Low dielectric loss and high thermal conductivity composite aramid insulation paper and preparation method thereof

Abstract

The application discloses a kind of low dielectric loss, high thermal conductivity's composite aramid insulation paper and preparation method thereof, belong to aramid fiber paper-based functional material technical field.Method includes: aramid short-cut fiber is gas-phase fluorination modification;Hydroxylated modification is carried out to boron nitride nanosheet;Hydroxylated boron nitride nanosheet is modified with the dispersion liquid of hydroxyl-terminated polydimethylsiloxane and amino silane coupling agent KH550 compounding gradient concentration insulation polymer, and polymer-coated boron nitride is obtained;Then it is grafted to fluorinated aramid short-cut fiber surface by silane coupling agent KH550;Finally, with aramid fibrid is blended, and is formed by wet laying and high temperature hot pressing.The application solves the problems that boron nitride nanosheet is easy to agglomerate and the interface of aramid matrix is poor in combination by multi-level interface design, and the obtained composite paper has high thermal conductivity, low dielectric loss and high breakdown strength, and is suitable for high-frequency high-voltage electrical equipment insulation heat dissipation.

Description

Technical Field

[0001] This invention belongs to the technical field of aramid fiber paper-based functional materials, specifically relating to a composite aramid insulating paper with low dielectric loss and high thermal conductivity and its preparation method. Background Technology

[0002] In the field of electrical insulation products such as motors and transformers, insulating paper made from aramid fibers has significant advantages over natural fiber products (such as cellulose-based insulating paper) in terms of thermal stability, electrical properties, and mechanical properties, which can significantly improve the service life and safety of electrical appliances. Aramid insulating paper has excellent mechanical properties, insulation properties, good high-temperature resistance, and chemical stability.

[0003] With the increasing number of devices operating in complex environments, there is a need to improve the thermal conductivity, mechanical strength, corona resistance, heat resistance, moisture resistance, and service life of traditional aramid insulation materials. Radar generates a large amount of heat during operation, placing new demands on the thermal conductivity of aramid. Aramid thermally conductive materials can achieve coordinated control of heat diffusion and electromagnetic wave transmission through a honeycomb structure design, ensuring the stable operation of radar and communication systems at high temperatures.

[0004] Aramid paper is a thin, lightweight insulating paper made primarily from chopped aramid fibers, precipitated aramid fibers, or pulp fibers through wet web forming and hot pressing post-treatment processes. It possesses excellent compressive strength, mechanical strength, and thermal stability, and is widely used in power facilities such as generators, motors, and transformers. Aramid fibers (AF), short for aromatic polyamide, are high-performance polymer fibers composed of benzene rings and amide groups linked together. They exhibit extremely high thermodynamic and insulating properties, making them one of the three major special fibers in modern industry, playing a crucial role in high-precision and cutting-edge fields such as military and aerospace. The basic unit composition and molecular structure of aramid fibers determine the performance of insulating paper made from aramid. This results in traditional aramid insulating paper having limited functionality, poor interfacial bonding, insufficient thermal conductivity, poor corona resistance, and temperature resistance that needs further improvement. Therefore, improving the thermal conductivity, corona resistance, and temperature resistance of aramid insulating paper is urgently needed.

[0005] Chinese patent CN 118621613 B discloses a high thermal conductivity aramid insulating paper for electrical equipment, its preparation method, and its application. The method first obtains GO / graphene fibers through wet spinning; then, using BNNT-NH2 spinning solution as the core layer and aramid nanofiber ANF spinning solution as the sheath layer, BNNT-NH2@ANF composite fibers are obtained through wet spinning; finally, GO / graphene fibers, BNNT-NH2@ANF composite fibers, and AF fiber pulps are thoroughly mixed to prepare AF / (GO / graphene) / BNNT-NH2@ANF insulating paper.

[0006] The aforementioned patent describes a method to increase the thermal conductivity of aramid composite insulating paper by adding thermally conductive materials such as GO, graphene, and BNNT-NH2 to the spinning solution and then wet-spinning thermally conductive fibers. However, GO and graphene are both excellent electrical conductors, which affects the insulation performance of the composite paper. Furthermore, wet spinning has drawbacks such as complex operation, significant pollution, and poor mechanical strength of the spun fibers. In practical production applications, it requires considerable time and is costly.

[0007] Furthermore, while existing technologies have attempted to incorporate boron nitride nanosheets (BNNS) as thermally conductive fillers into aramid paper, the high surface energy and tendency of BNNS to aggregate, coupled with weak interfacial bonding with the polymer matrix, often lead to discontinuous thermal conductivity pathways and decreased mechanical properties. Although hydroxylation can improve dispersibility, it remains difficult to maintain stable interfacial properties under high temperatures and high fields. In addition, the significant difference in thermal expansion coefficients between BNNS and aramid fibers makes them prone to interfacial debonding under long-term thermal cycling, affecting the durability of thermal conductivity and insulation properties.

[0008] On the other hand, while fluorination of aramid fibers can reduce dielectric loss, the surface polarity decreases after fluorination, further worsening its compatibility with inorganic fillers. Direct grafting of BNNS can easily lead to interfacial stress concentration, which is detrimental to long-term insulation reliability. Therefore, how to construct a stable composite structure that can strengthen the interface while maintaining high thermal conductivity and low dielectric loss remains a pressing technical challenge in this field.

[0009] In summary, existing high thermal conductivity composite aramid insulating paper still has many problems and drawbacks in actual production and application. Therefore, how to achieve rapid preparation and improve its thermal conductivity and dielectric properties while ensuring that the insulation performance is not affected is an important problem that needs to be solved for mass production. Summary of the Invention

[0010] This invention addresses the problems existing in the prior art by providing a method for preparing aramid composite paper with high thermal conductivity and stable insulation properties. By controlling the interfacial chemical reaction between the functional groups on the BNNS surface and the aramid precursor, uniform dispersion and strong interfacial bonding of boron nitride nanosheets in the fiber network are achieved. The boron nitride nanosheets, acting as a thermally conductive filler, form a thermally conductive network due to their alignment along the fiber direction, thereby enhancing the thermal conductivity of the aramid insulating paper. This solves the problems mentioned in the background.

[0011] To achieve the above-mentioned technical objectives, the technical solution adopted by the present invention is as follows:

[0012] A method for preparing a composite aramid insulating paper with low dielectric loss and high thermal conductivity includes the following steps:

[0013] (1) Fluoride modification of aramid short fibers to obtain fluorinated aramid short fibers;

[0014] (2) Boron nitride nanosheets were modified by hydroxylation with a strong base to obtain hydroxylated boron nitride nanosheets BNNS-OH;

[0015] (3) Hydroxylated boron nitride nanosheets BNNS-OH were modified with a gradient concentration of insulating polymer to obtain polymer-modified boron nitride nanosheets BNNS-OH@Polymer;

[0016] (4) Graft BNNS-OH@Polymer onto the surface of fluorinated aramid chopped fiber to obtain BNNS@Polymer-grafted-fluorinated aramid chopped fiber;

[0017] (5) Disperse the aramid precipitated fiber and BNNS@Polymer-grafted-fluorinated aramid chopped fiber in deionized water, transfer them to a fiber dissociator for decomposition, and obtain a mixed slurry after thorough and uniform mixing.

