A core-shell structure calcium carbonate aramid fiber composite paper and preparation method thereof

Abstract

The present invention relates to the technical field of high-performance paper-based materials, and particularly relates to a core-shell structured calcium carbonate aramid fiber composite paper and a preparation method thereof. The preparation method is as follows: calcium carbonate is uniformly dispersed in an aqueous chitosan acetate solution to obtain a dispersion; a flame retardant solution and an aqueous sodium silicate solution are added to the dispersion, and heating reaction is carried out. After the reaction is completed, core-shell structured calcium carbonate is obtained through solid-liquid separation; meta-aramid precipitated fibers, meta-aramid short-cut fibers, core-shell structured calcium carbonate and cationic chitosan are uniformly dispersed and then injected into a inclined screen forming device for papermaking to obtain a composite base paper; the composite base paper is subjected to multi-stage gradient hot pressing to obtain the core-shell structured calcium carbonate aramid fiber composite paper. The problems in the prior art that the aramid paper has insufficient mechanical properties due to weak fiber bonding force, poor dispersion of calcium carbonate fillers and weak interfacial bonding are solved, and at the same time, the defects of high energy consumption and easy fiber damage in the traditional hot pressing process are overcome.

Description

Technical Field

[0001] The present invention relates to a core-shell structure calcium carbonate aramid fiber composite paper and a preparation method thereof, belonging to the technical field of high-performance paper-based materials. Background Art

[0002] Aramid paper is a high-performance paper-based material prepared by wet papermaking and high-temperature hot pressing of aramid fibers, and has excellent properties such as high strength, high temperature resistance, high electrical insulation and flame retardancy. Due to its characteristics of light weight, high strength, high electrical insulation and flame retardancy, it can be used for aircraft honeycomb cores to meet the stringent requirements of lightweight and high compressive strength, and can also be used for transformer insulating paper, lithium battery diaphragms for new energy vehicles or gas diffusion layers for fuel cells. However, the industrial application of aramid paper still faces many difficulties. First, due to the smooth surface and chemical inertness of aramid fibers, the fibers are only bonded by van der Waals forces and a small amount of hydrogen bonds, resulting in low interlayer bonding strength and easy delamination. Second, aramid fibers are expensive, accounting for 60%-70% of the cost of aramid paper, and it is urgent to replace some fibers with low-cost fillers. Finally, traditional processes are difficult to achieve high strength, high flame retardancy and high aging resistance at the same time, and it is necessary to introduce multifunctional synergistic modification technology.

[0003] Calcium carbonate (CaCO3), as the most common inorganic filler in the paper industry, has an annual global consumption of more than 100 million tons, but it faces the following key problems when applied to aramid paper. First, conventional heavy calcium carbonate has a large particle size and a low specific surface area, and is easy to form "bridging" agglomeration between fibers, resulting in reduced paper formation. Second, the surface of calcium carbonate is hydrophilic, while aramid fibers are hydrophobic, and the interface between the two is weakly bonded, unable to effectively transfer stress. Finally, although amorphous calcium carbonate can improve the paper density, it is easy to crystallize and transform into calcite phase in a humid and hot environment, resulting in a decrease in mechanical properties.

[0004] At present, in the existing technologies, silane coupling agents (such as KH-550) are usually used to modify the surface of calcium carbonate. For example, in the patent with the publication number CN103772746B, silane coupling agent / hydroxystearic acid is used to modify the surface of calcium carbonate, making calcium carbonate more conducive to dispersion and compatibility in polymers, so as to achieve good filling and strengthening effects. However, this method has limitations. The addition amount of the coupling agent needs to be ≥3wt% to effectively improve the interfacial bonding. Excessive addition will lead to an increase in the brittleness of the paper, and the elongation at break will decrease significantly. Moreover, the silane coupling agent is easily decomposed during the high-temperature hot pressing process, resulting in a decrease in the interfacial bonding force, which cannot meet the requirements of the hot pressing process of aramid paper. At present, in the existing technologies, a multi-stage heating hot pressing process is usually adopted. For example, in the patent with the publication number CN 115159222B, the hot pressing is divided into low-temperature shaping, high-temperature hot pressing, and high-temperature shaping, which effectively reduces the thermal shrinkage rate of the paper and improves the flatness through this operation. However, high-temperature hot pressing easily causes the surface of aramid fibers to melt and reorganize, resulting in a serious decrease in the tear strength, and the hot pressing parameters fluctuate greatly between batches, easily leading to fluctuations in the strength of aramid paper and affecting the product consistency. At present, in the existing technologies, calcium carbonate is also the most common papermaking filler. For example, in the patent application with the publication number CN110080036A, microfibrillated cellulose (MFC) and inorganic fillers (such as calcium carbonate) are modified through anionic and cationic aids to prepare fiber composites. Through the synergistic effect of MFC and inorganic fillers, the dispersibility of the fillers is improved, so as to reduce the consumption of expensive fibers (such as aramid fibers) while maintaining a high filler content and reduce the cost. However, this scheme requires separate modification of MFC and inorganic fillers, with cumbersome steps, high energy consumption, and a high addition amount of aids, making it difficult to control the large-scale production cost.

