Aramid fiber insulation material with high dielectric strength and a method of making the same

Abstract

The application discloses a kind of high dielectric strength aramid fiber insulating materials and preparation method thereof, the insulating material is composed of aramid fiber framework, nanoscale dielectric filler and interface bonding agent, wherein aramid fiber framework includes para-aramid short-cut fiber and sedimentation fiber, nanoscale dielectric filler is boron nitride nanoplate or titanium dioxide nanotube, interface bonding agent is silane coupling agent. Fillers and coupling agents are dispersed to obtain functional dispersion, then uniformly mixed with aramid fiber to obtain composite slurry, and the finished product is obtained by forming, pressing dewatering, hot pressing and cooling. The application optimizes components and innovates process to ensure that fillers are uniformly dispersed and firmly combined in the fiber network. The material has excellent performance with dielectric strength not less than 60 kV / mm and heat resistance grade up to H level, suitable for insulation applications of high-voltage power equipment, including insulating paper and insulating film materials.

Description

Technical Field

[0001] This invention relates to the field of electrical insulation materials technology, and in particular to an aramid fiber-based insulation material with high dielectric strength for use in high-voltage power equipment and its preparation method. Background Technology

[0002] Aramid fibers, due to their excellent heat resistance, mechanical strength, and flame retardancy, have become one of the preferred materials for preparing high-end insulation materials. Currently, aramid insulation materials are mainly in the form of aramid paper, aramid nonwoven fabric, or laminated products, and are widely used in power equipment such as transformers and motors. However, existing aramid insulation materials still have some shortcomings. Although materials made purely of aramid fibers have better dielectric strength than ordinary materials, there is still room for improvement to meet the requirements of ultra-high voltage equipment. Secondly, in order to improve performance, it is often used to combine them with other films (such as polyimide films), but this introduces interface problems, increases process complexity and cost. Traditional processes also make it difficult to effectively introduce and disperse functional nanoparticles that can improve dielectric properties in the fiber network, and the interfacial bonding between particles and fibers is weak, affecting long-term reliability.

[0003] Therefore, it is of great significance to develop a single-material system of aramid insulating material with high bulk dielectric strength, uniform structure, stable performance and easy processing. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of existing technologies and provide an aramid fiber insulating material with high intrinsic dielectric strength and its preparation method. This method, through unique component design and preparation process, achieves uniform dispersion and strong bonding of nanofillers in the aramid fiber network, significantly improving the dielectric strength and overall performance of the material.

[0005] The technical solution of the present invention to solve the above-mentioned technical problems is as follows: A high dielectric strength aramid fiber insulating material is made of the following components: The aramid fiber skeleton is composed of para-aramid chopped fibers and para-aramid precipitated fibers in a mass ratio of (60-90):(40-10); the nanoscale dielectric filler is boron nitride nanosheets or titanium dioxide nanotubes, the content of which accounts for 1%-10% of the total mass of the aramid fiber skeleton; the interface binder is a silane coupling agent, the content of which accounts for 0.1%-2% of the total mass of the aramid fiber skeleton; wherein, the dielectric strength of the insulating material at 25℃ and 50Hz is not less than 60kV / mm, and the heat resistance level reaches H-class (180℃) or above.

[0006] In this invention, the para-aramid chopped fibers are preferably 3-8 mm in length and have a linear density of 1.0-2.5 dtex; the average length of the para-aramid precipitated fibers is preferably 0.5-3 mm.

[0007] In this invention, the preferred boron nitride nanosheets have a thickness of 1–10 nm and a lateral dimension of 0.5–5 μm; the titanium dioxide nanotubes have an outer diameter of 20–100 nm and a length of 0.5–10 μm.

[0008] In this invention, the preferred silane coupling agent is at least one of γ-aminopropyltriethoxysilane and γ-(2,3-epoxypropoxy)propyltrimethoxysilane.

