A core-shell structured flame retardant and its application in thermally conductive materials

Abstract

This invention discloses a core-shell structured flame retardant and its application method in thermally conductive materials. The method includes dispersing antimony phosphate oxyphosphate and boron nitride nanosheets in a liquid medium and subjecting them to a hydrothermal reaction to form a thermally conductive-flame-retardant composite core encapsulated by boron nitride nanosheets. After washing and drying, a core-shell structured thermally conductive flame retardant precursor is obtained. The precursor is dispersed in a solvent and sequentially coated with a borosilicate-modified phenolic resin inner shell through an in-situ polycondensation reaction, followed by a graphene nanosheet outer shell coating through a reduction deposition reaction. After separation and drying, a multifunctional core-shell structured thermally conductive flame retardant is obtained. After surface activation treatment, it is composited with a silicone matrix and thermally conductive auxiliary components, and directly molded into a high thermal conductivity and flame retardant heat dissipation interface material through a curing process. Through the multi-layer design and systematic application method of the core-shell structured flame retardant, a highly efficient synergy between the heat dissipation performance and fire safety of the thermally conductive material is achieved.

Description

Technical Field

[0001] This invention relates to the field of flame retardants, and more particularly to a core-shell structured flame retardant and its application method in thermally conductive materials. Background Technology

[0002] As high-power electronic devices, especially LED lighting, 5G communication devices, and power battery modules, develop towards higher integration and higher power density, the heat generated during operation increases dramatically. Heat dissipation has become a key factor affecting device performance, reliability, and lifespan. Thermal interface materials (such as thermal paste and thermal pads) are widely used to fill the micro-gaps between the heat source and the heat sink to improve heat transfer efficiency. However, the high-temperature environment inside the equipment and potential electrical faults also pose serious fire safety hazards. This makes the development of interface materials that combine high thermal conductivity with reliable flame retardancy an urgent technical need in this field.

[0003] Currently, the common method used in the industry to impart flame-retardant properties to thermally conductive materials is to directly add conventional flame-retardant fillers, such as aluminum hydroxide, magnesium hydroxide, or halogenated flame retardants, to thermally conductive matrices like silicone and epoxy resin. While this method can improve the flame-retardant rating of the material to some extent, it has a significant drawback: these added flame-retardant fillers form numerous phonon scattering interfaces within the matrix, severely disrupting the original continuous thermal conductivity network within the material, leading to a significant decrease in the overall thermal conductivity (K-value). In other words, existing technologies that add flame retardants through simple physical blending essentially sacrifice the material's core thermal conductivity for flame-retardant functionality, making it difficult to meet the stringent requirements of high-power equipment where "efficient heat dissipation" and "active fire protection" must coexist.

[0004] Therefore, how to achieve inherent flame retardancy of materials without compromising, or even simultaneously improving, thermal conductivity is a core technical challenge that urgently needs to be solved in this field. Summary of the Invention

[0005] The purpose of this invention is to provide a core-shell structured flame retardant and its application method in thermally conductive materials, thereby solving the above-mentioned technical problems.

[0006] To achieve this objective, the present invention adopts the following technical solution: A method for applying a core-shell structured flame retardant in a thermally conductive material includes the following steps: S1, phosphorus antimony oxyphosphate and boron nitride nanosheets are dispersed in a liquid medium and subjected to a hydrothermal reaction to form a thermally conductive and flame-retardant composite core encapsulated by boron nitride nanosheets. After washing and drying, a core-shell structured thermally conductive and flame-retardant precursor is obtained. S2, the core-shell structure thermally conductive flame retardant precursor is dispersed in a solvent, and then coated with borosilicate modified phenolic resin inner shell layer by in-situ polycondensation reaction, and coated with graphene nanosheet outer shell layer by reduction deposition reaction. After separation and drying, a multifunctional core-shell thermally conductive flame retardant with a three-layer structure is obtained. S3, after surface activation treatment, the multifunctional core-shell thermally conductive and flame-retardant agent is compounded with a silicone matrix and thermally conductive auxiliary components, and directly molded into a heat dissipation interface material with high thermal conductivity and flame retardancy through a curing process.

[0007] Optionally, the mass ratio of the antimony phosphate compound to the boron nitride nanosheets is 1:1 to 1:3, the hydrothermal reaction temperature is 160°C to 200°C, and the reaction time is 4 to 8 hours.

[0008] Optionally, when the in-situ polycondensation reaction coats the inner shell of borosilicate modified phenolic resin, the molar ratio of boric acid, phenol and formaldehyde is 1:2:4 to 1:4:6, and the reaction temperature is 80°C to 90°C; when the reduction deposition reaction coats the outer shell of graphene nanosheets, the reducing agent used is ascorbic acid or hydrazine hydrate.

[0009] Optionally, the surface activation treatment specifically involves mixing the multifunctional core-shell thermally conductive flame retardant with an aminosilane coupling agent at a mass ratio of 8:1 to 12:1, and stirring in an ethanol solvent at 60°C to 80°C for 1 to 3 hours.

[0010] Optionally, the composite processing specifically involves mixing the surface-activated multifunctional core-shell thermally conductive flame retardant with a silicone prepolymer at a ratio of 10% to 20%, and adding zinc oxide or aluminum oxide nanoparticles at a mass of 1% to 5% of the silicone prepolymer as a thermally conductive auxiliary component, followed by ultrasonic-mechanical synergistic dispersion for 0.5 to 1.5 hours.

