A sandwich-structured recycled aramid paper and its preparation method

Abstract

This invention belongs to the field of aramid paper recycling and high-end papermaking technology, specifically involving a sandwich-structured recycled aramid paper and its preparation method. The preparation method includes: pretreatment of waste aramid paper, solid-phase shearing and milling treatment, wet pulping and fibrillation treatment, wet papermaking, and composite of sandwich-structured recycled aramid paper. This preparation method overcomes the shortcomings of traditional mechanical crushing methods, such as large fiber damage, uneven length distribution, and performance degradation, through the synergistic process of solid-phase shearing and wet pulping and fibrillation treatment. It effectively preserves the quality of aramid fibers and fully recycles waste aramid paper. The sandwich-structured recycled aramid paper prepared by this invention has excellent performance, with tensile strength and tear strength close to that of virgin material. It also has high interlayer bonding strength, does not delaminate or deform, and has excellent dimensional stability under high temperature and high humidity environments, meeting the application requirements of high-end electrical insulation materials.

Description

Technical Field

[0001] This invention specifically relates to a sandwich-structured recycled aramid paper and its preparation method, belonging to the field of aramid paper recycling and high-end papermaking technology. Background Technology

[0002] Aramid paper, due to its high strength, high temperature resistance, and excellent insulation properties, has become a key material in aerospace, new energy, and rail transportation. However, the waste aramid paper (such as scraps) generated during its production and use is difficult to recycle, resulting in significant resource waste and environmental pressure. Existing recycling technologies have the following shortcomings:

[0003] (1) Severe fiber damage and significant performance degradation: The traditional recycling process uses "paper shredder coarse shredding + hydraulic pulper decomposition", resulting in random fiber breakage, which reduces the average length to 0.5~1.0 mm, and the specific surface area to only 4~6 m² / g. The surface polar groups are lost, and the tensile strength of recycled paper is only 50~60% of that of new material, which cannot meet the needs of mid-to-high-end scenarios.

[0004] (2) Low recycling ratio and poor resource utilization: In order to ensure the performance of recycled paper, existing technologies require mixing recycled fibers with more than 30% new materials. For example, the patent application with publication number CN114837012A discloses a method for recycling para-aramid paper waste, with a recycling ratio of 5-20%. When fully recycled, the paper is prone to delamination and has insufficient tear strength, making it difficult to apply industrially.

[0005] (3) The layered structure does not incorporate recycling requirements: For example, the patent application with publication number CN116905272A discloses a method for manufacturing a high stiffness and wear-resistant aramid paper. The layered structure of this aramid paper uses 100% new material and achieves functional zoning through “abrasion resistance of the surface layer + stiffness of the core layer”, without involving the high performance of recycled materials; the precipitated fiber is only used as a mixed filler (addition amount ≤20%), and its interface bridging effect is not maximized through structural design.

[0006] (4) Poor industrial adaptability: The fiber length distribution of traditional processes is uneven, and the white water loss rate during wet papermaking reaches 15-20%. Furthermore, there is a lack of targeted hot pressing processes, resulting in uneven basis weight and weak interlayer bonding in the paper.

[0007] In summary, existing technologies have obvious shortcomings. Therefore, it is of great significance to develop a technology for preparing aramid paper that achieves high recycling rate, low fiber damage, and high-performance composite while realizing the resource utilization of waste aramid paper, and making the performance of recycled paper close to that of virgin material, thus balancing environmental protection and industrial value. Summary of the Invention

[0008] This invention addresses the shortcomings of existing technologies by providing a method for preparing sandwich-structured recycled aramid paper. This method overcomes the drawbacks of traditional mechanical pulverization methods, such as significant fiber damage, uneven length distribution, and performance degradation, through a synergistic process of solid-phase shearing, wet refining, and fibrillation. It effectively preserves the quality of aramid fibers in waste aramid paper and fully recycles the waste aramid paper. The sandwich-structured recycled aramid paper prepared by this invention exhibits excellent performance, with tensile strength and tear strength approaching those of virgin material. It also demonstrates high interlayer bonding strength, does not delaminate or deform, and exhibits excellent dimensional stability under high temperature and humidity conditions, meeting the application requirements of high-end electrical insulation materials.

[0009] The technical solution of this invention to solve the above-mentioned technical problems is as follows: A method for preparing sandwich-structured recycled aramid paper, the preparation method comprising the following steps:

[0010] S1. Pre-treatment of waste aramid paper: Cut the scraps of waste aramid paper into fragments;

[0011] S2. Solid-phase shearing and grinding treatment: The waste aramid paper scraps pretreated in step S1 are fed into a disc-shaped mechanical chemical reactor for processing to obtain aramid short fibers.

[0012] S3. Wet refining and fibrillation treatment: The aramid short fibers obtained in step S2 are added to a refining machine for delamination and fibrillation treatment to obtain aramid pulp.

[0013] S4. Wet papermaking: Add precipitated fibers and mix with the aramid pulp obtained in step S3, wet papermaking, and then dry to obtain fully recycled aramid base paper;

[0014] S5. Composite of sandwich structure recycled aramid paper: Using the fully recycled aramid base paper obtained in step S4 as the core layer, and using virgin meta-aramid paper as the surface layer on both sides, a sandwich structure is composited. After staged hot pressing, sandwich structure recycled aramid paper is obtained.

[0015] Furthermore, in step S1, the waste aramid paper is waste aramid paper scraps obtained after wet papermaking, and the surface is not coated with resin or coating; the size of the fragments is 5~10mm.

[0016] Furthermore, in step S2, the grinding disc gap is 0.5~2mm, the rotation speed is 200~600rpm, and the number of processing times is 2~8; preferably, the grinding disc gap is 0.5~0.8mm and the rotation speed is 350~450rpm.

[0017] Furthermore, in step S3, during the disintegration stage, the rotation speed is 500~800 rpm, the disintegration time is 3~5 min, and the slurry concentration is controlled at 5~8%.

[0018] Furthermore, in step S3, during the fibrillation stage, the grinding disc gap is 0.1~0.3mm, the rotation speed is 800~1200rpm, and the time is 40~60min.

[0019] Furthermore, in step S3, during the fibrillation stage, the freeness is 60~80°SR.

[0020] Further, in step S4, the precipitated fibers are first added and mixed with the aramid pulp obtained in step S3, and then 0.3~0.5% of polyethylene oxide dispersant by weight of the total mass of the aramid pulp and precipitated fibers is added. The pulp is diluted with deionized water to a concentration of 0.01~0.03% before being fed to the grid. After forming, it is dewatered under vacuum, dewatered by pressing, and then dried at 60~80℃ to obtain fully recycled aramid base paper.

[0021] Furthermore, in step S4, the specific surface area of ​​the precipitated fiber is 20~30m / g, the average length is 0.1-0.5mm, and the degree of fibrillation is 50~70%.

