A light and thin p-aramid paper, a preparation method and application thereof

Abstract

This invention provides a thin, lightweight para-aramid paper, its preparation method, and its applications. The para-aramid paper prepared by this method has a basis weight of 5-30 g / m³. 2 The thickness is 0.01~0.04 mm. This method first prepares pulp A and pulp B, which are then processed to obtain the finished para-aramid paper. Pulp A directly uses oil-free para-aramid short fibers, avoiding the washing process of oily short fibers and reducing secondary pollution and wastewater disposal caused by washing oily short fibers. Pulp B uses specific types of nanofibers, and a binary retention aid composed of CPAM and ACPAM is added to these nanofibers. This facilitates the formation of micro-floc structures by the nanofibers, thereby improving the retention rate and filtration speed of the nanofibers, enabling continuous production of para-aramid paper. The addition of the aid enhances the hydrogen bonding between the nanofibers and chopped fibers, improving paper uniformity and thus increasing paper strength.

Description

Technical Field

[0001] This invention belongs to the field of papermaking technology, specifically relating to a thin and light para-aramid paper, its preparation method, and its application. Background Technology

[0002] With the transformation and upgrading of the global aviation industry and the rapid rise of the low-altitude economy, new transportation vehicles, represented by electric vertical takeoff and landing (EVTOL) aircraft and unmanned aerial vehicles (UAVs), are experiencing explosive growth. Against this backdrop, the aerospace manufacturing industry is placing increasingly stringent requirements on the performance of composite materials, and carbon reduction, cost reduction, and lightweighting have become core driving forces for industry development. Lightweighting not only directly improves the payload and range of aircraft but is also a key technological path to achieving energy conservation and emission reduction during aircraft operation and reaching "dual carbon" targets.

[0003] Aramid paper, as a core reinforcing material for preparing honeycomb sandwich composite structures, has become the preferred core material for secondary and primary load-bearing structural components such as aircraft interiors, radomes, ailerons, and rudders due to its excellent specific strength, specific stiffness, flame retardancy, heat resistance, and superior impact resistance. Furthermore, honeycomb composites made from para-aramid paper can achieve significant weight reduction (up to 30% or more) compared to traditional metal materials while maintaining structural mechanical properties, which is of great significance for improving the economy and environmental friendliness of aircraft.

[0004] However, with the pursuit of extreme efficiency in low-altitude economics and the deepening of structural-functional integration design in the aerospace field, the limitations of existing conventional aramid paper honeycomb materials are gradually becoming apparent. Especially in the design and application of complex curved surfaces, thin-walled sandwich structures, and ultra-lightweight structures, it is difficult to balance load-bearing capacity and weight reduction using traditional thickness meta-aramid paper substrates. To further reduce the structural weight coefficient, improve material utilization, and lower manufacturing costs, the demand for weight reduction in aerospace honeycomb structures is urgent, necessitating the development of ultra-thin, low-basis-weight, fully para-aramid paper that is thinner, has lower areal density, and does not degrade mechanical properties.

[0005] However, in para-aramid paper, the rigid molecular chain structure of aramid fibers and the chemical inertness of the fiber surface lead to a lack of effective bonding between fibers, resulting in many shortcomings in the finished paper, such as weak interlayer bonding, poor mechanical properties, and high production costs. To address these shortcomings, researchers have used reinforcing materials to prepare reinforced aramid paper. These reinforcing materials are generally selected from meta-aramid precipitated fibers or other reinforcing auxiliaries, primarily wet-strength agents (polyamine-epoxychloropropane). However, the resulting finished paper still suffers from weak interlayer bonding, poor paper uniformity, low mechanical strength, and high air permeability. Summary of the Invention

[0006] In view of this, the purpose of the present invention is to provide a thin and light para-aramid paper, its preparation method and application.

[0007] To achieve this objective, the present invention adopts the following technical solution:

[0008] In a first aspect, the present invention provides a method for preparing a thin and light para-aramid paper, comprising the following steps:

[0009] Para-aramid chopped fiber pulp and para-aramid nanofiber pulp are mixed in a certain proportion. The resulting mixed pulp is then formed, pressed, dried and hot-pressed to obtain a thin para-aramid paper.

[0010] The para-aramid nanofiber slurry comprises para-aramid nanofibers and a binary retention aid; the binary retention aid comprises cationic polyacrylamide and amphoteric polyacrylamide.

[0011] According to the present invention, para-aramid chopped fiber slurry and para-aramid nanofiber slurry are first mixed in a certain proportion.

[0012] The para-aramid chopped fiber slurry includes para-aramid chopped fibers and dispersing agents.

[0013] In this invention, the length of the para-aramid chopped fibers is 3-8 mm, specifically 3 mm, 4 mm, 5 mm, 6 mm, 7 mm, or 8 mm. In some embodiments of this invention, it is preferred to use a combination of para-aramid chopped fibers with two or more particle sizes. Studies have found that, compared to para-aramid chopped fibers with a single particle size, the para-aramid paper obtained by using a combination of three particle sizes or a combination of two particle sizes exhibits better performance. Among these, the para-aramid paper obtained by using a combination of three particle sizes of para-aramid chopped fibers has the optimal overall performance.

[0014] In some preferred embodiments of the present invention, the para-aramid chopped fibers include 3 mm chopped fibers, 6 mm chopped fibers, and 8 mm chopped fibers; wherein the mass percentage of 3 mm chopped fibers is 0-30%, and can be 0%, 5%, 10%, 15%, 20%, 25%, or 30%, etc.; the mass percentage of 6 mm chopped fibers is 30-90%, and can be 30%, 35%, 40%, 45%, 50%, 55%, or 60%, etc.; and the mass percentage of 8 mm chopped fibers is 0-20%, and can be 0%, 3%, 5%, 8%, 10%, 12%, 15%, 18%, or 20%, etc.

[0015] In this invention, the dispersing aid includes a dispersant and / or an antifoaming agent, preferably including both a dispersant and an antifoaming agent. The dispersant includes any one or more of water-soluble polymeric dispersants, anionic dispersants, or nonionic surfactants. Specifically, the water-soluble polymeric dispersant can be selected from any one or more of polyethylene oxide (PEO), polyacrylamide (PAM), carboxymethyl cellulose (CMC), polyvinyl alcohol (PVA), or sodium polyacrylate (PAAS); the anionic dispersant can be selected from alkylbenzene sulfonates and / or fatty alcohol sulfates; the nonionic surfactant can be selected from polyoxyethylene alkyl ethers and / or sorbitan esters. The antifoaming agent includes any one or more of silicone-based antifoaming agents, polyether-based antifoaming agents, or fatty alcohol-based antifoaming agents. This invention does not particularly limit the source of the antifoaming agent; commercially available products are acceptable.