[0011] Optionally, the curing process is a stepped temperature curing: first, pre-curing at 70°C to 90°C for 0.5 to 1.5 hours, and then curing at 110°C to 130°C for 1 to 3 hours.

[0012] Optionally, step S1 specifically includes: S11, to prepare an aqueous suspension of antimony phosphate oxidase, antimony phosphite powder was added to deionized water, and hydrogen peroxide with a mass fraction of 30% was added dropwise under stirring for oxidation treatment. The reaction was continued at 60°C for 2 hours to obtain a stable aqueous suspension of antimony phosphate oxidase. S12, construct a mixed precursor by mixing the phosphorus antimony oxide aqueous suspension with the boron nitride nanosheet suspension pre-dispersed with ethanol at a preset mass ratio, and adding polyethylene glycol as a dispersing aid. The first stage of synergistic dispersion treatment is carried out in an ultrasonic cell disruptor to obtain a uniform mixed precursor suspension. S13, perform interface-enhanced hydrothermal reaction, transfer the mixed precursor suspension to a high-pressure reactor, adjust the pH of the system to 4-5 using dilute nitric acid solution, and then carry out hydrothermal reaction in the temperature range of 160°C to 200°C, so that the boron nitride nanosheets are edge-activated in acidic medium and in situ encapsulated on the surface of phosphorus antimony oxide particles to form a preliminary composite core; S14. The product after hydrothermal reaction is centrifuged and then washed sequentially with deionized water, ethanol and acetone to thoroughly remove unreacted ions, organic dispersants and by-products, to obtain high-purity composite core wet material. S15, precursor forming: The composite core wet material is placed in a vacuum freeze dryer and pre-frozen at -50°C for 4 hours. Then, it is sublimated and dried under a vacuum of less than 10 Pa to finally obtain a core-shell structure thermally conductive flame retardant precursor with a loose structure and no hard agglomeration.

[0013] Optionally, step S2 specifically includes: S21, the core-shell structure thermally conductive flame retardant precursor is uniformly dispersed in ethanol solvent, and boric acid and phenyltrimethoxysilane are added as boron source and silicon source, respectively. At the same time, an appropriate amount of dispersant is added, and a uniform mixed dispersion is obtained by ultrasonic treatment. S22, formaldehyde solution and catalyst are added dropwise to the mixed dispersion. A stepwise heating program is adopted. First, an in-situ pre-condensation reaction is carried out at 60°C to 70°C for 1 to 2 hours, and then the temperature is raised to 80°C to 90°C to continue the reaction for 2 to 4 hours. Boron and silicon elements are effectively introduced into the phenolic resin network through the formation of BO-Si bonding and coated on the surface of the precursor to form an inner shell intermediate with high thermal stability. S23, Introduce graphene oxide and perform controlled reduction deposition. Add an aqueous dispersion containing graphene oxide to the reaction system of the inner shell intermediate, adjust the pH value of the system, and slowly add ascorbic acid solution as a reducing agent under stirring and gentle heating conditions. By controlling the dropping rate of the reducing agent and the reaction temperature, the graphene oxide is reduced in situ and gradually deposited onto the surface of the inner shell. S24. After the outer shell coating and post-treatment are completed, the graphene nanosheets are kept warm for 0.5 to 1.5 hours to ensure that the outer shell structure is dense and complete. Then the reaction is stopped and the resulting suspension is naturally cooled to room temperature. S25, the cooled suspension is centrifuged, and the resulting solid product is washed with deionized water and organic solvent in sequence to remove impurities. Finally, it is dried in a vacuum drying oven at 60°C to 80°C to obtain a multifunctional core-shell thermally conductive flame retardant with a three-layer structure of composite core / borosilicate phenolic resin inner shell / graphene nanosheet outer shell.

[0014] This invention also provides a core-shell structured flame retardant, applied to thermally conductive materials using the method described above. The core-shell structured flame retardant has a three-layer core-shell structure, comprising, from the inside out: The core is a thermally conductive and flame-retardant composite core, composed of antimony phosphorus oxide and boron nitride nanosheets coated on its surface; The inner shell layer is a borosilicate-modified phenolic resin layer that covers the outer surface of the composite core. The outer shell layer is a layer of graphene nanosheets that covers the outer surface of the inner shell layer.

[0015] Optionally, in the composite core, the mass ratio of antimony phosphate compound to boron nitride nanosheets is 1:1 to 1:3; in the borosilicate modified phenolic resin inner shell, the molar ratio of boron to silicon is 1:0.5 to 1:2; and the coating coverage of the graphene nanosheet outer shell is not less than 85%.

