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

Abstract

The invention relates to the technical field of high-performance paper-based materials, in particular to core-shell structure calcium carbonate aramid fiber composite paper and a preparation method thereof.The preparation method comprises the steps that calcium carbonate is evenly dispersed in a chitosan acetic acid aqueous solution to obtain dispersion liquid; adding a flame retardant solution and a sodium silicate aqueous solution into the dispersion liquid, carrying out heating reaction, and after the reaction is finished, carrying out solid-liquid separation to obtain core-shell structure calcium carbonate; the preparation method comprises the following steps: uniformly dispersing meta-aramid fibrid, meta-aramid chopped fiber, calcium carbonate with a core-shell structure and cationic chitosan, injecting into inclined wire forming equipment, and carrying out papermaking to obtain composite raw paper; and performing multi-stage gradient hot pressing on the composite raw paper to obtain the calcium carbonate aramid fiber composite paper with the core-shell structure. The problems of insufficient mechanical property, poor calcium carbonate filler dispersity and weak interface bonding caused by weak bonding force between fibers of aramid paper in the prior art are solved, and the defects of high energy consumption and easiness in fiber damage in a traditional hot pressing process are overcome.

Description

Technical Field

[0001] The invention relates to a core-shell structured 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 made of aramid fiber through wet papermaking and high-temperature hot pressing. It has excellent properties such as high strength, high temperature resistance, high electrical insulation and flame retardancy. Due to its light weight, high strength, high electrical insulation and flame retardancy, it can be used for aircraft honeycomb core materials to meet its stringent requirements of lightweight and high compressive strength. It can also be used for transformer insulation paper and new energy vehicle lithium battery diaphragm or fuel cell gas diffusion layer. However, the industrial application of aramid paper still faces many difficulties. First, due to the smooth surface and chemical inertness of aramid fiber, the fibers are only bonded by van der Waals force and a small amount of hydrogen bonds, resulting in low interlayer bonding strength and easy stratification. Secondly, aramid fiber is expensive, accounting for 60%-70% of the cost of aramid paper, and it is urgent to replace some fibers with low-cost fillers. Finally, it is difficult for traditional processes 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 (CaCO 3 ) As the most common inorganic filler in the papermaking industry, the global annual consumption exceeds 100 million tons, but it faces the following key problems when used in aramid paper. First, conventional heavy calcium carbonate has a large particle size and a low specific surface area, which easily forms "bridges" between fibers and agglomerates, resulting in reduced paper uniformity. Secondly, the surface of calcium carbonate is hydrophilic, while aramid fibers are hydrophobic, and the interface between the two is weak and cannot effectively transfer stress. Finally, although amorphous calcium carbonate can increase the density of paper, it is easy to crystallize and transform into the calcite phase in a hot and humid environment, resulting in a decrease in mechanical properties.

[0004] At present, in the existing technology, silane coupling agent (such as KH-550) is usually used to modify the surface of calcium carbonate. For example, in the patent with publication number CN103772746B, silane coupling agent / hydroxystearic acid is used to modify the surface of calcium carbonate, so that calcium carbonate is more conducive to dispersion and compatibility in the polymer, thereby achieving a good filling and reinforcement effect. However, this method has limitations. The amount of coupling agent added must be ≥3wt% to effectively improve the interface bonding. Excessive addition will increase the brittleness of the paper, which will be obvious at the elongation at break. In addition, the silane coupling agent is easy to decompose during the high-temperature hot pressing process, resulting in a decrease in the interfacial bonding force, which cannot meet the hot pressing process requirements of aramid paper. At present, in the existing technology, a multi-stage heating hot pressing process is usually used. For example, in the patent with publication number CN 115159222B, hot pressing is divided into low-temperature setting, high-temperature hot pressing and high-temperature setting. This operation effectively reduces the thermal shrinkage of the paper and improves the flatness. However, high-temperature hot pressing can easily cause the surface of aramid fibers to melt and reorganize, resulting in a serious decrease in tear strength. In addition, the hot pressing parameters fluctuate greatly between batches, which can easily cause fluctuations in the strength of aramid paper and affect product consistency. In the current existing technology, calcium carbonate is also the most common papermaking filler. For example, in the patent application with publication number CN110080036A, microfibrillated cellulose (MFC) and inorganic fillers (such as calcium carbonate) are modified by anionic and cationic additives to prepare fiber composite materials. The synergistic effect of MFC and inorganic fillers improves the dispersibility of the filler, thereby reducing the amount of expensive fibers (such as aramid fibers) while maintaining a high filler content, thereby reducing costs. However, this solution requires the modification of MFC and inorganic fillers separately, which is cumbersome, has high energy consumption, and requires a high amount of additives, making it difficult to control the cost of large-scale production.

