresses the technical shortcomings of existing epoxy resin materials in practical applications, such as high flammability, large smoke production during combustion, and significant decrease in the mechanical strength of the matrix material after adding traditional flame retardants. It provides a vanillin-based DOPS derivative flame retardant and its preparation method, and further provides a flame-retardant epoxy resin composite material containing the flame retardant and its preparation method.

[0006] This invention combines the DOPS structure, which has a synergistic flame-retardant effect of phosphorus and sulfur, with bio-based vanillin molecules through molecular design to prepare a functional molecule that has both flame-retardant and mechanical strengthening effects. The vanillin-based DOPS derivative flame retardant is molecularly dispersed in the epoxy resin matrix. By promoting the dehydration and char formation of the matrix and capturing gaseous free radicals during combustion, the epoxy resin composite material can achieve a high level of flame retardancy while maintaining a low addition amount. Furthermore, its rigid aromatic structure enhances the flexural strength and tensile strength of the composite material.

[0007] The present invention provides a vanillin-based DOPS derivative flame retardant, the chemical name of which is 6-[hydroxy(4-hydroxy-3-methoxyphenyl)methyl]-6H-dibenzo[c,e][1,2]oxophosphazene-6-sulfur oxide, abbreviated as VD, and its synthetic route is completed by the reaction of vanillin with DOPS.

[0008] The molecular structure of the flame retardant VD contains phenolic hydroxyl and methoxy groups introduced by vanillin, as well as a phosphorus-thiophene ring structure introduced by DOPS.

[0009] The preparation method of the above-mentioned vanillin-based DOPS derivative flame retardant is characterized by comprising the following specific steps: S1. Weigh the reaction raw materials according to the molar ratio of DOPS to VA of 1.1:1 to 1.3:1. DOPS is a phosphorus-sulfur-containing active component, and the phosphorus-hydrogen bond in its molecule has high reactivity with the aldehyde group in the vanillin molecule. S2. Place the weighed DOPS and VA together in a three-necked flask equipped with a reflux condenser, thermometer and magnetic stirrer, and add an organic solvent as the reaction medium. The organic solvent is toluene or xylene. Stir and heat to ensure that the raw materials are completely dissolved under heating to form a homogeneous reaction system. S3. Turn on the stirrer and heat to 105-115 °C. Reflux the reaction under normal pressure for 5-7 h. During this process, the phosphorus-hydrogen bonds in the DOPS molecules undergo nucleophilic addition with the aldehyde group of vanillin to form a stable... Bonds, thus constructing a structure containing free phenolic hydroxyl groups and The synergistic flame-retardant structure of the methylene hydroxyl group generates an intermediate structure containing hydroxyl groups. During the reaction, the color of the system gradually changes from light color to transparent or slightly yellow liquid. S4. After the reaction is complete, the reaction solution is transferred to a rotary evaporator and the organic solvent is removed under vacuum at 60-80 °C to obtain a viscous solid crude product with vanillin-based DOPS derivatives. S5. After cooling the crude product to room temperature, add ethyl acetate (EA) to dissolve it completely, and prepare a solution with a mass fraction of 30%-50%. Then, slowly add the solution dropwise to an excess of petroleum ether with a volume of 5-10 times its volume. The target product is precipitated by utilizing the difference in solvent polarity. After vacuum filtration, a solid filter cake is obtained. Repeat the above operation 3-5 times. S6. Place the washed filter cake in a vacuum drying oven and dry it at 45-55 ℃ for 8-12 h to obtain a white powdery solid, which is the vanillin-based DOPS derivative flame retardant VD.

[0010] This invention further provides a flame-retardant epoxy resin composite material prepared using the above-mentioned flame retardant, characterized in that the composite material is composed of the following components in parts by weight: 00 parts of epoxy resin matrix, 6-14 parts of vanillin-based DOPS derivative flame retardant VD, and 15-25 parts of curing agent. The epoxy resin matrix is ​​selected from E-51 bisphenol A type epoxy resin, and the curing agent is selected from 4,4'-diaminodiphenylmethane (DDM).