[0005] In recent years, the rapid development of fields such as aerospace and new energy has also put forward higher requirements for the mechanical properties, insulation properties, and flame retardant properties of aramid paper. Moreover, with the large-scale application of aramid paper, it is urgent to reduce the production cost through filler technology modification. However, the current technology is difficult to simultaneously solve the problems of filler dispersibility, interfacial bonding force, and functional design, resulting in limited improvement in the comprehensive performance of aramid paper.

[0006] Therefore, it is urgent to develop a new type of modified calcium carbonate, composite paper, and preparation method to break through the above technical bottlenecks and meet the performance requirements of aramid paper in high-end fields. Summary of the Invention

[0007] Aiming at the deficiencies of the existing technologies, the present invention provides a core-shell structure calcium carbonate aramid fiber composite paper and a preparation method, which solve the problems of insufficient mechanical properties, poor dispersibility of calcium carbonate fillers, and weak interfacial bonding in aramid paper due to weak bonding force between fibers in the existing technologies, and at the same time overcome the defects of high energy consumption and easy fiber damage in the traditional hot pressing process.

[0008] The technical solution for the present invention to solve the above technical problems is as follows: A preparation method of a core-shell structured calcium carbonate aramid fiber composite paper, and the preparation method is as follows:

[0009] S1. Uniformly disperse calcium carbonate in an aqueous chitosan acetate solution to obtain a dispersion; dissolve a flame retardant in deionized water to obtain a flame retardant solution;

[0010] S2. Add the flame retardant solution and an aqueous sodium silicate solution to the dispersion, heat and react. After the reaction ends, perform solid-liquid separation to obtain core-shell structured calcium carbonate;

[0011] S3. Uniformly disperse meta-aramid precipitated fibers, meta-aramid short cut fibers, core-shell structured calcium carbonate, and cationic chitosan, and then inject them into a inclined wire former for papermaking to obtain a composite base paper;

[0012] S4. Perform multi-stage gradient hot pressing on the composite base paper to obtain a core-shell structured calcium carbonate aramid fiber composite paper.

[0013] Further, in step S1, the pH of the aqueous chitosan acetate solution is 4.5 - 5, and the mass concentration of chitosan in the aqueous chitosan acetate solution is 1 - 2 wt%.

[0014] Further, in the dispersion, the mass concentration of calcium carbonate is 5 - 10 wt%, in the flame retardant solution, the mass concentration of the flame retardant is 10 - 20 wt%, and in the aqueous sodium silicate solution, the mass concentration of sodium silicate is 5 - 10 wt%.

[0015] Further, the mass ratio of sodium silicate to calcium carbonate is 1:(2 - 5), and the addition amount of the flame retardant is 3 - 5 wt% of the mass of the composite paper.

[0016] Further, in step S1, the flame retardant is ammonium polyphosphate and melamine polyphosphate, and the mass ratio of ammonium polyphosphate to melamine polyphosphate is (1.8 - 2.2):1.

[0017] Further, in step S2, gradually dropwise add the flame retardant solution and the aqueous sodium silicate solution to the dispersion, the heating reaction temperature is 60 - 80 °C, and the reaction time is 1.5 - 2.5 h.

[0018] Further, in step S3, the mass ratio of meta-aramid precipitated fibers, meta-aramid short cut fibers, and core-shell structured calcium carbonate is (1 - 3):(6 - 8):(1 - 2);

[0019] The meta-aramid precipitated fibers, meta-aramid short cut fibers, core-shell structured calcium carbonate, and cationic chitosan are uniformly dispersed in deionized water to form a slurry, and the solid content of the slurry is 0.02 - 0.05 ‰;

[0020] The addition amount of the cationic chitosan is 0.3-0.8 wt% of the total mass of the meta-aramid precipitated fiber, meta-aramid short fiber, and core-shell structured calcium carbonate. The addition of the cationic chitosan makes the Zeta potential of the slurry ≥ +40 mV.

[0021] Further, in step S3, during the papermaking process, the turbulence intensity is controlled to make the slurry disperse evenly, and after vacuum dewatering, pressing, and high-temperature drying, the composite base paper is obtained.

[0022] The turbulence intensity is 8-10 m / s, and the temperature of the high-temperature drying is 70-90 °C.

[0023] Further, in step S4, the gradient hot pressing process has three stages: the first stage has a temperature of 70-90 °C, a pressure of 0.5-1.0 MPa, and a time of 5-10 min; the second stage has a temperature of 150-180 °C, a pressure of 3.0-4.0 MPa, and a time of 10-15 min; the third stage has a temperature of 220-250 °C, a pressure of 5.0-8.0 MPa, and a time of 8-10 min.

[0024] The present invention also discloses a core-shell structured calcium carbonate aramid fiber composite paper, which is prepared according to the preparation method described in the present invention.