[0009] A method for manufacturing a high dielectric strength aramid fiber insulating material: (1) Pretreatment and dispersion: The nano-sized dielectric filler and silane coupling agent are dispersed together in an organic solvent, and then subjected to ultrasonic treatment and mechanical stirring to obtain a functional dispersion; (2) Slurry compounding: Add aramid short-cut fibers and aramid precipitated fibers to the functional dispersion obtained in step (1), and obtain a uniform composite slurry by mechanical stirring and ultrasonic treatment; (3) Forming and dewatering: The composite pulp is formed by wet papermaking, and then pressed and dewatered to obtain a wet paper sheet; (4) In-situ curing and hot pressing: The wet paper is subjected to two-stage hot pressing treatment. The first stage is treated at 1-3 MPa and 120-160℃, and the second stage is treated at 5-10 MPa and 240-300℃. (5) Post-processing: Cool and rewind the hot-pressed material to obtain the insulating material.

[0010] Preferably, in step (1), the power of the ultrasonic treatment is 200-500W and the treatment time is 30-90 minutes; the speed of the mechanical stirring is 1000-3000r / min and the stirring time is 60-120 minutes.

[0011] Preferably, in step (2), the slurry concentration is controlled to be 0.1% to 0.5%, the mechanical stirring speed is 500 to 1000 r / min, and the stirring time is 30 to 60 minutes; the ultrasonic treatment power is 100 to 300 W, and the treatment time is 10 to 30 minutes.

[0012] Preferably, in step (3), the pressing and dewatering pressure is 0.5 to 2.0 MPa, and the dryness of the wet paper sheet is controlled to be 35% to 45%.

[0013] Preferably, in step (4), the first stage processing time is 5 to 15 minutes, and the second stage processing time is 20 to 50 minutes; in step (5), the temperature is programmed to be cooled to below 50°C at a rate of 1 to 3°C / min.

[0014] Compared with the prior art, the technical solution of this application has the following beneficial technical effects: This invention constructs a stable three-dimensional fiber network framework by proportionally matching aramid chopped fibers and precipitated fibers. Through optimized nanofiller selection and interface design, a chemically bonded structure of fiber-coupling agent-filler is formed, effectively improving interfacial compatibility. A dispersion-then-composite process route is adopted, utilizing ultrasonic-mechanical stirring synergistic dispersion technology to solve the problem of easy agglomeration of nanofillers. A two-stage hot-pressing process achieves effective establishment of interfacial chemical bonds and optimized control of the material structure. The insulating material produced by this invention not only possesses high dielectric strength (≥60kV / mm) but also maintains excellent mechanical properties, heat resistance, and process adaptability, with overall performance significantly superior to existing products. Furthermore, this invention employs an aqueous phase system for processing, resulting in mild process conditions, facilitating large-scale production, and yielding significant economic and social benefits. Attached Figure Description

[0015] Figure 1 This is a process flow diagram of the preparation method of the present invention. Detailed Implementation

[0016] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Example 1

[0017] The raw materials prepared in this invention include para-aramid chopped fibers, para-aramid precipitated fibers, boron nitride nanosheets, γ-aminopropyltriethoxysilane (KH550), and N-methylpyrrolidone (NMP). The para-aramid chopped fibers have a length of 6 mm, a linear density of 1.67 dtex, a breaking strength ≥20 cN / dtex, and a moisture content controlled to ≤5%. The para-aramid precipitated fibers have an average length of 1.2 mm and a specific surface area of ​​8-12 m² / g. The boron nitride nanosheets have a thickness of 2-5 nm, a transverse dimension of 1-3 μm, a purity ≥99.5%, and a specific surface area of ​​280±20 m² / g. The interfacial binder is γ-aminopropyltriethoxysilane (KH550) with a purity ≥98%, and the organic solvent is N-methylpyrrolidone (NMP) with a purity ≥99.5% and a moisture content ≤0.05%.