[0005] In recent years, the rapid development of aerospace, new energy and other fields has also put forward higher requirements for the mechanical properties, insulation properties and flame retardant properties of aramid paper. With the large-scale application of aramid paper, it is urgent to reduce production costs through filler technology modification. However, it is difficult to solve the problems of filler dispersion, interface bonding and functional design at the same time with current technology, resulting in limited improvement in the comprehensive performance of aramid paper.

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

[0007] In view of the deficiencies in the prior art, the present invention provides a core-shell structured calcium carbonate aramid fiber composite paper and a preparation method thereof, which solves the problems of insufficient mechanical properties of aramid paper in the prior art due to weak inter-fiber bonding, poor dispersibility of calcium carbonate filler and weak interface bonding, while overcoming the defects of high energy consumption and easy damage to fibers in the traditional hot pressing process.

[0008] The technical solution of the present invention to solve the above technical problems is as follows: a method for preparing a core-shell structure calcium carbonate aramid fiber composite paper, the preparation method comprising: 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; 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 core-shell calcium carbonate by solid-liquid separation; 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; S4. The composite base paper is subjected to multi-stage gradient hot pressing to obtain a core-shell structured calcium carbonate aramid fiber composite paper.

[0009] Furthermore, in step S1, the pH of the chitosan acetic acid aqueous solution is 4.5-5, and the mass concentration of chitosan in the chitosan acetic acid aqueous solution is 1-2wt%.

[0010] Furthermore, 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%.

[0011] Furthermore, the mass ratio of the sodium silicate to the calcium carbonate is 1:(2-5), and the added amount of the flame retardant is 3-5wt% of the mass of the composite paper.

[0012] Furthermore, in step S1, the flame retardant is ammonium polyphosphate and melamine polyphosphate, and the mass ratio of the ammonium polyphosphate to the melamine polyphosphate is (1.8-2.2):1.

[0013] Furthermore, in step S2, the flame retardant solution and the sodium silicate aqueous solution are added dropwise to the dispersion, and the heating reaction temperature is 60-80° C. and the reaction time is 1.5-2.5 h.

[0014] Furthermore, 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 meta-aramid fibrids, meta-aramid chopped fibers, core-shell calcium carbonate, and cationic chitosan are uniformly dispersed in deionized water to form a slurry, wherein 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 fibrils, meta-aramid chopped fibers, and core-shell calcium carbonate. The addition of the cationic chitosan makes the slurry Zeta potential ≥+40mV.

[0015] Furthermore, 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℃.

[0016] Furthermore, 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.

[0017] The invention also discloses a core-shell structure calcium carbonate aramid fiber composite paper. The core-shell structure calcium carbonate aramid fiber composite paper is prepared according to the preparation method of the invention.

[0018] The beneficial effects of the present invention are: The composite paper of the present invention is composed of aramid short-cut fibers, aramid precipitated fibers, core-shell calcium carbonate and cationic chitosan. The core-shell calcium carbonate has calcium carbonate as the core, 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 of the present invention includes coating synthesis of core-shell calcium carbonate, ultra-low concentration oblique net forming, and three-stage gradient hot pressing. The present invention realizes the preparation of composite paper through material-process-function collaborative design. The core-shell calcium carbonate aramid fiber composite paper has both high strength and flame retardant functions, and the industrialization cost is reduced. DETAILED DESCRIPTION

[0019] The specific implementation of the present invention is described in detail below. The present invention can be implemented in many other ways than those described herein, and those skilled in the art can make similar improvements without violating the connotation of the present invention, so the present invention is not limited by the specific embodiments disclosed.