[0016] In some embodiments of the present invention, based on the oven-dry weight of para-aramid chopped fibers being 100%, the mass ratio of the para-aramid chopped fibers, dispersant, and defoamer is 1:(0.03~0.1):(0.03~0.1), such as 1:0.03:0.1, 1:0.03:0.06, 1:0.04:0.06, 1:0.05:0.06, 1:0.03:0.03, 1:0.06:0.03, 1:0.06:0.06, or 1:0.04:0.1, etc.

[0017] In some embodiments of the present invention, it is preferable to mix para-aramid chopped fibers and dispersing agents evenly to obtain para-aramid chopped fiber slurry.

[0018] Furthermore, in this invention, the para-aramid nanofiber slurry comprises para-aramid nanofibers and a binary retention aid; the binary retention aid comprises cationic polyacrylamide and amphoteric polyacrylamide. Wherein, based on the oven-dry weight of the para-aramid nanofibers as 100%, the mass ratio of the para-aramid nanofibers to the binary retention aid is 1:(0.001~0.01), such as 1:0.001, 1:0.003, 1:0.005, 1:0.08, or 1:0.01, etc.

[0019] In this invention, the para-aramid nanofibers are obtained by self-production. Specifically, the preparation method is as follows: The technology of our own patent "A Method for Preparing Para-aramid Nanofibers" (CN105153413B) can be used. The method includes the following steps: (1) Modified polymerization: Under nitrogen protection, the solvent that has been dehydrated is added to the reaction vessel. The co-solubilizing salt and surfactant are added under stirring, and the mixture is heated to 80~100℃ to dissolve the co-solubilizing salt and surfactant to obtain a solution of co-solubilizing salt and surfactant. The heating time is 45~60 min. The mixture is cooled to 0~15℃ in a cold water bath for 10 min. p-phenylenediamine is added to the reaction vessel. After the p-phenylenediamine dissolves, the reaction vessel is cooled to -15~0℃. Then terephthaloyl chloride is added, and the stirring speed is increased to 1000~2500. Continue stirring at r / min for 5-10 minutes at a temperature of -15 to 0°C. Stop stirring once gelation occurs in the reaction system to obtain a gel colloid. The molar concentration of p-phenylenediamine is 0.3-0.5 mol / L, and the molar ratio of terephthaloyl chloride to p-phenylenediamine is (1.007-1.012):1. The mass ratio of the co-solvent to p-phenylenediamine is (0.25-2):1. The mass ratio of the surfactant to p-phenylenediamine is (0.25-2):1. The solvent is one or more of N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, or hexamethylphosphoramide, mixed in any proportion. The co-solvent is one or more of calcium chloride, lithium chloride, or magnesium chloride, mixed in any proportion. The surfactant is a polyoxyethylene ether nonionic surfactant. The agent, the molecular weight of the polyoxyethylene ether nonionic surfactant is 500~6000, the end group of one or both ends of the polymer is methoxy, or it is a quaternary ammonium salt cationic surfactant, the quaternary ammonium salt cationic surfactant is hexadecyltrimethylammonium bromide or dodecyldimethylbenzylammonium chloride, or it is a heterocyclic cationic surfactant, the heterocyclic cationic surfactant is dodecylpyridine ammonium chloride or octadecyl cationic alkyl imidazoline, or it is a polymeric cationic surfactant, the polymeric cationic surfactant is polyvinylpyridine quaternary ammonium salt or cationic polyacrylamide; (2) Dispersion into fibers: add a dispersant to the gel in step (1) above, the amount of dispersant added is 5~50 times that of the solvent used in step (1) above, so that the gel swells, and the swollen gel is stirred into a homogeneous system by high speed stirring at a speed of 3000~5000. The homogeneous system was stirred at a speed of 4000–6000 r / min for 5 min, and a coagulant was added under vigorous stirring to obtain a uniform and stable suspension containing para-aramid nanofibers. Alternatively, the homogeneous system was injected into a high-speed stirring coagulation bath at a stirring speed of 4000–6000 r / min for 10 min to obtain a uniform and stable suspension containing para-aramid nanofibers, wherein the para-aramid nanofiber content was 0.0.1-1 wt% of a homogeneous and stable suspension containing para-aramid nanofibers is concentrated by heating at a temperature of 50-120°C for 1-6 hours to obtain a high-concentration para-aramid nanofiber suspension. The dispersant is one or more of N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, or hexamethylphosphoramide in any proportion. The coagulant is one or more of water, acetone, methanol, ethanol, propanol, or butanol in any proportion.

[0020] It should be noted that the self-made para-aramid nanofibers obtained in this invention need to have the following characteristics: specific viscosity of 2.0~3.8 dL / g, fiber diameter of 10~80 nm, and specific surface area ≥30 m². 2 / g, wet strength ≥2 MPa, possessing a three-dimensional network structure. This is because within this range, para-aramid nanofibers exhibit the best dispersibility, filtration properties, water permeability, and bonding strength.

[0021] In this invention, the binary retention aid is crucial because this retention aid system has a high compatibility with nanofibers, effectively improving nanofiber retention and reducing loss. The surface of para-aramid nanofibers is rich in polar groups and exhibits negative charge. The high charge density cationic component in the binary retention aid preferentially neutralizes the negative charge on the nanofiber surface through electrostatic adsorption and charge patching, eliminating electrostatic repulsion between particles and forming localized positively charged active sites on the fiber and nanofiber surfaces. Subsequently, the high molecular weight anionic component, relying on its long molecular chains, constructs bridging flocculation between different positively charged active sites, anchoring the free para-aramid nanofibers to the matrix fiber network, simultaneously forming a three-dimensional network flocculation structure to enhance the trapping effect. This significantly improves the retention rate of para-aramid nanofibers during the forming process and greatly reduces losses with white water.

[0022] It should be noted that the selection of the above-mentioned binary retention aids has been screened and optimized. If any substance is replaced or omitted, the above-mentioned synergistic retention mechanism will be destroyed, resulting in a significant deterioration of the retention behavior of para-aramid nanofibers. The nanofiber loss rate will increase significantly from about 20% in the original system to 40-50%, which will not only seriously affect the paper sheet uniformity and structural performance, but also cause increased raw material loss, significantly increase production costs, and make it difficult to meet the requirements of stable industrial production.