[0025] The beneficial effects of the present invention are as follows:

[0026] The composite paper described in the present invention is composed of aramid short fibers, aramid precipitated fibers, core-shell structured calcium carbonate, and cationic chitosan. The core-shell structured calcium carbonate takes calcium carbonate as the core, is coated with a chitosan-silicate composite layer, and silica particles are in-situ loaded on the surface. The flame retardant is synchronously loaded in the shell layer during the coating reaction. The preparation method described in the present invention includes the coating synthesis of the core-shell structured calcium carbonate, ultra-low concentration inclined wire forming, and three-stage gradient hot pressing. The present invention realizes the preparation of the composite paper through the collaborative design of materials - processes - functions. The core-shell structured calcium carbonate aramid fiber composite paper has both high strength and flame retardant functions, and the industrialization cost is reduced. Specific Embodiments

[0027] The following makes a detailed description of the specific embodiments of the present invention. The present invention can be implemented in many other ways different from those described herein. Those skilled in the art can make similar improvements without departing from the connotation of the present invention. Therefore, the present invention is not limited by the specific embodiments disclosed.

[0028] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the technical field to which the present invention belongs. The terms used are only for describing the specific embodiments and do not limit the present invention.

[0029] A preparation method of a core-shell structured calcium carbonate aramid fiber composite paper, the preparation method being as follows:

[0030] S1. Uniformly disperse calcium carbonate in an aqueous chitosan acetate solution to obtain a dispersion; dissolve a flame retardant in deionized water to obtain a flame retardant solution;

[0031] S2. Add the flame retardant solution and an aqueous sodium silicate solution to the dispersion, heat and react. After the reaction ends, perform solid-liquid separation to obtain core-shell structured calcium carbonate;

[0032] S3. Uniformly disperse meta-aramid precipitated fibers, meta-aramid short fibers, core-shell structured calcium carbonate and cationic chitosan, and then inject them into a inclined wire forming device for papermaking to obtain a composite base paper;

[0033] S4. Perform multi-stage gradient hot pressing on the composite base paper to obtain a core-shell structured calcium carbonate aramid fiber composite paper.

[0034] Specifically, in step S1, the pH of the aqueous chitosan acetate solution is 4.5 - 5, and the mass concentration of chitosan in the aqueous chitosan acetate solution is 1 - 2 wt%.

[0035] Specifically, the mass concentration of calcium carbonate in the dispersion is 5 - 10 wt%, the mass concentration of the flame retardant in the flame retardant solution is 10 - 20 wt%, and the mass concentration of sodium silicate in the aqueous sodium silicate solution is 5 - 10 wt%.

[0036] Specifically, the mass ratio of sodium silicate to calcium carbonate is 1:(2 - 5), and the addition amount of the flame retardant is 3 - 5 wt% of the mass of the composite paper.

[0037] Specifically, in step S1, the flame retardant is ammonium polyphosphate and melamine polyphosphate, and the mass ratio of ammonium polyphosphate to melamine polyphosphate is (1.8 - 2.2):1.

[0038] More specifically, add calcium carbonate to the aqueous chitosan acetate solution and obtain a dispersion through ultrasonic treatment. The ultrasonic treatment time is 30 - 60 min.

[0039] Specifically, in step S2, dropwise add the flame retardant solution and the aqueous sodium silicate solution to the dispersion. The heating reaction temperature is 60 - 80 °C, and the reaction time is 1.5 - 2.5 h.

[0040] More specifically, in step S2, after the reaction ends, perform centrifugation and drying to obtain core-shell structured calcium carbonate. The centrifugation time is 10 - 20 min, and the drying temperature is 50 - 60 °C.

[0041] Specifically, in step S3, the mass ratio of meta-aramid precipitated fiber, meta-aramid short cut fiber, and core-shell structured calcium carbonate is (1-3):(6-8):(1-2);

[0042] The meta-aramid precipitated fiber, meta-aramid short cut fiber, core-shell structured calcium carbonate, and cationic chitosan are uniformly dispersed in deionized water to form a slurry, and the solid content of the slurry is 0.02-0.05‰;

[0043] The addition amount of the cationic chitosan is 0.3-0.8 wt% of the total mass of the meta-aramid precipitated fiber, meta-aramid short cut fiber, and core-shell structured calcium carbonate, and the addition of the cationic chitosan makes the Zeta potential of the slurry ≥ +40 mV.

[0044] Specifically, in step S3, during the papermaking process, the turbulence intensity is controlled to make the slurry uniformly dispersed, and after vacuum dewatering, pressing, and high-temperature drying, a composite base paper is obtained;

[0045] The turbulence intensity is 8-10 m / s, and the temperature of the high-temperature drying is 70-90 °C. If the turbulence intensity < 8 m / s, the distribution of fibers and fillers is uneven, the density deviation of the formed paper increases, and the interlayer bonding strength decreases. High turbulence intensity can provide shear force to break the physical entanglement of fibers and fillers and ensure uniform dispersion.

[0046] Specifically, in step S4, the gradient hot pressing process has three stages: the first stage has a temperature of 70-90 °C, a pressure of 0.5-1.0 MPa, and a time of 5-10 min; the second stage has a temperature of 150-180 °C, a pressure of 3.0-4.0 MPa, and a time of 10-15 min; the third stage has a temperature of 220-250 °C, a pressure of 5.0-8.0 MPa, and a time of 8-10 min.

[0047] The present invention also discloses a core-shell structured calcium carbonate aramid fiber composite paper, which is prepared according to the preparation method described in the present invention.