[0020] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art to which the present invention belongs. The terms used are only for describing specific embodiments and are not intended to limit the present invention.

[0021] A method for preparing a core-shell structured calcium carbonate aramid fiber composite paper, the preparation method comprising: 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; 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 core-shell calcium carbonate by solid-liquid separation; 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; S4. The composite base paper is subjected to multi-stage gradient hot pressing to obtain a core-shell structured calcium carbonate aramid fiber composite paper.

[0022] Specifically, in step S1, the pH of the chitosan acetic acid aqueous solution is 4.5-5, and the mass concentration of chitosan in the chitosan acetic acid aqueous solution is 1-2wt%.

[0023] Specifically, 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%.

[0024] Specifically, the mass ratio of the sodium silicate to the calcium carbonate is 1:(2-5), and the added amount of the flame retardant is 3-5wt% of the mass of the composite paper.

[0025] Specifically, in step S1, the flame retardant is ammonium polyphosphate and melamine polyphosphate, and the mass ratio of the ammonium polyphosphate to the melamine polyphosphate is (1.8-2.2):1.

[0026] More specifically, calcium carbonate is added to the chitosan acetic acid aqueous solution and subjected to ultrasonic treatment to obtain a dispersion, and the ultrasonic treatment time is 30-60 minutes.

[0027] Specifically, in step S2, the flame retardant solution and the sodium silicate aqueous solution are added dropwise to the dispersion, the heating reaction temperature is 60-80° C., and the reaction time is 1.5-2.5 hours.

[0028] More specifically, in step S2, after the reaction is completed, centrifugation and drying are performed to obtain core-shell calcium carbonate, the centrifugation time is 10-20 min, and the drying temperature is 50-60°C.

[0029] Specifically, 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 meta-aramid fibrids, meta-aramid chopped fibers, core-shell calcium carbonate, and cationic chitosan are uniformly dispersed in deionized water to form a slurry, wherein 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 fibrils, meta-aramid chopped fibers, and core-shell calcium carbonate. The addition of the cationic chitosan makes the slurry Zeta potential ≥+40mV.

[0030] Specifically, 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℃. If the turbulence intensity is less than 8m / s, the fiber and filler are unevenly distributed, the paper density deviation increases, and the interlayer bonding strength decreases. High turbulence intensity can provide shear force to break the physical entanglement of fiber and filler and ensure uniform dispersion.

[0031] Specifically, 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 0.5-1.0 MPa, and the time is 5-10 min; the temperature of the second stage is 150-180°C, the pressure is 3.0-4.0 MPa, and the time is 10-15 min; the temperature of the third stage is 220-250°C, the pressure is 5.0-8.0 MPa, and the time is 8-10 min.

[0032] The invention also discloses a core-shell structure calcium carbonate aramid fiber composite paper. The core-shell structure calcium carbonate aramid fiber composite paper is prepared according to the preparation method of the invention.

[0033] More specifically, the specific information of the raw materials used in the examples of the present invention is as follows, but this is not a limitation of the present invention.

[0034] Calcium carbonate: particle size 50-100nm, purity > 99%; Chitosan: Deacetylation degree ≥ 90%, viscosity 100-200 mPa·s; Sodium silicate: modulus 3.2-3.5; Ammonium polyphosphate: degree of polymerization ≥1000; Melamine polyphosphate: purity> 99%; Meta-aramid fiber precipitation: specific surface area ≥ 20m 2 / g, average length 0.1-0.5mm, fibrillation degree ≥50%; Meta-aramid chopped fibers: length 3-5mm, diameter 10-15um; Cationic chitosan: quaternization degree ≥80%, substitution degree 0.8-1.2, molecular weight range 50-200 kDa.

