Preparation of aramid nanofiber from waste aramid paper and method thereof

Abstract

The application belongs to the technical field of resource recycling and nanotechnology, and particularly relates to a kind of aramid nanofiber prepared from waste aramid paper and a preparation method thereof.The preparation method is as follows: waste aramid paper scraps are soaked in a mixed solvent;then chemical treatment is carried out in a mixed solution of KOH and DMAC, and waste aramid paper slurry is obtained after centrifugal washing; then mechanical beating treatment is carried out on the waste aramid paper slurry to obtain fine waste aramid paper slurry; and finally, strong mechanical treatment is carried out to obtain aramid nanofiber.The preparation method provided by the application realizes the conversion of waste aramid paper into high-value nanofiber with the advantages of high efficiency and low energy consumption, and the performance of the fiber itself is little damaged during the preparation process, the fiber has good dispersibility, the size of the obtained regenerated nanofiber is uniform, the nanofiber has a small nanosize and a moderate aspect ratio, and the nanofiber can endow the material with higher performance.

Description

Technical Field

[0001] This invention specifically relates to a method for preparing aramid nanofibers from waste aramid paper, belonging to the technical field of resource recycling and nanotechnology intersection. Background Technology

[0002] Aramid paper, a high-performance insulating material made of aramid fibers, is widely used in power equipment, aerospace, and rail transportation due to its excellent thermal stability, mechanical strength, and electrical insulation properties. However, its production and use generate a large amount of waste aramid paper products, including scraps and defective products. If these wastes are disposed of using traditional landfill or incineration methods, it will not only waste resources but may also lead to environmental pollution.

[0003] Meanwhile, aramid nanofibers, as an emerging nanomaterial, have shown great application potential in high-performance composite materials, nanofiltration membranes, battery separators, and smart sensors due to their extremely high specific surface area, excellent mechanical properties, and good thermal stability. However, traditional methods for preparing aramid nanofibers typically use virgin aramid fibers as raw materials, which are costly and complex, limiting their large-scale application.

[0004] Several methods for recycling aramid paper waste exist in the prior art. For example, patent application CN102373639A discloses a method for recycling waste aramid paper. This method involves breaking down aramid paper into pulp, then mixing and soaking it with organic solvents and additives to dissolve it, ultimately producing aramid precipitated fibers. However, the final product of this process is aramid precipitated fibers, whose main use is as a binder in the manufacture of aramid paper, making it difficult to expand into high-value-added applications.

[0005] In summary, existing technologies have obvious shortcomings. Therefore, developing a low-cost recycling technology for waste aramid paper and simultaneously converting it into high-value aramid nanofibers is of great significance. This not only meets the needs of circular economy and sustainable development, but also provides a new approach to solving the problem of excessively high costs in the preparation of high-end nanomaterials, and has significant environmental, economic and social benefits. Summary of the Invention

[0006] This invention addresses the shortcomings of existing technologies by providing a method for preparing aramid nanofibers from waste aramid paper. This method achieves the transformation of waste aramid paper into high-value nanofibers with the advantages of high efficiency and low energy consumption. The preparation process causes minimal damage to the properties of the fibers themselves, and the fibers have good dispersion. The resulting recycled nanofibers are uniform in size, have small nanoscale dimensions and a moderate aspect ratio, and can endow the materials with higher performance.

[0007] The technical solution of this invention to solve the above-mentioned technical problems is as follows: A method for preparing aramid nanofibers from waste aramid paper, the method comprising the following steps:

[0008] S1. Soak waste aramid paper scraps in a mixed solvent;

[0009] S2. The waste aramid paper scraps processed in step S1 are chemically treated in a mixed solution of KOH and DMAC, and then centrifuged and washed to obtain waste aramid paper pulp.

[0010] S3. Mechanically beat the waste aramid paper pulp from step S2 to obtain fine waste aramid paper pulp.