[0022] Furthermore, in step S4, the molecular weight of the polyethylene oxide dispersant is 3~4×10⁻⁶. 6 g / mol;

[0023] Furthermore, in step S4, the content of the aramid pulp, by mass fraction, is 70-85 wt% of the total mass of the aramid pulp and the precipitated fibers, and the content of the precipitated fibers is 15-30 wt% of the total mass of the aramid pulp and the precipitated fibers. Preferably, the content of the aramid pulp is 80 wt% of the total mass of the aramid pulp and the precipitated fibers, and the content of the precipitated fibers is 20 wt% of the total mass of the aramid pulp and the precipitated fibers.

[0024] Furthermore, in step S5, the tensile strength of the virgin meta-aramid paper is 150~200MPa; the basis weight of the virgin meta-aramid paper is 30~40g / m².

[0025] Furthermore, the fully recycled aramid base paper accounts for 65-70.73% of the total mass of the sandwich structure recycled aramid paper.

[0026] Furthermore, in step S5, the staged hot pressing includes:

[0027] First stage: Temperature 160~180℃, pressure 1~2MPa, time 5~8min;

[0028] Second stage: temperature 220~250℃, pressure 5~8MPa, time 5~10min.

[0029] This invention also discloses a sandwich-structured recycled aramid paper, which is prepared according to the preparation method described in this invention; the sandwich-structured recycled aramid paper includes a surface layer and a core layer; the core layer is located between two surface layers; the core layer is a fully recycled aramid base paper, which includes recycled aramid fibers and precipitated fibers; the surface layer is a virgin meta-aramid paper.

[0030] The beneficial effects of this invention are:

[0031] (1) The sandwich structure recycled aramid paper provided by the present invention perfectly balances performance and cost. The sandwich composite structure design fully recycles and utilizes waste aramid paper. The core layer of the recycled aramid paper bears the main thickness and mechanical strength of the product. The new meta-aramid paper on the surface provides excellent appearance flatness, smoothness, wear resistance and other excellent properties. The sandwich structure recycled aramid paper obtained after the core layer and the surface layer are sandwich structured has excellent performance. Its tensile strength, tear strength and other properties are close to those of new material.

[0032] (2) The method for preparing sandwich-structured recycled aramid paper provided by this invention overcomes the shortcomings of traditional mechanical crushing methods, such as large fiber damage, uneven length distribution, and performance degradation, through the synergistic process of solid-phase shearing, wet refining, and fibrillation. It effectively preserves the quality of aramid fibers in waste aramid paper, providing fibers with good length retention and minimal damage. Solid-phase shearing is a mechanochemical treatment based on the combined effects of extrusion, shearing, and friction, providing fibers with good length retention and minimal damage. Wet refining and fibrillation greatly increase the specific surface area and hydrogen bonding points of the fibers, enhancing the compactness and interlayer bonding of subsequent papermaking, thus achieving high-value recycling of waste aramid paper. After wet refining and fibrillation, a hydrocyclone is introduced to remove short fibers <0.5mm, effectively removing the "weak points" that affect paper strength, while retaining the fiber body with moderate length and high strength, resulting in a more uniform core paper structure, higher strength, and fewer defects. The sandwich composite structure design, combined with an optimized staged hot-pressing process, ensures strong interlayer bonding and performance stability.

[0033] (3) The sandwich structure recycled aramid paper provided by the present invention has high interlayer bonding strength, does not delaminate or deform, and has excellent dimensional stability in high temperature and high humidity environments, meeting the application requirements of high-end electrical insulation materials. Detailed Implementation

[0034] The specific embodiments of the present invention will be described in detail below. The present invention can be implemented in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed herein.

[0035] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used is for describing particular embodiments only and is not intended to limit the invention.

[0036] A sandwich-structured recycled aramid paper includes a surface layer and a core layer; the core layer is located between two surface layers; the core layer is a fully recycled aramid base paper prepared by the preparation method of the present invention, the fully recycled aramid base paper including recycled aramid fibers and precipitated fibers; the surface layer is a virgin meta-aramid paper. In the embodiments of the present invention, the sandwich-structured recycled aramid paper obtained is a single-layer sandwich structure, specifically a "surface layer-core layer-surface layer" sandwich structure. However, this sandwich structure is not a limitation of the present invention. In specific implementation, the core layer and the surface layer can be compounded a limited number of times as needed to form a multi-layer composite material, such as five layers (surface layer-core layer-surface layer-core layer-surface layer), seven layers (surface layer-core layer-surface layer-core layer-surface layer-core layer-surface layer), nine layers (surface layer-core layer-surface layer-core layer-core layer-surface layer-core layer-surface layer), etc. Alternatively, the sandwich structure described in the present invention can be combined with other composite films as needed to form a multi-layered sandwich-structured recycled aramid paper.

[0037] A method for preparing sandwich-structured recycled aramid paper, the method comprising the following steps:

[0038] S1. Pre-treatment of waste aramid paper: Cut the scraps of waste aramid paper into fragments;

[0039] S2. Solid-phase shearing and grinding treatment: The waste aramid paper scraps pretreated in step S1 are fed into a disc-shaped mechanical chemical reactor for processing to obtain aramid short fibers.

[0040] S3. Wet refining and fibrillation treatment: The aramid short fibers obtained in step S2 are added to a refining machine for delamination and fibrillation treatment to obtain aramid pulp.

[0041] S4. Wet papermaking: Add precipitated fibers and mix with the aramid pulp obtained in step S3, wet papermaking, and then dry to obtain fully recycled aramid base paper;

[0042] S5. Composite of sandwich structure recycled aramid paper: Using the fully recycled aramid base paper obtained in step S4 as the core layer, and using virgin meta-aramid paper as the surface layer on both sides, a sandwich structure is composited. After staged hot pressing, sandwich structure recycled aramid paper is obtained.

[0043] The specific steps and process parameters involved in steps S1-S5 above are as follows:

[0044] S1. The specific steps for pretreatment of waste aramid paper are as follows:

[0045] Collect waste aramid paper scraps, remove impurities, and cut the waste aramid paper scraps into pieces with a size of 5~10mm. Avoid excessive energy consumption during subsequent grinding if the size is too large, and excessive fiber crushing if the size is too small.

[0046] Specifically, in step S1, the waste aramid paper is waste aramid paper scraps obtained after wet papermaking, and the surface is not coated with resin or coating.

[0047] S2. The specific steps of solid-phase shearing and grinding treatment are as follows:

[0048] The pretreated waste aramid paper scraps from step S1 are fed into a disc-type mechanical chemical reactor for processing. The disc gap is set to 0.5-2 mm, the rotation speed to 200-600 rpm, and the number of processing cycles to 2-8. The specific number of processing cycles is adjusted according to the fiber length to ensure that the fiber mass with a length of 1.5-3 mm accounts for at least 60% of the total fiber mass. Finally, aramid short fibers are obtained, wherein the average length of the obtained aramid short fibers is 1.5-3 mm and the specific surface area is 8-12 m² / g.

