Aramid fiber composite sheet prepared by high-pressure reverse diffusion dry method

Abstract

The invention discloses an aramid fiber composite sheet prepared by a high-pressure reverse diffusion dry method, the raw material of the aramid fiber composite sheet is a three-layer sandwich structure, and the aramid fiber composite sheet is composed of a top layer, a middle layer and a bottom layer; the top layer and the bottom layer are aramid fiber non-woven fabrics; the middle layer is a polyphenylene sulfide superfine fiber non-woven fabric; the raw materials are subjected to high-pressure hot-pressing treatment, the polyphenylene sulfide superfine fiber non-woven fabric of the middle layer is molten and permeates and fills pores of the aramid fiber non-woven fabric of the top layer and the aramid fiber non-woven fabric of the bottom layer, continuous polyphenylene sulfide films are formed on the surfaces of the top layer and the bottom layer, and the aramid fiber composite sheet of a five-layer structure is formed. The three layers are sequentially a polyphenylene sulfide film, an aramid fiber non-woven fabric and polyphenylene sulfide mixed layer, a polyphenylene sulfide layer, an aramid fiber non-woven fabric and polyphenylene sulfide mixed layer and a polyphenylene sulfide film. The cross section and the surface of the prepared aramid fiber composite sheet are free of obvious pores, the breakdown strength is larger than 50 kV / mm, the limit oxygen index is larger than 34% while the mechanical property of the material is kept, and the aramid fiber composite sheet has good application prospects.

Description

Technical Field

[0001] This application relates to the field of high-performance paper-based materials technology, specifically to an aramid composite sheet prepared by high-pressure reverse diffusion dry method. Background Technology

[0002] Aramid materials occupy an indispensable position in high-end fields such as aerospace, electrical insulation, and new energy due to their excellent high temperature resistance, good electrical insulation properties, and outstanding mechanical strength.

[0003] As high-end equipment develops towards lightweight and high reliability, higher comprehensive performance requirements are placed on aramid composite materials. The materials are required to have stable and high breakdown strength, maintain excellent insulation performance in high temperature and high humidity environments, and have long-term thermal stability and environmental barrier capabilities. These application scenarios require aramid composite materials to have excellent density in order to eliminate electric field concentration points, block breakdown paths, and prevent environmental media corrosion.

[0004] Traditional aramid paper is mostly prepared using a wet papermaking process, but this process has inherent defects, such as difficulty in fiber dispersion, high water consumption, high energy consumption, and the resulting aramid paper has a porous structure with a large number of micropores on both the cross-section and surface, which become weak points for electrical breakdown and environmental erosion.

[0005] To optimize the performance of aramid composite materials, existing technologies have developed composite solutions using polyphenylene sulfide (PPS) as the reinforcing phase. Chinese Patent Publication No. CN116749620A discloses an aramid composite insulating sheet for wet-type transformers and its preparation method. The method uses aramid sheets as the top and bottom layers and PPS microfiber nonwoven fabric as the intermediate layer. High-temperature hot pressing is used to make the intermediate PPS layer a dense film, and aramid fibers are embedded in the film to form an interwoven structure. The intermediate polyphenylene sulfide microfiber nonwoven fabric layer in this solution has no obvious porous structure.

[0006] However, the CN116749620A technical solution still has the following technical defects, making it difficult to meet the requirements of high-end applications for overall non-porous materials, as follows: PPS film formation is limited to the intermediate layer, while the aramid layer itself retains a porous structure. The hot pressing pressure used in this scheme is only 0.1-0.5 MPa. Under these conditions, although the PPS melt can melt and form a film, the film formation location is limited to the intermediate layer region where the original PPS nonwoven fabric is located. Due to the lack of sufficient driving force, the molten PPS cannot fully penetrate into the interior of the aramid layer to fill the pores between the fibers, resulting in a non-homogeneous structure of dense intermediate layer-porous aramid layer in the composite sheet. The porous aramid region becomes the weak link in the breakdown path.

[0007] The surface of the aramid nonwoven fabric is not covered by PPS, and there are open pores on the surface. In this solution, the PPS film is located in the interface area between the two aramid layers and has not fully migrated to the outermost surface of the composite sheet. Even if some PPS may migrate to the outermost surface of the composite sheet, it is a small amount and intermittent and discontinuous. The top and bottom surfaces are still the aramid nonwoven fabric body, and its fiber accumulation pores are completely exposed, forming open defects. These surface pores lead to problems such as electric field concentration, penetration of environmental media, and limited flame retardant performance. Furthermore, because the aramid layer maintains a porous structure and the surface pores are exposed, the surface open pores result in insufficient environmental barrier capacity.

[0008] In view of the above-mentioned deficiencies of the existing technology, there is an urgent need in the field to develop an aramid composite sheet and its preparation method that can achieve full penetration and filling of PPS, continuous and dense surface film, and excellent comprehensive performance, so as to meet the requirements of high-end application fields for material density, as well as the comprehensive performance of interface uniformity, breakdown strength, flame retardancy, mechanical properties and long-term reliability. Summary of the Invention

[0009] This application provides an aramid composite sheet prepared by high-pressure reverse diffusion dry method to solve or at least partially solve the above problems. The aramid composite sheet has high density, uniform interface, and better comprehensive performance in terms of tensile strength, flame retardancy and insulation.

[0010] To address the problems existing in the prior art, in the first aspect, this application provides an aramid composite sheet prepared by high-pressure reverse diffusion dry method. The raw material of the aramid composite sheet is a three-layer sandwich structure, consisting of a top layer, a middle layer, and a bottom layer; the top layer and the bottom layer are aramid nonwoven fabrics; the middle layer is polyphenylene sulfide microfiber nonwoven fabric. The raw materials of the aramid composite sheet are subjected to high-pressure hot pressing. The polyphenylene sulfide microfiber nonwoven fabric in the middle layer melts and permeates into the pores of the top and bottom aramid nonwoven fabrics, forming a continuous polyphenylene sulfide film on the surface of the top and bottom layers, thus forming a five-layer aramid composite sheet, which consists of a polyphenylene sulfide film, a mixed layer of aramid nonwoven fabric and polyphenylene sulfide, a polyphenylene sulfide layer, a mixed layer of aramid nonwoven fabric and polyphenylene sulfide, and a polyphenylene sulfide film.

[0011] Furthermore, the cross-section and surface of the aramid composite sheet have no obvious pores.

[0012] Furthermore, the basis weight of the aramid nonwoven fabric in the raw material of the aramid composite sheet is 10-200 g / m². 2 .