[0016] Compared with existing technologies, this invention has the following advantages: First, phosphorus antimony oxide and boron nitride nanosheets are combined in a liquid medium through a hydrothermal reaction to form a thermally conductive and flame-retardant composite core encapsulated by boron nitride nanosheets. After washing and drying, a core-shell structured thermally conductive and flame-retardant precursor is obtained. Subsequently, the precursor is dispersed in a solvent and coated with a borosilicate-modified phenolic resin inner shell layer through an in-situ polycondensation reaction, and then coated with a graphene nanosheet outer shell layer through a reduction deposition reaction. After separation and drying, a multifunctional core-shell thermally conductive and flame-retardant agent with a three-layer structure is obtained. After surface activation treatment, the flame retardant is composited with a silicone matrix and thermally conductive auxiliary components, and then directly molded into a high thermally conductive and flame-retardant heat dissipation interface material through a curing process, thereby completing the integrated application of the core-shell structured flame retardant in thermally conductive materials. This solution achieves efficient synergy between heat dissipation performance and fire safety in thermally conductive materials through the multi-layer design and systematic application method of the core-shell structured flame retardant. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] The structures, proportions, sizes, etc., shown in the accompanying drawings of this specification are only for the purpose of assisting those skilled in the art in understanding and reading the content disclosed in the specification, and are not intended to limit the conditions under which the present invention can be implemented. Therefore, they have no substantial technical significance. Any modifications to the structure, changes in the proportions, or adjustments to the size, without affecting the effects and objectives that the present invention can produce, should still fall within the scope of the technical content disclosed in the present invention.

[0019] Figure 1 This is a flowchart illustrating the application method of the core-shell structure flame retardant in thermally conductive materials according to Embodiment 1. Detailed Implementation

[0020] To make the objectives, features, and advantages of this invention more apparent and understandable, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the embodiments described below are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0021] In the description of this invention, it should be understood that the terms "upper," "lower," "top," "bottom," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing the invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the invention. It should be noted that when a component is considered to be "connected" to another component, it can be directly connected to the other component or there may be a component positioned centrally in the connection.

[0022] The technical solution of the present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0023] Example 1: Combination Figure 1 As shown, this embodiment of the invention provides a method for applying a core-shell structured flame retardant in a thermally conductive material, comprising the following steps: S1, phosphorus antimony oxide and boron nitride nanosheets are dispersed in a liquid medium and subjected to a hydrothermal reaction to form a thermally conductive and flame-retardant composite core encapsulated by boron nitride nanosheets. After washing and drying, a core-shell structured thermally conductive and flame-retardant precursor is obtained.

[0024] A hydrothermal synthesis method was used to achieve the chemical bonding and structural reorganization of antimony phosphorus oxide and boron nitride nanosheets at the molecular / nanoscale. Specifically, the two raw materials were strongly dispersed (e.g., by ultrasound) in a liquid medium (such as water or an alcohol-water system) to form a homogeneous suspension, and then placed in a sealed high-pressure reactor for hydrothermal reaction. Under the high temperature and high pressure hydrothermal environment, the surface activity of boron nitride nanosheets was significantly enhanced. The functional groups (such as -OH) at the edges of the sheets could interact strongly with the metal ions or oxygen-containing groups on the surface of the antimony phosphorus oxide particles (such as coordination and hydrogen bonding), thereby driving the boron nitride nanosheets to be in situ and directionally wrapped around the surface of the antimony phosphorus oxide particles. This process formed a "brick-and-mortar" composite core: the antimony phosphorus oxide acted as the flame-retardant bricks, while the highly thermally conductive boron nitride nanosheets acted as mortar and heat-conducting wires to wrap and connect them, achieving a native integrated design of flame-retardant function and thermal conductivity from the core.

[0025] S2, the core-shell structure thermally conductive flame retardant precursor is dispersed in a solvent, and then coated with borosilicate modified phenolic resin inner shell layer by in-situ polycondensation reaction, and coated with graphene nanosheet outer shell layer by reduction deposition reaction. After separation and drying, a multifunctional core-shell thermally conductive flame retardant with a three-layer structure is obtained. It should be noted that a step-by-step sequential coating strategy is adopted, in which shells with different functions are constructed sequentially on the composite core obtained in the early stage. First, the precursor is dispersed in a suitable solvent (such as ethanol), and a boron source (such as boric acid), a silicon source (such as a silane coupling agent), and phenolic resin monomers (phenol, formaldehyde) are added. By controlling the reaction conditions (temperature, pH, catalyst), an in-situ polycondensation reaction occurs on the surface of the composite core, generating a borosilicate-modified phenolic resin as the inner shell layer. The significance of this inner shell layer is as follows: First, it has excellent char-forming properties and thermal stability, and can serve as a highly efficient fire barrier; second, the boron and silicon elements in it can form chemical bonds with the core and subsequent outer shell, greatly enhancing the interfacial bonding force and thermomechanical stability of the entire core-shell structure, preventing the layers from peeling off during processing and use.

[0026] Building upon this foundation, the second step involves introducing a graphene oxide dispersion into the system. A mild chemical reduction method (such as using ascorbic acid) is then used to deposit the dispersion onto the surface of the particles already coated with the inner shell, reducing it to graphene nanosheets and forming the outermost shell. This graphene shell plays a crucial role: it constructs a highly conductive / thermally conductive outer network, significantly improving not only the thermal conductivity of the particles themselves but also acting as a thermal bridge in the final composite material, connecting adjacent filler particles and thus substantially enhancing the overall thermal conductivity of the material macroscopically. After separation and drying, a three-layer core-shell flame retardant with synergistic structure and performance is obtained, consisting of a core / borosilicate phenolic resin / graphene nanosheet composite.

[0027] S3 is a multifunctional core-shell thermally conductive and flame-retardant agent that is surface-activated, then composited with a silicone matrix and thermally conductive auxiliary components, and directly molded into a high thermal conductivity and flame-retardant heat dissipation interface material through a curing process.