[0018] The preparation process is as follows: (1) Pretreatment and dispersion: Accurately weigh 5.0 g of boron nitride nanosheets (5% of the total mass of aramid fibers) and 1.0 g of γ-aminopropyltriethoxysilane (KH550) (1% of the total mass of fibers). Measure 100 mL of N-methylpyrrolidone (NMP) solvent (20 times the mass of the filler). Add the above materials to a 500 mL beaker. Use an ultrasonic cell disruptor at 300 W power with a 2-second working and 1-second intermittent cycle for 60 minutes, while controlling the temperature to ≤35℃ using a water bath. Then, use a mechanical stirrer at 2000 r / min for 90 minutes to obtain a uniform functional dispersion.

[0019] (2) Slurry compounding: Accurately weigh 80.0g of aramid short-cut fibers and 20.0g of aramid precipitated fibers, and slowly add the fibers to the functional dispersion to control the total volume of the slurry to 33L and the concentration to 0.3%. First, use a stirrer to mechanically stir at 800r / min for 45 minutes, and then use an ultrasonic cleaner to ultrasonically assist dispersion at 200W power and 40kHz frequency for 20 minutes to obtain a uniform composite slurry.

[0020] (3) Forming and dewatering: The paper is formed using an inclined wire former with the wire speed controlled at 15m / min and the vacuum dewatering pressure at -0.05MPa. Subsequently, a two-roll press is used for pressing and dewatering with a linear pressure of 120kN / m and a pressing zone pressure of 1.0MPa, and the wet paper dryness is controlled at 40±2%.

[0021] (4) In-situ curing and hot pressing: The wet paper sheet is placed in a hot press and hot pressing is performed in two stages. The first stage is treated for 10 minutes at 2.0 MPa pressure and 140℃ temperature with a heating rate of 3℃ / min; the second stage is treated for 30 minutes at 8.0 MPa pressure and 260℃ temperature with a heating rate of 3℃ / min.

[0022] (5) Post-processing: In the post-processing stage, programmed cooling is used to cool the material down to 45°C at a cooling rate of 2°C / min. The tension is controlled at 1.0 N / cm using a tension control system. Finally, the material is wound up at a speed of 10 m / min using a winding machine to obtain the final product insulation material with a thickness of 0.08 ± 0.005 mm.

[0023] For performance index calculations, in terms of electrical performance, the prepared insulating material sample is placed between two electrodes of a specific shape, and an AC voltage is applied to the sample at a constant rate (e.g., 500 V / s) until the material breaks down. The breakdown voltage V is recorded, and the dielectric strength E is calculated using E=V / d, where d is the average thickness of the sample. Typically, multiple samples (e.g., 5-10) are tested, and the average value is taken as the final reported value. Volume resistivity is measured using a three-electrode system (measuring electrode, protective electrode, and high-voltage electrode) to eliminate the influence of surface leakage current. A stable DC voltage V is applied to the sample, and the current I flowing through the sample volume is measured. The volume resistance R=V / I, and the volume resistivity ρ=R×A / t, where A is the effective area of ​​the measuring electrode, and t is the sample thickness in meters. This value reflects the material's ability to resist volume current. The dielectric loss factor tanδ is a parameter directly measured by the instrument, representing the ratio of lost energy to stored energy. The smaller this value, the higher the efficiency of the insulating material. Tensile strength was determined by cutting the sample into a standard dumbbell shape and stretching it on a tensile testing machine. The tensile force F and elongation were continuously recorded until the sample broke. The tensile strength σ = F / (W×t), where F is the maximum tensile force, W is the sample width, and t is the thickness. For thermal properties, the average tensile strength TS1 of one set of original samples was tested first. Another set of samples was placed in an oven at 180℃ for 1000 hours. After removal, the average tensile strength TS2 of the aged samples was tested. Thermal stability = (TS2 / TS1)×100%.