[0023] In some embodiments of the present invention, the mass ratio of cationic polyacrylamide to zwitterionic polyacrylamide in the binary retention aid is 1:(0.8~1.2), such as 1:0.8, 1:0.9, 1:1.0, 1:1.1 or 1:1.2, etc.

[0024] The cationic polyacrylamide has a number-average molecular weight of 8 million to 16 million, preferably 12 million to 14 million; the zwitterionic polyacrylamide has a number-average molecular weight of 10 million to 20 million, preferably 13 million to 17 million.

[0025] After obtaining para-aramid chopped fiber slurry and para-aramid nanofiber slurry, the two are mixed in a certain proportion to obtain a mixed slurry.

[0026] In some embodiments of the present invention, the mass ratio of para-aramid chopped fiber in the para-aramid chopped fiber slurry to para-aramid nanofiber in the para-aramid nanofiber slurry is (30~70):(70~30), such as 30:70, 35:65, 40:60, 45:55, 50:50, 60:40, 65:35 or 70:30, etc.

[0027] Then, according to the present invention, the obtained mixed pulp is subjected to forming, pressing, drying and hot pressing to obtain a thin para-aramid paper. The present invention does not have any particular limitations on the above forming, pressing, drying and hot pressing, and can be carried out by means known to those skilled in the art.

[0028] In some preferred embodiments of the present invention, the concentration of the mixed slurry is adjusted to 0.01%~0.2wt% before molding, such as 0.01 wt%, 0.03 wt%, 0.05 wt%, 0.08 wt%, 0.1 wt%, 0.13 wt%, 0.15 wt%, 0.18 wt%, or 0.2 wt%, etc.

[0029] In this invention, the forming process can be achieved by wire mesh forming or copy forming, wherein the wire mesh forming process employs a single-layer or double-layer inclined wire mesh forming device.

[0030] In this invention, the drying temperature is 80~180℃, such as 80℃, 90℃, 100℃, 110℃, 120℃, 130℃, 140℃, 150℃, 160℃, 170℃ or 180℃, etc.; the time is 3~10 min, such as 3 min, 4 min, 5 min, 6 min, 7 min, 8 min, 9 min or 10 min, etc.

[0031] In this invention, the pressure of the hot pressing is 100~150 KN / m, such as 100 KN / m, 105 KN / m, 110 KN / m, 115 KN / m, 120 KN / m, 125 KN / m, 130 KN / m, 135 KN / m, 140 KN / m, 145 KN / m or 150 KN / m; the temperature is 220~330℃, such as 220℃, 230℃, 240℃, 250℃, 260℃, 270℃, 280℃, 290℃, 300℃, 310℃, 320℃ or 330℃, etc.

[0032] Secondly, the present invention provides a para-aramid paper prepared by the above-described preparation method. Testing shows that the basis weight of the para-aramid paper is 5-30 g / m³. 2 The thickness is 0.01~0.04 mm.

[0033] Thirdly, the present invention provides a honeycomb load-bearing product made of the para-aramid paper involved in the above-mentioned technical solution.

[0034] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0035] This invention provides a method for preparing thin para-aramid paper. The method first prepares para-aramid chopped fiber slurry A and para-aramid nanofiber slurry B, which are then subjected to molding, pressing, drying, and hot pressing to obtain the finished para-aramid paper. Specifically, para-aramid chopped fiber slurry A uses oil-free para-aramid short fibers, avoiding the washing process of oily short fibers and reducing secondary pollution and wastewater disposal caused by washing oily short fibers. Simultaneously, the addition of dispersants and defoamers to para-aramid chopped fiber slurry A increases the elimination of foaming between fibers and water, enhances the negative charge on the fiber surface, and increases the repulsive ability between fibers. Para-aramid nanofiber slurry B is prepared by diluting para-aramid nanofibers and adding CPAM (cationic polyacrylamide) and ACPAM (amphoteric polyacrylamide) to form a binary retention and filtration aid system. This facilitates the formation of micro-floc structures by nanofiber bonding, thereby improving the retention rate and filtration speed of nanofibers, enhancing their hydrogen bonding with chopped fibers, and ultimately improving the strength of the paper. Attached Figure Description

[0036] Figure 1 SEM image of the nanofibers obtained in Example 2. Detailed Implementation

[0037] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0038] To further illustrate the present invention, the following embodiments will be described in detail.

[0039] The experimental materials used in the following embodiments of the present invention are all commercially available products.

[0040] Among them, the preparation of PEO dispersant: Weigh 100 g of PEO and slowly add it to 100 L of water while stirring. After the addition is complete, stir for 30 min, filter it through a 100-mesh filter and set it aside for later use.

[0041] Preparation of CPAM retention aid: Weigh 100 g of CPAM and slowly add it to 100 L of water while stirring. After the addition is complete, stir for 60 min. Filter through a 100-mesh filter and set aside for later use.

[0042] Preparation of ACPAM retention aid: Weigh 100 g of ACPAM and slowly add it to 100 L of water while stirring. After the addition is complete, stir for 60 min. Filter through a 100-mesh filter and set aside for later use.

[0043] In the following examples, "short fiber" is an abbreviation for para-aramid short-cut fiber.

[0044] Preparation Example 1

[0045] This preparation example provides para-aramid chopped fibers with a length of 3-8 mm and a single filament diameter of 10-20 μm for use in subsequent examples and comparative examples, as follows:

[0046] Select commercially available oil-free aramid fibers with a strength ≥20 g / d, modulus ≥70 GPa, and monofilament diameter of 13 μm. Cut the fibers into short strands of 3 mm, 6 mm, and 8 mm as required.

[0047] Preparation Example 2

[0048] This preparation example provides para-aramid nanofibers for use in subsequent examples and comparative examples. The preparation method is as follows:

[0049] (1) Add N-methylpyrrolidone solvent to the reactor, and add a co-solubilizing salt while stirring under nitrogen protection. Heat to 80~100℃ to completely dissolve the co-solubilizing salt to obtain a first solution. The co-solubilizing salt is calcium chloride and the mass concentration of the co-solubilizing salt is 6.8%. Cool the first solution to 10℃ and add p-phenylenediamine to the first solution to obtain a second solution. The molar concentration of p-phenylenediamine in the second solution is 0.3 mol / L.