[0048] More specifically, the specific information of the raw materials used in the embodiments of the present invention is as follows, but this is not a limitation to the present invention.

[0049] Calcium carbonate: particle size 50-100 nm, purity > 99%;

[0050] Chitosan: deacetylation degree ≥ 90%, viscosity 100-200 mPa·s;

[0051] Sodium silicate: modulus 3.2-3.5;

[0052] Ammonium polyphosphate: degree of polymerization ≥ 1000;

[0053] Melamine polyphosphate: purity > 99%;

[0054] Meta-aramid precipitated fiber: specific surface area ≥ 20 m 2 / g, average length 0.1 - 0.5 mm, fibrillation degree ≥ 50%;

[0055] Meta-aramid short cut fiber: length 3 - 5 mm, diameter 10 - 15 um;

[0056] Cationic chitosan: degree of quaternization ≥ 80%, substitution degree 0.8 - 1.2, molecular weight range 50 - 200 kDa.

[0057] Example 1

[0058] Preparation of a core-shell structured calcium carbonate aramid fiber composite paper:

[0059] (1) Disperse precipitated calcium carbonate in an aqueous chitosan acetate solution with pH = 4.5 and ultrasonically treat for 30 min to obtain a dispersion with a calcium carbonate concentration of 5 wt%, and the mass concentration of chitosan in the aqueous chitosan acetate solution is 1 wt%.

[0060] (2) Dissolve ammonium polyphosphate and melamine polyphosphate with a mass ratio of 2:1 in deionized water to prepare a flame retardant solution with a concentration of 10 wt%.

[0061] (3) Dropwise add the flame retardant solution obtained in step (2) and a sodium silicate solution with a concentration of 8 wt% to the dispersion obtained in step (1), react at 70 °C for 2 h to obtain calcium carbonate with a silica / chitosan composite coating layer, the mass ratio of sodium silicate to calcium carbonate in the coating layer is 1:3, and the addition amount of the flame retardant is 4 wt% of the mass of the composite paper.

[0062] (4) Centrifuge and dry the calcium carbonate with a silica / chitosan composite coating layer obtained in step (3) at 50 °C for 15 min to obtain core-shell structured calcium carbonate.

[0063] (5) Uniformly disperse meta-aramid precipitated fiber, meta-aramid short cut fiber, and core-shell structured calcium carbonate in a weight ratio of 3:6:1, adjust the Zeta potential of the slurry to +50 mV by adding cationic chitosan, then inject it into a inclined wire forming device, control the turbulence intensity at 9 m / s to make the slurry disperse evenly, the solid content of the slurry is 0.02 ‰, and obtain a composite base paper through vacuum dewatering, pressing, and drying at 80 °C.

[0064] (6) The composite base paper obtained in step (5) is subjected to three-stage hot pressing: the temperature in the first stage is 70°C, the pressure is 0.8 MPa, and the time is 10 min; the temperature in the second stage is 160°C, the pressure is 3.5 MPa, and the time is 12 min; the temperature in the third stage is 240°C, the pressure is 6.0 MPa, and the time is 8 min. After the hot pressing treatment, a core-shell structured calcium carbonate aramid fiber composite paper is obtained.

[0065] Example 2

[0066] Preparation of a core-shell structured calcium carbonate aramid fiber composite paper:

[0067] (1) Precipitated calcium carbonate is dispersed in an aqueous chitosan acetate solution with pH = 4.5 and ultrasonicated for 30 min. A dispersion with a concentration of 5 wt% is obtained, and the mass concentration of chitosan in the aqueous chitosan acetate solution is 2 wt%.

[0068] (2) Ammonium polyphosphate and melamine polyphosphate with a mass ratio of 2:1 are dissolved in deionized water to prepare a flame retardant solution with a concentration of 10 wt%.

[0069] (3) The flame retardant solution obtained in step (2) and a sodium silicate solution with a concentration of 5 wt% are added dropwise to the dispersion obtained in step (1), and reacted at 70°C for 2 h to obtain calcium carbonate with a silica / chitosan composite coating layer. The mass ratio of sodium silicate to calcium carbonate in the coating layer is 1:2, and the addition amount of the flame retardant is 3 wt% of the mass of the composite paper.

[0070] (4) The calcium carbonate with a silica / chitosan composite coating layer obtained in step (3) is centrifugally dried at 50°C for 15 min to obtain core-shell structured calcium carbonate.

[0071] (5) Meta-aramid precipitated fibers, meta-aramid short cut fibers, and core-shell structured calcium carbonate are uniformly dispersed in a weight ratio of 3:6:1. By adding cationic chitosan, the Zeta potential of the slurry is adjusted to +50 mV, and then it is injected into a inclined screen forming device. The turbulence intensity is controlled at 9 m / s to make the slurry uniformly dispersed. The solid content of the slurry is 0.02‰. After vacuum dewatering, pressing, and drying at 80°C, a composite base paper is obtained.

[0072] (6) The composite base paper obtained in step (5) is subjected to three-stage hot pressing: the temperature in the first stage is 80°C, the pressure is 1.0 MPa, and the time is 8 min; the temperature in the second stage is 170°C, the pressure is 4.0 MPa, and the time is 15 min; the temperature in the third stage is 250°C, the pressure is 8.0 MPa, and the time is 10 min. After the hot pressing treatment, a core-shell structured calcium carbonate aramid fiber composite paper is obtained.