[0024] Performance test results: In terms of electrical properties, the dielectric strength is 72.5 kV / mm (ASTM D149), and the volume resistivity is 1.5 × 10¹. 6 Ω·m, tensile strength 48.3MPa (ASTM D828), thermal stability 95%, and dielectric loss factor 0.0018. Example 2

[0025] The selected raw materials include chopped para-aramid fibers, para-aramid precipitated fibers, titanium dioxide nanotubes, KH560 type γ-(2,3-epoxypropoxy)propyltrimethoxysilane, and N-methylpyrrolidone (NMP). Among these, the titanium dioxide nanotubes possess a unique tubular structure with an outer diameter of 50±10 nm, an inner diameter of 20±5 nm, a length of 2-5 μm, a specific surface area of ​​220±15 m² / g, and a purity ≥99.9%. Furthermore, the surface of the titanium dioxide nanotubes undergoes hydroxylation treatment, resulting in good hydrophilicity and dispersibility. KH560 type γ-(2,3-epoxypropoxy)propyltrimethoxysilane has an epoxy value of 0.42 mol / 100 g and a density of 0.94 g / mL. The mass ratio of chopped fibers to precipitated fibers is maintained at 75:25.

[0026] The preparation process is as follows: (1) Pretreatment and dispersion: The pretreatment and dispersion steps adopted a step-by-step process. First, 3.0 g of titanium dioxide nanotubes were slowly added to a beaker containing 60 mL of NMP solvent, and pretreated for 30 minutes at 250 W using an ultrasonic cell disruptor to ensure that the nanotubes were fully dispersed. Then, 1.2 g of KH560 coupling agent was added, and ultrasonic treatment was continued at 300 W for 40 minutes, during which the temperature was controlled not to exceed 40 °C using a circulating water bath system. The mixture was mechanically stirred at 1800 r / min for 75 minutes to obtain a uniform and stable functional dispersion.

[0027] (2) Slurry compounding: control the slurry concentration at 0.3%, first mechanically stir at 700r / min for 35 minutes, and then ultrasonically treat with 150W power for 15 minutes.

[0028] (3) Forming and dewatering: The paper is formed by using a slanted wire forming machine, and then dewatered by pressing under a pressure of 1.0 MPa, controlling the dryness of the wet paper sheet to 40%.

[0029] (4) In-situ curing and hot pressing: Place the wet paper sheet in a hot press. First stage: treat at 1.5MPa and 150℃ for 12 minutes; Second stage: treat at 7MPa and 280℃ for 25 minutes (heating rate 4℃ / min).

[0030] (5) Post-processing: Cool to 45°C at a rate of 2°C / min, control the tension at 1.0 N / cm, and roll up to obtain an insulating material with a thickness of 0.08 mm.

[0031] Performance test results: In terms of electrical properties, the dielectric strength is 68.7 kV / mm (ASTM D149), and the volume resistivity is 8.3 × 10¹. 5 The titanium dioxide nanotubes exhibit a tensile strength of 45.6 MPa (ASTM D828), a thermal stability of 93%, and a dielectric loss factor of 0.0021. These nanotubes are uniformly distributed within the aramid fiber network, forming a well-structured interconnected network. Example 3

[0032] Regarding raw material selection, a higher proportion of chopped fibers was chosen, with the mass ratio of chopped fibers to precipitated fibers adjusted to 85:15. Boron nitride nanosheets were still used as the nanofiller, but the addition amount was increased to 8.0% to study the material properties under high filler content. KH550 was selected as the silane coupling agent, and its addition amount was correspondingly increased to 1.5%.

[0033] In terms of the preparation process, during the pretreatment and dispersion steps, 8.0 g of boron nitride nanosheets and 1.5 g of KH550 were added to 160 mL of NMP solvent and ultrasonically treated with 350 W power for 70 minutes, followed by mechanical stirring at 2200 r / min for 100 minutes. During the pulp compounding process, due to the high proportion of chopped fibers, the mechanical stirring time was appropriately extended to 50 minutes, while the ultrasonic treatment time was maintained at 20 minutes. The forming and dewatering process parameters were the same as in Example 1, controlling the wet paper dryness at 40 ± 2%. The hot pressing process adopted a two-stage procedure: the first stage was treated at 2.5 MPa pressure and 130°C for 8 minutes, and the second stage was treated at 9 MPa pressure and 270°C for 35 minutes.