[0018] (6) The obtained mixed pulp is placed in a paper forming machine and wet paper web is obtained by wet paper forming technology;

[0019] (7) The obtained wet paper web is pressed and dried to obtain composite aramid base paper;

[0020] (8) The obtained composite aramid base paper is hot-pressed using a high-temperature roller hot press to obtain a composite aramid insulating paper product with high thermal conductivity and insulation.

[0021] Furthermore, the fluorination modification method in step (1) is to use gas-phase fluorination, placing the aramid chopped fibers in a 300Pa F2 / N2 mixed gas atmosphere, fluorinating at room temperature for 10 minutes, controlling the fluorine concentration to be 10%-30%, taking out the product and purging with inert gas to remove residual fluorine, thus obtaining fluorinated aramid chopped fibers. The remaining waste gas can be absorbed by alkaline solution.

[0022] Further, the method for hydroxylating and modifying boron nitride nanosheets in step (2) is hydrothermal modification: boron nitride nanosheets are dispersed in a 1-10M sodium hydroxide solution, placed in a reaction vessel, and reacted at 120-180℃ and 2-5MPa for 12-24 hours. After cooling, the nanosheets are centrifuged and washed until neutral, and then vacuum dried to obtain hydroxylated boron nitride nanosheets BNNS-OH.

[0023] Further, the gradient concentration insulating polymer modification treatment method in step (3) is as follows: First, hydroxylated boron nitride nanosheets BNNS-OH are dispersed in a polymer dispersion with a polyimide mass concentration of 1-2%. After ultrasonic and mechanical stirring at 55-60℃ for 2 hours, the product is centrifuged to obtain the preliminary modified product. Then, it is redispersed in a polymer dispersion with a polyimide mass concentration of 5-7%. After ultrasonic stirring at 55-60℃ for 2 hours, after centrifugation, it is redispersed in a polymer dispersion with a polyimide mass concentration of 10-12%. After reaction at 55-60℃ for 2 hours, the product is finally centrifuged, washed, and dried to obtain polymer-modified boron nitride nanosheets BNNS-OH@Polymer. The polymer dispersion is prepared by uniformly mixing polyimide, hydroxyl-terminated polydimethylsiloxane, and aminosilane coupling agent KH550 in a mass ratio of 3:1:1 and then dissolving them in N,N-dimethylacetamide.

[0024] Further, in step (4), the polymer-modified boron nitride nanosheets BNNS-OH@Polymer are dispersed in an ethanol solution of KH550, and fluorinated aramid chopped fibers are added. The reaction is carried out at 60-70℃ for 3-5 hours, so that the silanol groups generated by the hydrolysis of KH550 can undergo a condensation reaction with the hydroxyl groups on the surface of BNNS-OH@Polymer and the active sites on the surface of fluorinated aramid chopped fibers, thereby achieving effective grafting. After the reaction is completed, the fibers are filtered, washed and dried to obtain BNNS@Polymer-grafted-fluorinated aramid chopped fibers.

[0025] Furthermore, the mass concentration of the ethanol solution of KH550 is 2%-5%, the solid-liquid ratio of polymer-modified boron nitride nanosheets BNNS-OH@Polymer to the ethanol solution of KH550 is 1:10-1:20, and the solid-liquid ratio of fluorinated aramid short-cut fibers to the ethanol solution of KH550 is 1:15-1:25.

[0026] Furthermore, in step (5), the mass ratio of aramid precipitated fiber to BNNS@Polymer-grafted-fluorinated aramid chopped fiber is (1-3):(2-4); the total fiber concentration in the mixed slurry is 0.1-0.5wt%; and the fiber disintegrator desorption speed is 8000-12000r / min.

[0027] Furthermore, in step (7), the pressure during the pressing process is 0.5-1 MPa and the time is 5-10 min; during the drying process, the temperature is 80-100℃, the pressure is -0.05--0.1 MPa, and the time is 5-10 min.

[0028] Furthermore, in step (8), the temperature of the high-temperature roller hot press is 200-260℃, the pressure is 3-9Mpa, and the roller speed is 2-5m / min.

[0029] Compared with the prior art, the beneficial effects of the technical solution of the present invention are as follows:

[0030] (1) Aramid chopped fibers have good mechanical strength, high temperature resistance and anti-aging properties, but their fiber surface is smooth and has almost no active groups, resulting in poor interfacial bonding ability. After fluorination modification, active groups are added to the fiber surface, which not only improves the interfacial bonding ability, but also reduces the molecular polarizability and forms a "fluorine atom shielding effect", which directly leads to a reduction in dielectric loss and can remain stable in broadband and humid environments.

[0031] (2) Boron nitride nanosheets were selected as the thermally conductive filler due to their excellent thermal conductivity, approaching that of metals. Hydroxylation modification of the surface of the boron nitride nanosheets improved their interfacial bonding, providing more thermal pathways for heat transfer and better constructing a thermally conductive network, thereby enhancing the thermal conductivity of the composite aramid paper. In addition to its metallic thermal conductivity, boron nitride nanosheets also possess excellent insulation properties. As nanoscale ceramic particles, they not only facilitate heat transfer but also reduce potential current breakdown paths to a certain extent, increasing the electron trap density of the paper and improving the insulation performance of the composite aramid paper.

[0032] (3) In this invention, a polymer dispersion with gradually increasing concentration is used to modify boron nitride nanosheets (BNNS-OH). First, at a low concentration, polymer molecules are initially and uniformly adsorbed onto the surface of BNNS-OH to form "anchor points". Then, at medium and high concentrations, the polymer is gradually densified. This method effectively avoids the problems of rapid aggregation of polymer molecules, uneven coating, and even secondary aggregation caused by bridging multiple BNNS molecules due to a single high-concentration treatment.

[0033] Polymer modifiers consisting of polyimide, hydroxyl-terminated polydimethylsiloxane (OH-PDMS), and aminosilane coupling agent KH550 are used, with polyimide (PI) as the main material, providing excellent insulation, high-temperature resistance, and mechanical strength. Hydroxyl-terminated PDMS is introduced as a flexible segment; its low surface energy and high molecular chain flexibility help reduce the surface energy of the filler, improve dispersion, and, more importantly, effectively absorb and buffer the interfacial stress between BNNS and the aramid matrix caused by thermal expansion and contraction, thus enhancing the composite material's resistance to thermal fatigue. The aminosilane coupling agent KH550, after ethoxylation, can condense with the hydroxyl groups on the surface of BNNS-OH, while the amino groups can react with the carboxyl groups in the PI precursor (such as polyamic acid) or with KH550 in subsequent grafting processes. It plays a dual role in the system as both a "molecular bridge" and a "crosslinking point": firstly, it strengthens the chemical bond between the polymer coating layer itself and the BNNS substrate; secondly, in subsequent grafting steps, its exposed amino groups provide more active sites for the reaction with KH550 and fluorinated aramid. This treatment method achieves efficient modification of boron nitride, resulting in highly uniform dispersion of the modified BNNS-OH in the aramid matrix, significantly suppressing filler agglomeration and effectively constructing a continuous and stable thermally conductive network. Simultaneously, good interfacial compatibility is formed between the polymer coating layer and the aramid matrix, enhancing the heat transfer efficiency at the interface. The modified BNNS-OH not only improves the in-plane and longitudinal thermal conductivity of the composite aramid paper but also further optimizes the material's dielectric strength and partial discharge tolerance.