[0050] (2) After the p-phenylenediamine in the second solution from step (1) is completely dissolved, the temperature is lowered to -5℃, and terephthaloyl chloride melt is added to the second solution at a high temperature of 80~100℃. The molar ratio of terephthaloyl chloride to p-phenylenediamine is (1.007~1.012):1. A twin-screw reactor is used for reaction control, and the screw reaction temperature is controlled in sections at 40℃, 50℃, 70℃, 80℃, and 90℃ respectively. The polycondensation reaction temperature is maintained below 85℃, and the polymer outlet specific viscosity is controlled at 2.0~3.8. At this point, the reactants are in the form of a pale yellow, jelly-like gel. The gel is diluted with 5 times its weight of NMP to obtain a polymer with good flowability. The polymer and diluent are added to a high-speed fiber forming machine at a mass ratio of 1:10. The reactants are broken up and homogenized in a high-speed shearing machine to obtain a macroscopically uniform para-aramid nanofiber dispersion. The coagulant is a mixture of N-methylpyrrolidone and water at a mass ratio of 1:3. The high-speed shearing machine is a multi-stage emulsifying pump with adjustable gap, wherein the gap between the stator and the moving rotor of the high-speed shearing machine is less than 0.2 mm.

[0051] (3) The para-aramid nanofiber dispersion prepared in step (1) above is repeatedly washed with water using a high-speed centrifuge or a continuous countercurrent belt washer, and finally the para-aramid nanofiber is dispersed into a uniform slurry with deionized water under stirring conditions. The mass concentration of the para-aramid nanofiber slurry is (0.01~10)%.

[0052] The SEM image of the para-aramid nanofibers obtained in this preparation example is as follows: Figure 1 As shown, its characteristics are as follows: specific viscosity range of 2.0~3.8 dL / g, fiber diameter of 10~80 nm, and specific surface area ≥30 m². 2 / g, wet strength ≥2MPa, with a three-dimensional network structure.

[0053] Preparation Example 3

[0054] This preparation example provides para-aramid nanofibers for use in subsequent examples and comparative examples. The preparation method is as follows:

[0055] (1) Add N-methylpyrrolidone solvent to the reactor, and add a co-solubilizing salt while stirring under nitrogen protection. Heat to 80~100℃ to completely dissolve the co-solubilizing salt to obtain a first solution. The co-solubilizing salt is calcium chloride and the mass concentration of the co-solubilizing salt is 6.8%. Cool the first solution to 10℃ and add p-phenylenediamine to the first solution to obtain a second solution. The molar concentration of p-phenylenediamine in the second solution is 0.3 mol / L.

[0056] (2) After the p-phenylenediamine in the second solution from step (1) is completely dissolved, the temperature is lowered to -5℃, and terephthaloyl chloride melt is added to the second solution at a high temperature of 80~100℃. The molar ratio of terephthaloyl chloride to p-phenylenediamine is (1.007~1.012):1. A twin-screw reactor is used for reaction control, and the screw reaction temperature is controlled in sections at 40℃, 50℃, 60℃, 70℃, and 80℃ respectively. The polycondensation reaction temperature is maintained below 85℃, and the polymer outlet specific viscosity is controlled at 4.5. At this point, the reactants were in the form of a yellow, sludge-like gel. The gel was diluted with 5 times its weight of NMP to obtain a polymer with good flowability. The polymer and diluent were added to a high-speed fiber forming machine at a mass ratio of 1:10. The reactants were broken up and homogenized in a high-speed shearing machine to obtain a macroscopically uniform para-aramid nanofiber dispersion. The coagulant was a mixture of N-methylpyrrolidone and water at a mass ratio of 1:3. The high-speed shearing machine was a multi-stage emulsifying pump with adjustable gap, wherein the gap between the stator and the moving rotor of the high-speed shearing machine was less than 0.2 mm.

[0057] (3) The para-aramid nanofiber dispersion prepared in step (1) above is repeatedly washed with water using a high-speed centrifuge or a continuous countercurrent belt washer, and finally the para-aramid nanofiber is dispersed into a uniform slurry with deionized water under stirring conditions. The mass concentration of the para-aramid nanofiber slurry is (0.01~10)%.

[0058] The para-aramid nanofibers obtained in this preparation example have the following characteristics: specific viscosity range of 4.5 dL / g, fiber diameter of 10~80 nm, and specific surface area of ​​10~30 m². 2 / g, low wet strength, and has a three-dimensional network structure.

[0059] Preparation Example 4

[0060] This preparation example provides para-aramid nanofibers for use in subsequent examples and comparative examples. The preparation method is as follows:

[0061] (1) Add N-methylpyrrolidone solvent to the reactor, and add a co-solubilizing salt while stirring under nitrogen protection. Heat to 80~100℃ to completely dissolve the co-solubilizing salt to obtain a first solution. The co-solubilizing salt is calcium chloride and the mass concentration of the co-solubilizing salt is 6.8%. Cool the first solution to 10℃ and add p-phenylenediamine to the first solution to obtain a second solution. The molar concentration of p-phenylenediamine in the second solution is 0.3 mol / L.

[0062] (2) After the p-phenylenediamine in the second solution from step (1) is completely dissolved, the temperature is lowered to -5℃, and terephthaloyl chloride melt is added to the second solution at a high temperature of 80~100℃ to obtain the terephthaloyl chloride melt. The molar ratio of terephthaloyl chloride to p-phenylenediamine is (1.007~1.012):1. A twin-screw reactor is used for reaction control, and the screw reaction temperature is controlled in sections at 40℃, 50℃, 50℃, 60℃, and 70℃ respectively. The specific viscosity of the polymer outlet is controlled at 1.5. At this point, the reactants are in a syrupy colloid state. The polymer is diluted with 5 times its weight of NMP to obtain a polymer with good flowability. The polymer and diluent are added to a high-speed fiber forming machine at a mass ratio of 1:10. The reactants are broken up and homogenized in a high-speed shearing machine to obtain a macroscopically uniform para-aramid nanofiber dispersion. The coagulant is a mixture of N-methylpyrrolidone and water at a mass ratio of 1:3. The high-speed shearing machine is a multi-stage emulsifying pump with adjustable gap, wherein the gap between the stator and the moving rotor of the high-speed shearing machine is less than 0.2 mm.

[0063] (3) The para-aramid nanofiber dispersion prepared in step (1) above is repeatedly washed with water using a high-speed centrifuge or a continuous countercurrent belt washer, and finally the para-aramid nanofiber is dispersed into a uniform slurry with deionized water under stirring conditions. The mass concentration of the para-aramid nanofiber slurry is (0.01~10)%.

[0064] The para-aramid nanofibers obtained in this preparation example have the following characteristics: specific viscosity range of 1.5 dL / g, fiber diameter of 20~30 nm, and specific surface area ≥80 m². 2 / g, wet strength ≥2 MPa, with a three-dimensional network structure.