[0011] The preparation method of the above-mentioned flame-retardant epoxy resin composite material is characterized by the following process steps: SS1. Place the epoxy resin matrix in a beaker and preheat it to 60-80 ℃ to reduce the viscosity of the system. Then, further heat it to 170-180 ℃. Under this high temperature condition, slowly add the weighed vanillin-based DOPS derivative flame retardant VD to the epoxy resin and keep stirring at high speed for 10 min. Use the energy provided by the high temperature to overcome the lattice energy of VD molecules, so that it is completely dissolved and uniformly dispersed in the epoxy resin until the system presents a completely transparent and uniform phase without any suspended particles. SS2. Reduce the temperature of the above mixture to 95-105 ℃, add curing agent DDM according to the predetermined ratio, and perform degassing treatment for 3-5 min under the condition of maintaining a vacuum degree of -0.08MPa to -0.1MPa to eliminate the micro bubbles generated during the mixing process and prevent stress concentration points from being generated inside the cured product. SS3. Pour the degassed mixture into a polytetrafluoroethylene mold preheated to 80-100 ℃, and place the mold in an oven for segmented curing: First, maintain at 100 ℃ for 2 h to allow the epoxy resin system to undergo initial cross-linking and form a gel network, which plays a role in fixing the position of flame retardant molecules; then raise the temperature to 150 ℃ and continue curing for 3 h to allow the epoxy groups and amino groups to fully react and construct a three-dimensional network structure with high cross-linking density; SS4. After curing, allow the epoxy resin to cool naturally to room temperature in the oven, then demold to obtain the finished flame-retardant epoxy resin composite material.

[0012] In the above technical solution, the amount of vanillin-based DOPS derivative flame retardant VD added has a decisive influence on the performance of the final composite material. When the amount of VD added is 6%, the limiting oxygen index (LOI) of the composite material increases from 24.4% for pure epoxy resin to over 27.5%; when the amount added increases to 14 wt%, the LOI value of the composite material reaches 32.8%, and it achieves the V-0 level in the UL-94 vertical burning test, exhibiting extremely excellent flame retardant properties. This improvement in flame retardant performance stems from the synergistic effect of phosphorus and sulfur elements in the VD molecular structure: during combustion, a continuous and dense char layer is formed on the polymer surface, playing a physical barrier role in heat insulation, oxygen isolation, and inhibiting the escape of combustible gases; at the same time, the introduction of sulfur elements enhances the graphitization degree and thermal stability of the char layer, reducing the oxidation loss of the char layer; in addition, the phosphorus-containing free radicals generated by the phosphorus-phenanthroline structure in the gas phase capture H· and HO· free radicals in the combustion chain reaction, interrupting the combustion reaction and achieving two-phase flame retardancy in both the condensed phase and the gas phase.

[0013] In terms of mechanical properties, the technical solution described in this invention achieves reinforcement of the epoxy resin matrix by introducing VD molecules with rigid biphenyl and benzene ring structures. Unlike traditional additive flame retardants, which often lead to material embrittlement or strength reduction, the aromatic ring structure in the VD molecule can effectively fill the free volume of the epoxy resin crosslinking network, playing a role in transferring loads during stress. Experimental test results show that when the VD addition amount is 14 wt%, the flexural strength of the resulting composite material reaches 135-145 MPa, which is more than 20% higher than that of pure epoxy resin; the tensile strength reaches 75-85 MPa, which is more than 10% higher than that of pure epoxy resin. This simultaneous improvement in mechanical properties solves the contradiction of "flame retardancy versus mechanical properties" commonly found in flame retardant modification, making the material more suitable for electronic packaging and aerospace components with strict structural strength requirements.

[0014] Furthermore, the high-temperature mixing step of 170-180 °C in the preparation process of this invention is crucial to ensuring the optimal performance of the flame retardant. Due to the high melting point and molecular rigidity of VD, it is difficult to achieve molecular-level compatibility with epoxy resin at conventional temperatures. By increasing the mixing temperature to approximately 175 °C, the thermal motion at high temperature promotes the interpenetration of flame retardant molecules and epoxy resin molecules. Combined with the subsequent gradient curing process, this ensures that the flame retardant molecules are firmly locked within the cross-linked network of the epoxy resin during curing, preventing migration and precipitation of the flame retardant during long-term use, thereby guaranteeing the durability of the flame retardant effect.