[0049] Specifically, in step S2, the preferred process parameters are: grinding disc gap 0.5~0.8mm; rotation speed 350~450rpm.

[0050] Specifically, in step S2, the millstone-shaped mechanochemical reactor is a high-efficiency processing equipment developed by the State Key Laboratory of Polymer Materials Engineering (Sichuan University) based on the principles of polymer mechanochemistry, drawing on the traditional stone mill structure and innovating its design, to realize the crushing, mixing and mechanochemical reaction of polymers and fillers.

[0051] S3. The specific steps for wet pulping and fibrillation treatment are as follows:

[0052] The aramid short fibers obtained in step S2 are added to a refiner for staged delamination and fibrillation. The delamination process parameters are set as follows: rotation speed 500~800 rpm, delamination time 3~5 min, and pulp concentration controlled at 5~8%. The fibrillation process parameters are set as follows: grinding disc gap 0.1~0.3 mm, rotation speed 800~1200 rpm, and time 40~60 min. After fibrillation, the pulp is passed through a hydrocyclone separator with a separation factor of 1000~1500 to remove short fibers smaller than 0.5 mm, finally obtaining aramid pulp with a freeness of 60~80°SR and a fiber length distribution of 1~3 mm.

[0053] In this step, the de-fibrillation process can break up the fiber bundles, and fibrillation can peel off nanoscale microfibers from the fiber surface.

[0054] Specifically, in step S3, the refiner is a conventional PFI refiner.

[0055] S4. The specific steps of wet papermaking are as follows:

[0056] Add the precipitated fibers and mix with the aramid pulp obtained in step S3. Then add 0.3-0.5% of polyethylene oxide dispersant by weight of the total mass of aramid pulp and precipitated fibers to disperse them evenly to obtain pulp. Dilute the pulp with deionized water to a wire-on concentration of 0.01-0.03%. Transport the pulp to a wire forming machine for forming. After vacuum dewatering, press dewatering, and drying at 60-80℃, obtain fully recycled aramid base paper with a basis weight of 140-160 g / m².

[0057] Specifically, in step S4, the molecular weight of the polyethylene oxide dispersant is 3~4×10⁻⁶. 6 g / mol.

[0058] Specifically, in step S4, the specific surface area of ​​the precipitated fiber is 20~30m / g, the average length is 0.1-0.5mm, and the degree of fibrillation is 50~70%.

[0059] Specifically, in step S4, the content of the aramid pulp is 70-85 wt% of the total mass of the aramid pulp and the precipitated fiber, and the content of the precipitated fiber is 15-30 wt% of the total mass of the aramid pulp and the precipitated fiber.

[0060] More specifically, preferably, the content of the aramid pulp is 80 wt% of the total mass of the aramid pulp and the precipitated fiber, and the content of the precipitated fiber is 20 wt% of the total mass of the aramid pulp and the precipitated fiber.

[0061] Specifically, in step S4, the vacuum degree of vacuum dehydration is controlled at 0.1-1 MPa.

[0062] The specific steps for S5, sandwich structure recycled aramid paper composite are as follows:

[0063] Using the fully recycled aramid base paper obtained in step S4 as the core layer and the virgin meta-aramid paper as the surface layer, the surface layer, core layer, and surface layer are stacked sequentially to ensure alignment deviation ≤ 0.5 mm; then, hot pressing is performed in two stages: the first stage: temperature 160~180℃, pressure 1~2MPa, time 5~8min; the second stage: temperature 220~250℃, pressure 5~8MPa, time 5~10min; after cooling and setting, a sandwich structure recycled aramid paper is obtained.

[0064] In step S5 of this invention, the first stage of the phased hot pressing is pre-compression fusion, which aims to soften the surface fibers, promote diffusion at the interface between the core and the surface, achieve preliminary interface fusion and sufficient expulsion of water molecules, and avoid defects such as delamination and blistering caused by rapid evaporation of water vapor in the subsequent high-temperature stage. The second stage of the phased hot pressing is shaping and densification. The second stage hot pressing process is carried out under high pressure shaping at a temperature higher than the glass transition temperature of aramid, which promotes the formation of strong hydrogen bonds and mechanical interlocking between the surface and core fibers, and at the same time, allows the fibers themselves to complete heat shaping and eliminate internal stress. In this stage, excessive compression should be avoided to prevent the core layer from becoming brittle.

[0065] Specifically, in step S5, the tensile strength of the virgin meta-aramid paper is 150~200MPa; the basis weight of the virgin meta-aramid paper is 30~40g / m².

[0066] More specifically, the fully recycled aramid base paper accounts for 65-70.73% of the total mass of the sandwich structure recycled aramid paper.

[0067] Example 1

[0068] A sandwich-structured recycled aramid paper, the raw material of which is waste aramid paper scraps, wherein the waste aramid paper scraps are scraps obtained after wet papermaking, and the surface is not coated with resin or coating. The specific preparation method includes the following steps:

[0069] S1. Pretreatment of waste aramid paper:

[0070] Collect waste aramid paper scraps, remove impurities, and cut the waste aramid paper scraps into 8mm pieces.

[0071] S2, Solid-phase shearing and grinding treatment:

[0072] The waste aramid paper scraps pretreated in step S1 were fed into a grinding disc mechanical chemical reactor for processing. The grinding disc gap was set to 0.6 mm, the rotation speed was 400 rpm, and the process was repeated 5 times to finally obtain aramid short fibers.

[0073] S3, Wet pulping and fibrillation treatment:

[0074] The aramid short fibers obtained in step S2 are added to a PFI refiner for staged delamination and fibrillation. The delamination process parameters are set as follows: rotation speed 600 rpm, delamination time 4 min, and pulp concentration controlled at 6%. The fibrillation process parameters are set as follows: grinding disc gap 0.2 mm, rotation speed 1000 rpm, and time 50 min. After fibrillation, the pulp is passed through a hydrocyclone separator with a separation factor of 1000~1500 to remove short fibers smaller than 0.5 mm, finally obtaining aramid pulp with a freeness of 70°SR.

[0075] S4. Wet papermaking:

[0076] The aramid pulp obtained in step S3 is mixed with precipitated fibers at a mass ratio of 4:1. At this time, the content of aramid pulp is 80wt% of the total mass of aramid pulp and precipitated fibers, and the content of precipitated fibers is 20wt% of the total mass of aramid pulp and precipitated fibers. Then, 0.4% of polyethylene oxide dispersant is added to the total mass of aramid pulp and precipitated fibers to make them evenly dispersed to obtain pulp. The pulp is diluted with deionized water to a wire-on-grid concentration of 0.02%. The pulp is then transported to a slanted wire forming machine for forming. After vacuum dewatering, press dewatering, and drying at 80℃, fully recycled aramid base paper with a basis weight of 150g / m² is obtained.