[0065] Example 1

[0066] Para-aramid short-cut fibers (6 mm in length) and para-aramid nanofibers (specific viscosity 2.6 dL / g, specific surface area 50.2 m²) were taken respectively. 2 / g, wet strength 2.5 MPa), and prepared into pulps at a ratio of 30%:70%, with the sum of the mass percentages of all components being 100%. Pulp A preparation: Short fibers, PEO dispersant, and silicone defoamer are added to a hydraulic pulper for dispersion. The amount of PEO dispersant added is 0.5 wt% (relative to the oven-dry weight of the short fibers), the amount of silicone defoamer stock solution added is 0.8 wt% (relative to the oven-dry weight of the short fibers), and the short fiber dispersion concentration is 0.1 wt%. Dispersion is carried out for 30 min and set aside. Pulp B preparation: Para-aramid nanofibers are dispersed, the concentration is adjusted to 0.1 wt%, and CPAM retention aid 0.5 wt% (relative to the oven-dry weight of the nanofibers) and ACPAM retention aid 0.3 wt% (relative to the oven-dry weight of the nanofibers) are added. The prepared pulps A and B are mixed in a mass ratio of 30% to 70% of para-aramid short fibers and para-aramid nanofibers. The mixture is then formed by a sheeter, pressed at 2 MPa for 1 min, dried at 150℃ for 5 min, and hot-pressed at 290℃ and 120 KN / m to obtain the finished paper.

[0067] Example 2

[0068] Para-aramid short-cut fibers (6 mm in length) and para-aramid nanofibers (specific viscosity 2.6 dL / g, specific surface area 50.2 m²) were taken respectively. 2 / g, wet strength 2.5 MPa), and mix the pulps in a 50%:50% ratio, with the sum of the mass percentages of all components being 100%. Pulp A preparation: Add short fibers, PEO dispersant, and silicone defoamer to a hydraulic pulper for dispersion. The amount of dispersant added is 0.5 wt% (relative to the oven-dry weight of the short fibers), the amount of silicone defoamer stock solution added is 0.8 wt% (relative to the oven-dry weight of the short fibers), and the short fiber dispersion concentration is 0.1 wt%. Disperse for 30 min and set aside. Pulp B preparation: Disperse para-aramid nanofibers, adjust the concentration to 0.1 wt%, and add 0.5 wt% (relative to the oven-dry weight of the nanofibers) of CPAM retention aid and 0.3 wt% (relative to the oven-dry weight of the nanofibers) of ACPAM retention aid. The prepared pulps A and B are mixed in a mass ratio of 50%:50% for para-aramid short fibers and para-aramid nanofibers. The mixture is then formed by a sheeter, pressed at 2MPa for 1 min, dried at 150℃ for 5 min, and hot-pressed at 290℃ and 120 KN / m to obtain the finished paper.

[0069] Example 3

[0070] Short-cut para-aramid fibers (6 mm in length) and para-aramid nanofibers (specific viscosity 2.6 dL / g, specific surface area 50.2 m²) were obtained. 2 / g, wet strength 2.5MPa), and mix the pulps in a 60%:40% ratio, with the sum of the mass percentages of all components being 100%. Pulp A preparation: Add short fibers, PEO dispersant, and silicone defoamer to a hydraulic pulper for dispersion. The amount of dispersant added is 0.5 wt% (relative to the oven-dry weight of short fibers), the amount of silicone defoamer stock solution added is 0.8 wt% (relative to the oven-dry weight of short fibers), and the short fiber dispersion concentration is 0.1 wt%. Disperse for 30 min and set aside. Pulp B preparation: Disperse para-aramid nanofibers, adjust the concentration to 0.1%, and add 0.5 wt% (relative to the oven-dry weight of nanofibers) of CPAM retention aid and 0.3 wt% (relative to the oven-dry weight of nanofibers) of ACPAM retention aid. Pulps A and B were mixed in a mass ratio of 60% chopped para-aramid fibers to 40% para-aramid nanofibers. The mixture was then formed in a sheeter, pressed at 2 MPa for 1 min, dried at 150℃ for 5 min, and hot-pressed at 290℃ and 120 KN / m to obtain the finished paper. The resulting paper was then tested.

[0071] Example 4

[0072] Para-aramid short-cut fibers (6 mm:3 mm = 75:25, mass ratio) and para-aramid nanofibers (specific viscosity 2.6 dL / g, specific surface area 50.2 m²) were taken separately. 2 / g, wet strength 2.5 MPa), and mix them in a 50%:50% ratio, with the sum of the mass percentages of all components being 100%. Pulp A preparation: Add short fibers, PEO dispersant, and silicone defoamer to a hydraulic pulper for dispersion. The dispersant addition is 0.5% (relative to the oven-dry weight of the short fibers), the silicone defoamer concentrate addition is 0.8% (relative to the oven-dry weight of the short fibers), and the short fiber dispersion concentration is 0.1%. Disperse for 30 min and set aside. Pulp B preparation: Disperse para-aramid nanofibers, adjust the concentration to 0.1%, and add 0.5% CPAM retention aid (relative to the oven-dry weight of the nanofibers) and 0.3% ACPAM retention aid (relative to the oven-dry weight of the nanofibers). Pulps A and B were mixed in a 50%:50% mass ratio of chopped para-aramid fibers to para-aramid nanofibers. The mixture was then formed in a sheeter, pressed at 2 MPa for 1 min, dried at 150℃ for 5 min, and hot-pressed at 290℃ and 120 KN / m to obtain the finished paper. The resulting paper was then tested.

[0073] Example 5

[0074] Para-aramid short-cut fibers (3 mm:8 mm = 75:25, mass ratio) and para-aramid nanofibers (specific viscosity 2.6 dL / g, specific surface area 50.2 m²) were taken separately.2 / g, wet strength 2.5 MPa), and mix them in a 50%:50% ratio, with the sum of the mass percentages of all components being 100%. Pulp A preparation: Add short fibers, PEO dispersant, and silicone defoamer to a hydraulic pulper for dispersion. The dispersant addition is 0.5% (relative to the oven-dry weight of the short fibers), the silicone defoamer concentrate addition is 0.8% (relative to the oven-dry weight of the short fibers), and the short fiber dispersion concentration is 0.1%. Disperse for 30 min and set aside. Pulp B preparation: Disperse para-aramid nanofibers, adjust the concentration to 0.1%, and add 0.5% CPAM retention aid (relative to the oven-dry weight of the nanofibers) and 0.3% ACPAM retention aid (relative to the oven-dry weight of the nanofibers). Pulps A and B were mixed in a 50%:50% mass ratio of chopped para-aramid fibers to para-aramid nanofibers. The mixture was then formed in a sheeter, pressed at 2 MPa for 1 min, dried at 150℃ for 5 min, and hot-pressed at 290℃ and 120 KN / m to obtain the finished paper. The resulting paper was then tested.