[0028] First, the core-shell flame retardant undergoes surface activation treatment (typically using silane coupling agents) to graft chemically compatible organic functional groups onto its graphene shell. This fundamentally solves the industry problem of easy agglomeration and uneven dispersion of nanofillers in the matrix, ensuring that each functional particle can effectively perform its function. Next, the surface-activated flame retardant is composited with a silicone matrix (such as vinyl silicone oil) and thermally conductive auxiliary components (such as a small amount of alumina and zinc oxide microspheres to further fill voids and optimize the thermal conductivity network). This process employs a dispersion technique combining high-shear mechanical stirring and ultrasonic cavitation to break down residual soft agglomerates, achieving nanoscale uniform dispersion of the functional filler in the matrix and forming a stable composite slurry.

[0029] Finally, the slurry is set through a curing process (such as heating to induce an addition vulcanization reaction in the silicone). During this process, the well-dispersed core-shell particles not only provide flame retardancy and thermal conductivity, but their surface graphene layers also interlock within the cured silicone three-dimensional network, forming a continuous, highly efficient thermally conductive and flame-retardant barrier network. Ultimately, this results in a solid thermal interface paste or gasket material with excellent thermal conductivity, high flame retardancy, and stable reliability, which can be directly applied to the thermal management of high-power electronic devices.

[0030] The working principle of this invention is as follows: First, phosphorus antimony oxide and boron nitride nanosheets are combined in a liquid medium through a hydrothermal reaction to form a thermally conductive and flame-retardant composite core encapsulated by boron nitride nanosheets. After washing and drying, a core-shell structured thermally conductive and flame-retardant precursor is obtained. Subsequently, the precursor is dispersed in a solvent and coated with a borosilicate-modified phenolic resin inner shell layer through an in-situ polycondensation reaction, and then coated with a graphene nanosheet outer shell layer through a reduction deposition reaction. After separation and drying, a multifunctional core-shell thermally conductive and flame-retardant agent with a three-layer structure is obtained. After surface activation treatment, the flame retardant is composited with a silicone matrix and thermally conductive auxiliary components, and then directly molded into a high thermally conductive and flame-retardant heat dissipation interface material through a curing process, thereby completing the integrated application of the core-shell structured flame retardant in thermally conductive materials. This solution achieves efficient synergy between heat dissipation performance and fire safety in thermally conductive materials through the multi-layer design and systematic application method of the core-shell structured flame retardant.

[0031] In this embodiment, the mass ratio of antimony phosphate oxyphosphate to boron nitride nanosheets is 1:1 to 1:3, the hydrothermal reaction temperature is 160°C to 200°C, and the reaction time is 4 to 8 hours.

[0032] The hydrothermal temperature and time window are designed to provide sufficient energy to drive the edge activation and orientation of boron nitride nanosheets, enabling them to adhere firmly to the core surface through chemical action. If the temperature is too low or the time too short, the reaction will be incomplete, resulting in loose encapsulation; if the temperature is too high, it may damage the boron nitride structure or lead to side reactions.

[0033] In this embodiment, when the borosilicate-modified phenolic resin inner shell is coated by in-situ polycondensation reaction, the molar ratio of boric acid, phenol and formaldehyde is 1:2:4 to 1:4:6, and the reaction temperature is 80°C to 90°C; when the graphene nanosheet outer shell is coated by reduction deposition reaction, the reducing agent used is ascorbic acid or hydrazine hydrate.

[0034] It should be noted that for the in-situ polycondensation reaction, the examples specify a molar ratio of boric acid, phenol, and formaldehyde of 1:2:4 to 1:4:6, and a reaction temperature of 80°C to 90°C. This ratio aims to precisely control the degree of crosslinking of the phenolic resin and the amount of boron and silicon introduced. Sufficient phenol and formaldehyde ensure the formation of a complete three-dimensional resin network, while the addition of boric acid and silicon sources allows the resin to form BO-Si bonds with higher heat resistance during curing, significantly enhancing the thermal stability and char-forming properties of the inner shell. This temperature range is the ideal range for the polycondensation of phenolic resin prepolymers, ensuring the reaction rate while preventing excessively vigorous reactions that could lead to uncontrolled gelation or uneven coating. For the construction of the graphene outer shell, ascorbic acid or hydrazine hydrate is chosen as a reducing agent because they can gently and effectively reduce graphene oxide and restore its sp. 2 The conjugated structure (the basis of high thermal conductivity) and the slow reduction process on the particle surface facilitate the deposition and overlapping of graphene nanosheets in a more ordered manner, forming a continuous and robust conductive and thermally conductive outer shell, rather than a chaotic attachment.

[0035] As a preferred embodiment, the surface activation treatment specifically involves mixing a multifunctional core-shell thermally conductive flame retardant with an aminosilane coupling agent at a mass ratio of 8:1 to 12:1, and then stirring the mixture in an ethanol solvent at 60°C to 80°C for 1 to 3 hours.

[0036] As a preferred embodiment, the examples specify the parameters for surface activation treatment in detail: the mass ratio of aminosilane coupling agent to multifunctional core-shell thermally conductive flame retardant is 8:1 to 12:1, the treatment temperature is 60°C to 80°C, and the time is 1 to 3 hours. The core purpose of this step is to construct a strong "molecular bridge" between the inorganic / graphene shell and the organosilicon matrix. The selected mass ratio ensures that the amount of coupling agent is sufficient to form an effective monomolecular modification layer on the surface of the flame retardant particles; too little will result in incomplete modification, while too much may lead to multilayer physical adsorption, which will negatively affect performance. Treatment in ethanol solvent at 60°C to 80°C is beneficial for the hydrolysis of the coupling agent and its condensation reaction with hydroxyl and other groups on its surface, forming a stable chemical bond.