[0077] S5, Composite of sandwich-structured recycled aramid paper:

[0078] Using the fully recycled aramid base paper obtained in step S4 as the core layer and the virgin meta-aramid paper as the surface layer (basis weight 35 g / m²), the surface layer, core layer, and surface layer are stacked sequentially; then, hot pressing is performed in two stages, namely: first stage: temperature 170℃, pressure 1.5MPa, time 6min; second stage: temperature 230℃, pressure 6MPa, time 8min; after cooling and shaping, sandwich structure recycled aramid paper is obtained.

[0079] In this embodiment, fully recycled aramid base paper accounts for 68.18% of the total mass of sandwich structure recycled aramid paper; recycled waste aramid paper scraps account for 54.55% of the mass of sandwich structure recycled aramid paper.

[0080] Example 2

[0081] A sandwich-structured recycled aramid paper, the raw material of which is waste aramid paper scraps, wherein the waste aramid paper scraps are scraps obtained after wet papermaking, and the surface is not coated with resin or coating. The specific preparation method includes the following steps:

[0082] S1. Pretreatment of waste aramid paper:

[0083] Collect waste aramid paper scraps, remove impurities, and cut the waste aramid paper scraps into 8mm pieces.

[0084] S2, Solid-phase shearing and grinding treatment:

[0085] The waste aramid paper scraps pretreated in step S1 were fed into a grinding disc mechanical chemical reactor for processing. The grinding disc gap was set to 0.6 mm, the rotation speed was 400 rpm, and the process was repeated 3 times to finally obtain aramid short fibers.

[0086] S3, Wet pulping and fibrillation treatment:

[0087] The aramid short fibers obtained in step S2 are added to a PFI refiner for staged delamination and fibrillation. The delamination process parameters are set as follows: rotation speed 600 rpm, delamination time 4 min, and pulp concentration controlled at 6%. The fibrillation process parameters are set as follows: grinding disc gap 0.2 mm, rotation speed 1000 rpm, and time 50 min. After fibrillation, the pulp is passed through a hydrocyclone separator with a separation factor of 1000~1500 to remove short fibers smaller than 0.5 mm, finally obtaining aramid pulp with a freeness of 70°SR.

[0088] S4. Wet papermaking:

[0089] The aramid pulp obtained in step S3 is mixed with precipitated fibers at a mass ratio of 4:1. At this time, the content of aramid pulp is 80wt% of the total mass of aramid pulp and precipitated fibers, and the content of precipitated fibers is 20wt% of the total mass of aramid pulp and precipitated fibers. Then, 0.3% of polyethylene oxide dispersant is added to the total mass of aramid pulp and precipitated fibers to make them evenly dispersed to obtain pulp. The pulp is diluted with deionized water to a wire mesh concentration of 0.02%. The pulp is then transported to an inclined wire forming machine for forming. After vacuum dewatering, press dewatering, and drying at 70℃, fully recycled aramid base paper with a basis weight of 150g / m² is obtained.

[0090] S5, Composite of sandwich-structured recycled aramid paper:

[0091] Using the fully recycled aramid base paper obtained in step S4 as the core layer and the virgin meta-aramid paper as the surface layer (basis weight 35g / m²), the surface layer, core layer, and surface layer are stacked in sequence; then, hot pressing is carried out in two stages, which are: first stage: temperature 160℃, pressure 1MPa, time 6min; second stage: temperature 240℃, pressure 6MPa, time 6min; after cooling and shaping, sandwich structure recycled aramid paper is obtained.

[0092] In this embodiment, fully recycled aramid base paper accounts for 68.18% of the total mass of sandwich structure recycled aramid paper; recycled waste aramid paper scraps account for 54.55% of the mass of sandwich structure recycled aramid paper.

[0093] Example 3

[0094] A sandwich-structured recycled aramid paper, the raw material of which is waste aramid paper scraps, wherein the waste aramid paper scraps are scraps obtained after wet papermaking, and the surface is not coated with resin or coating. The specific preparation method includes the following steps:

[0095] S1. Pretreatment of waste aramid paper:

[0096] Collect waste aramid paper scraps, remove impurities, and cut the waste aramid paper scraps into 8mm pieces.

[0097] S2, Solid-phase shearing and grinding treatment:

[0098] The waste aramid paper scraps pretreated in step S1 were fed into a grinding disc mechanical chemical reactor for processing. The grinding disc gap was set to 0.6 mm, the rotation speed was 400 rpm, and the process was repeated 5 times to finally obtain aramid short fibers.

[0099] S3, Wet pulping and fibrillation treatment:

[0100] The aramid short fibers obtained in step S2 are added to a PFI refiner for staged delamination and fibrillation. The delamination process parameters are set as follows: rotation speed 600 rpm, delamination time 4 min, and pulp concentration controlled at 6%. The fibrillation process parameters are set as follows: grinding disc gap 0.2 mm, rotation speed 1000 rpm, and time 60 min. After fibrillation, the pulp is passed through a hydrocyclone separator with a separation factor of 1000~1500 to remove short fibers smaller than 0.5 mm, finally obtaining aramid pulp with a freeness of 78°SR.

[0101] S4. Wet papermaking:

[0102] The aramid pulp obtained in step S3 is mixed with precipitated fibers at a mass ratio of 4:1. At this time, the content of aramid pulp is 80wt% of the total mass of aramid pulp and precipitated fibers, and the content of precipitated fibers is 20wt% of the total mass of aramid pulp and precipitated fibers. Then, 0.4% of polyethylene oxide dispersant is added to the total mass of aramid pulp and precipitated fibers to make them evenly dispersed to obtain pulp. The pulp is diluted with deionized water to a wire-on-grid concentration of 0.02%. The pulp is then transported to a slanted wire forming machine for forming. After vacuum dewatering, press dewatering, and drying at 80℃, fully recycled aramid base paper with a basis weight of 150g / m² is obtained.

[0103] S5, Composite of sandwich-structured recycled aramid paper:

[0104] Using the fully recycled aramid base paper obtained in step S4 as the core layer and the virgin meta-aramid paper as the surface layer (basis weight 35 g / m²), the surface layer, core layer, and surface layer are stacked sequentially; then, hot pressing is performed in two stages, namely: first stage: temperature 170℃, pressure 1.5MPa, time 6min; second stage: temperature 230℃, pressure 6MPa, time 8min; after cooling and shaping, sandwich structure recycled aramid paper is obtained.

[0105] In this embodiment, fully recycled aramid base paper accounts for 68.18% of the total mass of sandwich structure recycled aramid paper; recycled waste aramid paper scraps account for 54.55% of the mass of sandwich structure recycled aramid paper.