[0034] Furthermore, the modified BNNS-OH@Polymer, with its surface rich in active groups (-OH, -NH2), can form strong chemical bonds and physical interlocks with the microgroove structure and active sites on the surface of fluorinated aramid chopped fibers through fluorination etching using the silane coupling agent KH550. The low dielectric loss characteristics brought by the fluorination layer, combined with the strong interfacial bonding and stress buffering characteristics brought by the polymer coating layer, significantly improve the dielectric stability and thermal conductivity durability of the composite aramid paper under high-frequency and high-voltage environments.

[0035] (4) In summary, this invention achieves uniform dispersion and strong bonding of boron nitride nanosheets in an aramid matrix through multi-scale interface design and stepwise modification strategies, significantly improving the thermal conductivity, insulation, and mechanical properties of the composite paper. This material has broad application prospects in the field of insulation and heat dissipation of high-temperature and high-frequency electronic devices, providing a new approach for the development of high-performance insulating materials. Therefore, this material is very suitable for cutting-edge fields such as high-frequency communication equipment (e.g., radomes), high-power-density motors and electrical appliances, and advanced electronic packaging, where heat dissipation, insulation, and reliability requirements are extremely stringent. Attached Figure Description

[0036] Figure 1This is a scanning electron microscope (SEM) image of the BNNS@Polymer-grafted-fluorinated aramid short-cut fibers obtained in Example 1 of the present invention.

[0037] Figure 2 This is a surface SEM image of the aramid insulating paper obtained in Example 1;

[0038] Figure 3 The image shows a surface SEM image of the aramid insulating paper obtained in Comparative Example 1.

[0039] Figure 4 The image shows a surface SEM image of the aramid insulating paper obtained in Comparative Examples 2-7. Detailed Implementation

[0040] The technical solution of the present invention will be further described below with reference to specific embodiments, but it is not limited thereto.

[0041] Example 1

[0042] A method for preparing a composite aramid insulating paper with low dielectric loss and high thermal conductivity includes the following steps:

[0043] (1) Fluoride modification of aramid short fibers to obtain fluorinated aramid short fibers;

[0044] (2) Boron nitride nanosheets were modified by hydroxylation with a strong base to obtain hydroxylated boron nitride nanosheets BNNS-OH;

[0045] (3) Hydroxylated boron nitride nanosheets BNNS-OH were modified with a gradient concentration of insulating polymer to obtain polymer-modified boron nitride nanosheets BNNS-OH@Polymer;

[0046] (4) Graft BNNS-OH@Polymer onto the surface of fluorinated aramid chopped fiber to obtain BNNS@Polymer-grafted-fluorinated aramid chopped fiber;

[0047] (5) Disperse the aramid precipitated fiber and BNNS@Polymer-grafted-fluorinated aramid chopped fiber in deionized water, transfer them to a fiber dissociator for decomposition, and obtain a mixed slurry after thorough and uniform mixing.

[0048] (6) The obtained mixed pulp is placed in a paper forming machine and wet paper web is obtained by wet paper forming technology;

[0049] (7) The obtained wet paper web is pressed and dried to obtain composite aramid base paper;

[0050] (8) The obtained composite aramid base paper is hot-pressed using a high-temperature roller hot press to obtain a composite aramid insulating paper product with high thermal conductivity and insulation.

[0051] Step (1) The fluorination modification method is to use gas phase fluorination. The aramid short-cut fibers are placed in an F2 / N2 mixed gas atmosphere of 300 Pa and fluorinated at room temperature for 10 min. The fluorine concentration is controlled at 10%. The product is taken out and the residual fluorine is removed by purging with inert gas to obtain fluorinated aramid short-cut fibers. The remaining waste gas can be absorbed by alkaline solution.

[0052] Step (2) The method for hydroxylating and modifying boron nitride nanosheets is hydrothermal modification: boron nitride nanosheets are dispersed in 1M sodium hydroxide solution, placed in a reaction vessel, and reacted at 120℃ and 2MPa for 12 hours. After cooling, the nanosheets are centrifuged and washed until neutral, and then vacuum dried to obtain hydroxylated boron nitride nanosheets BNNS-OH.

[0053] Step (3) Gradient concentration insulating polymer modification method is as follows: First, hydroxylated boron nitride nanosheets BNNS-OH are dispersed in a polymer dispersion with a polyimide mass concentration of 1-2%. After ultrasonic and mechanical stirring at 55-60℃ for 2 hours, the product is centrifuged to obtain the preliminary modified product. Then, it is redispersed in a polymer dispersion with a polyimide mass concentration of 5-7%. After ultrasonic stirring at 55-60℃ for 2 hours, it is centrifuged and then redispersed in a polymer dispersion with a polyimide mass concentration of 10-12%. The reaction is carried out at 55-60℃ for 2 hours. Finally, it is centrifuged, washed and dried to obtain polymer-modified boron nitride nanosheets BNNS-OH@Polymer. The polymer dispersion is prepared by uniformly mixing polyimide, hydroxyl-terminated polydimethylsiloxane and aminosilane coupling agent KH550 in a mass ratio of 3:1:1 and then dissolving them in N,N-dimethylacetamide.

[0054] Step (4) The polymer-modified boron nitride nanosheets BNNS-OH@Polymer were dispersed in an ethanol solution of KH550, and fluorinated aramid chopped fibers were added. The reaction was carried out at 60-70℃ for 3 hours, so that the silanol groups generated by the hydrolysis of KH550 could undergo a condensation reaction with the hydroxyl groups on the surface of BNNS-OH@Polymer and the active sites on the surface of fluorinated aramid chopped fibers, thus achieving effective grafting. After the reaction was completed, the fibers were filtered, washed and dried to obtain BNNS@Polymer-grafted-fluorinated aramid chopped fibers.

[0055] The mass concentration of the ethanol solution of KH550 is 2%. The solid-liquid ratio of the polymer-modified boron nitride nanosheets BNNS-OH@Polymer to the ethanol solution of KH550 is 1:10. The solid-liquid ratio of the fluorinated aramid short-cut fibers to the ethanol solution of KH550 is 1:15.

[0056] In step (5), the mass ratio of aramid precipitated fiber to BNNS@Polymer-grafted-fluorinated aramid chopped fiber is 1:2; the total fiber concentration in the mixed slurry is 0.1wt%; and the fiber dissociation speed is 8000r / min.