[0035] Example 1 Preparation of a core-shell structured calcium carbonate aramid fiber composite paper: (1) Precipitated calcium carbonate was dispersed in a chitosan acetic acid aqueous solution at pH = 4.5 and subjected to ultrasonic treatment for 30 min to obtain a dispersion having a calcium carbonate concentration of 5 wt %. The chitosan mass concentration in the chitosan acetic acid aqueous solution was 1 wt %.

[0036] (2) Ammonium polyphosphate and melamine polyphosphate in a mass ratio of 2:1 were dissolved in deionized water to prepare a flame retardant solution with a concentration of 10 wt %.

[0037] (3) Add the flame retardant solution obtained in step (2) and a sodium silicate solution with a concentration of 8 wt% dropwise to the dispersion obtained in step (1), and react at a high temperature of 70° C. for 2 h to obtain a calcium carbonate coating of a silicon dioxide / chitosan composite. The mass ratio of sodium silicate to calcium carbonate in the coating is 1:3, and the amount of flame retardant added is 4 wt% of the mass of the composite paper.

[0038] (4) The calcium carbonate with the silica / chitosan composite coating obtained in step (3) is centrifugally dried at 50° C. for 15 min to obtain a core-shell structured calcium carbonate.

[0039] (5) The meta-aramid fiber precipitation, meta-aramid short fiber and core-shell calcium carbonate were dispersed in a weight ratio of 3:6:1. The Zeta potential of the slurry was increased to +50 mV by adding cationic chitosan. The slurry was then injected into an inclined screen forming device. The turbulence intensity was controlled at 9 m / s to make the slurry evenly dispersed. The solid content of the slurry was 0.02‰. The composite base paper was obtained by vacuum dehydration, pressing and high temperature drying at 80°C.

[0040] (6) The composite base paper obtained in step (5) is subjected to three-stage hot pressing treatment: the first stage is at a temperature of 70°C, a pressure of 0.8 MPa, and a time of 10 min; the second stage is at a temperature of 160°C, a pressure of 3.5 MPa, and a time of 12 min; the third stage is at a temperature of 240°C, a pressure of 6.0 MPa, and a time of 8 min. After the hot pressing treatment, a core-shell structured calcium carbonate aramid fiber composite paper is obtained.

[0041] Example 2 Preparation of a core-shell structured calcium carbonate aramid fiber composite paper: (1) Precipitated calcium carbonate was dispersed in a chitosan acetic acid aqueous solution at pH = 4.5 and ultrasonically treated for 30 minutes to obtain a dispersion with a concentration of 5 wt %, wherein the mass concentration of chitosan in the chitosan acetic acid aqueous solution was 2 wt %.

[0042] (2) Ammonium polyphosphate and melamine polyphosphate in a mass ratio of 2:1 were dissolved in deionized water to prepare a flame retardant solution with a concentration of 10 wt %.

[0043] (3) Add the flame retardant solution obtained in step (2) and a 5 wt % sodium silicate solution dropwise to the dispersion obtained in step (1), and react at 70° C. for 2 h to obtain a calcium carbonate coating of a silicon dioxide / chitosan composite. The mass ratio of sodium silicate to calcium carbonate in the coating is 1:2, and the amount of flame retardant added is 3 wt % of the mass of the composite paper.

[0044] (4) The calcium carbonate with the silica / chitosan composite coating obtained in step (3) is centrifugally dried at 50° C. for 15 min to obtain a core-shell structured calcium carbonate.

[0045] (5) The meta-aramid fiber precipitation, meta-aramid short fiber and core-shell calcium carbonate were dispersed in a weight ratio of 3:6:1. The Zeta potential of the slurry was increased to +50 mV by adding cationic chitosan. The slurry was then injected into an inclined screen forming device. The turbulence intensity was controlled at 9 m / s to make the slurry evenly dispersed. The solid content of the slurry was 0.02‰. The composite base paper was obtained by vacuum dehydration, pressing and high temperature drying at 80°C.