[0075] Example 6

[0076] Para-aramid short-cut fibers (6 mm:3 mm:8 mm = 75:20:5, mass ratio) and para-aramid nanofibers (specific viscosity 2.6 dL / g, specific surface area 50.2 m²) were taken separately. 2 / g, wet strength 2.5 MPa), and mix them in a 50%:50% ratio, with the sum of the mass percentages of all components being 100%. Pulp A preparation: Add short fibers, PEO dispersant, and silicone defoamer to a hydraulic pulper for dispersion. The dispersant addition is 0.5% (relative to the oven-dry weight of the short fibers), the silicone defoamer concentrate addition is 0.8% (relative to the oven-dry weight of the short fibers), and the short fiber dispersion concentration is 0.1%. Disperse for 30 min and set aside. Pulp B preparation: Disperse para-aramid nanofibers, adjust the concentration to 0.1%, and add 0.5% CPAM retention aid (relative to the oven-dry weight of the nanofibers) and 0.3% ACPAM retention aid (relative to the oven-dry weight of the nanofibers). Pulps A and B were mixed in a 50%:50% mass ratio of chopped para-aramid fibers to para-aramid nanofibers. The mixture was then formed in a sheeter, pressed at 2 MPa for 1 min, dried at 150℃ for 5 min, and hot-pressed at 290℃ and 120 KN / m to obtain the finished paper. The resulting paper was then tested.

[0077] Example 7

[0078] Para-aramid short-cut fibers (6 mm:3 mm:8 mm = 75:20:5) and para-aramid nanofibers (specific viscosity 3.2 dL / g, specific surface area 45.6 m²) were selected, respectively. 2 / g, wet strength 2.2 MPa), and mix them in a 50%:50% ratio, with the sum of the mass percentages of all components being 100%. Pulp A preparation: Add short fibers, dispersant PEO, and silicone defoamer to a hydraulic pulper for dispersion. The amount of dispersant added is 0.5% (relative to the oven-dry weight of short fibers), the amount of silicone defoamer stock solution added is 0.8% (relative to the oven-dry weight of short fibers), the short fiber dispersion concentration is 0.1%, disperse for 30 min, and set aside. Pulp B preparation: Disperse para-aramid nanofibers, adjust the concentration to 0.3%, and add 0.5% CPAM retention aid (relative to the oven-dry weight of nanofibers) and 0.1% ACPAM retention aid (relative to the oven-dry weight of nanofibers). The prepared pulps A and B are mixed in a mass ratio of 50%:50% for para-aramid short fibers and para-aramid nanofibers. The mixture is then formed in a sheeter, pressed at 2MPa for 1 min, dried at 150℃ for 5 min, and hot-pressed at 290℃ and 120 KN / m to obtain the finished paper.

[0079] Example 8

[0080] Para-aramid short-cut fibers (6mm:3mm:8mm=75:20:5) and para-aramid nanofibers (specific viscosity 3.2 dL / g, specific surface area 45.6 m²) were taken respectively. 2 / g, wet strength 2.2 MPa), and mix the pulps in a 50%:50% ratio, with the sum of the mass percentages of all components being 100%. Pulp A preparation: Add short fibers, dispersant PEO, and silicone defoamer to a hydraulic pulper for dispersion. The amount of dispersant added is 0.1% (relative to the oven-dry weight of the short fibers), and the amount of silicone defoamer stock solution added is 2% (relative to the oven-dry weight of the short fibers). The short fiber dispersion concentration is 0.1%, and disperse for 30 min. Pulp B preparation: Disperse para-aramid nanofibers, adjust the concentration to 0.1%, and add 0.3% CPAM retention aid (relative to the oven-dry weight of the nanofibers) and 0.5% ACPAM retention aid (relative to the oven-dry weight of the nanofibers). Pulps A and B were mixed in a 50%:50% mass ratio of chopped para-aramid fibers to para-aramid nanofibers. The mixture was then formed in a sheeter, pressed at 2 MPa for 1 min, dried at 150℃ for 5 min, and hot-pressed at 290℃ and 120 KN / m to obtain the finished paper. The resulting paper was then tested.

[0081] Example 9

[0082] Para-aramid short-cut fibers (6mm:3mm:8mm=75:20:5) and para-aramid nanofibers (specific viscosity 3.2 dL / g, specific surface area 45.6 m²) were taken respectively. 2 / g, wet strength 2.2 MPa), and mix them in a 50%:50% ratio, with the sum of the mass percentages of all components being 100%. Pulp A preparation: Add short fibers, PEO dispersant, and silicone defoamer to a hydraulic pulper for dispersion. The dispersant addition is 0.7% (relative to the oven-dry weight of the short fibers), the silicone defoamer concentrate addition is 3% (relative to the oven-dry weight of the short fibers), and the short fiber dispersion concentration is 0.1%. Disperse for 30 min and set aside. Pulp B preparation: Disperse para-aramid nanofibers, adjust the concentration to 0.1%, and add 0.2% CPAM retention aid (relative to the oven-dry weight of the nanofibers) and 0.6% ACPAM retention aid (relative to the oven-dry weight of the nanofibers). Pulps A and B were mixed in a 50%:50% mass ratio of chopped para-aramid fibers to para-aramid nanofibers. The mixture was then formed in a sheeter, pressed at 2 MPa for 1 min, dried at 150℃ for 5 min, and hot-pressed at 270℃ and 120 KN / m to obtain the finished paper. The resulting paper was then tested.