[0057] In step (7), the pressure during pressing is 0.5 MPa and the time is 5 min; during drying, the temperature is 80-100℃, the pressure is -0.05 MPa, and the time is 5 min.

[0058] In step (8), the temperature of the high-temperature roller hot press is 200℃, the pressure is 3Mpa, and the roller speed is 2m / min.

[0059] Example 2

[0060] A method for preparing a composite aramid insulating paper with low dielectric loss and high thermal conductivity includes the following steps:

[0061] (1) Fluoride modification of aramid short fibers to obtain fluorinated aramid short fibers;

[0062] (2) Boron nitride nanosheets were modified by hydroxylation with a strong base to obtain hydroxylated boron nitride nanosheets BNNS-OH;

[0063] (3) Hydroxylated boron nitride nanosheets BNNS-OH were modified with a gradient concentration of insulating polymer to obtain polymer-modified boron nitride nanosheets BNNS-OH@Polymer;

[0064] (4) Graft BNNS-OH@Polymer onto the surface of fluorinated aramid chopped fiber to obtain BNNS@Polymer-grafted-fluorinated aramid chopped fiber;

[0065] (5) Disperse the aramid precipitated fiber and BNNS@Polymer-grafted-fluorinated aramid chopped fiber in deionized water, transfer them to a fiber dissociator for decomposition, and obtain a mixed slurry after thorough and uniform mixing.

[0066] (6) The obtained mixed pulp is placed in a paper forming machine and wet paper web is obtained by wet paper forming technology;

[0067] (7) The obtained wet paper web is pressed and dried to obtain composite aramid base paper;

[0068] (8) The obtained composite aramid base paper is hot-pressed using a high-temperature roller hot press to obtain a composite aramid insulating paper product with high thermal conductivity and insulation.

[0069] Step (1) The fluorination modification method is to use gas phase fluorination. The aramid short-cut fibers are placed in an F2 / N2 mixed gas atmosphere of 300 Pa and fluorinated at room temperature for 10 min. The fluorine concentration is controlled at 20%. The product is taken out and the residual fluorine is removed by purging with inert gas to obtain fluorinated aramid short-cut fibers. The remaining waste gas can be absorbed by alkaline solution.

[0070] Step (2) The method for hydroxylating and modifying boron nitride nanosheets is hydrothermal modification: boron nitride nanosheets are dispersed in a 4M sodium hydroxide solution, placed in a reaction vessel, and reacted at 140℃ and 3MPa for 16 hours. After cooling, the nanosheets are centrifuged and washed until neutral, and then vacuum dried to obtain hydroxylated boron nitride nanosheets BNNS-OH.

[0071] Step (3) Gradient concentration insulating polymer modification method is as follows: First, hydroxylated boron nitride nanosheets BNNS-OH are dispersed in a polymer dispersion with a polyimide mass concentration of 1-2%. After ultrasonic and mechanical stirring at 55-60℃ for 2 hours, the product is centrifuged to obtain the preliminary modified product. Then, it is redispersed in a polymer dispersion with a polyimide mass concentration of 5-7%. After ultrasonic stirring at 55-60℃ for 2 hours, it is centrifuged and then redispersed in a polymer dispersion with a polyimide mass concentration of 10-12%. The reaction is carried out at 55-60℃ for 2 hours. Finally, it is centrifuged, washed and dried to obtain polymer-modified boron nitride nanosheets BNNS-OH@Polymer. The polymer dispersion is prepared by uniformly mixing polyimide, hydroxyl-terminated polydimethylsiloxane and aminosilane coupling agent KH550 in a mass ratio of 3:1:1 and then dissolving them in N,N-dimethylacetamide.

[0072] Step (4) The polymer-modified boron nitride nanosheets BNNS-OH@Polymer were dispersed in an ethanol solution of KH550, and fluorinated aramid chopped fibers were added. The reaction was carried out at 60-70℃ for 3 hours, so that the silanol groups generated by the hydrolysis of KH550 could undergo a condensation reaction with the hydroxyl groups on the surface of BNNS-OH@Polymer and the active sites on the surface of fluorinated aramid chopped fibers, thus achieving effective grafting. After the reaction was completed, the fibers were filtered, washed and dried to obtain BNNS@Polymer-grafted-fluorinated aramid chopped fibers.

[0073] The mass concentration of the ethanol solution of KH550 is 3%. The solid-liquid ratio of the polymer-modified boron nitride nanosheets BNNS-OH@Polymer to the ethanol solution of KH550 is 1:13. The solid-liquid ratio of the fluorinated aramid short-cut fibers to the ethanol solution of KH550 is 1:18.

[0074] In step (5), the mass ratio of aramid precipitated fiber to BNNS@Polymer-grafted-fluorinated aramid chopped fiber is 1:3; the total fiber concentration in the mixed slurry is 0.2wt%; and the fiber dissociation speed is 10000r / min.

[0075] In step (7), the pressure during pressing is 0.5 MPa and the time is 8 min; during drying, the temperature is 80-100℃, the pressure is -0.05 MPa, and the time is 8 min.

[0076] In step (8), the temperature of the high-temperature roller hot press is 220℃, the pressure is 5Mpa, and the roller speed is 3m / min.

[0077] Example 3

[0078] A method for preparing a composite aramid insulating paper with low dielectric loss and high thermal conductivity includes the following steps:

[0079] (1) Fluoride modification of aramid short fibers to obtain fluorinated aramid short fibers;

[0080] (2) Boron nitride nanosheets were modified by hydroxylation with a strong base to obtain hydroxylated boron nitride nanosheets BNNS-OH;

[0081] (3) Hydroxylated boron nitride nanosheets BNNS-OH were modified with a gradient concentration of insulating polymer to obtain polymer-modified boron nitride nanosheets BNNS-OH@Polymer;

[0082] (4) Graft BNNS-OH@Polymer onto the surface of fluorinated aramid chopped fiber to obtain BNNS@Polymer-grafted-fluorinated aramid chopped fiber;

[0083] (5) Disperse the aramid precipitated fiber and BNNS@Polymer-grafted-fluorinated aramid chopped fiber in deionized water, transfer them to a fiber dissociator for decomposition, and obtain a mixed slurry after thorough and uniform mixing.

[0084] (6) The obtained mixed pulp is placed in a paper forming machine and wet paper web is obtained by wet paper forming technology;

[0085] (7) The obtained wet paper web is pressed and dried to obtain composite aramid base paper;

[0086] (8) The obtained composite aramid base paper is hot-pressed using a high-temperature roller hot press to obtain a composite aramid insulating paper product with high thermal conductivity and insulation.

[0087] Step (1) The fluorination modification method is to use gas phase fluorination. The aramid short-cut fibers are placed in an F2 / N2 mixed gas atmosphere of 300 Pa and fluorinated at room temperature for 10 min. The fluorine concentration is controlled at 30%. The product is taken out and the residual fluorine is removed by purging with inert gas to obtain fluorinated aramid short-cut fibers. The remaining waste gas can be absorbed by alkaline solution.