[0073] Example 3

[0074] Preparation of a core-shell structured calcium carbonate aramid fiber composite paper:

[0075] (1) Disperse precipitated calcium carbonate in an aqueous chitosan acetate solution with pH = 4.5 and ultrasonically treat it for 30 min to obtain a dispersion with a concentration of 5 wt%. The mass concentration of chitosan in the aqueous chitosan acetate solution is 1.5 wt%.

[0076] (2) Dissolve ammonium polyphosphate and melamine polyphosphate with a mass ratio of 2:1 in deionized water to prepare a flame retardant solution with a concentration of 10 wt%.

[0077] (3) Dropwise add the flame retardant solution obtained in step (2) and a sodium silicate solution with a concentration of 10 wt% to the dispersion obtained in step (1), and react at 70 °C for 2 h to obtain calcium carbonate with a silica / chitosan composite coating layer. The mass ratio of sodium silicate to calcium carbonate in the coating layer is 1:5, and the addition amount of the flame retardant is 5 wt% of the mass of the composite paper.

[0078] (4) Centrifuge and dry the calcium carbonate with a silica / chitosan composite coating layer obtained in step (3) at 60 °C for 15 min to obtain core-shell structured calcium carbonate.

[0079] (5) Uniformly disperse meta-aramid precipitated fibers, meta-aramid short cut fibers, and core-shell structured calcium carbonate in a weight ratio of 1:8:1. Add cationic chitosan to make the Zeta potential of the slurry reach +45 mV, then inject it into an inclined screen forming device, control the turbulence intensity at 9 m / s to make the slurry disperse evenly. The solid content of the slurry is 0.05‰, and after vacuum dewatering, pressing, and drying at 80 °C, a composite base paper is obtained.

[0080] (6) Subject the composite base paper obtained in step (5) to three-stage hot pressing: the first stage temperature is 75 °C, the pressure is 0.7 MPa, and the time is 12 min; the second stage temperature is 155 °C, the pressure is 3.2 MPa, and the time is 13 min; the third stage temperature is 230 °C, the pressure is 5.5 MPa, and the time is 9 min. After hot pressing, a core-shell structured calcium carbonate aramid fiber composite paper is obtained.

[0081] Example 4

[0082] Preparation of a core-shell structured calcium carbonate aramid fiber composite paper:

[0083] (1) Disperse precipitated calcium carbonate in an aqueous chitosan acetate solution with pH = 5.0 and ultrasonically treat it for 60 min to obtain a dispersion with a calcium carbonate concentration of 10 wt%. The mass concentration of chitosan in the aqueous chitosan acetate solution is 2 wt%.

[0084] (2) Ammonium polyphosphate and melamine polyphosphate with a mass ratio of 2.2:1 were dissolved in deionized water to prepare a flame retardant solution with a concentration of 20 wt%.

[0085] (3) The flame retardant solution obtained in step (2) and a sodium silicate solution with a concentration of 8 wt% were added dropwise to the dispersion obtained in step (1), and reacted at a high temperature of 80 °C for 1.5 h to obtain calcium carbonate with a silica / chitosan composite coating layer. The mass ratio of sodium silicate to calcium carbonate in the coating layer was 1:4, and the addition amount of the flame retardant was 4 wt% of the mass of the composite paper.

[0086] (4) The calcium carbonate with a silica / chitosan composite coating layer obtained in step (3) was centrifugally dried at 60 °C for 10 min to obtain core-shell structured calcium carbonate.

[0087] (5) Meta-aramid precipitated fibers, meta-aramid short cut fibers, and core-shell structured calcium carbonate were uniformly dispersed in a weight ratio of 3:6:1. By adding cationic chitosan, the Zeta potential of the slurry was adjusted to +50 mV, and then the slurry was injected into an inclined wire forming device. The turbulence intensity was controlled at 10 m / s to make the slurry evenly dispersed. The solid content of the slurry was 0.04‰. After vacuum dewatering, pressing, and drying at a high temperature of 90 °C, a composite base paper was obtained.

[0088] (6) The composite base paper obtained in step (5) was subjected to three-stage hot pressing treatment: the first stage temperature was 90 °C, the pressure was 0.5 MPa, and the time was 5 min; the second stage temperature was 150 °C, the pressure was 3.5 MPa, and the time was 13 min; the third stage temperature was 220 °C, the pressure was 7.0 MPa, and the time was 9 min. After the hot pressing treatment, a core-shell structured calcium carbonate aramid fiber composite paper was obtained.

[0089] Example 5

[0090] Preparation of a core-shell structured calcium carbonate aramid fiber composite paper:

[0091] (1) Precipitated calcium carbonate was dispersed in a chitosan acetic acid aqueous solution with pH = 5.0 and ultrasonicated for 30 min to obtain a dispersion with a calcium carbonate concentration of 6 wt%. The mass concentration of chitosan in the chitosan acetic acid aqueous solution was 1 wt%.

[0092] (2) Ammonium polyphosphate and melamine polyphosphate with a mass ratio of 1.8:1 were dissolved in deionized water to prepare a flame retardant solution with a concentration of 20 wt%.