[0015] In this preparation method, precise control of the raw material ratio is also crucial. Maintaining a molar ratio of DOPS to VA of approximately 1.2:1 ensures complete conversion of the aldehyde groups, preventing residual aldehyde groups from causing side reactions or increasing the material's hygroscopicity during subsequent curing. Simultaneously, excess DOPS is effectively removed after the reaction via a petroleum ether washing process, ensuring the purity of the flame retardant VD.

[0016] The beneficial effects of this invention are reflected in the following aspects: Firstly, this invention utilizes the bio-based raw material vanillin as the framework structure of the flame retardant, and introduces phosphorus and sulfur flame-retardant elements through a chemical reaction to prepare an environmentally friendly and highly efficient flame retardant. This flame retardant does not produce toxic and harmful gases such as hydrogen halides during combustion, meeting the requirements of modern industry for green and environmentally friendly materials.

[0017] Secondly, the flame-retardant epoxy resin composite material prepared by this invention can achieve a high flame-retardant rating with a relatively low phosphorus content. Due to the presence of phosphorus-sulfur double bonds (P=S) in the DOPS molecule, its thermal stability is superior to the traditional phosphorus-oxygen double bond (P=O) structure. The phosphoric acid derivatives generated at high temperatures can more effectively promote char formation in the epoxy resin matrix. Experimental data show that the char residue rate formed is significantly higher than that of the unmodified system at 600 °C. This high-quality char layer is the core guarantee for achieving the UL-94 V-0 flame-retardant rating.

[0018] Third, this invention achieves synergistic enhancement of flame retardancy and mechanical properties. The polycyclic aromatic structure in the VD molecule endows the modified epoxy resin with higher rigidity and modulus, enabling it to exhibit superior resistance to deformation under bending and tensile loads. Specifically, the flexural strength increases from approximately 110 MPa in pure EP to over 140 MPa, and the tensile strength also increases simultaneously.

[0019] Fourth, the preparation process of this invention is simple, the raw materials are widely available, and the cost is controllable. The flame retardant is synthesized using a one-step addition method, which has a high yield and does not involve complex separation and purification in the post-processing. The preparation of the composite material is fully compatible with existing epoxy resin processing equipment and has the potential for large-scale industrial production.

[0020] Fifth, due to the good compatibility between VD molecules and epoxy resin matrix, the cured composite material has a smooth surface, a dense internal structure, and no obvious phase separation phenomenon. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the synthetic route for the vanillin-based DOPS derivative flame retardant VD of the present invention. Figure 2 FTIR(a) for VD. 1 H NMR (b) 31 P NMR (c) and HR-MS (d) spectra. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0023] This invention provides a preparation process for a vanillin-based DOPS derivative flame retardant, VD. The core principle lies in utilizing the highly reactive phosphorus-hydrogen bonds in the DOPS molecule to undergo a nucleophilic addition reaction with the aldehyde group in the VA molecule. (See attached reference.) Figure 1 The synthetic route shown describes a reaction that, through intermolecular collisions and group recombination, introduces a phosphorus-sulfur synergistic structure into the bio-based vanillin framework, thereby constructing a novel flame-retardant molecule containing hydroxyl, methoxy, and phosphorus-sulfur phenanthrene groups. In the specific preparation process, a 500 mL three-necked flask equipped with a reflux condenser, thermometer, and magnetic stirrer is first prepared. 200 mL of toluene, the organic solvent used as the reaction medium, is added to the flask. Then, 10.03 g (approximately 0.066 mol) of vanillin (VA) and 19.02 g (approximately 0.082 mol) of DOPS are accurately weighed and added to the system. In this step, the molar ratio of DOPS to VA is set at 1.2:1. The excess DOPS ensures that the aldehyde groups in the vanillin molecule are completely converted, thereby eliminating the adverse effects of residual aldehyde groups on the subsequent epoxy resin curing process.

[0024] The magnetic stirrer was started and the stirring speed was set to 250 r / min. The heating mantle was turned on to raise the temperature to 110 °C and maintain a constant temperature. At this temperature, toluene was under reflux, and the reaction system gradually changed from an initial heterogeneous suspension to a clear and transparent liquid. The reaction continued for 6 h. During this period, phosphorus-hydrogen bonds continuously attacked the carbon atoms in the aldehyde group, and the generated intermediate formed a stable vanillin-based DOPS derivative, VD, through proton transfer. After the reaction was completed, heating was stopped and the system was allowed to cool naturally to room temperature. The reaction solution was then transferred to a rotary evaporator and concentrated by rotary evaporation at 60 °C and -0.095 MPa. Most of the toluene solvent was removed by vacuum distillation, and finally, a viscous solid crude product, VD, was obtained at the bottom of the flask.