[0013] Furthermore, the basis weight of the polyphenylene sulfide microfiber nonwoven fabric in the raw material of the aramid composite sheet is 20-100 g / m². 2 .

[0014] Furthermore, the mass percentage of polyphenylene sulfide microfiber nonwoven fabric in the raw materials of the aramid composite sheet is 30-70%.

[0015] Furthermore, the aramid nonwoven fabric accounts for 30-70% of the mass of the raw materials of the aramid composite sheet.

[0016] The mass ratio of polyphenylene sulfide microfiber nonwoven fabric is precisely controlled by adjusting the basis weight of polyphenylene sulfide microfiber nonwoven fabric and aramid nonwoven fabric.

[0017] Furthermore, the polyphenylene sulfide microfiber nonwoven fabric in the raw material of the aramid composite sheet has a fiber diameter of 1-10 μm.

[0018] Furthermore, the top and bottom aramid nonwoven fabrics are single-layer or multi-layer stacked structures; preferably, the top and bottom aramid nonwoven fabrics are 1-5 layer stacked structures.

[0019] Furthermore, the polyphenylene sulfide microfiber nonwoven fabric of the intermediate layer is a single-layer or multi-layer stacked structure; preferably, the polyphenylene sulfide microfiber nonwoven fabric of the intermediate layer is a 1-5 layer stacked structure.

[0020] Furthermore, the aramid composite sheet has a breakdown strength greater than 50 kV / mm and a limiting oxygen index greater than 34%.

[0021] Furthermore, the high-pressure hot pressing is performed using a roller press with a temperature of 90-220℃, a pressure of 2-15 MPa, and a linear speed of 0.2-3.8 m / min.

[0022] Secondly, this application provides a method for preparing aramid composite sheets using a high-pressure reverse diffusion dry method, for preparing the aramid composite sheets described in the first aspect, comprising the following steps: S1: Aramid fibers are made into aramid nonwoven fabric through needle punching or hydroentangling processes; S2: Polyphenylene sulfide is processed into polyphenylene sulfide microfiber nonwoven fabric through meltblowing process; S3: Using the aramid nonwoven fabric prepared in step S1 as the top and bottom layers, and the polyphenylene sulfide microfiber nonwoven fabric prepared in step S2 as the middle layer, they are stacked to form a three-layer sandwich structure of aramid nonwoven fabric-polyphenylene sulfide microfiber nonwoven fabric-aramid nonwoven fabric, thus obtaining the raw material for aramid composite sheet. S4: The raw material of the aramid composite sheet obtained in step S3 is pre-pressed and flattened, then subjected to high-pressure hot pressing and cooling to obtain the aramid composite sheet.

[0023] Furthermore, the aramid fiber mentioned in step S1 is a para-aramid fiber or a meta-aramid fiber, and the length of the aramid fiber is 20-120 mm and the fiber diameter is 5-20 μm.

[0024] Furthermore, the para-aramid fiber is poly(p-phenylene terephthalamide) fiber; the meta-aramid fiber is poly(m-phenylene isophthalamide) fiber.

[0025] The type of aramid fiber can be flexibly selected according to the mechanical properties and insulation performance requirements of the product.

[0026] Furthermore, the needle-punching process has a needle-punching density of 200-500 needles / cm²; the hydroentangling process has a hydroentangling pressure of 0.3-0.8MPa, ensuring tight bonding between aramid fibers while retaining an appropriate amount of pores for reverse diffusion of polyphenylene sulfide meltblown fibers.

[0027] Furthermore, the basis weight of the aramid nonwoven fabric mentioned in step S1 is 10-200 g / m². 2 .

[0028] Furthermore, the specific steps of step S1 are as follows: Aramid fibers are opened, carded, and laid into a web, and then reinforced by needle punching or hydroentangling processes to produce aramid nonwoven fabric.

[0029] By opening, carding, and web-laying aramid fibers, entanglement of aramid fibers can be effectively avoided, allowing the aramid fibers to be evenly arranged to form a fiber web. The carding and web-laying processes ensure that the aramid fibers are evenly distributed in the nonwoven fabric, which can replace the traditional wet papermaking process and solve the problems of entanglement, delamination, and high energy consumption of aramid fibers and polyphenylene sulfide pulp in wet papermaking.

[0030] Furthermore, the parameters for the meltblown process in step S2 are as follows: The melt-blowing temperature is 280-320℃, the spinneret diameter is 0.2-0.5mm, and the receiving distance is 10-20cm.

[0031] The polyphenylene sulfide ultrafine fibers prepared by meltblown process have the characteristics of fine diameter, large specific surface area and good dispersibility, which can effectively solve the problem of uneven mixing of aramid fibers and polyphenylene sulfide resin, and lay the foundation for uniform mixing of the two in the future.

[0032] Furthermore, the polyphenylene sulfide mentioned in step S2 is a linear polyphenylene sulfide with a melting point of 280-290℃.

[0033] Linear polyphenylene sulfide resin possesses excellent thermal stability and insulation properties, and its performance is highly compatible with that of aramid fiber, enabling synergistic effects between the two and further enhancing the overall performance of the product.

[0034] Furthermore, the fiber diameter of the polyphenylene sulfide microfiber nonwoven fabric described in step S2 is 1-10 μm.

[0035] Furthermore, the basis weight of the polyphenylene sulfide microfiber nonwoven fabric mentioned in step S2 is 20-100 g / m². 2 .

[0036] The fine diameter of polyphenylene sulfide microfiber nonwoven fabric allows it to quickly and evenly penetrate into the pores between aramid fibers, further solving the problem of uneven mixing between the two.

[0037] Furthermore, the polyphenylene sulfide microfiber nonwoven fabric accounts for 30-70% of the mass of the aramid composite sheet.

[0038] By precisely adjusting the basis weight of aramid nonwoven fabric and polyphenylene sulfide meltblown nonwoven fabric, the quality ratio of polyphenylene sulfide microfiber nonwoven fabric can be accurately controlled, adapting to the performance requirements of different application scenarios, while ensuring a uniform mixing ratio of polyphenylene sulfide microfiber and aramid fiber.

[0039] Furthermore, the top and bottom aramid nonwoven fabrics in step S3 are single-layer or multi-layer stacked structures; preferably, the top and bottom aramid nonwoven fabrics are 1-5 layer stacked structures.

[0040] Furthermore, the polyphenylene sulfide microfiber nonwoven fabric of the intermediate layer in step S3 is a single-layer or multi-layer stacked structure; preferably, the polyphenylene sulfide microfiber nonwoven fabric of the intermediate layer is a 1-5 layer stacked structure.