[0088] Step (2) The method for hydroxylating and modifying boron nitride nanosheets is hydrothermal modification: boron nitride nanosheets are dispersed in a 10M sodium hydroxide solution, placed in a reaction vessel, and reacted at 180℃ and 5MPa for 24 hours. After cooling, the nanosheets are centrifuged and washed until neutral, and then vacuum dried to obtain hydroxylated boron nitride nanosheets BNNS-OH.

[0089] Step (3) Gradient concentration insulating polymer modification method is as follows: First, hydroxylated boron nitride nanosheets BNNS-OH are dispersed in a polymer dispersion with a polyimide mass concentration of 1-2%. After ultrasonic and mechanical stirring at 55-60℃ for 2 hours, the product is centrifuged to obtain the preliminary modified product. Then, it is redispersed in a polymer dispersion with a polyimide mass concentration of 5-7%. After ultrasonic stirring at 55-60℃ for 2 hours, it is centrifuged and then redispersed in a polymer dispersion with a polyimide mass concentration of 10-12%. The reaction is carried out at 55-60℃ for 2 hours. Finally, it is centrifuged, washed and dried to obtain polymer-modified boron nitride nanosheets BNNS-OH@Polymer. The polymer dispersion is prepared by uniformly mixing polyimide, hydroxyl-terminated polydimethylsiloxane and aminosilane coupling agent KH550 in a mass ratio of 3:1:1 and then dissolving them in N,N-dimethylacetamide.

[0090] Step (4) The polymer-modified boron nitride nanosheets BNNS-OH@Polymer are dispersed in an ethanol solution of KH550, and fluorinated aramid chopped fibers are added. The reaction is carried out at 60-70℃ for 5 hours, so that the silanol groups generated by the hydrolysis of KH550 can undergo a condensation reaction with the hydroxyl groups on the surface of BNNS-OH@Polymer and the active sites on the surface of fluorinated aramid chopped fibers, thus achieving effective grafting. After the reaction is completed, the mixture is filtered, washed and dried to obtain BNNS@Polymer-grafted-fluorinated aramid chopped fibers.

[0091] The mass concentration of the ethanol solution of KH550 is 5%. The solid-liquid ratio of the polymer-modified boron nitride nanosheets BNNS-OH@Polymer to the ethanol solution of KH550 is 1:20. The solid-liquid ratio of the fluorinated aramid short-cut fibers to the ethanol solution of KH550 is 1:25.

[0092] In step (5), the mass ratio of aramid precipitated fiber to BNNS@Polymer-grafted-fluorinated aramid chopped fiber is 3:4; the total fiber concentration in the mixed slurry is 0.5wt%; and the fiber dissociation speed is 12000r / min.

[0093] In step (7), the pressure during pressing is 1 MPa and the time is 10 min; during drying, the temperature is 80-100℃, the pressure is -0.1 MPa, and the time is 10 min.

[0094] In step (8), the temperature of the high-temperature roller hot press is 260℃, the pressure is 9Mpa, and the roller speed is 5m / min.

[0095] Example 4

[0096] A method for preparing a composite aramid insulating paper with low dielectric loss and high thermal conductivity includes the following steps:

[0097] (1) Fluoride modification of aramid short fibers to obtain fluorinated aramid short fibers;

[0098] (2) Boron nitride nanosheets were modified by hydroxylation with a strong base to obtain hydroxylated boron nitride nanosheets BNNS-OH;

[0099] (3) Hydroxylated boron nitride nanosheets BNNS-OH were modified with a gradient concentration of insulating polymer to obtain polymer-modified boron nitride nanosheets BNNS-OH@Polymer;

[0100] (4) Graft BNNS-OH@Polymer onto the surface of fluorinated aramid chopped fiber to obtain BNNS@Polymer-grafted-fluorinated aramid chopped fiber;

[0101] (5) Disperse the aramid precipitated fiber and BNNS@Polymer-grafted-fluorinated aramid chopped fiber in deionized water, transfer them to a fiber dissociator for decomposition, and obtain a mixed slurry after thorough and uniform mixing.

[0102] (6) The obtained mixed pulp is placed in a paper forming machine and wet paper web is obtained by wet paper forming technology;

[0103] (7) The obtained wet paper web is pressed and dried to obtain composite aramid base paper;

[0104] (8) The obtained composite aramid base paper is hot-pressed using a high-temperature roller hot press to obtain a composite aramid insulating paper product with high thermal conductivity and insulation.

[0105] Step (1) The fluorination modification method is to use gas phase fluorination. The aramid chopped fibers are placed in an F2 / N2 mixed gas atmosphere of 300 Pa and fluorinated at room temperature for 10 min. The fluorine concentration is controlled at 25%. The product is taken out and the residual fluorine is removed by purging with inert gas to obtain fluorinated aramid chopped fibers. The remaining waste gas can be absorbed by alkaline solution.

[0106] Step (2) The method for hydroxylating and modifying boron nitride nanosheets is hydrothermal modification: boron nitride nanosheets are dispersed in an 8M sodium hydroxide solution, placed in a reaction vessel, and reacted at 180℃ and 5MPa for 22 hours. After cooling, the nanosheets are centrifuged and washed until neutral, and then vacuum dried to obtain hydroxylated boron nitride nanosheets BNNS-OH.

[0107] Step (3) Gradient concentration insulating polymer modification method is as follows: First, hydroxylated boron nitride nanosheets BNNS-OH are dispersed in a polymer dispersion with a polyimide mass concentration of 1-2%. After ultrasonic and mechanical stirring at 55-60℃ for 2 hours, the product is centrifuged to obtain the preliminary modified product. Then, it is redispersed in a polymer dispersion with a polyimide mass concentration of 5-7%. After ultrasonic stirring at 55-60℃ for 2 hours, it is centrifuged and then redispersed in a polymer dispersion with a polyimide mass concentration of 10-12%. The reaction is carried out at 55-60℃ for 2 hours. Finally, it is centrifuged, washed and dried to obtain polymer-modified boron nitride nanosheets BNNS-OH@Polymer. The polymer dispersion is prepared by uniformly mixing polyimide, hydroxyl-terminated polydimethylsiloxane and aminosilane coupling agent KH550 in a mass ratio of 3:1:1 and then dissolving them in N,N-dimethylacetamide.

[0108] Step (4) The polymer-modified boron nitride nanosheets BNNS-OH@Polymer are dispersed in an ethanol solution of KH550, and fluorinated aramid chopped fibers are added. The reaction is carried out at 60-70℃ for 4 hours, so that the silanol groups generated by the hydrolysis of KH550 can undergo a condensation reaction with the hydroxyl groups on the surface of BNNS-OH@Polymer and the active sites on the surface of fluorinated aramid chopped fibers, thus achieving effective grafting. After the reaction is completed, the mixture is filtered, washed and dried to obtain BNNS@Polymer-grafted-fluorinated aramid chopped fibers.