[0046] (6) The composite base paper obtained in step (5) is subjected to three-stage hot pressing treatment: the first stage is at a temperature of 80°C, a pressure of 1.0 MPa, and a time of 8 min; the second stage is at a temperature of 170°C, a pressure of 4.0 MPa, and a time of 15 min; the third stage is at a temperature of 250°C, a pressure of 8.0 MPa, and a time of 10 min. After the hot pressing treatment, a core-shell structured calcium carbonate aramid fiber composite paper is obtained.

[0047] Example 3 Preparation of a core-shell structured calcium carbonate aramid fiber composite paper: (1) Precipitated calcium carbonate was dispersed in a chitosan acetic acid aqueous solution at pH = 4.5 and ultrasonically treated for 30 minutes to obtain a dispersion with a concentration of 5 wt %, wherein the mass concentration of chitosan in the chitosan acetic acid aqueous solution was 1.5 wt %.

[0048] (2) Ammonium polyphosphate and melamine polyphosphate in a mass ratio of 2:1 were dissolved in deionized water to prepare a flame retardant solution with a concentration of 10 wt %.

[0049] (3) Add the flame retardant solution obtained in step (2) and a 10 wt % sodium silicate solution dropwise to the dispersion obtained in step (1), and react at 70° C. for 2 h to obtain a calcium carbonate coating of a silicon dioxide / chitosan composite. The mass ratio of sodium silicate to calcium carbonate in the coating is 1:5, and the amount of flame retardant added is 5 wt % of the mass of the composite paper.

[0050] (4) The calcium carbonate with the silica / chitosan composite coating obtained in step (3) is centrifugally dried at 60° C. for 15 min to obtain a core-shell structured calcium carbonate.

[0051] (5) The meta-aramid fiber precipitation, meta-aramid short fiber and core-shell calcium carbonate were dispersed in a weight ratio of 1:8:1, and the Zeta potential of the slurry was increased to +45 mV by adding cationic chitosan. The slurry was then injected into an inclined screen forming device, and the turbulence intensity was controlled at 9 m / s to make the slurry evenly dispersed. The solid content of the slurry was 0.05‰. The composite base paper was obtained by vacuum dehydration, pressing and high temperature drying at 80°C.

[0052] (6) The composite base paper obtained in step (5) is subjected to three-stage hot pressing treatment: the temperature of the first stage is 75°C, the pressure is 0.7 MPa, and the time is 12 min; the temperature of the second stage is 155°C, the pressure is 3.2 MPa, and the time is 13 min; the temperature of the third stage is 230°C, the pressure is 5.5 MPa, and the time is 9 min, and a core-shell structure calcium carbonate aramid fiber composite paper is obtained after hot pressing.

[0053] Example 4 Preparation of a core-shell structured calcium carbonate aramid fiber composite paper: (1) Precipitated calcium carbonate was dispersed in a chitosan acetic acid aqueous solution at pH = 5.0 and subjected to ultrasonic treatment for 60 min to obtain a dispersion having a calcium carbonate concentration of 10 wt %. The mass concentration of chitosan in the chitosan acetic acid aqueous solution was 2 wt %.

[0054] (2) Ammonium polyphosphate and melamine polyphosphate in a mass ratio of 2.2:1 were dissolved in deionized water to prepare a flame retardant solution with a concentration of 20 wt %.

[0055] (3) Add the flame retardant solution obtained in step (2) and a sodium silicate solution with a concentration of 8 wt% dropwise to the dispersion obtained in step (1), and react at a high temperature of 80° C. for 1.5 h to obtain a calcium carbonate coating of a silicon dioxide / chitosan composite. The mass ratio of sodium silicate to calcium carbonate in the coating is 1:4, and the amount of flame retardant added is 4 wt% of the mass of the composite paper.

[0056] (4) The calcium carbonate with the silica / chitosan composite coating obtained in step (3) is centrifugally dried at 60° C. for 10 min to obtain a core-shell structured calcium carbonate.

[0057] (5) The meta-aramid fiber precipitation, meta-aramid short fiber and core-shell calcium carbonate were dispersed in a weight ratio of 3:6:1. The Zeta potential of the slurry was increased to +50 mV by adding cationic chitosan. The slurry was then injected into an inclined screen 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‰. The composite base paper was obtained by vacuum dehydration, pressing and high temperature drying at 90°C.