[0106] Example 4

[0107] A sandwich-structured recycled aramid paper, the raw material of which is waste aramid paper scraps, wherein the waste aramid paper scraps are scraps obtained after wet papermaking, and the surface is not coated with resin or coating. The specific preparation method includes the following steps:

[0108] S1. Pretreatment of waste aramid paper:

[0109] Collect waste aramid paper scraps, remove impurities, and cut the waste aramid paper scraps into 6mm pieces.

[0110] S2, Solid-phase shearing and grinding treatment:

[0111] The waste aramid paper scraps pretreated in step S1 were fed into a grinding disc mechanical chemical reactor for processing. The grinding disc gap was set to 0.5 mm, the rotation speed was 500 rpm, and the process was repeated 6 times to finally obtain aramid short fibers.

[0112] S3, Wet pulping and fibrillation treatment:

[0113] The aramid short fibers obtained in step S2 are added to a PFI refiner for staged delamination and fibrillation. The delamination process parameters are set as follows: rotation speed 600 rpm, delamination time 4 min, and pulp concentration controlled at 8%. The fibrillation process parameters are set as follows: grinding disc gap 0.15 mm, rotation speed 1100 rpm, and time 60 min. After fibrillation, the pulp is passed through a hydrocyclone separator with a separation factor of 1000~1500 to remove short fibers smaller than 0.5 mm, finally obtaining aramid pulp with a freeness of 78°SR.

[0114] S4. Wet papermaking:

[0115] The aramid pulp obtained in step S3 is mixed with precipitated fibers at a mass ratio of 4:1. At this time, the content of aramid pulp is 80wt% of the total mass of aramid pulp and precipitated fibers, and the content of precipitated fibers is 20wt% of the total mass of aramid pulp and precipitated fibers. Then, 0.4% of polyethylene oxide dispersant is added to the total mass of aramid pulp and precipitated fibers to make them evenly dispersed to obtain pulp. The pulp is diluted with deionized water to a wire mesh concentration of 0.02%. The pulp is then transported to a slanted wire forming machine for forming. After vacuum dewatering, press dewatering, and drying at 80℃, fully recycled aramid base paper with a basis weight of 145g / m² is obtained.

[0116] S5, Composite of sandwich-structured recycled aramid paper:

[0117] Using the fully recycled aramid base paper obtained in step S4 as the core layer and the virgin meta-aramid paper as the surface layer (basis weight 30 g / m²), the surface layer, core layer, and surface layer are stacked sequentially; then, hot pressing is performed in two stages, namely: first stage: temperature 170℃, pressure 1.5MPa, time 6min; second stage: temperature 230℃, pressure 6MPa, time 8min; after cooling and shaping, sandwich structure recycled aramid paper is obtained.

[0118] In this embodiment, fully recycled aramid base paper accounts for 70.73% of the total mass of sandwich structure recycled aramid paper; recycled waste aramid paper scraps account for 56.59% of the mass of sandwich structure recycled aramid paper.

[0119] Example 5

[0120] A sandwich-structured recycled aramid paper, the raw material of which is waste aramid paper scraps, wherein the waste aramid paper scraps are scraps obtained after wet papermaking, and the surface is not coated with resin or coating. The specific preparation method includes the following steps:

[0121] S1. Pretreatment of waste aramid paper:

[0122] Collect waste aramid paper scraps, remove impurities, and cut the waste aramid paper scraps into 5mm pieces.

[0123] S2, Solid-phase shearing and grinding treatment:

[0124] The waste aramid paper scraps pretreated in step S1 were fed into a grinding disc mechanical chemical reactor for processing. The grinding disc gap was set to 0.5 mm, the rotation speed was 200 rpm, and the process was repeated 8 times to finally obtain aramid short fibers.

[0125] S3, Wet pulping and fibrillation treatment:

[0126] The aramid short fibers obtained in step S2 are added to a PFI refiner for staged delamination and fibrillation. The delamination process parameters are set as follows: rotation speed 500 rpm, delamination time 5 min, and pulp concentration controlled at 5%. The fibrillation process parameters are set as follows: grinding disc gap 0.1 mm, rotation speed 1200 rpm, and time 40 min. After fibrillation, the pulp is passed through a hydrocyclone separator with a separation factor of 1000~1500 to remove short fibers smaller than 0.5 mm, finally obtaining aramid pulp with a freeness of 70°SR.

[0127] S4. Wet papermaking:

[0128] The aramid pulp obtained in step S3 is mixed with precipitated fibers at a mass ratio of 2.3:1. At this point, the content of aramid pulp is 70 wt% of the total mass of aramid pulp and precipitated fibers, and the content of precipitated fibers is 30 wt% of the total mass of aramid pulp and precipitated fibers. Then, 0.5% of polyethylene oxide dispersant is added to the total mass of aramid pulp and precipitated fibers to make them evenly dispersed to obtain pulp. The pulp is diluted with deionized water to a wire-on-grid concentration of 0.01%. The pulp is then transported to an inclined wire forming machine for forming. After vacuum dewatering, press dewatering, and drying at 80°C, fully recycled aramid base paper with a basis weight of 150 g / m² is obtained.

[0129] S5, Composite of sandwich-structured recycled aramid paper:

[0130] Using the fully recycled aramid base paper obtained in step S4 as the core layer and the virgin meta-aramid paper as the surface layer (basis weight 35g / m²), the surface layer, core layer, and surface layer are stacked in sequence; then, hot pressing is performed in two stages, which are: first stage: temperature 180℃, pressure 2MPa, time 5min; second stage: temperature 220℃, pressure 5MPa, time 10min; after cooling and shaping, sandwich structure recycled aramid paper is obtained.

[0131] In this embodiment, fully recycled aramid base paper accounts for 68.18% of the total mass of sandwich structure recycled aramid paper; recycled waste aramid paper scraps account for 47.73% of the mass of sandwich structure recycled aramid paper.

[0132] Example 6

[0133] A sandwich-structured recycled aramid paper, the raw material of which is waste aramid paper scraps, wherein the waste aramid paper scraps are scraps obtained after wet papermaking, and the surface is not coated with resin or coating. The specific preparation method includes the following steps:

[0134] S1. Pretreatment of waste aramid paper:

[0135] Collect waste aramid paper scraps, remove impurities, and cut the waste aramid paper scraps into 8mm pieces.

[0136] S2, Solid-phase shearing and grinding treatment:

[0137] The waste aramid paper scraps pretreated in step S1 were fed into a grinding disc mechanical chemical reactor for processing. The grinding disc gap was set to 2 mm, the rotation speed was 600 rpm, and the process was repeated 8 times to finally obtain aramid short fibers.