[0083] Example 10

[0084] Para-aramid short-cut fibers (6 mm in length) and para-aramid nanofibers (specific viscosity 4.5 dL / g, specific surface area 26.8 m²) were taken respectively. 2 / g, wet strength 0.5MPa), and mix them in a 50%:50% ratio, with the sum of the mass percentages of all components being 100%. Pulp A preparation: Add short fibers, PEO dispersant, and silicone defoamer to a hydraulic pulper for dispersion. The amount of PEO dispersant added is 0.5 wt% (relative to the oven-dry weight of the short fibers), the amount of silicone defoamer stock solution added is 0.8 wt% (relative to the oven-dry weight of the short fibers), and the short fiber dispersion concentration is 0.1 wt%. Disperse for 30 min and set aside. Pulp B preparation: Disperse para-aramid nanofibers, adjust the concentration to 0.1 wt%, and add 0.4 wt% CPAM retention aid (relative to the oven-dry weight of the nanofibers) and 0.3 wt% ACPAM retention aid (relative to the oven-dry weight of the nanofibers). The prepared pulps A and B were mixed in a mass ratio of 50%:50% for para-aramid chopped fibers and para-aramid nanofibers. The mixture was then formed using a sheet forming machine. However, because the nanofibers of this type have a high specific viscosity, i.e., a high molecular weight, the molecular chains coiled during the fiber formation process. This resulted in nanofibers with a small aspect ratio, which prevented the formation of a good network structure between the fiber clusters. Consequently, the wet strength was low, and the wet paper could not be transferred. Therefore, no sample was produced.

[0085] Example 11

[0086] Para-aramid short-cut fibers (6 mm in length) and para-aramid nanofibers (specific viscosity 1.5 dL / g, specific surface area 88.5 m²) were taken respectively. 2 / g, wet strength 3.5 MPa), and mix them in a 50%:50% ratio, with the sum of the mass percentages of all components being 100%. Pulp A preparation: Add short fibers, PEO dispersant, and silicone defoamer to a hydraulic pulper for dispersion. The amount of PEO dispersant added is 0.5 wt% (relative to the oven-dry weight of the short fibers), the amount of silicone defoamer stock solution added is 0.8 wt% (relative to the oven-dry weight of the short fibers), and the short fiber dispersion concentration is 0.1 wt%. Disperse for 30 min and set aside. Pulp B preparation: Disperse para-aramid nanofibers, adjust the concentration to 0.1 wt%, and add 0.5 wt% CPAM retention aid (relative to the oven-dry weight of the nanofibers) and 0.3 wt% ACPAM retention aid (relative to the oven-dry weight of the nanofibers). The prepared slurries A and B were mixed in a 50%:50% mass ratio of para-aramid chopped fibers to para-aramid nanofibers. The mixture was then formed using a sheet forming machine. Because this type of nanofiber has a low specific viscosity, i.e., a small molecular weight, there are more exposed -COOH groups during end-capping, resulting in good hydrophilicity. Even with the addition of a binary retention aid, it was still impossible to form large flocs. Therefore, during the sheet forming process, the high-density forming wire used was too small, and the nanofibers could not be effectively retained in the wet paper sheet, thus failing to form a bridging effect on the chopped fibers and hindering the transfer of the wet paper sheet. Consequently, no sample was produced.

[0087] Comparative Example 1 - No binary retention aid

[0088] Para-aramid chopped fibers (6 mm in length) and para-aramid nanofibers (specific viscosity 3.5 dL / g) were prepared into pulps at a ratio of 50%:50%, with the sum of the mass percentages of all components being 100%. Pulp A was prepared by dispersing the chopped fibers with PEO dispersant at a rate of 0.1 wt% (relative to the oven-dry weight of the chopped fibers), achieving a dispersion concentration of 0.05 wt%, and dispersing for 30 min. Pulp B was prepared by dispersing the nanofibers and adjusting the concentration to 0.1 wt%. Pulps A and B were then mixed at a mass ratio of 50%:50% para-aramid chopped fibers to para-aramid nanofibers. The mixture was formed using a sheet forming machine, pressed at 2 MPa for 1 min, dried at 150℃ for 5 min, and hot-pressed at 290℃ and 120 kN / m to obtain the finished paper.

[0089] Comparative Example 2 - Single CPAM Retention Aid

[0090] Para-aramid chopped fibers (6 mm in length) and para-aramid nanofibers were taken separately and mixed, with the sum of the mass percentages of the components being 100%. Pulp A was prepared by dispersing the chopped fibers with PEO dispersant at a rate of 0.5 wt% (relative to the oven-dry weight of the chopped fibers), achieving a dispersion concentration of 0.1 wt%, and dispersing for 30 min. Pulp B was prepared by dispersing the nanofibers, adjusting the concentration to 0.1 wt%, and adding 0.5 wt% CPAM retention aid (relative to the oven-dry weight of the nanofibers), where the molecular weight of CPAM was 8 million to 10 million. Pulps A and B were mixed in a 50%:50% mass ratio of para-aramid chopped fibers to para-aramid nanofibers. The mixture was then formed using a sheeter, pressed at 2 MPa for 1 min, dried at 150℃ for 5 min, and hot-pressed at 290℃ and 120 kN / m to obtain the finished paper.

[0091] Comparative Example 3 - Single CPAM Retention Aid

[0092] Para-aramid short-cut fibers (6 mm in length) and para-aramid nanofibers (specific viscosity 3.5 dL / g) were mixed in a 50%:50% ratio to prepare slurry. The sum of the mass percentages of all components was 100%. Slurry A was prepared by dispersing the short fibers with PEO dispersant at a rate of 0.5 wt% (relative to the oven-dry weight of the short fibers), achieving a dispersion concentration of 0.1 wt%, and dispersing for 30 min. Slurry B was prepared by dispersing the nanofibers to a concentration of 0.1 wt%, and adding 0.5% CPAM retention aid (relative to the oven-dry weight of the nanofibers). The CPAM had a molecular weight of 16 million to 18 million. The prepared pulps A and B are mixed in a mass ratio of 50%:50% for para-aramid short fibers and para-aramid nanofibers. The mixture is then formed by a sheeter, pressed at 2 MPa for 1 min, dried at 150℃ for 5 min, and hot-pressed at 290℃ and 120 kN / m to obtain the finished paper.

[0093] Comparative Example 4 - Single ACPAM Retention Aid

[0094] Para-aramid short-cut fibers (6 mm in length) and para-aramid nanofibers (specific viscosity 3.5 dL / g) were mixed in a 50%:50% ratio to prepare slurries, with the sum of the mass percentages of all components being 100%. Slurry A was prepared by dispersing the short fibers with PEO dispersant at a rate of 0.5 wt% (relative to the oven-dry weight of the short fibers), achieving a dispersion concentration of 0.1 wt%, and dispersing for 30 minutes. Slurry B was prepared by dispersing the nanofibers, adjusting the concentration to 0.1 wt%, and adding 0.5 wt% ACPAM retention aid (relative to the oven-dry weight of the nanofibers), wherein the molecular weight of ACPAM was 6-8 million. The prepared pulps A and B are mixed in a mass ratio of 50%:50% for para-aramid short fibers and para-aramid nanofibers. The mixture is then formed by a slanted wire paper machine, pressed at 2 MPa for 1 min, dried at 150℃ for 5 min, and hot-pressed at 290℃ and 120 kN / m to obtain the finished paper.