[0109] The mass concentration of the ethanol solution of KH550 is 5%. The solid-liquid ratio of the polymer-modified boron nitride nanosheets BNNS-OH@Polymer to the ethanol solution of KH550 is 1:18. The solid-liquid ratio of the fluorinated aramid short-cut fibers to the ethanol solution of KH550 is 1:20.

[0110] In step (5), the mass ratio of aramid precipitated fiber to BNNS@Polymer-grafted-fluorinated aramid chopped fiber is 1:3; the total fiber concentration in the mixed slurry is 0.3wt%; and the fiber dissociation speed is 8000r / min.

[0111] In step (7), the pressure during pressing is 0.5 MPa and the time is 5 min; during drying, the temperature is 80-100℃, the pressure is -0.05 MPa, and the time is 10 min.

[0112] In step (8), the temperature of the high-temperature roller hot press is 260℃, the pressure is 9Mpa, and the roller speed is 5m / min.

[0113] Comparative Example 1

[0114] Compared to Example 1, this comparative example uses unfluorinated aramid chopped fibers and aramid precipitated fibers as raw materials, and does not use boron nitride nanosheets. All other process parameters are the same as in Example 1, specifically:

[0115] A method for preparing composite aramid insulating paper includes the following steps:

[0116] (1) Disperse aramid precipitated fibers and aramid chopped fibers in deionized water, transfer them to a fiber deionizer for disintegration, and mix them thoroughly to obtain a mixed slurry.

[0117] (2) The obtained mixed pulp is placed in a paper forming machine and wet paper web is obtained by wet paper forming technology;

[0118] (3) The obtained wet paper web is pressed and dried to obtain composite aramid base paper;

[0119] (4) The obtained composite aramid base paper is hot-pressed using a high-temperature roller hot press to obtain a composite aramid insulating paper product with high thermal conductivity and insulation.

[0120] Comparative Example 2

[0121] Compared to Example 1, only hydroxylated boron nitride nanosheets were used. All other process parameters were the same as in Example 1, specifically:

[0122] A method for preparing composite aramid insulating paper includes the following steps:

[0123] (1) Fluoride modification of aramid short fibers to obtain fluorinated aramid short fibers;

[0124] (2) Boron nitride nanosheets were modified by hydroxylation with a strong base to obtain hydroxylated boron nitride nanosheets BNNS-OH;

[0125] (3) Grafting BNNS-OH onto the surface of fluorinated aramid chopped fibers to obtain BNNS-grafted-fluorinated aramid chopped fibers;

[0126] (4) Disperse the aramid precipitated fiber and BNNS-grafted-fluorinated aramid short fiber in deionized water, transfer them to a fiber dissociator for decomposition, and mix them thoroughly to obtain a mixed slurry.

[0127] (5) The obtained mixed pulp is placed in a paper forming machine and wet paper web is obtained by wet paper forming technology;

[0128] (6) The obtained wet paper web is pressed and dried to obtain composite aramid base paper;

[0129] (7) The obtained composite aramid base paper is hot-pressed using a high-temperature roller hot press to obtain a composite aramid insulating paper product with high thermal conductivity and insulation.

[0130] Step (1) The fluorination modification method is to use gas phase fluorination. The aramid short-cut fibers are placed in an F2 / N2 mixed gas atmosphere of 300 Pa and fluorinated at room temperature for 10 min. The fluorine concentration is controlled at 10%. The product is taken out and the residual fluorine is removed by purging with inert gas to obtain fluorinated aramid short-cut fibers. The remaining waste gas can be absorbed by alkaline solution.

[0131] Step (2) The method for hydroxylating and modifying boron nitride nanosheets is hydrothermal modification: boron nitride nanosheets are dispersed in 1M sodium hydroxide solution, placed in a reaction vessel, and reacted at 120℃ and 2MPa for 12 hours. After cooling, the nanosheets are centrifuged and washed until neutral, and then vacuum dried to obtain hydroxylated boron nitride nanosheets BNNS-OH.

[0132] Comparative Example 3

[0133] Compared to Example 1, except for the use of a single concentration treatment (polymer dispersion concentration of 1-2%) during the modification of boron nitride with the insulating polymer, all other process parameters are the same as in Example 1, specifically:

[0134] A method for preparing composite aramid insulating paper includes the following steps:

[0135] (1) Fluoride modification of aramid short fibers to obtain fluorinated aramid short fibers;

[0136] (2) Boron nitride nanosheets were modified by hydroxylation with a strong base to obtain hydroxylated boron nitride nanosheets BNNS-OH;

[0137] (3) Hydroxylated boron nitride nanosheets BNNS-OH were modified with insulating polymers to obtain polymer-modified boron nitride nanosheets BNNS-OH@Polymer;

[0138] (4) Graft BNNS-OH@Polymer onto the surface of fluorinated aramid chopped fiber to obtain BNNS@Polymer-grafted-fluorinated aramid chopped fiber;

[0139] (5) Disperse the aramid precipitated fiber and BNNS@Polymer-grafted-fluorinated aramid chopped fiber in deionized water, transfer them to a fiber dissociator for decomposition, and obtain a mixed slurry after thorough and uniform mixing.

[0140] (6) The obtained mixed pulp is placed in a paper forming machine and wet paper web is obtained by wet paper forming technology;

[0141] (7) The obtained wet paper web is pressed and dried to obtain composite aramid base paper;

[0142] (8) The obtained composite aramid base paper is hot-pressed using a high-temperature roller hot press to obtain a composite aramid insulating paper product with high thermal conductivity and insulation.

[0143] Step (3) The method for modifying the insulating polymer is as follows: First, hydroxylated boron nitride nanosheets BNNS-OH are dispersed in a polymer dispersion with a polyimide mass concentration of 1-2%. After ultrasonic and mechanical stirring at 55-60℃ for 6 hours, the mixture is finally centrifuged, washed and dried to obtain polymer-modified boron nitride nanosheets BNNS-OH@Polymer. The polymer dispersion is prepared by uniformly mixing polyimide, hydroxyl-terminated polydimethylsiloxane and aminosilane coupling agent KH550 in a mass ratio of 3:1:1 and then dissolving them in N,N-dimethylacetamide.

[0144] Comparative Example 4

[0145] Compared to Example 1, except that a single concentration treatment was used when modifying boron nitride with the insulating polymer (the polymer dispersion concentration was 5-7%), all other process parameters were the same as in Example 1, specifically:

[0146] Step (3) The method for modifying the insulating polymer is as follows: First, hydroxylated boron nitride nanosheets BNNS-OH are dispersed in a polymer dispersion with a polyimide mass concentration of 5-7%. After ultrasonication and mechanical stirring at 55-60℃ for 6 hours, the nanosheets are finally centrifuged, washed and dried to obtain polymer-modified boron nitride nanosheets BNNS-OH@Polymer. The polymer dispersion is prepared by uniformly mixing polyimide, hydroxyl-terminated polydimethylsiloxane and aminosilane coupling agent KH550 in a mass ratio of 3:1:1 and then dissolving them in N,N-dimethylacetamide.