[0037] In this embodiment, the composite processing specifically involves mixing the surface-activated multifunctional core-shell thermally conductive flame retardant with the silicone prepolymer at a ratio of 10% to 20%, and adding zinc oxide or aluminum oxide nanoparticles at a mass of 1% to 5% of the silicone prepolymer as a thermally conductive auxiliary component, followed by ultrasonic-mechanical synergistic dispersion for 0.5 to 1.5 hours.

[0038] It should be noted that the addition amount of 10% to 20% is a balanced performance range: within this range, the core-shell flame retardant is sufficient to form a preliminary thermally conductive and flame-retardant network in the matrix, while avoiding excessive addition that would lead to a sharp increase in slurry viscosity, processing difficulties, or embrittlement of mechanical properties. The additional addition of a small amount of zinc oxide / alumina nanoparticles fills any tiny gaps that may exist between the core-shell particles, further optimizing the thermal conductivity pathway and improving the overall thermal conductivity. Ultrasonic-mechanical synergistic dispersion is a key process: high-speed mechanical shearing primarily breaks down large-sized soft agglomerates, while the ultrasonic cavitation effect further separates nanoscale agglomerates. A dispersion time of 0.5 to 1.5 hours is sufficient to achieve a near-monodispersed state of the functional filler in the matrix while avoiding material degradation caused by prolonged ultrasonication, thus forming a uniform and dense composite slurry.

[0039] In this embodiment, it is further explained that the curing process is a stepped temperature curing: first, pre-curing at 70°C to 90°C for 0.5 to 1.5 hours, and then curing at 110°C to 130°C for 1 to 3 hours.

[0040] The process involves pre-curing at 70°C to 90°C for 0.5 to 1.5 hours, followed by curing at 110°C to 130°C for 1 to 3 hours. This design has a clear physicochemical purpose. The first stage (pre-curing) has a relatively low temperature, and its main function is to allow the silicone prepolymer to begin slow cross-linking, gradually increasing the system viscosity. This fixes the already uniformly dispersed functional filler network, preventing it from migrating or settling due to convection or other factors during subsequent heating, which could lead to uneven distribution. Simultaneously, this stage also facilitates the gradual escape of small molecule volatiles (such as residual solvents), reducing bubble defects. The subsequent second stage (main curing) is carried out at a higher temperature. Its purpose is to provide sufficient energy to activate and complete the deep cross-linking reaction of the silicone matrix, enabling the composite material to achieve its final mechanical strength, thermal stability, and long-term reliability.

[0041] In this embodiment, step S1 specifically includes: S11, to prepare an aqueous suspension of antimony phosphate oxidase, antimony phosphite powder was added to deionized water, and hydrogen peroxide with a mass fraction of 30% was added dropwise under stirring for oxidation treatment. The reaction was continued at 60°C for 2 hours to obtain a stable aqueous suspension of antimony phosphate oxidase. A mild oxidation method was employed to convert antimony phosphite powder, serving as a precursor, into the target antimony phosphorus oxide in an aqueous phase. Specifically, under stirring conditions, 30% hydrogen peroxide (by mass) was added dropwise to an aqueous suspension of antimony phosphite. Hydrogen peroxide, as a clean oxidant, partially oxidized the phosphorus and antimony elements in the phosphite ions to higher valence states, generating a more complex antimony phosphorus oxide with superior thermal stability and flame-retardant catalytic efficacy. The reaction temperature was controlled at 60°C for 2 hours to ensure a complete, uniform, and controllable oxidation reaction, avoiding excessive particle growth or agglomeration caused by vigorous reactions.

[0042] S12, construct a mixed precursor by mixing an aqueous suspension of antimony phosphate oxyphosphate with a boron nitride nanosheet suspension pre-dispersed in ethanol at a preset mass ratio, and adding polyethylene glycol as a dispersing agent. The first stage of synergistic dispersion is carried out in an ultrasonic cell disruptor to obtain a uniform mixed precursor suspension. Hydrophobic boron nitride nanosheets were first ultrasonically dispersed in ethanol to form a stable suspension; while antimony phosphorus oxide remained stable in the aqueous suspension. When mixing the two at a predetermined mass ratio, the introduction of polyethylene glycol (PEG) as a dispersant was crucial. PEG is an amphiphilic polymer whose long chains can simultaneously adsorb onto the surfaces of both types of particles, effectively preventing heterogeneous aggregation between different particles through steric hindrance. Subsequently, a high-intensity synergistic dispersion was performed using an ultrasonic cell disruptor. The resulting cavitation effect and microjets provided enormous instantaneous energy, powerfully breaking down all soft aggregates and "dispersing" and "stirring" the boron nitride nanosheets and antimony phosphorus oxide particles to a molecular-level contact state.