[0025] To obtain a high-purity flame retardant product, this embodiment employs a refined recrystallization and precipitation washing process. An appropriate amount of EA was added to the crude product to dissolve it, preparing a saturated solution. This solution was then slowly added dropwise to a beaker containing 300 mL of petroleum ether while maintaining rapid stirring. Because the target product VD has extremely low solubility in petroleum ether, while residual DOPS and byproducts have some solubility, a large amount of white flocculent material rapidly precipitated from the system during the dropwise addition process. The solid filter cake was collected by vacuum filtration and washed repeatedly with fresh petroleum ether at least three times. Finally, the obtained solid was placed in a vacuum drying oven and dried at 50 °C for 8 h to completely remove residual trace solvents. The final VD product was a white powdery solid with a precisely determined melting point range of 172.9–174.2 °C, a yield of 80.5%, and a purity greater than 99.0% as determined by high-performance liquid chromatography. Figure 2 The characterization methods shown, including the disappearance of the aldehyde absorption peak in the infrared spectrum and the appearance of the characteristic shift signal in the nuclear magnetic resonance spectrum, confirm that the target product VD has been successfully synthesized.

[0026] In the preparation of flame-retardant epoxy resin composites, this invention fully utilizes the physicochemical compatibility between VD molecules and the epoxy resin matrix. Taking the preparation of a flame-retardant epoxy resin (EP / VD-14) with a flame retardant addition of 14 wt% as an example, the specific process is as follows: First, the polytetrafluoroethylene mold and EP are placed in a vacuum drying oven at 100 ℃ for preheating treatment. Heating reduces the viscosity of EP and removes moisture from the mold surface. 100 g of the preheated EP is weighed and placed in a 250 mL beaker, and the heating and stirring platform is turned on. When the temperature reaches 175 ℃, 20.4 g of vanillin-based DOPS derivative flame retardant VD is slowly added in batches. At the high temperature of 175 ℃, VD molecules gain sufficient kinetic energy to overcome intermolecular forces, thereby achieving molecular-level dissolution and uniform distribution in the epoxy resin matrix. Stirring continues until the system exhibits a completely transparent and homogeneous state. This high-temperature blending step ensures the high dispersion of the flame retardant in the matrix through molecular diffusion, laying the foundation for the uniformity of subsequent material properties.

[0027] After VD is completely dissolved, the system is cooled to 100 °C, at which point 25.3 g of curing agent DDM is added. During stirring, the amino groups of DDM begin to undergo a preliminary addition reaction with the epoxy groups in EP. To prevent bubble defects inside the final sample, the mixture is quickly transferred to a vacuum drying oven and treated under a vacuum of -0.1 MPa for 3-5 min. The pressure difference drives the tiny bubbles inside the system to rise and break. After degassing, the mixture is carefully poured into a preheated mold. The curing process is carried out in three stages: the first stage is pre-curing at room temperature for 30 min, which locks the spatial distribution of the flame retardant through preliminary gelation; the second stage is isothermal curing at 100 °C for 2 h, at which temperature the reaction rate is moderate, releasing some of the heat of reaction by building a preliminary cross-linking network to prevent material cracking; the third stage is high-temperature curing at 150 °C for 3 h, where the high-temperature environment further increases the cross-linking density, so that VD molecules are firmly locked in the three-dimensional network of epoxy resin. After curing, allow it to cool naturally to room temperature, and then demold to obtain flame-retardant epoxy resin samples.