[0041] The raw material for the three-layer sandwich structure of aramid nonwoven fabric-polyphenylene sulfide microfiber nonwoven fabric-aramid nonwoven fabric obtained in step S3 is such that the polyphenylene sulfide microfiber is wrapped in aramid fiber, providing a uniform diffusion channel for subsequent high-pressure reverse diffusion. This ensures that the polyphenylene sulfide fiber can penetrate into the pores of the aramid nonwoven fabric in all directions and achieve uniform mixing with the aramid fiber, avoiding the problems of polyphenylene sulfide resin agglomeration or aramid fiber exposure.

[0042] Furthermore, in step S4, the high-pressure hot pressing is carried out using a roller press with a temperature of 90-220℃, a pressure of 2-15 MPa, and a linear speed of 0.2-3.8 m / min.

[0043] In the technical solution of this application, the high-pressure driven permeation mechanism reduces the sensitivity to the basis weight of the raw materials, and aramid composite sheets with a dense structure can be stably obtained in a wide range of PPS nonwoven fabric basis weights of 20-100 g / m² and aramid nonwoven fabric basis weights of 10-200 g / m².

[0044] Furthermore, in step S4, the pre-pressing and flattening involves placing the raw material of the aramid composite sheet in an anti-sticking interlayer structure and pressing it under pressure at room temperature.

[0045] Furthermore, the anti-stick interlayer structure includes two layers of isolation film and two layers of rigid plate, with the raw material of the aramid composite sheet placed between the two layers of isolation film, and a rigid plate placed on the outer side of each of the two layers of isolation film.

[0046] Furthermore, the separating membrane is a polyimide membrane, and the rigid plate is a metal plate.

[0047] Furthermore, in step S4, the pressing pressure is 0.3-1.0 MPa, and the holding time is 10-30 seconds.

[0048] Furthermore, the roller press is a two-roller press.

[0049] Since both aramid nonwoven fabric and polyphenylene sulfide microfiber nonwoven fabric are loose structures made of stacked fibers, and the three layers are only physically stacked together, there are problems such as uneven thickness, weak interlayer bonding, and uneven surface. In addition, when using a roller press for high-pressure hot pressing, the roller press applies point pressure. If high-pressure hot pressing is performed directly, the roller press applies line contact progressive pressure to the material. The loose raw material is violently compressed the moment it enters the roller gap, which can easily cause the polyphenylene sulfide fibers in the middle layer to migrate laterally and be unevenly distributed. At the same time, slippage and misalignment may occur between the layers, resulting in defects such as large thickness deviation and insufficient local penetration after hot pressing, which seriously affects product quality and consistency.

[0050] By pre-compressing the raw material of the three-layer sandwich aramid composite sheet under a pressure of 0.3-1.0 MPa, the fluffy fiber layers can be initially compressed at room temperature, making the three-layer structure tightly bonded and uniform in thickness. At the same time, the air between the layers is eliminated. Pre-compression at room temperature will not damage the fiber structure. This provides a structurally stable and uniformly thick blank for the uniform melting and penetration of polyphenylene sulfide during the subsequent high-pressure hot pressing process, ensuring that the final product has no obvious pores and excellent performance.

[0051] The preparation method of the aramid composite sheet in this application adopts a high-pressure reverse diffusion dry method. High-pressure reverse diffusion involves using a three-layer sandwich structure as a carrier, applying a directional pressure of 2-15 MPa through a roller press, combined with a hot-pressing temperature of 90-220℃ and a linear velocity of 0.2-3.8 m / min. This drives the molten polyphenylene sulfide (PPS) ultrafine fibers with a diameter of 1-10 μm in the middle layer to break through the conventional penetration direction and undergo bidirectional reverse diffusion into the pores of the top and bottom aramid nonwoven fabrics. The hot-pressing temperature can be flexibly adjusted according to the softening temperature of the PPS resin to ensure that the PPS meltblown fibers soften smoothly and diffuse in reverse under high pressure, while avoiding damage to the properties of the aramid fibers caused by high temperature.

[0052] The core characteristic of this process is: Reverse diffusion direction: Unlike the traditional hot pressing mode where the binder phase penetrates shallowly in one direction, the molten polyphenylene sulfide fiber diffuses bidirectionally from the middle layer to the two aramid nonwoven fabric layers, achieving simultaneous filling of the pores of the top and bottom aramid nonwoven fabrics.

[0053] Pressure-driven dominance: High pressure breaks the high viscosity limitation of polyphenylene sulfide melt, forcing it to penetrate into the micropores between aramid fibers, rather than just staying at the interlayer interface, forming a polyphenylene sulfide fiber-aramid fiber interpenetrating network structure.

[0054] Completeness of synergistic effect: Hot pressing temperature ensures that polyphenylene sulfide is fully melted to have flow and diffusion capabilities, and linear velocity matches the time window of melting, penetration and curing. Pressure, temperature and linear velocity work together to achieve uniform distribution of polyphenylene sulfide in aramid matrix, ultimately making the cross section and surface of aramid composite sheet free of obvious pores, and forming a continuous polyphenylene sulfide film on the top and bottom surfaces.

[0055] High-pressure reverse diffusion is the core process mechanism for achieving full penetration of polyphenylene sulfide and high densification of aramid composite sheets in this application. Its essence is a dry composite diffusion technology that is pressure-driven, thermally assisted, and bidirectionally penetrated, which is a key innovation that distinguishes it from existing traditional hot pressing and wet composite processes.

[0056] Compared with the prior art, this application includes the following beneficial technical effects: This application employs a high-pressure reverse diffusion dry process to prepare aramid composite sheets. High-pressure hot pressing is performed using a roller press at 90-220℃, 0.2-3.8 m / min, and 2-15 MPa. This process allows polyphenylene sulfide (PPS) to fill the numerous voids in the aramid nonwoven fabric, forming an interpenetrating network structure. This tightly binds the aramid fibers with PPS, creating a dense structure. Furthermore, the PPS melt is reverse-extruded onto the surface of the aramid composite sheet to form a thin film. After cooling, the resulting aramid composite sheet is obtained using this high-pressure reverse diffusion dry process. The overall structure of the aramid composite sheet is free of significant voids. Driven by a high pressure of 2-15 MPa, the molten PPS not only forms a dense intermediate layer but also diffuses and fills all the pores of the aramid nonwoven fabric on both sides, simultaneously forming a continuous PPS film on the surface. The final aramid composite sheet has no significant pores in its cross-section or surface, eliminating electric field concentration points and breakdown paths.