[0147] Comparative Example 5

[0148] Compared with Example 1, except that a single concentration treatment was used when modifying boron nitride with the insulating polymer (the polymer dispersion concentration was 10-12%), all other process parameters were the same as in Example 1, specifically:

[0149] Step (3) The method for modifying the insulating polymer is as follows: First, hydroxylated boron nitride nanosheets BNNS-OH are dispersed in a polymer dispersion with a polyimide mass concentration of 10-12%. After ultrasonication and mechanical stirring at 55-60℃ for 6 hours, the nanosheets are finally centrifuged, washed and dried to obtain polymer-modified boron nitride nanosheets BNNS-OH@Polymer. The polymer dispersion is prepared by uniformly mixing polyimide, hydroxyl-terminated polydimethylsiloxane and aminosilane coupling agent KH550 in a mass ratio of 3:1:1 and then dissolving them in N,N-dimethylacetamide.

[0150] Comparative Example 6

[0151] Compared to Example 1, except that only hydroxyl-terminated polydimethylsiloxane was used in the treatment of boron nitride with insulating polymer modification, all other process parameters were the same as in Example 1, specifically:

[0152] Step (3) Gradient concentration insulating polymer modification method is as follows: First, hydroxylated boron nitride nanosheets BNNS-OH are dispersed in a polymer dispersion with a polyimide mass concentration of 1-2%. After ultrasonic and mechanical stirring at 55-60℃ for 2 hours, the product is centrifuged to obtain the preliminary modified product. Then, it is redispersed in a polymer dispersion with a polyimide mass concentration of 5-7%. After ultrasonic stirring at 55-60℃ for 2 hours, it is centrifuged and then redispersed in a polymer dispersion with a polyimide mass concentration of 10-12%. The reaction is carried out at 55-60℃ for 2 hours. Finally, it is centrifuged, washed and dried to obtain polymer-modified boron nitride nanosheets BNNS-OH@Polymer. The polymer dispersion is prepared by uniformly mixing polyimide and hydroxyl-terminated polydimethylsiloxane at a mass ratio of 3:1 and dissolving them in N,N-dimethylacetamide.

[0153] Comparative Example 7

[0154] Compared to Example 1, except that only the aminosilane coupling agent KH550 was used in the treatment of boron nitride with insulating polymer modification, all other process parameters were the same as in Example 1, specifically:

[0155] Step (3) Gradient concentration insulating polymer modification method is as follows: First, hydroxylated boron nitride nanosheets BNNS-OH are dispersed in a polymer dispersion with a polyimide mass concentration of 1-2%. After ultrasonic and mechanical stirring at 55-60℃ for 2 hours, the product is centrifuged to obtain the preliminary modified product. Then, it is redispersed in a polymer dispersion with a polyimide mass concentration of 5-7%. After ultrasonic stirring at 55-60℃ for 2 hours, it is centrifuged and then redispersed in a polymer dispersion with a polyimide mass concentration of 10-12%. The reaction is carried out at 55-60℃ for 2 hours. Finally, it is centrifuged, washed and dried to obtain polymer modified boron nitride nanosheets BNNS-OH@Polymer. The polymer dispersion is prepared by uniformly mixing polyimide and aminosilane coupling agent KH550 at a mass ratio of 3:1 and dissolving them in N,N-dimethylacetamide.

[0156] Performance testing

[0157] Insulating paper samples were prepared according to the examples and comparative methods, and their insulation performance, thermal conductivity, and mechanical strength were tested. Five samples were repeated for each experiment, and the average of all results was taken. The test methods are as follows:

[0158] Microscopic morphological characterization:

[0159] The surface of the experimental samples was observed using a scanning electron microscope (SEM). The samples were sputter-coated with gold. The scanning voltage was 10 kV and the scanning speed was 60 mm / min.

[0160] Mechanical property testing:

[0161] The experimental samples were placed in an international constant temperature and humidity environment (23±0.5)℃ and (50±5)% for 12 h to equilibrate. Mechanical properties were tested according to the following national standards: tensile strength (GB / T 12914-2018) and tear strength (GB / T 455-2002(9)). Electrical insulation performance was tested.

[0162] Tests shall be conducted at high frequencies according to the following national standards: breakdown strength (GB / T 1408.1-2016), dielectric constant and dielectric loss factor (GB / T 1409-2006). Thermal conductivity testing:

[0163] Calculate the thermal conductivity of the sample using the following formula.

[0164] λ = α × Cp × ρ

[0165] Where: α is the thermal diffusivity coefficient of aramid paper, m² / s, measured at 25℃ using a Netzsch LFA 467 laser flash tester; Cp is the specific heat capacity of aramid paper, J / (kg·K), measured by the sapphire method using a PerkinElmer DSC8000 differential scanning calorimeter; ρ is the density of aramid paper, kg / m³, calculated from mass (m) and volume (v) using ρ = m / v. Table 1 Performance test results

[0166]

[0167] As shown in Table 1, the composite aramid insulating paper prepared in Examples 1-4 outperforms Comparative Examples 1-7 in key performance indicators such as thermal conductivity, breakdown strength, and tensile strength. Specifically, Example 4 exhibits a thermal conductivity of 9.754 W / (m·K) and a breakdown strength of 19.009 kV / mm, significantly higher than Comparative Example 1 (thermal conductivity 5.204 W / (m·K), breakdown strength 9.764 kV / mm), which is unfluorinated and does not contain boron nitride nanosheets. This indicates that the synergistic effect of fluorinated modified aramid chopped fibers and gradient polymer-modified boron nitride nanosheets effectively improves the thermal conductivity and insulation strength of the material. Meanwhile, Comparative Example 2, using only hydroxylated boron nitride nanosheets, shows lower thermal conductivity (6.102 W / (m·K)) and breakdown strength (11.245 kV / mm) than Example 1, suggesting that polymer-modified boron nitride nanosheets are more beneficial for improving interfacial compatibility and promoting the construction of heat conduction pathways. Comparing Examples 3-5, it can be seen that Example 1, which uses a gradient concentration polymer to modify boron nitride nanosheets, exhibits superior overall performance compared to the examples treated with a single concentration, especially in terms of dielectric loss tangent (0.0096) and breakdown strength (14.793 kV / mm). This is likely due to the gradient interface structure formed by the gradient modification reducing thermal resistance and defects. The gradient modification strategy can more effectively achieve uniform and dense coating of the polymer on the BNNS surface, avoiding agglomeration and optimizing the interface, which is key to improving the synergy between thermal conductivity and insulation. Furthermore, Comparative Examples 6 and 7, due to the lack of certain components in the polymer dispersion, showed a decrease in thermal conductivity and insulation performance, further verifying the importance of the compound system of hydroxyl-terminated polydimethylsiloxane and aminosilane coupling agent in improving material performance. The hydroxyl-terminated polydimethylsiloxane OH-PDMS (providing flexibility and stress buffer) and the aminosilane coupling agent KH550 (providing reaction sites and bridging) are indispensable and work synergistically to achieve strong interfacial bonding, low internal stress, and high reactivity. Although Comparative Example 1 exhibits higher tensile strength (4.17 kN / m) and tear strength (1076 mN), its overall practicality is inferior to that of the Example samples due to its poor thermal conductivity and insulation properties. This indicates that while the introduction of BNNS and the chemical modification of the fiber significantly improve thermal conductivity and insulation properties, they also have a slight impact on the mechanical integrity of the fiber itself. However, the Example samples still maintain high mechanical strength (>3.5 kN / m), which is acceptable in most application scenarios, demonstrating a good balance of overall performance. From the scanning electron microscope images of the Examples and Comparative Examples, we can see that in Example 1, the aramid chopped fibers and boron nitride nanosheets are uniformly dispersed and tightly bonded at the interface. In contrast, in Comparative Examples 2-7, due to poor interfacial compatibility or filler agglomeration, the boron nitride is unevenly dispersed, exhibiting obvious pores and interfacial debonding, resulting in poor fiber morphology. This further confirms the significant advantages of gradient polymer modification in improving filler dispersion and interfacial bonding strength.