[0034] Performance test results: In terms of electrical properties, the dielectric strength is 70.2 kV / mm (ASTM D149), and the volume resistivity is 9.8 × 10¹. 5 The material exhibits a tensile strength of 46.8 MPa (ASTM D828), a thermal stability of 94%, and a dielectric loss factor of 0.0019. A denser three-dimensional network structure is formed within the material, with boron nitride nanosheets acting as effective bridging agents between the fibers.

[0035] In Comparative Example 1, the blank control experiment, no nanofillers or silane coupling agents were added; only aramid fiber raw materials were used, with the mass ratio of chopped fibers to precipitated fibers maintained at 80:20. In the preparation process, 100g of aramid fiber was directly dispersed in 100mL of NMP solvent, using the same stirring and ultrasonic treatment parameters as in Example 1. The forming and dehydration processes were kept consistent, controlling the wet paper sheet dryness at 40%. The hot-pressing process employed the same two-stage procedure to eliminate the influence of process parameters.

[0036] The performance test results show that the material in Comparative Example 1 has significantly worse performance. In terms of electrical properties, its dielectric strength is 48.2 kV / mm (ASTM D149), and its volume resistivity is 2.1 × 10¹. 5 The material exhibits a tensile strength of 42.1 MPa (ASTM D828), a thermal stability of 90%, and a dielectric loss factor of 0.0035. However, the presence of numerous pores and defects within the material, coupled with an insufficiently dense fiber network structure, directly impacts its various properties.

[0037] Comparative Example 2 simulates a commonly used filler addition method in the industry. Micron-sized alumina filler with an average particle size of 5 μm and a purity ≥99.6% was selected, and the addition amount was 5.0%. No silane coupling agent was used; a traditional mechanical mixing process was employed, directly mixing the alumina filler with aramid fibers in NMP solvent. During the preparation process, a mechanical stirrer was used to stir at 1500 rpm for 60 minutes; ultrasonic-assisted dispersion was not used. The hot-pressing process adopted a traditional single-stage procedure, directly heating from room temperature to 260°C at a rate of 5°C / min, and then holding at 8 MPa pressure for 30 minutes.

[0038] Performance test results showed a significant deterioration in performance. In terms of electrical properties, the dielectric strength was 52.3 kV / mm (ASTM D149), and the volume resistivity was 3.5 × 10¹. 5 The material exhibits a tensile strength of 38.5 MPa (ASTM D828), a thermal stability of 88%, and a dielectric loss factor of 0.0028. The micron-sized alumina filler shows severe agglomeration within the fiber network, with significant interfacial gaps between the filler and the fiber matrix. These structural defects directly lead to a decline in material properties.

[0039] Comparative Example 3 uses the same raw material ratio and basic process parameters as Example 1, namely 80.0g of para-aramid chopped fiber, 20.0g of para-aramid precipitated fiber, 5.0g of boron nitride nanosheets, and 1.0g of KH550 silane coupling agent.

[0040] Strictly following the method described in Example 1, 5.0 g of boron nitride nanosheets and 1.0 g of KH550 were added to 100 mL of NMP and treated with the same ultrasonic and stirring parameters as in step (1) of Example 1, followed by drying to obtain modified BN powder. 80.0 g of para-aramid chopped fibers and 20.0 g of para-aramid precipitated fibers were dispersed in an NMP / water mixture as in step (2) of Example 1 to form a fiber slurry. The modified BN powder was added to the fiber slurry and mixed evenly with the same stirring and ultrasonic parameters as in step (2) of Example 1. The forming, dehydration, hot pressing, and cooling steps are the same as in Example 1. Specifically, 5.0g of boron nitride nanosheets and 1.0g of KH550 are added to 100mL of MP solvent. The mixture is then treated for 60 minutes using an ultrasonic cell disruptor at 300W power for 2 seconds followed by a 1-second pause, with the temperature controlled at ≤35℃. Subsequently, the mixture is mechanically stirred at 2000r / min for 90 minutes to obtain a functional dispersion. Then, 80.0g of para-aramid chopped fibers and 20.0g of para-aramid precipitated fibers are added to the dispersion, and water is added to bring the total volume to 33L, resulting in a pulp concentration of 0.3%. The mixture is first mechanically stirred at 800r / min for 45 minutes, followed by ultrasonic treatment at 200W for 20 minutes to obtain a composite pulp. The pulp is then formed using a slanted wire forming machine, vacuum dehydrated at -0.05MPa, and then dehydrated by pressing at 1.0MPa, controlling the wet paper dryness at 40±2%. The material undergoes two stages of hot pressing: the first stage is treated at 2.0 MPa and 140°C for 10 minutes, and the second stage is treated at 8.0 MPa and 260°C for 30 minutes; finally, it is cooled to 45°C at a rate of 2°C / min and then wound up to obtain the insulating material.