[0058] (6) The composite base paper obtained in step (5) is subjected to three-stage hot pressing treatment: the first stage is at a temperature of 90°C, a pressure of 0.5 MPa, and a time of 5 min; the second stage is at a temperature of 150°C, a pressure of 3.5 MPa, and a time of 13 min; the third stage is at a temperature of 220°C, a pressure of 7.0 MPa, and a time of 9 min. After the hot pressing treatment, a core-shell structured calcium carbonate aramid fiber composite paper is obtained.

[0059] Example 5 Preparation of a core-shell structured calcium carbonate aramid fiber composite paper: (1) Precipitated calcium carbonate was dispersed in a chitosan acetic acid aqueous solution at pH = 5.0 and subjected to ultrasonic treatment for 30 min to obtain a dispersion having a calcium carbonate concentration of 6 wt %. The chitosan mass concentration in the chitosan acetic acid aqueous solution was 1 wt %.

[0060] (2) Ammonium polyphosphate and melamine polyphosphate in a mass ratio of 1.8:1 were dissolved in deionized water to prepare a flame retardant solution with a concentration of 20 wt %.

[0061] (3) Add the flame retardant solution obtained in step (2) and a 7 wt % sodium silicate solution dropwise to the dispersion obtained in step (1), and react at 60° C. for 2.5 h to obtain a calcium carbonate coating of a silicon dioxide / chitosan composite. The mass ratio of sodium silicate to calcium carbonate in the coating is 1:3, and the amount of flame retardant added is 4 wt % of the mass of the composite paper.

[0062] (4) The calcium carbonate with the silica / chitosan composite coating obtained in step (3) is centrifugally dried at 60° C. for 10 min to obtain a core-shell structured calcium carbonate.

[0063] (5) The meta-aramid fiber precipitation, meta-aramid short fiber and core-shell calcium carbonate were dispersed in a weight ratio of 2:6:2, and the Zeta potential of the slurry was increased to +50 mV by adding cationic chitosan. The slurry was then injected into an inclined screen forming device, and the turbulence intensity was controlled at 8 m / s to make the slurry evenly dispersed. The solid content of the slurry was 0.03‰. The composite base paper was obtained by vacuum dehydration, pressing and high temperature drying at 70°C.

[0064] (6) The composite base paper obtained in step (5) is subjected to three-stage hot pressing treatment: the first stage is at a temperature of 70°C, a pressure of 0.8 MPa, and a time of 8 min; the second stage is at a temperature of 180°C, a pressure of 3.0 MPa, and a time of 10 min; the third stage is at a temperature of 240°C, a pressure of 5.0 MPa, and a time of 10 min. After the hot pressing treatment, a core-shell structured calcium carbonate aramid fiber composite paper is obtained.

[0065] Comparative Example 1 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 1, the pH of the chitosan acetic acid aqueous solution was 3.

[0066] Comparative Example 2 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 2, the pH of the chitosan acetic acid aqueous solution was 5.8.

[0067] Comparative Example 3 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 3, the dosage ratio of sodium silicate was increased, and the mass ratio of sodium silicate to calcium carbonate was 1:1.

[0068] Comparative Example 4 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 4, the reaction temperature of step (3) was reduced to 40°C.

[0069] Comparative Example 5 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 of step (3) was increased to 80°C.

[0070] Comparative Example 6 The core-shell structured calcium carbonate aramid fiber composite paper was prepared by the same method as in Example 1, except that in Comparative Example 6, the amount of the flame retardant added was reduced to 2 wt % of the mass of the composite paper.

[0071] Comparative Example 7 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.

[0072] Comparative Example 8 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 of step (6) were changed, specifically: the temperature of the first stage was 60°C, the pressure was 0.3 MPa, and the time was 10 min; the temperature of the second stage was 120°C, the pressure was 2 MPa, and the time was 12 min; the temperature of the third stage was 200°C, the pressure was 4.0 MPa, and the time was 8 min.