[0095] Performance testing

[0096] The finished paper obtained from the examples and comparative examples was tested using the following methods:

[0097] Basis weight: Refer to GB / T 451.2-2002, Paper and paperboard - Determination of basis weight;

[0098] Thickness: Refer to GB / T 451.3-2002, Paper and Paperboard - Determination of Thickness;

[0099] Tear strength: Refer to GB / T 455-2002, Paper and Paperboard - Determination of tear strength;

[0100] Tensile strength: Refer to GB / T 12914-2008, Paper and paperboard - Determination of tensile strength;

[0101] Air permeability: Refer to GB / T 458-2008, Paper and Paperboard - Determination of air permeability;

[0102] Water absorption: Refer to GB / T 1540-2002, Determination of water absorption of paper and paperboard (Cobb method);

[0103] Retention rate: Calculated using the solids content in white water and pulp concentration, referring to GB 5399-2004-T, Determination of pulp concentration.

[0104] The test results are shown in Table 1:

[0105] Table 1

[0106]

[0107] As can be seen from the comparison between Comparative Example 1 and the Examples, the addition of the CPAM and ACPAM dual retention aid system significantly improved both the basis weight and strength of the paper. This is mainly because the addition of CPAM can cause nanofibers to form micro-flocs, thereby improving the retention rate of nanofibers. The reasons are as follows: CPAM quickly neutralizes the negative charge on the surface of nanofibers, destabilizing them and initially agglomerating them into small flocs, which are adsorbed on the surface of fibers and initially agglomerated nanofiber flocs, providing cationic sites. ACPAM, on the other hand, combines with cationic polymers through anionic groups to form a three-dimensional network structure; or it can encapsulate the flocs through its huge specific surface area to form larger and stronger flocs, constructing a spatial network to capture tiny particles that have not been agglomerated by the first component, reducing the loss of nanofibers in subsequent processes, and ultimately improving the nanofiber retention rate, thereby improving the basis weight of the paper.

[0108] As can be seen from the comparison of Comparative Examples 1-4 and Examples, by using the two retention aids alone and in combination, the retention rate of using only one retention aid is significantly improved compared to the scheme without a retention aid, but the retention rate of a single retention aid is still at a low level.

[0109] As can be seen from the comparison between Examples 6-9 and Examples 1-3 and Examples 4-5, using para-aramid chopped fibers of three lengths can form a gradient distribution structure during the paper forming process, compared with para-aramid chopped fibers of a single length or a combination of two lengths. This enables the orderly interweaving, nesting, and interlocking of long and short fibers, significantly improving the internal density and structural uniformity of the paper, while also improving the bonding strength between fibers and the overall mechanical properties. It effectively reduces porosity defects during the forming process, taking into account the paper's strength, uniformity, and dimensional stability.

[0110] As can be seen from the examples and comparative examples, increasing the proportion of nanofibers in paper can coat short fibers, forming more hydrogen bonds, thereby improving the mechanical strength of the paper.

[0111] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for preparing a thin, lightweight para-aramid paper, characterized in that, Includes the following steps: Para-aramid chopped fiber slurry and para-aramid nanofiber slurry are mixed in a certain proportion. The resulting mixed slurry is then formed, pressed, dried and hot-pressed to obtain para-aramid paper. The para-aramid nanofiber slurry comprises para-aramid nanofibers and a binary retention aid; the binary retention aid comprises cationic polyacrylamide and amphoteric polyacrylamide.

2. The preparation method according to claim 1, characterized in that, The mass ratio of the cationic polyacrylamide to the amphoteric polyacrylamide is 1:(0.8~1.2); The number-average molecular weight of the cationic polyacrylamide is 8 million to 16 million. The number-average molecular weight of the zwitterionic polyacrylamide is 10 million to 20 million.

3. The preparation method according to claim 1 or 2, characterized in that, With the oven-dry weight of para-aramid nanofibers being 100%, the mass ratio of the para-aramid nanofibers to the binary retention aid is 1:(0.001~0.01).

4. The preparation method according to any one of claims 1 to 3, characterized in that, The para-aramid nanofibers have the following characteristics: Specific viscosity 2.0~3.8 dL / g, fiber diameter 10~80 nm, specific surface area ≥30 m² 2 / g, wet strength ≥2 MPa, with a three-dimensional network structure.

5. The preparation method according to any one of claims 1 to 4, characterized in that, The para-aramid chopped fiber slurry comprises para-aramid chopped fibers and a dispersing agent; The length of the para-aramid chopped fibers is 3~8 mm; The dispersing aid includes dispersants and / or defoamers; The dispersant includes any one or more of water-soluble polymeric dispersants, anionic dispersants, or nonionic surfactants; The defoamer includes any one or more of the following: silicone defoamers, polyether defoamers, or fatty alcohol defoamers.

6. The preparation method according to claim 5, characterized in that, Based on an oven-dry weight of 100% for para-aramid chopped fibers, the mass ratio of the para-aramid chopped fibers, dispersant, and defoamer is 1:(0.03~0.1):(0.03~0.1). The para-aramid chopped fibers include 3 mm chopped fibers, 6 mm chopped fibers and 8 mm chopped fibers; wherein the mass percentage of 3 mm chopped fibers is 0~30%, the mass percentage of 6 mm chopped fibers is 30~90%, and the mass percentage of 8 mm chopped fibers is 0~20%.

7. The preparation method according to any one of claims 1 to 6, characterized in that, The mass ratio of para-aramid chopped fiber in the para-aramid chopped fiber slurry to para-aramid nanofiber in the para-aramid nanofiber slurry is (30~70):(70~30).

8. The preparation method according to any one of claims 1 to 7, characterized in that, Before molding, the concentration of the mixed slurry is adjusted to 0.01%~0.2 wt%; The drying temperature is 80~180℃, and the time is 3~10 min; The hot pressing pressure is 100~150 kN / m, and the temperature is 220~330℃.

9. The thin para-aramid paper prepared by the method according to any one of claims 1 to 8, characterized in that, The basis weight of the thin para-aramid paper is 5~30 g / m³. 2 The thickness is 0.01~0.04 mm.

10. A honeycomb load-bearing product, characterized in that, It is made of the thin para-aramid paper prepared by the preparation method of any one of claims 1 to 8 or the thin para-aramid paper of claim 9.

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