[0041] The difference between Comparative Example 3 and Example 1 lies in the contact method between the filler and the fiber. In Example 1, the filler is directly and uniformly mixed with the fiber in the liquid phase in a functionalized state, while in Comparative Example 3, the filler is first dried into powder and then added to the fiber slurry in solid form. Performance test results show a significant deterioration in performance. Regarding electrical properties, the dielectric strength is 58.6 kV / mm (ASTM D149), and the volume resistivity is 4.2 × 10¹. 5 Ω·m, tensile strength 40.1MPa (ASTM D828), thermal stability 88%, and dielectric loss factor 0.0026.

[0042] Comparative Example 4 used the same raw material ratios and basic process parameters as Example 1, namely 80.0 g of para-aramid chopped fibers, 20.0 g of para-aramid precipitated fibers, 5.0 g of boron nitride nanosheets, and 1.0 g of KH550 silane coupling agent. Except for shortening the second-stage hot-pressing time to 5 minutes, the raw materials, ratios, and process steps were the same as in Example 1. The first stage of hot pressing was performed at 2.0 MPa and 140°C for 10 minutes; the second stage was performed at 8.0 MPa and 260°C for 5 minutes.

[0043] Performance test results showed a significant deterioration in performance. In terms of electrical properties, the dielectric strength was 56.8 kV / mm (ASTM D149), and the volume resistivity was 3.8 × 10¹. 5 Ω·m, tensile strength 39.5MPa (ASTM D828), thermal stability 87%, and dielectric loss factor 0.0029.

[0044] Analysis of the experimental results shows that Examples 1-3 all exhibit significantly better overall performance than the comparative examples. The synergistic effect of the nanofiller and the silane coupling agent effectively improves the filler dispersion and interfacial bonding strength, while the two-stage hot-pressing process optimizes the microstructure of the material. Particularly in terms of dielectric properties and thermal stability, the technical solution provided by this invention demonstrates significant technical advantages, offering a new technical approach for the development of high-performance insulating materials.

[0045] Analysis of the experimental results shows that Examples 1-3 all exhibit significantly better overall performance than the comparative examples. The synergistic effect of the nanofiller and the silane coupling agent effectively improves the filler dispersion and interfacial bonding strength, while the two-stage hot-pressing process optimizes the microstructure of the material. Particularly in terms of dielectric properties and thermal stability, the technical solution provided by this invention demonstrates significant technical advantages, offering a new technical approach for the development of high-performance insulating materials.

[0046] Table 1 compares the dielectric strength and performance test results of the examples and comparative examples:

[0047] The testing standards used in the embodiments of the present invention are as follows: Dielectric strength is measured according to standard ASTM D149, which is officially titled "Standard Test Methods for Dielectric Breakdown Voltage and Dielectric Strength of Solid Electrical Insulators at Commercial Power Frequency". Volume resistivity is measured using ASTM D257, which defines the resistance of the material per unit cross-sectional area and per unit length. Tensile strength is measured according to standard ASTM D828, which is officially titled "Standard Test Method for Determining Tensile Properties of Plastics Using Micro-Tension Specimens"; Thermal stability is determined according to IEC 60085 "Classification of heat resistance of electrical insulation materials".