[0073] Comparative Example 9 The core-shell structured calcium carbonate aramid fiber composite paper was prepared by the same method as in Example 1, except that in Comparative Example 9, only ammonium polyphosphate was used as the flame retardant, and melamine polyphosphate was not added.

[0074] Comparative Example 10 The same method as in Example 1 was adopted to prepare core-shell structured calcium carbonate aramid fiber composite paper, except that in Comparative Example 9, only melamine polyphosphate was used as the flame retardant, and no ammonium polyphosphate was added.

[0075] The core-shell calcium carbonate aramid fiber composite paper prepared in the above embodiments and comparative examples was subjected to performance tests. The specific test results are shown in Table 1 below, wherein the test methods involved are: after the composite paper is subjected to constant temperature and humidity treatment, the quantitative test standard is GB / T451.2~2023; the thickness test standard is GB / T451.3~2002; the uniformity test method is to use a 3D paper analyzer, fix the paper on a rotating glass drum, and install the light source and the photodiode detector on both sides of the drum respectively, and move synchronously in the synchronous direction, thereby completing the scanning of the paper; 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 / T14656~2009; the compressive strength test standard is GB / T 1408.1~2016.

[0076] Table 1 Performance test results of core-shell calcium carbonate aramid fiber composite paper

[0077] From the above table data, it can be seen that the core-shell structure calcium carbonate aramid fiber composite paper prepared by the preparation method of the present invention in Examples 1 to 5 can have both flame retardant properties and high strength properties, thereby solving the problems of insufficient mechanical properties, poor dispersibility of calcium carbonate fillers and weak interface bonding of aramid paper in the prior art due to weak bonding between fibers.

[0078] From the comparison of the experimental results of Comparative Example 1, Comparative Example 2 and Example 1, it can be seen that when the pH of the chitosan acetic acid aqueous solution is less than 4.5 or greater than 5.0, the protonation degree of chitosan is insufficient, the electrostatic adsorption is weakened, the coating layer is uneven, the Zeta potential of the core-shell calcium carbonate is reduced, the filler agglomeration size is increased, and the interface bonding strength is reduced. When the pH of the chitosan acetic acid aqueous solution is 4.5-5.0, the amino group is fully protonated to -NH 3+ , combined with the negative charge (-OH) on the surface of calcium carbonate through electrostatic attraction, pH deviation will lead to a reduction in the adsorption amount, resulting in a discontinuous coating layer, and ultimately affecting the strength and compressive strength properties of the composite paper.

[0079] From the comparison of the experimental results of Comparative Example 3 and Example 1, it can be seen that if the dosage ratio of sodium silicate increases, SiO 2The particles are generated too quickly, forming independent agglomerates, resulting in uneven coating thickness. Of course, if the amount of sodium silicate is too small, it will cause SiO 2 Insufficient loading, too thin coating layer, weak interface bonding, will eventually affect the strength and compressive strength of the composite paper. It can be seen that the concentration and dosage of sodium silicate directly affect the hydrolysis and condensation rate. Excessive amount leads to particle agglomeration, and insufficient amount cannot form a continuous protective layer. The use of the sodium silicate dosage specified in the present invention is more conducive to obtaining composite paper with excellent comprehensive performance.

[0080] From the comparison of the experimental results of Comparative Example 4, Comparative Example 5 and Example 1, it can be seen that if the core-shell reaction temperature is too low or too high, the strength and compressive strength of the final composite paper will decrease. 2 The particles are not fully formed, the coating layer is loose, and the filler has poor dispersion. When the temperature is greater than 80°C, chitosan decomposes, the coating layer carbonizes, and the Zeta potential decreases. In the core-shell reaction process, temperature and time control the hydrolysis and condensation reaction rate. Too low temperature or insufficient time will lead to incomplete reaction, while too high temperature will destroy the organic-inorganic hybrid structure.