[0138] S3, Wet pulping and fibrillation treatment:

[0139] The aramid short fibers obtained in step S2 are added to a PFI refiner for staged delamination and fibrillation. The delamination process parameters are set as follows: rotation speed 800 rpm, delamination time 3 min, and pulp concentration controlled at 6%. The fibrillation process parameters are set as follows: grinding disc gap 0.3 mm, rotation speed 800 rpm, and time 50 min. After fibrillation, the pulp is passed through a hydrocyclone separator with a separation factor of 1000~1500 to remove short fibers smaller than 0.5 mm, finally obtaining aramid pulp with a freeness of 70°SR.

[0140] S4. Wet papermaking:

[0141] The aramid pulp obtained in step S3 is mixed with precipitated fibers at a mass ratio of 5.67:1. At this point, the content of aramid pulp is 85 wt% of the total mass of aramid pulp and precipitated fibers, and the content of precipitated fibers is 15 wt% of the total mass of aramid pulp and precipitated fibers. Then, 0.4% of polyethylene oxide dispersant is added to the total mass of aramid pulp and precipitated fibers to make them uniformly dispersed to obtain pulp. The pulp is diluted with deionized water to a wire-on-grid concentration of 0.03%. The pulp is then transported to an inclined wire forming machine for forming. After vacuum dewatering, press dewatering, and drying at 80°C, fully recycled aramid base paper with a basis weight of 150 g / m² is obtained.

[0142] S5, Composite of sandwich-structured recycled aramid paper:

[0143] Using the fully recycled aramid base paper obtained in step S4 as the core layer and the virgin meta-aramid paper as the surface layer (basis weight 40 g / m²), the surface layer, core layer, and surface layer are stacked in sequence; then, hot pressing is carried out in two stages, which are: first stage: temperature 160℃, pressure 2MPa, time 8min; second stage: temperature 250℃, pressure 8MPa, time 5min; after cooling and shaping, sandwich structure recycled aramid paper is obtained.

[0144] In this embodiment, fully recycled aramid base paper accounts for 65.22% of the total mass of sandwich structure recycled aramid paper; recycled waste aramid paper scraps account for 55.43% of the mass of sandwich structure recycled aramid paper.

[0145] Comparative Example 1

[0146] A sandwich structure recycled aramid paper. This Comparative Example 1 uses the same method as Example 1, except that this Comparative Example 1 does not pretreat the waste aramid paper in the manner described in step S1 of Example 1, nor does it perform solid-phase shearing and grinding treatment. Instead, a paper shredder is used to coarsely shred the waste aramid paper scraps into 2mm fragments. In the wet pulping stage, a hydraulic pulper is used to decompose the pulp, with a rotation speed of 1500 rpm and a duration of 30 minutes, and the pulp concentration is controlled at 5%.

[0147] Comparative Example 2

[0148] A sandwich structure recycled aramid paper, Comparative Example 2 adopts the same method as Example 1, except that no precipitated fibers are added in step S4.

[0149] Comparative Example 3

[0150] A sandwich structure recycled aramid paper. This comparative example 3 uses the same method as example 1, except that in step S2, the grinding disc gap is set to 1.5 mm, the rotation speed is 650 rpm, and the processing is performed 5 times.

[0151] Comparative Example 4

[0152] A sandwich structure recycled aramid paper. This comparative example 4 uses the same method as example 1, except that the hot pressing composite process parameters in step S5 are different from those in example 1. The hot pressing composite is carried out in two stages: first stage: temperature 100℃, pressure 1.5MPa; second stage: temperature 200℃, pressure 4MPa.

[0153] Comparative Example 5

[0154] A sandwich structure recycled aramid paper, this comparative example 5 uses the same method as example 1, except that in step S2, the grinding disc gap of the solid phase shearing and grinding treatment is set to 0.3 mm.

[0155] Comparative Example 6

[0156] A sandwich structure recycled aramid paper, this comparative example 6 uses the same method as example 1, except that in step S2, the grinding disc gap of the solid phase shearing and grinding treatment is set to 2.4 mm.

[0157] Phenomenon: After processing, the output material contained not only short fibers but also small pieces of paper and fiber clumps that were not completely separated, so it was not processed further. This is because the gap between the grinding discs was too large, and the shearing force of the grinding disc teeth on the material was insufficient to effectively "tear apart" the intertwined aramid fiber web.

[0158] Comparative Example 7

[0159] A sandwich structure recycled aramid paper, Comparative Example 7 adopts the same method as Example 1, except that in step S2, the rotation speed of the solid phase shear milling treatment is set to 150 rpm.

[0160] Phenomenon: After processing, the output material contained a large number of small pieces of paper and fiber clumps that were not completely separated, so further processing was not carried out. This is because when the rotation speed was set to 150 rpm, the mechanical energy was insufficient to effectively break the hydrogen bonds and mechanical entanglement between fibers in the waste aramid paper, resulting in uneven fiber length and morphological distribution in the product.

[0161] Comparative Example 8

[0162] A sandwich structure recycled aramid paper, this comparative example 8 uses the same method as example 1, except that fibrillation treatment is not performed in step S3.

[0163] Comparative Example 9

[0164] A sandwich structure recycled aramid paper, Comparative Example 9 adopts the same method as Example 1, except that in step S5, the hot pressing temperature of the first stage is 100°C.

[0165] Comparative Example 10

[0166] A sandwich structure recycled aramid paper, Comparative Example 10 adopts the same method as Example 1, except that in step S5, the hot pressing temperature of the second stage is 200°C.

[0167] Comparative Example 11

[0168] A sandwich structure recycled aramid paper, this Comparative Example 11 uses the same method as Example 1, except that in step S5, the hot pressing temperature of the second stage is 300°C.

[0169] The tests involved in the preparation of the sandwich-structured recycled aramid paper in the embodiments and comparative examples of this invention are as follows:

[0170] Tensile strength: Standard adopted: GB / T 12914-2018, Instrument: Tensile strength tester, Test method: A 15mm wide and 20cm long aramid paper standard sample is broken at a rate of 100mm / min, and the tensile strength is obtained from the tensile curve.

[0171] Tear strength: Standard adopted: GB / T 455-2022; Instrument: Tear strength tester. Test method: A 63mm wide and 10cm long aramid paper standard sample is fixed in the instrument, and the instrument automatically records the maximum force during the tearing process.

[0172] Interlaminar bond strength: Standard adopted: GB / T 26203-2023; Instrument: Interlaminar bond strength tester. Test method: A 25.4mm×25.4mm aramid paper sample assembly is fixed on the fixture of the testing machine. After the pendulum impacts the sample, the instrument automatically calculates the energy absorbed when the sample fails.

[0173] Withstand voltage: Standard adopted: GB / T 29627.2-2013; Instrument: Breakdown voltage tester; Test method: Aramid paper is subjected to a voltage of 5kV and a rate of 50V / s for about 10-20s to break down, and the breakdown voltage can be measured. The breakdown voltage divided by the thickness of the aramid paper is the withstand voltage of the aramid paper. The average value is taken for 3 measurements.