[0057] In the technical solution of this application, the PPS melt breaks through the interface limitation under high pressure and forms an interpenetrating network structure with aramid fibers across the entire thickness range, rather than the fiber embedding limited to the shallow surface layer of the interface in the prior art. This three-dimensional reinforcement structure significantly improves the interlayer bonding strength and greatly enhances the tensile strength.

[0058] Existing technologies only achieve densification of the intermediate layer, while the surface aramid layer retains a porous structure. This application uses high-pressure reverse diffusion to enable PPS to simultaneously complete surface film formation and internal filling, forming a functional structure of a dense surface barrier, internal interpenetrating reinforcement, and intermediate barrier. Both the surface and the interior are densified, resulting in a comprehensive improvement in environmental barrier capability and electrical insulation performance.

[0059] The aramid composite sheet provided in this application has a breakdown strength that is consistently greater than 50 kV / mm; and PPS is continuously distributed in the material to form a three-dimensional flame-retardant network with a limiting oxygen index greater than 34%, meeting the requirements for high-end insulation and flame retardancy.

[0060] The preparation process of the aramid composite sheet in this application does not require aqueous phase dispersion or chemical additives, thus avoiding the high water consumption, high energy consumption and wastewater treatment problems of wet processes, which is in line with the trend of green manufacturing. Attached Figure Description

[0061] The present application will be described in further detail below with reference to the accompanying drawings and preferred embodiments. However, those skilled in the art will understand that these drawings are drawn only for the purpose of explaining the preferred embodiments and therefore should not be construed as limiting the scope of this disclosure. Furthermore, unless specifically indicated, the drawings are only schematic representations of the composition or structure of the described objects and may contain exaggerated depictions, and the drawings are not necessarily drawn to scale.

[0062] Figure 1 This application presents a schematic diagram of the high-pressure reverse diffusion dry process unit.

[0063] Figure 2 Scanning electron microscope (SEM) images of the morphology of aramid composite sheets prepared by high-pressure reverse diffusion dry method, wherein (a) is a surface SEM image of the aramid composite sheet; and (b) is a cross-sectional SEM image of the aramid composite sheet.

[0064] Figure 3 The images show photographs of the aramid composite insulating sheet prepared in Comparative Example 11 and the aramid composite sheet prepared in Example 4, wherein (a) is a photograph of the aramid composite insulating sheet prepared in Comparative Example 11; and (b) is a photograph of the aramid composite sheet prepared in Example 4. Detailed Implementation

[0065] The following is in conjunction with the appendix Figures 1 to 3 This application will be described in detail.

[0066] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0067] Chemicals and reagents Unless otherwise specified, all other reagents used in the embodiments of this application are from conventional commercially available products.

[0068] Detection methods Tensile strength: The prepared aramid composite sheet was cut into strips 10mm wide and 10cm long, and the tensile strength was tested by an electronic universal testing machine. The average value of the test data was taken from multiple samples.

[0069] Breakdown strength: The prepared aramid composite sheet was cut into strips 10cm wide and 10cm long. The strips were immersed in a constant temperature water bath filled with silicone oil. The breakdown strength was tested by an ultra-high pressure withstand tester. The average value of the test data was taken from multiple test points.

[0070] Limiting oxygen index: The prepared aramid composite sheet is cut into strips 6.5 mm wide and 13 cm long. The strips are fixed in a mixed gas flow. By adjusting the flow rates of nitrogen and oxygen, the sample is made to automatically extinguish within a specified time or distance. The highest oxygen ratio reached by the mixed gas flow is the limiting oxygen index. The final result is the highest value that the sample passes multiple tests.

[0071] Fiber diameter of polyphenylene sulfide microfiber nonwoven fabric: The diameter of the fibers is determined by measuring the scanning electron microscope image of the polyphenylene sulfide microfiber nonwoven fabric and statistically analyzing its diameter distribution.

[0072] The specific implementation method of this application is as follows.

[0073] Example 1 Combination Figure 1 The schematic diagram of the high-pressure reverse diffusion dry process apparatus shown illustrates how this embodiment provides an aramid composite sheet prepared by high-pressure reverse diffusion dry process, comprising the following steps: S1: Select para-aramid short-cut fibers with a length of 60mm and a diameter of 12μm. After opening, carding, and web laying, reinforce them by needle punching with a needle punching density of 300 needles / cm² to produce aramid nonwoven fabric with a basis weight of 70 g / m².

[0074] S2: Polyphenylene sulfide is processed into polyphenylene sulfide microfiber nonwoven fabric using a melt-blowing process. The basis weight of the polyphenylene sulfide microfiber nonwoven fabric is 60 g / m². 2 The mass percentage of polyphenylene sulfide microfiber nonwoven fabric in aramid composite sheet is 30%.

[0075] S3: Using the aramid nonwoven fabric prepared in step S1 as the top and bottom layers, and the polyphenylene sulfide microfiber nonwoven fabric prepared in step S2 as the middle layer, they are stacked to form a three-layer sandwich structure of aramid nonwoven fabric-polyphenylene sulfide microfiber nonwoven fabric-aramid nonwoven fabric, thus obtaining the raw material for aramid composite sheet. The middle layer of polyphenylene sulfide microfiber nonwoven fabric, the top layer and the bottom layer of aramid nonwoven fabric are all single-layer structures.

[0076] S4: The raw material of the aramid composite sheet obtained in step S3 is pre-pressed and flattened. The raw material of the three-layer sandwich structure aramid composite sheet is placed between two polyimide films, and a flat iron plate is placed on the outside of each of the two polyimide films to form a sandwich structure. Then, at room temperature, a mechanical pressure of 0.5MPa is applied and the pressure is held for 15s to press and smooth the flatness of the three-layer sandwich structure. The pre-pressed and flattened three-layer sandwich structure is then rolled by a double-roll press at a linear speed of 1.0 m / min, a temperature of 190℃, and a pressure of 2.5 MPa. The pressure drives the polyphenylene sulfide fibers in the three-layer sandwich structure to diffuse in the reverse direction into the pores of the aramid nonwoven fabric, achieving microscale penetration and densification. A polyphenylene sulfide film is formed on the top and bottom surfaces of the aramid composite sheet. After cooling, the aramid composite sheet is obtained. The prepared aramid composite sheet has a five-layer structure, consisting of a polyphenylene sulfide film, a mixed layer of aramid nonwoven fabric and polyphenylene sulfide, a polyphenylene sulfide layer, a mixed layer of aramid nonwoven fabric and polyphenylene sulfide, and a polyphenylene sulfide film.