[0028] The flame-retardant mechanism of the composite material described in this invention is based on the synergistic flame-retardant effect of phosphorus and sulfur. When the material is heated in contact with a fire source, the P=S bonds in the molecular structure break and recombine. In the gas phase, the phosphorus-containing free radical fragments generated by its decomposition capture H· and HO· in the combustion chain reaction, thereby diluting the concentration of combustible gases and terminating the free radical reaction, thus playing a gas-phase flame-retardant role. In the condensed phase, the phosphoric acid derivatives generated by phosphorus catalyze the dehydration of EP to char at high temperatures and undergo isomerization reactions to form a continuous, dense, and highly thermally stable char layer. The introduction of sulfur enhances the graphitization degree of the char layer, giving it a stronger physical barrier effect at high temperatures, preventing the diffusion of oxygen into the internal matrix and the escape of internal combustible gases. Experimental data show that the LOI value of pure EP is only 24.4% and has no flame-retardant rating. However, the LOI value of the EP / VD-14 sample is increased to 32.8%, and it successfully passed the UL-94 V-0 rating test. Throughout the test, due to the formation of a dense carbon layer, the material did not produce any molten droplets, effectively preventing the secondary spread of the fire through a physical isolation mechanism.

[0029] The bending, tensile and impact properties of the composite material EP / VD were tested, and the results are shown in Table 1.

[0030] Table 1 Mechanical property test data of EP and EP / VD ; As shown in Table 1, the flexural strength and tensile strength of EP / VD both increase with the increase of VD content. Compared with pure EP, the flexural strength of EP / VD-14 increased from 113.48 MPa to 152.81 MPa, an increase of 25.74%; the tensile strength increased from 59.31 MPa to 66.61 MPa, an increase of 10.96%. The improvement in flexural and tensile strength may be attributed to the following two aspects: Firstly, the introduction of VD as a macromolecular flame retardant may reduce the crosslinking density of EP, increasing the free volume of the crosslinking network, thereby providing more deformation space for chain segment movement under external force; secondly, the phosphenanthrene groups and benzene rings in the VD molecular structure have rigid characteristics, which help to improve the strength and hardness of the material.

[0031] The flame retardant properties of EP and its composites were evaluated using LOI and UL-94 tests, and the results are shown in Table 2. The LOI value of pure EP was 24.4%, and it did not have a flame retardant rating. With increasing VD content, the LOI value of the composite material showed a continuous upward trend. When the VD content increased to 14 wt%, the LOI value increased from 24.4% to 32.8%, and it also passed the UL-94 V-0 rating test.

[0032] Table 2. LOI and UL-94 test results of EP and its composite material EP / VD ; The flame-retardant system provided by this invention exhibits significant reliability in practical applications. For example, in the field of electronic packaging, epoxy resin composite materials need to simultaneously possess excellent insulation, flame retardancy, and mechanical reliability. The composite material EP / VD-14 prepared in this embodiment utilizes its high LOI value and V-0 flame-retardant properties to block the thermal runaway path through the rapid formation of a carbon layer on the surface when the circuit experiences overload and overheating. Simultaneously, its improved flexural strength resists warping deformation caused by thermal stress, ensuring the structural integrity of electronic components. This dual-effect solution of flame retardancy and reinforcement, through the organic combination of chemical synthesis and physical blending, solves the problem of material embrittlement and strength reduction caused by traditional flame retardants.

[0033] To meet different industrial needs, this invention also provides various formulation schemes. EP and EP / VD samples are prepared according to the formulations in Table 3. First, the mold and EP are preheated at 100 °C in a vacuum drying oven. Then, 100 g of EP is weighed and poured into a 250 mL beaker, heated and stirred to 175 °C, and then the flame retardant VD is added. After the system becomes transparent and homogeneous, it is cooled to 100 °C and the curing agent DDM is added. Once the DDM is completely dissolved, the mixture is quickly transferred to a vacuum drying oven for degassing under reduced pressure. Finally, the homogeneously mixed epoxy curing material is poured into a preheated mold, pre-cured at room temperature for 30 min, then cured at 100 °C for 2 h, followed by heating to 150 °C and continuing curing for 3 h. After natural cooling, the sample is demolded to obtain the flame-retardant EP sample.

[0034] Table 3 Formulation table of EP and its composite material EP / VD ; Examples EP / VD-6, EP / VD-8, EP / VD-10, and EP / VD-12 correspond to phosphorus contents ranging from 0.47 wt% to 0.94 wt%. Experimental observations revealed that even at lower addition levels (e.g., 6 wt%), the LOI value reached 28.7%, achieving a V1 flame retardant rating. This demonstrates precise control over flame retardant performance and cost by adjusting the VD addition ratio. The preparation of these samples with different formulations strictly adhered to the aforementioned 175℃ blending and segmented curing process, ensuring the structural regularity of each sample. Through this standardized preparation process, VD molecules maintained good dispersion at different concentrations.