[0093] (3) Add dropwise the flame retardant solution obtained in step (2) and the sodium silicate solution with a concentration of 7 wt% to the dispersion obtained in step (1), and react at a high temperature of 60 °C for 2.5 h to obtain calcium carbonate with a silica / chitosan composite coating layer. The mass ratio of sodium silicate to calcium carbonate in the coating layer is 1:3, and the addition amount of the flame retardant is 4 wt% of the mass of the composite paper.

[0094] (4) Centrifuge and dry the calcium carbonate with a silica / chitosan composite coating layer obtained in step (3) at 60 °C for 10 min to obtain core-shell structured calcium carbonate.

[0095] (5) Uniformly disperse meta-aramid precipitated fibers, meta-aramid short cut fibers, and core-shell structured calcium carbonate in a weight ratio of 2:6:2. Add cationic chitosan to make the Zeta potential of the slurry reach +50 mV, and then inject it into a inclined wire forming device. Control the turbulence intensity at 8 m / s to make the slurry disperse evenly. The solid content of the slurry is 0.03‰. After vacuum dewatering, pressing, and drying at a high temperature of 70 °C, a composite base paper is obtained.

[0096] (6) Subject the composite base paper obtained in step (5) to three-stage hot pressing treatment: the temperature in the first stage is 70 °C, the pressure is 0.8 MPa, and the time is 8 min; the temperature in the second stage is 180 °C, the pressure is 3.0 MPa, and the time is 10 min; the temperature in the third stage is 240 °C, the pressure is 5.0 MPa, and the time is 10 min. After the hot pressing treatment, a core-shell structured calcium carbonate aramid fiber composite paper is obtained.

[0097] Comparative Example 1

[0098] Prepare a core-shell structured calcium carbonate aramid fiber composite paper by the same method as in Example 1, except that in this Comparative Example 1, the pH of the chitosan acetic acid aqueous solution is 3.

[0099] Comparative Example 2

[0100] Prepare a core-shell structured calcium carbonate aramid fiber composite paper by the same method as in Example 1, except that in this Comparative Example 2, the pH of the chitosan acetic acid aqueous solution is 5.8.

[0101] Comparative Example 3

[0102] Prepare a core-shell structured calcium carbonate aramid fiber composite paper by the same method as in Example 1, except that in this Comparative Example 3, the dosage ratio of sodium silicate is increased, and the mass ratio of sodium silicate to calcium carbonate is 1:1.

[0103] Comparative Example 4

[0104] Prepare a core-shell structured calcium carbonate aramid fiber composite paper by the same method as in Example 1, except that in this Comparative Example 4, the reaction temperature in step (3) is reduced, and the reaction temperature is 40 °C.

[0105] Comparative Example 5

[0106] The core-shell structured calcium carbonate aramid fiber composite paper was prepared by the same method as in Example 1, except that in this Comparative Example 5, the reaction temperature in step (3) was increased to 80 °C.

[0107] Comparative Example 6

[0108] The core-shell structured calcium carbonate aramid fiber composite paper was prepared by the same method as in Example 1, except that in this Comparative Example 6, the addition amount of the flame retardant was reduced, and the addition amount of the flame retardant was 2 wt% of the mass of the composite paper.

[0109] Comparative Example 7

[0110] The core-shell structured calcium carbonate aramid fiber composite paper was prepared by the same method as in Example 1, except that in this Comparative Example 7, the Zeta potential of the slurry in step (5) was 30 mV.

[0111] Comparative Example 8

[0112] The core-shell structured calcium carbonate aramid fiber composite paper was prepared by the same method as in Example 1, except that in this Comparative Example 8, the gradient hot pressing parameters in step (6) were changed. Specifically: the temperature in the first stage was 60 °C, the pressure was 0.3 MPa, and the time was 10 min; the temperature in the second stage was 120 °C, the pressure was 2 MPa, and the time was 12 min; the temperature in the third stage was 200 °C, the pressure was 4.0 MPa, and the time was 8 min.

[0113] Comparative Example 9

[0114] The core-shell structured calcium carbonate aramid fiber composite paper was prepared by the same method as in Example 1, except that in this Comparative Example 9, only ammonium polyphosphate was used as the flame retardant, and melamine polyphosphate was not added.

[0115] Comparative Example 10

[0116] The core-shell structured calcium carbonate aramid fiber composite paper was prepared by the same method as in Example 1, except that in this Comparative Example 9, only melamine polyphosphate was used as the flame retardant, and ammonium polyphosphate was not added.

[0117] The core-shell structured calcium carbonate aramid fiber composite papers prepared in the above examples and comparative examples were subjected to performance tests. The specific test results are shown in Table 1 below. The test methods involved are as follows: After subjecting the composite paper to constant temperature and humidity treatment, the basis weight test standard is GB / T 451.2~2023; the thickness test standard is GB / T 451.3~2002; the formation test method is to use a 3D paper sheet analyzer. The paper sheet is fixed on a rotating glass drum, and the light source and the photodiode detector are respectively installed on both sides of the drum and move synchronously in the same direction to complete the scanning of the paper sheet; the tensile strength test standard is GB / T 12914~2018; the interlayer bonding strength test standard is TAPPI / ANSI T 569 om-22; the flame retardant performance test standard is GB / T 14656~2009; the compressive strength test standard is GB / T 1408.1~2016.