[0081] From the comparison of the experimental results of Comparative Example 6 and Example 1, it can be seen that if the amount of flame retardant is reduced, LOI will be less than 28%, which cannot meet the flame retardant standard. In addition, it is also found in the experiment that if the amount of flame retardant is too high, the mobility of the flame retardant will increase and the loss of tensile strength will be large. This is because excessive flame retardant destroys the shell structure and causes interface defects; insufficient flame retardant efficiency does not meet the standard. Therefore, the use of the flame retardant amount defined in the present invention is more conducive to obtaining composite paper with excellent comprehensive performance.

[0082] From the comparison of the experimental results of Comparative Example 7 and Example 1, it can be seen that if the Zeta potential is less than +40 mV, the electrostatic repulsion of the particles is insufficient, the slurry uniformity is poor, and the tensile strength of the paper is reduced. The slurry with a high Zeta potential (≥+40mV) can stabilize the dispersion system through electrostatic repulsion, avoid the flocculation of fillers and fibers, and ultimately make the composite paper have good flame retardant properties while improving the strength properties of the composite paper.

[0083] From the comparison of the experimental results of Comparative Example 8 and Example 1, it can be seen that if the step hot pressing conditions specified in the present invention are not adopted, the strength performance of the composite paper will be significantly reduced. When the temperature of the first stage is less than 70°C: the chitosan molecular chain is not activated and cannot promote the initial bonding of the fiber and the filler, the pressure is less than 0.5 MPa: the fiber and the filler are not in sufficient contact, and the interface porosity increases. The temperature of the second stage is less than 150°C: the chemical cross-linking reaction is not triggered, the three-dimensional network structure is not formed, and the pressure is less than 3.0 MPa: the fiber-filler interface bonding is weak, and the tensile strength decreases. The temperature of the third stage is less than 220°C: the material is not fully shaped, and the strength retention rate after wet heat aging is less than 85%, and the pressure is less than 5.0 MPa: the structural density is insufficient and the dielectric strength decreases.

[0084] From the comparison of the experimental results of Comparative Example 9, Comparative Example 5 and Example 1, it can be seen that when ammonium polyphosphate and melamine polyphosphate are used together as flame retardants, it is more conducive to improving the performance of composite paper, because ammonium polyphosphate has insufficient thermal stability, is easy to decompose at high temperature, and the flame retardant effect drops sharply, and the carbon layer formed is loose and porous and easy to be penetrated by flames. If the addition amount is increased, the brittleness of the material will increase. Melamine polyphosphate mainly relies on gas phase flame retardancy, has weak condensed phase carbon formation ability, has insufficient inhibitory effect on high ignition energy materials, and is expensive. Compared with using it alone, the flame retardant performance is further improved after composite use, the density of the carbon layer is increased, the total amount of the composite system can be reduced by 30%, and the CO release is reduced.

[0085] The technical features of the above-described embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above-described embodiments are exhaustively listed. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0086] For those skilled in the art, several modifications and improvements may be made without departing from the concept of the present invention, all of which belong to the protection scope of the present invention. The protection scope of the present invention shall be based on the attached 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; 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 core-shell calcium carbonate by solid-liquid separation; 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; S4. The composite base paper is subjected to multi-stage gradient hot pressing to obtain a core-shell structured calcium carbonate aramid fiber composite paper.

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 pH of the chitosan acetic acid aqueous solution is 4.5-5, and 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: The mass ratio of the sodium silicate to the calcium carbonate is 1:(2-5), and the added amount of the flame retardant is 3-5wt% of the composite paper mass.

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

1.

6. The method for preparing a core-shell structured 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 heating reaction temperature is 60-80° C. and the reaction time is 1.5-2.5 hours.

7. The method for preparing a core-shell 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 meta-aramid fibrids, meta-aramid chopped fibers, core-shell calcium carbonate, and cationic chitosan are uniformly dispersed in deionized water to form a slurry, wherein 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 fibrils, meta-aramid chopped fibers, and core-shell calcium carbonate. The addition of the cationic chitosan makes the slurry Zeta potential ≥+40mV.

8. 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℃.

9. The method for preparing a core-shell calcium carbonate aramid fiber composite paper according to claim 1, characterized in that: 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.

10. 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 1-9.

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