[0043] S13, an interface-enhanced hydrothermal reaction is carried out. The mixed precursor suspension is transferred to a high-pressure reactor. The pH of the system is adjusted to 4-5 using dilute nitric acid solution. Then, a hydrothermal reaction is carried out in the temperature range of 160°C to 200°C, so that the boron nitride nanosheets are edge-activated in the acidic medium and in situ encapsulated on the surface of the antimony phosphate oxide particles to form a preliminary composite core. By controlling the hydrothermal chemical environment, boron nitride nanosheets are driven to be in-situ, oriented, and firmly coated onto the surface of antimony phosphate oxide. The innovative operation involves adjusting the pH of the system to a weakly acidic range of 4-5 using dilute nitric acid solution. Under these acidic conditions, the -NH2 groups at the edges of the boron nitride nanosheets become protonated and positively charged, while the surface of the antimony phosphate oxide particles may become negatively charged. The introduction of electrostatic attraction greatly promotes the close proximity and interfacial bonding between the two. Subsequently, in a high-temperature, high-pressure hydrothermal environment of 160°C to 200°C, the highly reactive water molecules further activate the edges of the boron nitride nanosheets, exposing more active sites. Simultaneously, they provide sufficient energy to drive these active sites to undergo dehydroxylation condensation and other chemical reactions with the metal hydroxyl groups on the surface of the antimony phosphate oxide, forming strong chemical bonds such as BO-Sb or BOP. This interfacial chemical bonding achieved under specific pH and temperature conditions is far stronger than simple physical adsorption, resulting in a preliminary composite core with excellent structural stability and interfacial thermal conductivity.

[0044] S14. The product after hydrothermal reaction is centrifuged and then washed sequentially with deionized water, ethanol and acetone to thoroughly remove unreacted ions, organic dispersants and by-products, to obtain high-purity composite core wet material. After the hydrothermal reaction, in addition to the target complex nucleus, there are still unreacted ions (such as H+) remaining in the system. + NO3 - Small molecules from the decomposition or unadsorbed dispersing agent (polyethylene glycol), as well as other possible byproducts, cannot be completely removed by a simple single water wash, especially organic matter embedded in particle gaps or weakly adsorbed.

[0045] Therefore, this scheme adopts a sequential washing method based on solvent polarity gradient: first, washing with deionized water is used to remove most of the inorganic salt ions and water-soluble impurities; then, washing with ethanol is used, taking advantage of its neutral polarity and good organic dissolving ability to remove most of the residual organic dispersants and some hydrophobic impurities; finally, washing with acetone is used. As a low-boiling-point, highly soluble organic solvent, acetone can effectively extract and remove trace organic matter that could not be removed in the first two rounds of washing, and its volatility facilitates subsequent drying.

[0046] This three-step gradient washing process—water-alcohol-ketone—is like fine filtration, progressing step by step to ensure that the final composite core wet material has the highest chemical purity. This prevents impurities from decomposing or damaging the interface during subsequent high-temperature applications, thus affecting the material's ultimate performance.

[0047] S15, precursor molding, the composite core wet material is placed in a vacuum freeze dryer and pre-frozen at -50°C for 4 hours, and then sublimated and dried under a vacuum of less than 10Pa to finally obtain a core-shell structure thermally conductive flame retardant precursor with a loose structure and no hard agglomeration.

[0048] In conventional drying processes, the evaporation of liquid (water or solvent) between particles generates significant capillary forces at the gas-liquid interface. This force forces the moistened nanoparticles to approach and compress each other, eventually "welding" together as the liquid disappears, forming hard agglomerates that are difficult to redisperse. This severely damages the established core-shell structure and creates defects in the final composite material. Freeze-drying perfectly avoids this problem: First, the wet material is rapidly frozen at -50°C, causing all liquid water to solidify into solid ice crystals. The supporting effect of the ice crystals keeps the particles in a dispersed state. Subsequently, the system is slowly heated under a high vacuum environment below 10 Pa, allowing the ice crystals to sublimate directly into water vapor without passing through a liquid phase and be removed. Since there is no liquid phase present, the capillary forces are completely eliminated, allowing the particles to maintain their loose, frozen state. The resulting core-shell structured thermally conductive flame retardant precursor is a fluffy, flocculent or powdery form with almost no hard agglomerates, a large specific surface area, and high activity.

[0049] In this embodiment, step S2 specifically includes: S21, the core-shell structure thermally conductive flame retardant precursor is uniformly dispersed in ethanol solvent, and boric acid and phenyltrimethoxysilane are added as boron source and silicon source, respectively. At the same time, an appropriate amount of dispersant is added, and a uniform mixed dispersion is obtained by ultrasonic treatment. Ethanol, with its moderate polarity and boiling point, is an ideal medium for achieving thorough dispersion of the precursor and uniformity of subsequent reactions. The introduction of boric acid and phenyltrimethoxysilane precisely provides the boron and silicon sources for constructing the BO-Si network at the molecular level. The combination of an appropriate amount of dispersant and ultrasonic treatment aims to completely destroy the soft agglomerates of the precursor, forming a highly homogeneous dispersion system with sufficient contact between all components.

[0050] S22, formaldehyde solution and catalyst are added dropwise to the mixed dispersion. A stepwise heating program is adopted. First, an in-situ pre-condensation reaction is carried out at 60°C to 70°C for 1 to 2 hours. Then, the temperature is raised to 80°C to 90°C and the reaction is continued for 2 to 4 hours. Boron and silicon elements are effectively introduced into the phenolic resin network through the formation of BO-Si bonding and coated on the surface of the precursor to form an inner shell intermediate with high thermal stability. Prepolymerization is first performed at a lower temperature to generate a prepolymer with a suitable molecular weight and good flowability, enabling it to effectively wet and adsorb onto the core surface. Subsequently, the temperature is increased to complete deep curing, which promotes the tight cross-linking of the resin network and drives boron and silicon elements to be firmly embedded in the polymer backbone through the formation of BO-Si chemical bonds.