[0035] In summary, this invention synthesizes a vanillin-based DOPS derivative (VD) with a specific chemical structure, then adds it to epoxy resin (EP), and solves the compatibility problem of the flame retardant in EP through high-temperature melting. A high-performance cross-linked network is constructed using a segmented curing process. During combustion, the dense char layer formed through the phosphorus-sulfur synergistic flame retardant mechanism acts as a physical barrier for heat and oxygen insulation; under stress, the structural reinforcement of rigid molecules enhances the mechanical strength of the material. This invention not only provides a green and efficient method for preparing bio-based flame retardants, but also achieves simultaneous improvement in the flame retardant and mechanical properties of epoxy resin composites through specific technical means, possessing high engineering application value.

Claims

1. A vanillin-based DOPS derivative flame retardant VD, characterized in that, Its chemical name is 6-[hydroxy(4-hydroxy-3-methoxyphenyl)methyl]-6H-dibenzo[c,e][1,2]oxophosphazene-6-sulfur oxide, and its molecular structure contains a phenolic hydroxyl group and a methoxy group introduced by vanillin, as well as a phosphorus sulfide ring structure introduced by 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-sulfide.

2. The method for preparing vanillin-based DOPS derivative flame retardant VD as described in claim 1, characterized in that, Includes the following steps: S1. Weigh the reaction raw materials 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-sulfide and vanillin; S2. Place the weighed raw materials into a three-necked flask and add an organic solvent to form a reaction system; S3. Start stirring and heat to carry out reflux reaction, so that the phosphorus hydrogen bond in the 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-sulfide molecule undergoes nucleophilic addition reaction with the aldehyde group of vanillin. S4. After the reaction is complete, the organic solvent is removed by rotary evaporation to obtain a viscous solid crude product. S5. Dissolve the crude product in ethyl acetate to prepare a solution, then add the solution dropwise to petroleum ether to precipitate a solid. After vacuum filtration, obtain a solid filter cake and wash it with petroleum ether. S6. Vacuum dry the washed filter cake to obtain a powdered solid.

3. The method for preparing vanillin-based DOPS derivative flame retardant VD as described in claim 2, characterized in that... Further specifying that in step S1, the molar ratio of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-sulfide to vanillin is 1.1:1 to 1.3:

1.

4. The method for preparing vanillin-based DOPS derivative flame retardant VD as described in claim 2, characterized in that... The organic solvent in step S2 is further specified as toluene, and the reflux reaction temperature in step S3 is 105 °C to 115 °C, and the reflux reaction time is 5 h to 7 h.

5. A flame-retardant epoxy resin composite material, characterized in that, It is composed of the following components in parts by weight: 100 parts of epoxy resin matrix (1), 6 to 14 parts of vanillin-based DOPS derivative flame retardant VD (2) as described in claim 1, and 15 to 25 parts of curing agent DDM (3).

6. The flame-retardant epoxy resin composite material as described in claim 5, characterized in that, The epoxy resin matrix (1) is E51 bisphenol A type epoxy resin.

7. The method for preparing the flame-retardant epoxy resin composite material as described in claim 5, characterized in that, Includes the following steps: S1. Heat the epoxy resin matrix (1) to 170 ℃ to 180 ℃, add vanillin-based DOPS derivative flame retardant VD (2), and keep stirring for 10 min until the system becomes transparent. S2. Reduce the temperature of the mixture to 95 ℃ to 105 ℃, add curing agent DDM (3), and perform degassing treatment under vacuum conditions of -0.08 MPa to -0.1 MPa; S3. Pour the degassed mixture into the preheated mold for segmented curing. S4. After curing, cool to room temperature in the oven and demold.

8. The method for preparing the flame-retardant epoxy resin composite material as described in claim 7, characterized in that... The segmented curing process in step S3 is further defined as follows: first, maintain at 100 ℃ for 2 h, then raise the temperature to 150 ℃ and continue curing for 3 h.

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