[0077] Based on Example 1, this application adjusts the basis weight of aramid nonwoven fabric and the basis weight of polyphenylene sulfide microfiber nonwoven fabric to adjust the mass ratio of polyphenylene sulfide microfiber nonwoven fabric. Other parameters and processes are the same as in Example 1. The performance of the prepared aramid composite sheet was tested, and the specific parameters and test results are shown in Table 1.

[0078] Table 1 As shown in Table 1, with the increase of the mass ratio of polyphenylene sulfide (PPS) microfiber nonwoven fabric, the tensile strength, breakdown strength, and limiting oxygen index of the aramid composite sheet prepared by high-pressure reverse diffusion dry method all show a trend of first increasing and then decreasing. When the content of PPS microfiber nonwoven fabric is low, it effectively fills the pores between aramid fibers as a thermoplastic binder, enhancing fiber bonding and structural density, allowing the high strength, high insulation, and high flame retardancy of aramid to be fully utilized, and producing a synergistic flame retardant effect. However, when the content of PPS microfiber nonwoven fabric exceeds 61.9%, its relatively low mechanical strength, electrical strength, and limiting oxygen index become the dominant factors, leading to a decline in the overall performance of the composite material.

[0079] This application performed electron microscopy scanning on the aramid composite sheet prepared in Example 1, and the results are as follows: Figure 2 As shown, by Figure 2 As can be seen, the aramid composite sheet prepared by high-pressure reverse diffusion dry method in Example 1 of this application has a homogeneous structure, with polyphenylene sulfide uniformly distributed in the pores of para-aramid fibers and forming a continuous film on the aramid surface.

[0080] Based on Examples 1-5, this application pre-presses and flattens the three-layer sandwich structure of aramid nonwoven fabric-polyphenylene sulfide microfiber nonwoven fabric-aramid nonwoven fabric obtained in step S3, and then hot-presses it using the traditional hot-pressing method at a temperature of 190°C and a pressure of 2.5 MPa to prepare aramid composite sheets, as shown in Comparative Examples 1-5.

[0081] Comparative Example 1 S1: Para-aramid fibers are opened, carded, and laid into a web, then needle-punched to produce aramid nonwoven fabric with a basis weight of 70 g / m². 2 .

[0082] S2: Polyphenylene sulfide is processed into polyphenylene sulfide microfiber nonwoven fabric using a melt-blowing process. The basis weight of the polyphenylene sulfide microfiber nonwoven fabric is 60 g / m². 2 The mass percentage of polyphenylene sulfide microfiber nonwoven fabric in aramid composite sheet is 30%.

[0083] S3: Using the aramid nonwoven fabric prepared in step S1 as the top and bottom layers, and the polyphenylene sulfide microfiber nonwoven fabric prepared in step S2 as the middle layer, they are stacked to form a three-layer sandwich structure of aramid nonwoven fabric-polyphenylene sulfide microfiber nonwoven fabric-aramid nonwoven fabric, thus obtaining the raw material for aramid composite sheet. The middle layer of polyphenylene sulfide microfiber nonwoven fabric, the top layer and the bottom layer of aramid nonwoven fabric are all single-layer structures.

[0084] S4': After the raw material of the aramid composite sheet is pre-pressed and flattened, it is hot-pressed using a flatbed hot press, wherein the hot pressing temperature is 190℃ and the pressure is 2.5 MPa.

[0085] The parameters for Comparative Examples 2-5 are shown in Table 2, and the process steps are the same as those for Comparative Example 1.

[0086] Table 2 The properties of the aramid composite sheets prepared in Comparative Examples 1-5 were tested in this application, and the results are shown in Table 3.

[0087] Table 3 Table 3 shows that the tensile strength of the aramid composite sheets prepared in this application is significantly higher than that of the traditional hot pressing method, indicating that the high-pressure reverse diffusion dry method used in this application has a significant effect on improving mechanical properties.

[0088] Based on Example 5, this application maintains the basis weight of polyphenylene sulfide microfiber nonwoven fabric and studies the effect of the number of layers of the intermediate polyphenylene sulfide microfiber nonwoven fabric on the performance of aramid composite sheets. At the same time, the basis weight of aramid nonwoven fabric is adjusted so that the proportion of aramid nonwoven fabric is between 30% and 70%. The specific parameters are shown in Table 4. The other parameters and processes are the same as in Example 5. The performance of the prepared aramid composite sheets is tested, and the results are shown in Table 4.

[0089] Table 4 Table 4 shows the results of the study on the fixed basis weight (10 g / m²) of aramid nonwoven fabric. 2 In the case of [missing information], the effect of increasing the number of layers of polyphenylene sulfide (PPS) microfiber nonwoven fabric on the performance of the composite sheet is as follows: With the increase in the number of PPS microfiber nonwoven fabric layers, the proportion of PPS microfiber nonwoven fabric gradually decreases from 69.2% to approximately 36%, while the tensile strength increases from 64.8 MPa to approximately 74.8 MPa before slightly decreasing. This indicates that appropriately increasing the number of PPS microfiber nonwoven fabric layers can improve mechanical properties, but excessive layers may weaken the overall strength due to the relatively low strength of the PPS microfiber nonwoven fabric itself. Both breakdown strength and limiting oxygen index decrease with increasing PPS microfiber nonwoven fabric layers, indicating that reducing the proportion of PPS microfiber nonwoven fabric weakens the insulation and flame retardant properties, but they remain at a high level: breakdown strength > 57 kV / mm and limiting oxygen index > 34%.

[0090] The weight study of aramid nonwoven fabric in Table 4 was conducted using a needle-punching process with a needle-punching density of 350 needles / cm². The needle-punching density in this application needs to be controlled between 200-500 needles / cm². If the needle-punching density is lower than this range, the aramid fibers will not be sufficiently entangled, resulting in a loose nonwoven fabric structure. During pre-pressing and hot-pressing, the fiber layers are prone to slippage and misalignment, leading to deformation and uneven thickness of the three-layer structure of the raw material. The flow path of the polyphenylene sulfide melt during reverse diffusion will be inconsistent, and the tensile strength, puncture strength, and limiting oxygen index of the composite sheet will be significantly deteriorated, and delamination defects are likely to occur. If the needle punching density exceeds this range, the excessive needle punching density will cause excessive entanglement and breakage of aramid fibers, resulting in an overly dense nonwoven fabric structure and a significant reduction in porosity. This hinders the penetration of polyphenylene sulfide melt into the pores of aramid fibers during the subsequent high-pressure reverse diffusion process, causing insufficient resin wetting and weakened interfacial bonding. The surface of the prepared aramid composite sheet lacks the gloss of a continuous polyphenylene sulfide film, ultimately leading to a decrease in the tensile strength, breakdown strength, and limiting oxygen index of the composite sheet.