[0051] S23, Introduce graphene oxide and perform controlled reduction deposition. Add an aqueous dispersion containing graphene oxide to the reaction system of the inner shell intermediate, adjust the pH value of the system, and slowly add ascorbic acid solution as a reducing agent under stirring and gentle heating conditions. By controlling the dropping rate of the reducing agent and the reaction temperature, the graphene oxide is reduced in situ and gradually deposited on the surface of the inner shell. By adjusting the pH of the system and controlling the dropping rate of the reducing agent and the reaction temperature, a slow and gentle in-situ reduction was achieved. This strategy is crucial: it allows the reduced graphene nanosheets to be deposited layer by layer on the surface of the inner shell in a more ordered and dense manner, rather than rapidly and disorderly agglomerating and precipitating. This controllable deposition is beneficial for forming a complete, continuous, and firmly bonded graphene conductive and thermally conductive outer shell.

[0052] S24. After the outer shell coating and post-treatment are completed, the graphene nanosheets are kept warm for 0.5 to 1.5 hours to ensure that the outer shell structure is dense and complete. Then the reaction is stopped and the resulting suspension is naturally cooled to room temperature. This process can effectively repair microscopic defects that may occur during deposition, promote the overlap and fusion between graphene sheets, thereby ensuring the density and integrity of the outer shell structure and improving its overall performance as a physical barrier and heat conduction network.

[0053] S25, the cooled suspension is centrifuged, and the resulting solid product is washed with deionized water and organic solvent in sequence to remove impurities. Finally, it is dried in a vacuum drying oven at 60°C to 80°C to obtain a multifunctional core-shell thermally conductive flame retardant with a three-layer structure of composite core / borosilicate phenolic resin inner shell / graphene nanosheet outer shell.

[0054] A gradient washing process using deionized water and organic solvents (such as ethanol) was employed to efficiently remove water-soluble salts and organic reaction residues, respectively. Finally, medium-temperature vacuum drying was chosen to ensure complete solvent removal while avoiding excessively high temperatures that could damage the constructed fine core-shell structure, particularly preventing oxidation of the graphene layer or excessive pyrolysis of the phenolic resin.

[0055] Example 2: This invention also provides a core-shell structured flame retardant, applied to a thermally conductive material using the method described in Example 1. The core-shell structured flame retardant has a three-layer core-shell structure, comprising, from the inside out: The core is a thermally conductive and flame-retardant composite core, composed of antimony phosphorus oxide and boron nitride nanosheets coated on its surface; The inner shell is a borosilicate-modified phenolic resin layer that covers the outer surface of the composite core. The outer shell layer consists of graphene nanosheets that coat the outer surface of the inner shell layer.

[0056] In this embodiment, the mass ratio of antimony phosphate compound to boron nitride nanosheets is 1:1 to 1:3; the molar ratio of boron to silicon in the borosilicate modified phenolic resin inner shell is 1:0.5 to 1:2; and the coating coverage of the graphene nanosheet outer shell is not less than 85%.

[0057] The above-described embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for applying a core-shell structured flame retardant in a thermally conductive material, characterized in that, Includes the following steps: S1, phosphorus antimony oxyphosphate and boron nitride nanosheets are dispersed in a liquid medium and subjected to a hydrothermal reaction to form a thermally conductive and flame-retardant composite core encapsulated by boron nitride nanosheets. After washing and drying, a core-shell structured thermally conductive and flame-retardant precursor is obtained. S2, the core-shell structure thermally conductive flame retardant precursor is dispersed in a solvent, and then coated with borosilicate modified phenolic resin inner shell layer by in-situ polycondensation reaction, and coated with graphene nanosheet outer shell layer by reduction deposition reaction. After separation and drying, a multifunctional core-shell thermally conductive flame retardant with a three-layer structure is obtained. S3, after surface activation treatment, the multifunctional core-shell thermally conductive and flame-retardant agent is compounded with a silicone matrix and thermally conductive auxiliary components, and directly molded into a heat dissipation interface material with high thermal conductivity and flame retardancy through a curing process.

2. The method for applying the core-shell structured flame retardant in thermally conductive materials according to claim 1, characterized in that, The mass ratio of the phosphorus antimony oxide to the boron nitride nanosheets is 1:1 to 1:3, the hydrothermal reaction temperature is 160°C to 200°C, and the reaction time is 4 to 8 hours.

3. The method for applying the core-shell structured flame retardant in thermally conductive materials according to claim 1, characterized in that, When the in-situ polycondensation reaction coats the inner shell of borosilicate modified phenolic resin, the molar ratio of boric acid, phenol and formaldehyde is 1:2:4 to 1:4:6, and the reaction temperature is 80°C to 90°C; when the reduction deposition reaction coats the outer shell of graphene nanosheets, the reducing agent used is ascorbic acid or hydrazine hydrate.

4. The method for applying the core-shell structured flame retardant in thermally conductive materials according to claim 1, characterized in that, The surface activation treatment specifically involves mixing the multifunctional core-shell thermally conductive flame retardant with an aminosilane coupling agent at a mass ratio of 8:1 to 12:1, and then stirring the mixture in an ethanol solvent at 60°C to 80°C for 1 to 3 hours.