[0118] Table 1 Performance test results of the core-shell structured calcium carbonate aramid fiber composite paper

[0119]

[0120] It can be seen from the above table data that: The core-shell structured calcium carbonate aramid fiber composite papers prepared by the preparation method described in the present invention in Examples 1-5 can simultaneously have flame retardant performance and high strength performance. It solves the problems of insufficient mechanical properties, poor dispersion of calcium carbonate fillers and weak interfacial bonding in aramid papers in the prior art due to weak fiber-fiber bonding force.

[0121] It can be seen from the comparison of the experimental results of Comparative Example 1, Comparative Example 2 and Example 1 that: When the pH of the chitosan acetic acid aqueous solution is <4.5 or >5.0, the degree of protonation of chitosan is insufficient, the electrostatic adsorption is weakened, the coating layer is uneven, the Zeta potential of the core-shell structured calcium carbonate decreases, the agglomeration size of the filler increases, and the interfacial bonding strength decreases. When the pH of the chitosan acetic acid aqueous solution is 4.5-5.0, the amino group is fully protonated to -NH 3+ , and it combines with the negative charge (-OH) on the surface of calcium carbonate through electrostatic attraction. Deviation of pH will lead to a decrease in the adsorption amount, resulting in a discontinuous coating layer, and ultimately affecting the strength performance and compressive strength performance of the composite paper.

[0122] From the comparison of the experimental results between Comparative Example 3 and Example 1, it can be seen that: if the dosage ratio of sodium silicate increases, the generation rate of SiO2 particles is too fast, forming independent aggregates, resulting in uneven coating thickness. Of course, if the dosage ratio of sodium silicate is too small, it will lead to insufficient SiO2 loading, too thin coating layer, and weak interfacial bonding. Eventually, both will affect the strength performance and compressive strength performance of the composite paper. It can be seen from this that: the concentration and dosage of sodium silicate directly affect the hydrolysis and condensation rate. Excess leads to particle aggregation, and insufficiency fails to form a continuous protective layer. Using the dosage of sodium silicate defined in the present invention is more conducive to obtaining a composite paper with excellent comprehensive performance.

[0123] From the comparison of the experimental results between Comparative Example 4, Comparative Example 5 and Example 1, it can be seen that: too low or too high core-shell reaction temperature will lead to a decrease in the strength performance and compressive strength performance of the final composite paper. When the temperature < 60 °C, SiO2 particles are not fully formed, the coating layer is loose, and the filler dispersion is poor; when the temperature > 80 °C, chitosan decomposes, the coating layer carbonizes, and the Zeta potential decreases. During the core-shell reaction process, temperature and time control the hydrolysis and condensation reaction rate. Too low temperature or insufficient time leads to incomplete reaction, and too high temperature destroys the organic-inorganic hybrid structure.

[0124] From the comparison of the experimental results between Comparative Example 6 and Example 1, it can be seen that: reducing the dosage of the flame retardant will result in LOI < 28%, which cannot meet the flame retardant standard. In addition, it was also found in the experiment that if the dosage of the flame retardant is too high, the migration rate of the flame retardant increases, and the tensile strength loss is large. Because excessive flame retardant destroys the shell structure, resulting in interfacial defects; insufficiency leads to unqualified flame retardant efficiency. Therefore, using the dosage of the flame retardant defined in the present invention is more conducive to obtaining a composite paper with excellent comprehensive performance.

[0125] From the comparison of the experimental results between Comparative Example 7 and Example 1, it can be seen that: if the Zeta potential < +40 mV, the electrostatic repulsion of the particles is insufficient, the slurry uniformity is poor, and the tensile strength of the formed paper decreases. The slurry with a high Zeta potential (≥ +40 mV) can stabilize the dispersion system through electrostatic repulsion, avoid the flocculation of the filler and fiber, and finally make the composite paper have good flame retardant performance while improving the strength performance of the composite paper.

[0126] It can be seen from the comparison of the experimental results of Comparative Example 8 and Example 1 that: if the stepped hot pressing conditions defined in the present invention are not adopted, the strength performance of the composite paper will decrease significantly. When the temperature in the first stage < 70 °C: the molecular chains of chitosan are not activated, and the preliminary combination of fibers and fillers cannot be promoted. When the pressure < 0.5 MPa: the contact between fibers and fillers is insufficient, and the interfacial porosity increases. When the temperature in the second stage < 150 °C: the chemical cross-linking reaction is not triggered, and the three-dimensional network structure is not formed. When the pressure < 3.0 MPa: the fiber-filler interfacial bonding is weak, and the tensile strength decreases. When the temperature in the third stage < 220 °C: the material is not completely shaped, and the strength retention rate after wet heat aging < 85%. When the pressure < 5.0 MPa: the structural compactness is insufficient, and the dielectric strength decreases.