[0174] The performance test data of the sandwich structure recycled aramid paper prepared in the embodiments and comparative examples of the present invention are shown in Table 1.

[0175] Table 1. Performance test data of sandwich-structured recycled aramid paper prepared in the embodiments and comparative examples of the present invention.

[0176]

[0177] According to the performance test data of the sandwich-structured recycled aramid paper prepared in Examples 1-6 of Table 1, the sandwich-structured recycled aramid paper prepared by the method described in this invention exhibits excellent performance. Its tensile strength and tear strength are close to those of virgin material. Furthermore, the composite paper has high interlayer bonding strength, does not delaminate or deform, and demonstrates excellent dimensional stability under high temperature and humidity conditions, meeting the application requirements of high-end electrical insulation materials. This is because the sandwich-structured recycled aramid paper provided by this invention perfectly balances performance and cost. The preparation method of the sandwich-structured recycled aramid paper provided by this invention overcomes the shortcomings of traditional mechanical crushing methods, such as significant fiber damage, uneven length distribution, and performance degradation, through a synergistic process of solid-phase shearing, wet pulping, and fibrillation. It effectively preserves the quality of aramid fibers in waste aramid paper. Solid-phase shearing and milling is a mechanochemical treatment based on the combined effects of extrusion, shearing, and friction, providing fibers with good length retention and minimal damage. Wet pulping and fibrillation significantly increase the specific surface area and hydrogen bonding points of the fibers, enhancing the compactness and interlayer bonding of the subsequent paper, thus achieving high-value recycling of waste aramid paper. After wet pulping and fibrillation, a hydraulic separator is introduced to remove short fibers <0.5mm, effectively eliminating "weak points" that affect paper strength while retaining the main fiber body of moderate length and high strength. This results in a more uniform core layer paper structure with higher strength and fewer defects. During the wet papermaking process, precipitated fibers are added to enhance the bonding force between fibers. The sandwich composite structure design, combined with an optimized staged hot-pressing process, ensures interlayer bonding strength and performance stability. The first-stage hot-pressing process achieves preliminary interface fusion and sufficient water molecule removal, avoiding defects such as delamination and blistering caused by rapid water vapor evaporation in the subsequent high-temperature stage. The second-stage hot-pressing process uses high-pressure setting at conditions higher than the glass transition temperature of aramid fibers, promoting the formation of strong hydrogen bonds and mechanical interlocking between the surface and core fibers. At the same time, it allows the fibers themselves to complete heat setting, eliminating internal stress. This results in high interlayer bonding strength, no delamination or deformation, and excellent dimensional stability under high temperature and humidity conditions, meeting the application requirements of high-end electrical insulation materials.

[0178] Based on the data in Table 1, a comparison of the experimental results of Comparative Example 1 and Example 1 shows that the performance of the sandwich-structured recycled aramid composite paper prepared in Example 1 is significantly better than that of the composite paper prepared in Comparative Example 1. Its tensile strength, tear strength, interlayer bonding strength, and compressive strength are all significantly higher than those of Comparative Example 1. This is because Comparative Example 1 uses a traditional paper shredder to coarsely shred waste aramid paper scraps into 2mm fragments, resulting in severe fiber length loss. Short fibers cannot form an effective network during the papermaking process. In the wet pulping stage, a hydraulic pulper is used for decomposition. Its main function is to rely on hydraulic eddies to break up the cut fiber clumps and disperse them evenly in the water. It does not have the ability to longitudinally tear or separate individual fibers. The lack of fibrillation results in smooth fiber surfaces, weak bonding strength, low paper density, and poor interlayer bonding strength.

[0179] Based on the data in Table 1, a comparison of the experimental results of Comparative Example 2 and Example 1 shows that the performance of the sandwich-structured recycled aramid composite paper prepared in Example 1 is significantly better than that of the composite paper prepared in Comparative Example 2. This is because, lacking the bonding of precipitated fibers, the pulp fiber network is loose and cannot effectively transfer and disperse stress.

[0180] Based on the data in Table 1, a comparison of the experimental results of Comparative Example 3 and Example 1 shows that the performance of the sandwich-structured recycled aramid composite paper prepared in Example 1 is significantly better than that of the composite paper prepared in Comparative Example 3. This is because the strong shear force and frictional heat generated by excessively high rotational speed will forcibly cut the fibers. The "shear-dissociation" action of the grinding disc teeth on the fibers becomes "impact-cutting," which not only shortens the fiber length but also generates a large number of microcracks on the surface, further weakening the fiber's own strength.

[0181] According to the data in Table 1, a comparison of the experimental results of Comparative Example 4 and Example 1 shows that the sandwich-structured recycled aramid composite paper prepared in Example 1 has a higher yield than the composite paper prepared in Comparative Example 4. This is because, in step S5, in the first stage, when the hot-pressing temperature is 100℃, the temperature is lower than the temperature set in this invention. The aramid polymer chain segment mobility is very weak, and the fibers and precipitated fibers are still in a "hard glass state." The surface layer and the core layer are only in physical contact and have not formed an effective initial fusion, resulting in a very weak foundation for interlayer bonding. In the second stage, when the hot-pressing temperature is 200℃, the temperature is too low. The precipitated fibers have not reached their optimal softening state, have poor fluidity, and cannot effectively achieve the bonding function. The aramid fiber body is still very rigid and cannot form a tight bond with adjacent fibers through chain segment movement, resulting in a serious deficiency in the number and quality of bonding points between fibers. At the same time, the low temperature means that the internal stress generated during processing cannot be effectively eliminated, which will lead to dimensional shrinkage and deformation during use.

[0182] According to the data in Table 1, a comparison of the experimental results of Comparative Example 5 and Example 1 shows that the performance of the sandwich-structured recycled aramid composite paper prepared in Example 1 is significantly better than that of the composite paper prepared in Comparative Example 5. This is because, in step S2, the gap between the grinding discs in the solid-phase shearing milling process is too small, the fibers are strongly crushed and cut, and the fiber properties are severely damaged. In the PFI refiner, these short fibers are difficult to further fibrillate due to insufficient length and damaged structure, resulting in a low freeness of the final pulp. The pulp lacks the necessary microfibrils to form hydrogen bonds, which in turn affects the performance of the final sandwich-structured recycled aramid paper.

[0183] Based on the data in Table 1, a comparison of the experimental results of Comparative Example 8 and Example 1 shows that the performance of the sandwich-structured recycled aramid composite paper prepared in Example 1 is significantly better than that of the composite paper prepared in Comparative Example 8. This is because, in step S3 of Comparative Example 8, no fibrillation treatment was performed, and a large number of microfibrils did not split off from the fiber surface. The effective contact area was small, and there were very few sites available for forming hydrogen bonds, making it impossible to form a strong mechanical interlocking structure, which affected the binding effect of the precipitated fibers.