[0091] Based on Example 5, this application maintains the basis weight of the aramid nonwoven fabric and studies the effect of the number of aramid nonwoven fabric layers on the performance of the aramid composite sheet. At the same time, the basis weight of the polyphenylene sulfide microfiber nonwoven fabric is adjusted so that the mass ratio of the polyphenylene sulfide microfiber nonwoven fabric is between 30% and 70%. The specific parameters are shown in Table 5. The remaining parameters and processes are the same as in Example 5. The performance of the prepared aramid composite sheet is tested, and the results are shown in Table 5.

[0092] Table 5 Table 5 shows the effect of increasing the total number of layers of top and bottom aramid nonwoven fabric on performance when the basis weight of polyphenylene sulfide (PPS) microfiber nonwoven fabric is constant. The mass percentage of PPS microfiber nonwoven fabric decreases with increasing aramid layer number, while the tensile strength increases significantly from 64.8 MPa to approximately 94 MPa, indicating that increasing the number of aramid nonwoven fabric layers significantly enhances mechanical properties. The breakdown strength and limiting oxygen index initially decrease slightly and then increase significantly. When the number of aramid nonwoven fabric layers is 6, the breakdown strength reaches a peak of 96.5 kV / mm and the limiting oxygen index reaches 42.4%, indicating that appropriately increasing the number of aramid nonwoven fabric layers is beneficial for improving insulation and flame retardant properties.

[0093] Based on Example 4, this application studies the temperature of high-pressure hot pressing. Other parameters and processes are the same as in Example 4. The properties of aramid composite materials prepared at different temperatures are tested, and the results are shown in Table 6.

[0094] Table 6 Based on Example 4, this application studies the pressure of high-pressure hot pressing. Other parameters and processes are the same as in Example 4. The properties of aramid composite materials prepared under different pressures are tested, and the results are shown in Table 7.

[0095] Table 7 Based on Example 4, this application studies the linear velocity of high-pressure hot pressing. Other parameters and processes are the same as in Example 4. The properties of aramid composite materials prepared at different linear velocities are tested, and the results are shown in Table 8.

[0096] Table 8 Table 6-8 shows that, regarding the roll forming process, when the temperature is below 190℃, the pressure is below 2.5 MPa, and the linear speed is below 1.0 m / min, the polyphenylene sulfide (PPS) microfiber nonwoven fabric exhibits insufficient fiber melting, resulting in poor wetting of the aramid nonwoven fabric fibers. This leads to internal pores and weak interfacial bonding, resulting in suboptimal performance. Excessive linear speed can cause over-crystallization and coarsening of the PPS fibers due to prolonged thermal action, leading to performance degradation. However, when the roll forming temperature is 190℃, the pressure is 2.5 MPa, and the linear speed is 1.0 m / min, the PPS microfiber nonwoven fabric fibers are fully melted and completely wetted under suitable pressure, expelling pores and forming a dense and uniform microstructure. Furthermore, the fibers are undamaged, the material does not degrade, and the mechanical, electrical, and flame-retardant properties reach a synergistic peak. When the rolling temperature exceeds 190℃, the pressure exceeds 2.5 MPa, and the linear speed exceeds 1.0 m / min, excessively high temperatures can easily lead to excessive crystallization and coarsening of the polyphenylene sulfide microfiber nonwoven fabric fibers; excessively high pressure can easily cause mechanical damage to the aramid nonwoven fabric fibers; and excessively high linear speeds can lead to insufficient wetting and curing. Excessively high temperature, pressure, and linear speed together cause material structure deterioration, interface damage, and defects, resulting in a decline in overall performance.

[0097] Furthermore, while performance begins to decline at 220℃, it remains superior to the group with temperatures below 90℃ (breakdown strength greater than 50 kV / mm, limiting oxygen index at least greater than 34%). Above 220℃ (e.g., 300℃), polyphenylene sulfide fibers not only undergo excessive crystallization but may also experience thermal degradation, leading to a sharp deterioration in performance, with a tensile strength of only 45.3 MPa. When pressure exceeds 2.5 MPa, in addition to mechanical damage to the aramid nonwoven fibers, excessive extrusion and loss of the polyphenylene sulfide melt may occur, resulting in insufficient local binder phase and affecting interfacial bonding. Therefore, although performance remains relatively high (e.g., tensile strength of 99.3 MPa at 10 MPa), it is slightly below the optimal value. At a linear velocity of 0.2 m / min, despite a longer thermal treatment time, severe over-crystallization of the polyphenylene sulfide fibers does not occur, and performance remains at a relatively high level, with a tensile strength of 83.8 MPa.

[0098] It is important to note that, based on the study of rolling pressure in Table 7, the high-pressure reverse diffusion in this application is not a simple physical penetration, but a non-equilibrium melt transport process under a high-pressure field of 2-15 MPa. The core is to overcome the high viscosity resistance of polyphenylene sulfide (PPS) melt and the surface tension barrier of aramid fibers through high pressure, while simultaneously constructing a high shear rate to achieve melt shear thinning. Ultimately, this drives the PPS melt to reversely force-fill from the middle layer to the two aramid layers. The high shear rate of the PPS melt refers to the rapid flow of molten PPS in the micron-sized pores / fiber gaps of the aramid nonwoven fabric during high-pressure rolling, resulting in high-speed shear deformation caused by relative motion with the fibers and the rolling surface. This is the key to achieving melt shear thinning and improving fluidity.

[0099] In existing technologies, under conventional low pressure (0.1-0.5 MPa), the flow of PPS melt is driven by capillary force and gravity. However, the viscosity of PPS after melting is still relatively high. The aramid nonwoven fabric has a complex pore structure and high flow resistance. The driving force provided by low pressure cannot push the melt into the micro-nano pores of aramid, nor can it form an effective high shear rate. The melt cannot be sheared and thinned, and can only wet the shallow interlayer. It cannot penetrate to the surface of aramid and cannot expel the air between the fibers. In the end, a heterogeneous structure of dense core layer and porous surface layer is formed, and the pores become weak points in insulation and corrosion resistance.