5. The method for applying the core-shell structured flame retardant in thermally conductive materials according to claim 1, characterized in that, The composite processing specifically involves mixing the surface-activated multifunctional core-shell thermally conductive flame retardant with a silicone prepolymer at a ratio of 10% to 20%, and adding zinc oxide or aluminum oxide nanoparticles at a mass of 1% to 5% of the silicone prepolymer as a thermally conductive auxiliary component, followed by ultrasonic-mechanical synergistic dispersion for 0.5 to 1.5 hours.

6. The method for applying the core-shell structured flame retardant in thermally conductive materials according to claim 1, characterized in that, The curing process is a stepped temperature curing process: first, pre-curing at 70°C to 90°C for 0.5 to 1.5 hours, and then curing at 110°C to 130°C for 1 to 3 hours.

7. The method for applying the core-shell structured flame retardant in thermally conductive materials according to claim 1, characterized in that, Step S1 specifically includes: S11, to prepare an aqueous suspension of antimony phosphate oxidase, antimony phosphite powder was added to deionized water, and hydrogen peroxide with a mass fraction of 30% was added dropwise under stirring for oxidation treatment. The reaction was continued at 60°C for 2 hours to obtain a stable aqueous suspension of antimony phosphate oxidase. S12, construct a mixed precursor by mixing the phosphorus antimony oxide aqueous suspension with the boron nitride nanosheet suspension pre-dispersed with ethanol at a preset mass ratio, and adding polyethylene glycol as a dispersing aid. The first stage of synergistic dispersion treatment is carried out in an ultrasonic cell disruptor to obtain a uniform mixed precursor suspension. S13, perform interface-enhanced hydrothermal reaction, transfer the mixed precursor suspension to a high-pressure reactor, adjust the pH of the system to 4-5 using dilute nitric acid solution, and then carry out hydrothermal reaction in the temperature range of 160°C to 200°C, so that the boron nitride nanosheets are edge-activated in acidic medium and in situ encapsulated on the surface of phosphorus antimony oxide particles to form a preliminary composite core; S14. The product after hydrothermal reaction is centrifuged and then washed sequentially with deionized water, ethanol and acetone to thoroughly remove unreacted ions, organic dispersants and by-products, to obtain high-purity composite core wet material. S15, precursor forming: The composite core wet material is placed in a vacuum freeze dryer and pre-frozen at -50°C for 4 hours. Then, it is sublimated and dried under a vacuum of less than 10 Pa to finally obtain a core-shell structure thermally conductive flame retardant precursor with a loose structure and no hard agglomeration.

8. The method for applying the core-shell structured flame retardant in thermally conductive materials according to claim 1, characterized in that, Step S2 specifically includes: S21, the core-shell structure thermally conductive flame retardant precursor is uniformly dispersed in ethanol solvent, and boric acid and phenyltrimethoxysilane are added as boron source and silicon source, respectively. At the same time, an appropriate amount of dispersant is added, and a uniform mixed dispersion is obtained by ultrasonic treatment. S22, formaldehyde solution and catalyst are added dropwise to the mixed dispersion. A stepwise heating program is adopted. First, an in-situ pre-condensation reaction is carried out at 60°C to 70°C for 1 to 2 hours, and then the temperature is raised to 80°C to 90°C to continue the reaction for 2 to 4 hours. Boron and silicon elements are effectively introduced into the phenolic resin network through the formation of BO-Si bonding and coated on the surface of the precursor to form an inner shell intermediate with high thermal stability. S23, Introduce graphene oxide and perform controlled reduction deposition. Add an aqueous dispersion containing graphene oxide to the reaction system of the inner shell intermediate, adjust the pH value of the system, and slowly add ascorbic acid solution as a reducing agent under stirring and gentle heating conditions. By controlling the dropping rate of the reducing agent and the reaction temperature, the graphene oxide is reduced in situ and gradually deposited onto the surface of the inner shell. S24. After the outer shell coating and post-treatment are completed, the graphene nanosheets are kept warm for 0.5 to 1.5 hours to ensure that the outer shell structure is dense and complete. Then the reaction is stopped and the resulting suspension is naturally cooled to room temperature. S25, the cooled suspension is centrifuged, and the resulting solid product is washed with deionized water and organic solvent in sequence to remove impurities. Finally, it is dried in a vacuum drying oven at 60°C to 80°C to obtain a multifunctional core-shell thermally conductive flame retardant with a three-layer structure of composite core / borosilicate phenolic resin inner shell / graphene nanosheet outer shell.

9. A core-shell structured flame retardant, characterized in that, The application method as described in any one of claims 1 to 8 is used in thermally conductive materials, wherein the core-shell structured flame retardant has a three-layer core-shell structure, comprising, from the inside out: The core is a thermally conductive and flame-retardant composite core, composed of antimony phosphorus oxide and boron nitride nanosheets coated on its surface; The inner shell layer is a borosilicate-modified phenolic resin layer that covers the outer surface of the composite core. The outer shell layer is a layer of graphene nanosheets that covers the outer surface of the inner shell layer.

10. The core-shell structured flame retardant according to claim 9, characterized in that, In the composite core, the mass ratio of antimony phosphate compound to boron nitride nanosheets is 1:1 to 1:3; in the borosilicate modified phenolic resin inner shell, the molar ratio of boron to silicon is 1:0.5 to 1:2; and the coating coverage of the graphene nanosheet outer shell is not less than 85%.

Images