[0184] Based on the data in Table 1, a comparison of the experimental results of Comparative Example 9 and Example 1 shows that the performance of the sandwich-structured recycled aramid composite paper prepared in Example 1 is significantly better than that of the composite paper prepared in Comparative Example 9. This is because, in step S5, during the first stage, when the hot-pressing temperature is 100°C, the temperature is lower than the temperature set in this invention. The aramid polymer chain segments have very weak mobility, and the fibers and precipitated fibers remain in a "hard glassy state." The surface layer and the core layer are only in physical contact, without forming an effective initial fusion, resulting in a weak foundation for interlayer bonding.

[0185] According to the data in Table 1, a comparison of the experimental results of Comparative Example 10 and Example 1 shows that the performance of the sandwich-structured recycled aramid composite paper prepared in Example 1 is significantly better than that of the composite paper prepared in Comparative Example 10. This is because, in step S5, the second stage, when the hot-pressing temperature is 200°C, the temperature is too low. The precipitated fibers do not reach their optimal softening state, resulting in poor fluidity and an inability to effectively achieve the bonding function. The aramid fiber body remains very rigid and cannot form a tight bond with adjacent fibers through chain segment movement, resulting in a severe deficiency in both the number and quality of bonding points between fibers. At the same time, the low temperature prevents the effective elimination of internal stress generated during processing, leading to dimensional shrinkage and deformation during use.

[0186] Based on the data in Table 1, a comparison of the experimental results of Comparative Example 11 and Example 1 shows that the sandwich-structured recycled aramid paper material obtained in Comparative Example 11 becomes brittle and its performance deteriorates. This is because when the hot-pressing temperature in the second stage is 300°C, the precipitated fibers undergo secondary crystallization during the hot-pressing process. Excessive temperature leads to overgrowth of the crystalline region, resulting in overly regular molecular chain arrangement, significantly increasing the material's rigidity while decreasing its toughness.

[0187] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are exhaustively listed. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0188] For those skilled in the art, various modifications and improvements can be made without departing from the concept of the present invention, and these all fall within the scope of protection of the present invention. The scope of protection of the present invention is defined by the appended claims.

Claims

1. A method for preparing a sandwich-structured recycled aramid paper, characterized in that, The preparation method includes the following steps: S1. Pre-treatment of waste aramid paper: Cut the scraps of waste aramid paper into pieces with a size of 5~10mm; S2. Solid-phase shearing and grinding treatment: The waste aramid paper scraps pretreated in step S1 are put into a disc-shaped mechanical chemical reactor. The disc gap is set to 0.5~2mm, the rotation speed is 200~600rpm, and the number of processing times is 2~8. The specific number of processing times is adjusted according to the fiber length to ensure that the fiber mass with a fiber length of 1.5~3mm accounts for at least 60% of the total fiber mass. Aramid short fibers are obtained, wherein the average length of the obtained aramid short fibers is 1.5~3mm and the specific surface area is 8~12m² / g. S3. Wet refining and fibrillation: The aramid short fibers obtained in step S2 are added to a refiner for delamination and fibrillation. During the delamination stage, the rotation speed is 500-800 rpm, and during the fibrillation stage, the disc gap is 0.1-0.3 mm and the beating degree is 60-80°SR, to obtain aramid pulp. S4. Wet papermaking: Add precipitated fibers and mix with the aramid pulp obtained in step S3. The content of aramid pulp is 70-85 wt% of the total mass of the aramid pulp and the precipitated fibers, and the content of precipitated fibers is 15-30 wt% of the total mass of the aramid pulp and the precipitated fibers. After wet papermaking, dry to obtain fully recycled aramid base paper. S5. Composite of sandwich structure recycled aramid paper: Using the fully recycled aramid base paper obtained in step S4 as the core layer, and using virgin meta-aramid paper as the surface layer on both sides, a sandwich structure is composited and hot-pressed in stages to obtain sandwich structure recycled aramid paper. The waste aramid paper scraps are waste aramid paper scraps obtained after wet papermaking, and the surface is not coated with resin or coating. In step S4, the specific surface area of ​​the precipitated fibers is 20~30m². 2 / g, average length is 0.1-0.5mm, fibrillation degree is 50-70%; In step S5, the staged hot pressing includes: the first stage: temperature 160~180℃, pressure 1~2MPa; the second stage: temperature 220~250℃, pressure 5~8MPa. The sandwich-structured recycled aramid paper must simultaneously meet the following requirements: tensile strength of 158~180 N / cm, tear strength of 4.1~4.8 N, and interlayer bonding strength of 6.8~7.5 N / cm. 2 .

2. The method for preparing a sandwich-structured recycled aramid paper according to claim 1, characterized in that, In step S3, the disintegration time is 3-5 minutes, and the slurry concentration is controlled at 5-8%. During the fibrillation stage, the rotation speed is 800~1200 rpm and the time is 40~60 min.

3. The method for preparing a sandwich-structured recycled aramid paper according to claim 1, characterized in that, In step S4, the precipitated fibers are first added and mixed with the aramid pulp obtained in step S3. Then, 0.3-0.5% of polyethylene oxide dispersant by weight of the total mass of the aramid pulp and precipitated fibers is added. The pulp is diluted with deionized water to a concentration of 0.01-0.03% before being fed to the grid. After forming, the pulp is dewatered under vacuum, dewatered by pressing, and then dried at 60-80°C to obtain fully recycled aramid base paper.

4. The method for preparing a sandwich-structured recycled aramid paper according to claim 3, characterized in that, In step S4, the molecular weight of the polyethylene oxide dispersant is 3 × 10⁻⁶. 6 ~4×10 6 g / mol.

5. The method for preparing a sandwich-structured recycled aramid paper according to claim 1, characterized in that, In step S5, the tensile strength of the new meta-aramid paper is 150~200MPa, and the basis weight of the new meta-aramid paper is 30~40g / m². The fully recycled aramid base paper accounts for 65-70.73% of the total mass of the sandwich structure recycled aramid paper.

6. The method for preparing a sandwich-structured recycled aramid paper according to claim 1, characterized in that, In step S5, the first stage of the phased hot pressing lasts for 5-8 minutes, and the second stage lasts for 5-10 minutes.

7. A sandwich-structured recycled aramid paper, characterized in that, The sandwich-structured recycled aramid paper is prepared according to the preparation method described in any one of claims 1-6; The sandwich-structured recycled aramid paper includes a surface layer and a core layer; The core layer is located between the two surface layers; The core layer is a fully recycled aramid base paper, which includes recycled aramid fibers and precipitated fibers. The surface layer is made of virgin meta-aramid paper; The sandwich-structured recycled aramid paper must simultaneously meet the following requirements: tensile strength of 158~180 N / cm, tear strength of 4.1~4.8 N, and interlayer bonding strength of 6.8~7.5 N / cm. 2 .

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