[0100] This application employs a high-pressure (2-15 MPa) driven reverse diffusion mechanism. The 2-15 MPa high pressure, in synergy with the aramid micro / nano pores, triggers a change in the system's properties, achieving reverse diffusion. The core process is as follows: High shear rate formation and shear thinning: High pressure provides a strong driving force for the melt, propelling it to migrate rapidly into the aramid pores. The 1-10μm micron-sized fiber gaps and pores in the aramid nonwoven fabric form a narrow microscale flow space, amplifying the shear effect and increasing the melt shear rate to tens of times higher than at low pressure. As a pseudoplastic non-Newtonian fluid, PPS experiences a significant reduction in apparent viscosity due to the high shear rate, resulting in an exponential increase in fluidity and overcoming high viscosity resistance.

[0101] Forced impregnation depth filling: High pressure driving force combined with low viscosity melt makes the melt decoupled from capillary action dependence, forced impregnation, breaking through the surface tension barrier, penetrating deep into the micro-nano pores of aramid fiber bundles, and achieving full-dimensional pore filling.

[0102] Reverse penetration and surface film formation: Under bidirectional high pressure extrusion, the molten PPS in the intermediate layer migrates in reverse from the inside to the outside along the pressure gradient. The melt breaks through the pore barrier of the surface aramid and spreads evenly on the outer surface of the material. After cooling, it solidifies in situ to form a continuous and dense PPS film, achieving a surface without obvious voids.

[0103] Pore ​​elimination and three-dimensional interpenetrating network construction: High pressure drives the melt to penetrate longitudinally and promotes its full lateral spread, expelling air between fibers and eliminating micropores; high shear rate allows PPS and aramid fibers to fully contact and mix, and after cooling, a three-dimensional interpenetrating network structure is formed, realizing the transformation from physical stacking to mechanical interlocking.

[0104] 2-15 MPa is the critical pressure for reverse diffusion. Below this range, an effective high shear rate cannot be formed, the melt cannot be sheared and thinned, and the reverse diffusion mechanism does not exist. Above this range (e.g., >20 MPa), mechanical damage to aramid fibers and excessive loss of PPS melt can easily occur, damaging the material structure and properties.

[0105] It should be noted that the 2-15 MPa high-pressure parameters selected in this application are not obtained by those skilled in the art through conventional optimization of existing low-pressure technologies (0.1-0.5 MPa). Traditional hot-pressing processes for composite materials generally believe that high pressure easily leads to resin loss and fiber breakage, and low pressure (<1 MPa) is conventionally preferred. It is difficult to foresee that increasing the pressure by tens of times to 2-15 MPa can not only maintain structural integrity but also trigger the critical permeation threshold. For aramid / PPS systems, the conventional optimization approach of technicians is to adjust the temperature and linear velocity, rather than increasing the pressure by 30-150 times to more than 10 MPa to achieve melt shear thinning by constructing a high shear rate. This application finds that 2 MPa is the qualitative change point for PPS melt from interlayer filling to through-type reverse diffusion, and the material structure undergoes a qualitative change from porous layering to overall dense interpenetration. Existing technologies (such as CN116749620A) do not involve technical solutions for forming high shear rates under high pressure to achieve surface self-sealing films.

[0106] Under high pressure of 2-15 MPa, the aramid composite sheet prepared by achieving melt shear thinning and reverse diffusion through high shear rate achieves a qualitative leap in performance compared to low-pressure processes, rather than a simple improvement. The structural morphology is fundamentally changed. The low-pressure sample has a layered structure with a porous surface, while the high-pressure sample forms a uniform five-layer gradient interpenetrating structure, and a continuous and dense PPS film spontaneously forms on the surface. This surface self-sealing effect is a unique result that the low-pressure process cannot achieve. The three-dimensional interpenetrating network significantly improves the interlayer shear strength, which is impossible to achieve with the low-pressure process. Within the pressure range of 2-15 MPa, the material can stably form a high shear rate and achieve reverse diffusion, maintaining excellent density and performance stability, and widening the process window. When the pressure increases from 2 MPa to 15 MPa, the breakdown strength jumps to a stable >50 kV / mm (maximum 98.2 kV / mm), which far exceeds the expected improvement in density and eliminates the problem of electric field concentration caused by pores.

[0107] In summary, the 2-15 MPa pressure selection breaks through the traditional understanding of the process. By constructing a high shear rate under high pressure, it solves the industry problem of PPS melt's difficulty in deeply penetrating the pores of aramid fibers, achieving full densification of the material and producing comprehensive performance that cannot be achieved by low-pressure processes.

[0108] As shown in Tables 1 and 4-8, the fiber diameter data of polyphenylene sulfide (PPS) microfiber nonwoven fabrics demonstrate that the meltblown process can precisely control the fiber diameter, ensuring that it meets the structural requirements of microfibers. Microfibers with a diameter of 1-10 μm have a larger specific surface area, allowing for rapid melting during hot pressing, reducing melt viscosity, and facilitating penetration into the micropores of aramid nonwoven fabric under high pressure, achieving reverse diffusion. If the fiber diameter is too large (>10 μm), the specific surface area decreases, resulting in slow melting speed, poor melt flowability, and difficulty in fully filling the aramid pores; if the diameter is too small (<1 μm), the fiber strength is insufficient, making it prone to breakage during hot pressing, affecting the overall mechanical properties of the material. Therefore, 1-10 μm is the optimal range balancing penetration effect and structural strength.

[0109] To verify the necessity of the pre-compression leveling process, this application sets up comparative examples 6-10. The preparation process of comparative examples 6-10 is basically the same as that of examples 1-5, except that the pre-compression leveling step in step S4 is omitted, and the three-layer sandwich structure raw material prepared in step S3 is directly subjected to high-pressure hot pressing according to the high-pressure hot pressing process in step S4. The remaining experimental parameters and processes are the same as those of the corresponding examples. Among them, comparative example 6 corresponds to example 1, comparative example 7 corresponds to example 2, comparative example 8 corresponds to example 3, comparative example 9 corresponds to example 4, and comparative example 10 corresponds to example 5. The research parameters and performance comparisons between comparative examples 6-10 and examples 1-5 are shown in Table 9.