[0048] This invention is not limited to the above embodiments. Any modifications, alterations, simplifications, combinations, or substitutions that do not depart from the essence and principle of this invention are within the protection scope of this patent.

Claims

1. A high dielectric strength aramid fiber insulating material, characterized in that, It is composed of a para-aramid fiber skeleton, nano-scale dielectric filler and interface binder. The nano-scale dielectric filler is chemically bonded to the aramid fiber skeleton through the interface binder to form a stable fiber-coupling agent-filler composite structure. The para-aramid fiber skeleton is composed of para-aramid chopped fibers and para-aramid precipitated fibers in a mass ratio of (60-90):(40-10); The nanoscale dielectric filler is boron nitride nanosheets or titanium dioxide nanotubes, and its content accounts for 1% to 10% of the total mass of the aramid fiber skeleton; wherein, the thickness of the boron nitride nanosheets is 1 to 10 nm and the transverse dimension is 0.5 to 5 μm; the outer diameter of the titanium dioxide nanotubes is 20 to 100 nm and the length is 0.5 to 10 μm. The interface binder is a silane coupling agent, and its content accounts for 0.1% to 2% of the total mass of the aramid fiber skeleton; The insulating material is prepared by a method comprising the following steps: the nanoscale dielectric filler and the silane coupling agent are ultrasonically and mechanically dispersed in an organic solvent to form a functional dispersion; then, the para-aramid chopped fibers and para-aramid precipitated fibers are added to the dispersion, and the mixture is stirred and ultrasonically treated to obtain a composite slurry; this slurry is then formed by papermaking, dehydrated by pressing, and subjected to a two-stage hot-pressing process, comprising: a first stage of treatment at 1–3 MPa and 120–160°C for 5–15 minutes; and a second stage of treatment at 5–10 MPa and 240–300°C for 20–50 minutes; finally, the insulating material is obtained by cooling. The dielectric strength of the insulating material is not less than 60kV / mm under the conditions of 25℃ and 50Hz, and the heat resistance level reaches H class (180℃) or above.

2. The insulating material according to claim 1, characterized in that: The length of the para-aramid chopped fibers is 3-8 mm, and the linear density is 1.0-2.5 dtex; the average length of the para-aramid precipitated fibers is 0.5-3 mm.

3. The insulating material according to claim 1, characterized in that: The silane coupling agent is at least one of γ-aminopropyltriethoxysilane and γ-(2,3-epoxypropoxy)propyltrimethoxysilane.

4. A method for preparing the high dielectric strength aramid fiber insulating material as described in any one of claims 1 to 3, characterized in that, Includes the following steps: (1) The nano-sized dielectric filler and silane coupling agent are dispersed together in an organic solvent, and then subjected to ultrasonic treatment and mechanical stirring to obtain a functional dispersion; (2) Add aramid short-cut fibers and aramid precipitated fibers to the functional dispersion obtained in step (1), and obtain a uniform composite slurry by mechanical stirring and ultrasonic treatment; (3) The composite pulp is formed by wet papermaking, and then pressed and dewatered to obtain a wet paper sheet; (4) Perform a two-stage hot pressing treatment on the wet paper sheets; (5) Cool the hot-pressed material to below 50°C at a rate of 1-3°C / min, and then wind it up to obtain the insulating material.

5. The method according to claim 4, characterized in that: In step (1), the ultrasonic treatment power is 200-500W and the treatment time is 30-90 minutes; the mechanical stirring speed is 1000-3000r / min and the stirring time is 60-120 minutes.

6. The method according to claim 4, characterized in that: In step (2), the slurry concentration is controlled at 0.1% to 0.5%, the mechanical stirring speed is 500 to 1000 r / min, and the stirring time is 30 to 60 minutes; the ultrasonic treatment power is 100 to 300 W, and the treatment time is 10 to 30 minutes.

7. The method according to claim 4, characterized in that: In step (3), the pressure for pressing and dewatering is 0.5 to 2.0 MPa, and the dryness of the wet paper sheet is controlled at 35% to 45%.

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