[0127] It can be seen from the comparison of the experimental results of Comparative Example 9, Comparative Example 5 and Example 1 that: when ammonium polyphosphate and melamine polyphosphate are used in combination as flame retardants, it is more conducive to improving the performance of the composite paper. Because ammonium polyphosphate has insufficient thermal stability and is easily decomposed at high temperatures, the flame retardant effect drops sharply, and the formed carbon layer is loose and porous and easily penetrated by flames. If the addition amount is increased, it will lead to an increase in the brittleness of the material. Melamine polyphosphate mainly relies on gas-phase flame retardancy, has weak ability to form carbon by condensation, has insufficient inhibitory effect on materials with high ignition energy, and is expensive. After being used in combination, compared with being used alone, the flame retardant performance is further improved, the density of the carbon layer increases, the total dosage of the composite system can be reduced by 30%, and the CO release amount is reduced.

[0128] The technical features of the above-described embodiments can be combined arbitrarily. For the sake of brevity of description, all possible combinations of the various technical features in the above embodiments are not exhaustively listed. However, as long as there is no contradiction in the combination of these technical features, it should be considered to be within the scope described in this specification.

[0129] For those of ordinary skill in the art, without departing from the concept of the present invention, several deformations and improvements can still be made, and these all belong to the protection scope of the present invention. The protection scope of the present invention is subject to the appended claims.

Claims

1. A method for preparing a core-shell structured calcium carbonate aramid fiber composite paper, characterized in that: The preparation method is: S1, dispersing calcium carbonate uniformly in a chitosan acetic acid aqueous solution to obtain a dispersion; dissolving a flame retardant in deionized water to obtain a flame retardant solution; the pH of the chitosan acetic acid aqueous solution is 4.5-5, and the flame retardant is ammonium polyphosphate and melamine polyphosphate; S2, adding a flame retardant solution and a sodium silicate aqueous solution to the dispersion, heating for reaction, and after the reaction is completed, obtaining a core-shell structure calcium carbonate by solid-liquid separation; the heating reaction temperature is 60-80°C; the mass ratio of the sodium silicate to the calcium carbonate is 1:(2-5), and the addition amount of the flame retardant is 3-5wt% of the mass of the composite paper; S3, uniformly dispersing meta-aramid fibrils, meta-aramid chopped fibers, core-shell calcium carbonate and cationic chitosan, and injecting them into an inclined mesh forming device for papermaking to obtain composite base paper; The meta-aramid fiber precipitation, the meta-aramid short fibers, the core-shell calcium carbonate and the cationic chitosan are uniformly dispersed in deionized water to form a slurry, and the addition of the cationic chitosan makes the slurry Zeta potential ≥ +40mV; S4, subjecting the composite base paper to multi-stage gradient hot pressing to obtain a core-shell structured calcium carbonate aramid fiber composite paper; In step S4, the gradient hot pressing process is provided with three stages: the temperature of the first stage is 70-90°C, the pressure is 0.5-1.0MPa, and the time is 5-10min; the temperature of the second stage is 150-180°C, the pressure is 3.0-4.0MPa, and the time is 10-15min; the temperature of the third stage is 220-250°C, the pressure is 5.0-8.0MPa, and the time is 8-10min.

2. The method for preparing a core-shell calcium carbonate aramid fiber composite paper according to claim 1, characterized in that: In step S1, the mass concentration of chitosan in the chitosan acetic acid aqueous solution is 1-2wt%.

3. The method for preparing a core-shell calcium carbonate aramid fiber composite paper according to claim 1, characterized in that: The mass concentration of calcium carbonate in the dispersion is 5-10wt%, the mass concentration of the flame retardant in the flame retardant solution is 10-20wt%, and the mass concentration of sodium silicate in the sodium silicate aqueous solution is 5-10wt%.

4. The method for preparing a core-shell calcium carbonate aramid fiber composite paper according to claim 1, characterized in that: In step S1, the mass ratio of the ammonium polyphosphate to the melamine polyphosphate is (1.8-2.2):

1.

5. The method for preparing a core-shell calcium carbonate aramid fiber composite paper according to claim 1, characterized in that: In step S2, the flame retardant solution and the sodium silicate aqueous solution are added dropwise to the dispersion, and the reaction time is 1.5-2.5 hours.

6. The method for preparing a core-shell structured calcium carbonate aramid fiber composite paper according to claim 1, characterized in that: In step S3, the mass ratio of meta-aramid fibrils, meta-aramid chopped fibers, and core-shell calcium carbonate is (1-3): (6-8): (1-2); The solid content of the slurry is 0.02-0.05‰; The amount of the cationic chitosan added is 0.3-0.8wt% of the total mass of the meta-aramid fibrids, meta-aramid chopped fibers, and core-shell calcium carbonate.

7. The method for preparing a core-shell calcium carbonate aramid fiber composite paper according to claim 1, characterized in that: In step S3, during the papermaking process, the turbulence intensity is controlled to make the slurry dispersed evenly, and the composite base paper is obtained by vacuum dehydration, pressing, and high-temperature drying; The turbulence intensity is 8-10m / s, and the high temperature drying temperature is 70-90℃.

8. A core-shell structured calcium carbonate aramid fiber composite paper, characterized in that: The core-shell structured calcium carbonate aramid fiber composite paper is prepared according to the preparation method according to any one of claims 1-7.

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