[0168] In summary, this invention successfully prepared a composite aramid insulating paper with high thermal conductivity (>9.7 W / (m·K)), low dielectric loss (<0.01), and high breakdown strength (>19 kV / mm) through a multi-level interface design involving aramid fiber fluorination, BNNS hydroxylation, polymer gradient coating, and silane coupling grafting. This paper is suitable for applications with stringent requirements for thermal management and electrical insulation, such as high-voltage insulation, high-power motors, and electronic devices, and possesses broad application prospects and industrialization value.

[0169] It should be noted that the above embodiments are merely some preferred embodiments of the present invention, and not all embodiments. Obviously, based on the above embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

Claims

1. A method for preparing a composite aramid insulating paper with low dielectric loss and high thermal conductivity, characterized in that, Includes the following steps: (1) Fluoride modification of aramid short fibers to obtain fluorinated aramid short fibers; (2) Boron nitride nanosheets were modified by hydroxylation with a strong base to obtain hydroxylated boron nitride nanosheets BNNS-OH; (3) Hydroxylated boron nitride nanosheets BNNS-OH were modified with a gradient concentration of insulating polymer to obtain polymer-modified boron nitride nanosheets BNNS-OH@Polymer; (4) Graft BNNS-OH@Polymer onto the surface of fluorinated aramid chopped fiber to obtain BNNS@Polymer-grafted-fluorinated aramid chopped fiber; (5) Disperse the aramid precipitated fiber and BNNS@Polymer-grafted-fluorinated aramid chopped fiber in deionized water, transfer them to a fiber dissociator for decomposition, and obtain a mixed slurry after thorough and uniform mixing. (6) The obtained mixed pulp is placed in a paper forming machine and wet paper web is obtained by wet paper forming technology; (7) The obtained wet paper web is pressed and dried to obtain composite aramid base paper; (8) The obtained composite aramid base paper is hot-pressed using a high-temperature roller hot press to obtain a composite aramid insulating paper product with high thermal conductivity and insulation. Step (3) Gradient concentration insulating polymer modification method is as follows: First, hydroxylated boron nitride nanosheets BNNS-OH are dispersed in a polymer dispersion with a polyimide mass concentration of 1-2%. After ultrasonic and mechanical stirring at 55-60℃ for 2 hours, the product is centrifuged to obtain the preliminary modified product. Then, it is redispersed in a polymer dispersion with a polyimide mass concentration of 5-7%. After ultrasonic stirring at 55-60℃ for 2 hours, it is centrifuged and then redispersed in a polymer dispersion with a polyimide mass concentration of 10-12%. The reaction is carried out at 55-60℃ for 2 hours. Finally, it is centrifuged, washed and dried to obtain polymer-modified boron nitride nanosheets BNNS-OH@Polymer. The polymer dispersion is prepared by uniformly mixing polyimide, hydroxyl-terminated polydimethylsiloxane and aminosilane coupling agent KH550 in a mass ratio of 3:1:1 and then dissolving them in N,N-dimethylacetamide.

2. The method for preparing the low dielectric loss, high thermal conductivity composite aramid insulating paper according to claim 1, characterized in that, Step (1) The fluorination modification method is to use gas phase fluorination. The aramid short-cut fibers are placed in an F2 / N2 mixed gas atmosphere of 300Pa and fluorinated at room temperature for 10min. The fluorine concentration is controlled at 10%-30%. The product is taken out and the residual fluorine is removed by purging with inert gas to obtain fluorinated aramid short-cut fibers.

3. The method for preparing the low dielectric loss, high thermal conductivity composite aramid insulating paper according to claim 1, characterized in that, Step (2) The method for hydroxylating and modifying boron nitride nanosheets is hydrothermal modification: disperse boron nitride nanosheets in a 1-10M sodium hydroxide solution, place them in a reaction vessel, and react for 12-24 hours at 120-180℃ and 2-5MPa. After cooling, centrifuge and wash until neutral, and vacuum dry to obtain hydroxylated boron nitride nanosheets BNNS-OH.

4. The method for preparing the low dielectric loss, high thermal conductivity composite aramid insulating paper according to claim 1, characterized in that, Step (4) The polymer-modified boron nitride nanosheets BNNS-OH@Polymer were dispersed in an ethanol solution of KH550, and fluorinated aramid short fibers were added. The reaction was carried out at 60-70℃ for 3-5 hours. After the reaction was completed, the mixture was filtered, washed and dried to obtain BNNS@Polymer-grafted-fluorinated aramid short fibers.

5. The method for preparing the low dielectric loss, high thermal conductivity composite aramid insulating paper according to claim 4, characterized in that, The mass concentration of the ethanol solution of KH550 is 2%-5%. The solid-liquid ratio of polymer-modified boron nitride nanosheets BNNS-OH@Polymer to the ethanol solution of KH550 is 1:10-1:

20. The solid-liquid ratio of fluorinated aramid short-cut fibers to the ethanol solution of KH550 is 1:15-1:

25.

6. The method for preparing the low dielectric loss, high thermal conductivity composite aramid insulating paper according to claim 1, characterized in that, In step (5), the mass ratio of aramid precipitated fiber to BNNS@Polymer-grafted-fluorinated aramid chopped fiber is (1-3):(2-4); the total fiber concentration in the mixed slurry is 0.1-0.5wt%; and the fiber disintegration speed is 8000-12000r / min.

7. The method for preparing the low dielectric loss, high thermal conductivity composite aramid insulating paper according to claim 1, characterized in that, In step (7), the pressure during pressing is 0.5-1 MPa and the time is 5-10 min; during drying, the temperature is 80-100℃, the pressure is -0.05--0.1 MPa, and the time is 5-10 min.

8. The method for preparing the low dielectric loss, high thermal conductivity composite aramid insulating paper according to claim 1, characterized in that, In step (8), the temperature of the high-temperature roller hot press is 200-260℃, the pressure is 3-9Mpa, and the roller speed is 2-5m / min.

9. A low dielectric loss, high thermal conductivity composite aramid insulating paper obtained by the preparation method according to any one of claims 1-8.

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