[0110] Table 9 As shown in Table 9, among all corresponding groups, the pre-compression leveled sample significantly outperformed the uncompression leveled comparative sample in all three key properties: tensile strength, breakdown strength, and limiting oxygen index. Taking Example 4 and Comparative Example 9 as examples, pre-compression leveling increased the tensile strength from 98.7 MPa to 105.1 MPa (an increase of 6.4 MPa, or 6.1%), the breakdown strength from 95.7 kV / mm to 98.2 kV / mm (an increase of 2.5 kV / mm, or 2.5%), and the limiting oxygen index from 41.1% to 42.4% (an increase of 1.3 percentage points). The other corresponding groups also showed the same upward trend. This indicates that pre-pressing and leveling can effectively prevent interlayer displacement and fiber migration caused by the line contact progressive pressure of the roller press during high-pressure hot pressing, ensuring that the three-layer structure is tightly bonded and of uniform thickness before entering high-pressure hot pressing, providing structurally stable blanks for the uniform penetration of PPS melt, and is a necessary process for obtaining high-performance aramid composite sheets.

[0111] Comparative Example 11 This comparative example describes the preparation of aramid composite insulating sheets using the method described in Chinese Patent Publication No. CN116749620A.

[0112] The products of the aramid composite insulating sheet prepared in Comparative Example 11 and the aramid composite sheet prepared in Example 4 are as follows: Figure 3 As shown, Figure 3 Image a is a photograph of the aramid composite insulating sheet prepared in this comparative example. Figure 3 Image b is a photograph of the aramid composite sheet prepared in Example 4. Figure 3 As can be seen, the aramid composite sheet prepared in Example 4 has a smooth and continuous surface, exhibiting obvious polyphenylene sulfide film coverage characteristics; while the aramid composite insulating sheet prepared in Comparative Example 11 has a rough surface, no continuous film structure is observed, and it still maintains the original fiber stacking morphology of the aramid nonwoven fabric. This indicates that this application has successfully achieved continuous film formation of polyphenylene sulfide on the surface of aramid nonwoven fabric through a high-pressure reverse diffusion process, forming a product structure significantly different from the prior art.

[0113] The present application has been described in detail above. Specific examples have been used to illustrate the principles and implementation methods of the present application. The descriptions of the embodiments above are only for the purpose of helping to understand the present disclosure. It should be noted that those skilled in the art can make several improvements and modifications to the present application without departing from the principles of the present application, and these improvements and modifications also fall within the protection scope of the claims of the present application.

Claims

1. An aramid composite sheet prepared by high-pressure reverse diffusion dry method, characterized in that, The aramid composite sheet is made of a three-layer sandwich structure, consisting of a top layer, a middle layer, and a bottom layer; the top and bottom layers are aramid nonwoven fabrics; and the middle layer is polyphenylene sulfide microfiber nonwoven fabric. The raw materials of the aramid composite sheet are subjected to high-pressure hot pressing. The polyphenylene sulfide microfiber nonwoven fabric in the middle layer melts and permeates into the pores of the top and bottom aramid nonwoven fabrics, forming a continuous polyphenylene sulfide film on the surface of the top and bottom layers, thus forming a five-layer aramid composite sheet, which consists of a polyphenylene sulfide film, a mixed layer of aramid nonwoven fabric and polyphenylene sulfide, a polyphenylene sulfide layer, a mixed layer of aramid nonwoven fabric and polyphenylene sulfide, and a polyphenylene sulfide film.

2. The aramid composite sheet prepared by the high-pressure reverse diffusion dry method as described in claim 1, characterized in that, The cross-section and surface of the aramid composite sheet have no obvious pores.

3. The aramid composite sheet prepared by the high-pressure reverse diffusion dry method as described in claim 1, characterized in that, The aramid composite sheet material contains aramid nonwoven fabric with a basis weight of 10-200 g / m². 2 The basis weight of polyphenylene sulfide microfiber nonwoven fabric is 20-100 g / m². 2 ; The mass percentage of polyphenylene sulfide microfiber nonwoven fabric in the raw materials of the aramid composite sheet is 30-70%.

4. The aramid composite sheet prepared by the high-pressure reverse diffusion dry method as described in claim 1, characterized in that, The top and bottom layers of aramid nonwoven fabric are single-layer or multi-layer stacked structures. The intermediate layer of polyphenylene sulfide microfiber nonwoven fabric has a single-layer or multi-layer stacked structure.

5. The aramid composite sheet prepared by the high-pressure reverse diffusion dry method according to any one of claims 1-4, characterized in that, The aramid composite sheet has a breakdown strength greater than 50 kV / mm and a limiting oxygen index greater than 34%.

6. A method for preparing aramid composite sheets using a high-pressure reverse diffusion dry process, for preparing the aramid composite sheets according to any one of claims 1-4, characterized in that, Includes the following steps: S1: Aramid fibers are made into aramid nonwoven fabric through needle punching or hydroentangling processes; S2: Polyphenylene sulfide is processed into polyphenylene sulfide microfiber nonwoven fabric through meltblowing process; S3: Using the aramid nonwoven fabric prepared in step S1 as the top and bottom layers, and the polyphenylene sulfide microfiber nonwoven fabric prepared in step S2 as the middle layer, they are stacked to form a three-layer sandwich structure of aramid nonwoven fabric-polyphenylene sulfide microfiber nonwoven fabric-aramid nonwoven fabric, thus obtaining the raw material for aramid composite sheet. S4: The raw material of the aramid composite sheet obtained in step S3 is pre-pressed and flattened, then subjected to high-pressure hot pressing and cooling to obtain the aramid composite sheet.

7. The method for preparing aramid composite sheets by high-pressure reverse diffusion dry method as described in claim 6, characterized in that, The aramid fiber mentioned in step S1 has a length of 20-120 mm and a fiber diameter of 5-20 μm; the needle punching density of the needle punching process is 200-500 needles / cm²; and the hydroentangling pressure of the hydroentangling process is 0.3-0.8 MPa. In step S4, the high-pressure hot pressing is performed using a roller press with a temperature of 90-220℃, a pressure of 2-15 MPa, and a linear speed of 0.2-3.8 m / min.

8. The method for preparing aramid composite sheets by high-pressure reverse diffusion dry method as described in claim 6, characterized in that, In step S4, the pre-pressing and flattening involves placing the raw material of the aramid composite sheet in an anti-stick interlayer structure and pressing it under pressure at room temperature.

9. The method for preparing aramid composite sheets by high-pressure reverse diffusion dry method as described in claim 8, characterized in that, The anti-stick interlayer structure includes two layers of isolation film and two layers of rigid plate. The raw material of the aramid composite sheet is placed between the two layers of isolation film, and a rigid plate is placed on the outside of each of the two layers of isolation film.

10. The method for preparing aramid composite sheets by high-pressure reverse diffusion dry method as described in claim 9, characterized in that, The separator is a polyimide membrane, the rigid plate is a metal plate, the pressing pressure is 0.3-1.0 MPa, and the holding time is 10-30 s.

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