[0011] S4. The waste aramid paper pulp from step S3 is subjected to strong mechanical treatment to obtain aramid nanofibers.

[0012] Furthermore, the waste aramid paper scraps include chopped aramid fibers and precipitated aramid fibers.

[0013] Furthermore, in step S1, the waste aramid paper scraps are scraps obtained after wet papermaking, and the surface is not coated with resin or coating.

[0014] Furthermore, in step S1, the mass ratio of waste aramid paper scraps to the mixed solvent is 1:(10~20); in step S1, the mixed solvent is ethanol and deionized water, wherein the volume ratio of ethanol to deionized water is 1:(1~3); in step S1, the soaking time is 6~12h. In step S2, the mass ratio of waste aramid paper scraps treated in S1 to the mixed solution is 1:(80~120); the mass ratio of KOH to DMAC is 1:200~500; in step S2, the chemical treatment temperature is 60~80℃, and the chemical treatment time is 1~2h.

[0015] Furthermore, in step S1, the waste aramid paper scraps are waste aramid paper with a resin or coating on the surface, and the coating thickness is 30~100μm.

[0016] Furthermore, in step S1, the mass ratio of waste aramid paper scraps to the mixed solvent is 1:(10~20). In step S1, the mixed solvent is methanol and dichloromethane, wherein dichloromethane accounts for 60%~80% of the total volume of the mixed solvent, and methanol accounts for 20%~40% of the total volume of the mixed solvent, preferably 80% of the total volume of the mixed solvent and 20% of the total volume of the mixed solvent. In step S1, the soaking time is determined according to the coating thickness of the surface coating resin or coating layer, and the ratio between soaking time and coating thickness is (0.1~0.2)h:1μm. In step S2, the mass ratio of waste aramid paper scraps treated in S1 to the mixed solution is 1:(80~120); the mass ratio of KOH to DMAC is 1:200~500; the chemical treatment temperature is 60~80℃, and the chemical treatment time is (0.2~0.5) times the soaking time in step S1.

[0017] Furthermore, in step S1, the surface of the soaked waste aramid paper scraps is scraped and cleaned.

[0018] Furthermore, in step S2, the mixed solution of KOH and DMAC needs to be stirred during the mixing process for 0.5 to 2 hours. The stirring time here is not a limitation of the technology of this invention, as long as it can be stirred until KOH is completely dissolved.

[0019] Furthermore, in step S2, the mass concentration of the obtained waste aramid pulp is adjusted to 5-8 wt%; the average length of the mixed fibers in the pulp is 0.5-1 mm, and the average diameter is 20-30 μm.

[0020] Furthermore, the mechanical beating in step S3 is carried out in a PFI disc mill with a mechanical beating speed of 20,000 to 80,000 r and a beating time of 15 to 30 min. The freeness of the waste aramid paper pulp is 30 to 60°SR. The concentration of the obtained waste aramid paper fine pulp is adjusted to 0.5 to 0.8 wt%. The average length of the mixed fibers in the waste aramid paper fine pulp is 10 to 100 μm and the average diameter is 5 to 10 μm.

[0021] Furthermore, in step S4, the strong mechanical treatment is a mechanical homogenization treatment, with a homogenization pressure of 1000~1200 bar and a homogenization time of 1~2 h; the resulting aramid nanofibers have an average length of 1~10 μm and an average diameter of 50~300 nm.

[0022] The present invention also discloses an aramid nanofiber, which is prepared by the preparation method of aramid nanofiber from waste aramid paper as described in the present invention; the aramid nanofiber has an average length of 1~10μm and an average diameter of 50~300nm.

[0023] The beneficial effects of this invention are:

[0024] (1) The aramid nanofibers prepared from waste aramid paper provided by the present invention have uniform size, small nanoscale size and moderate aspect ratio. The average length of the aramid nanofibers is 1~10μm and the average diameter is 50~300nm, which makes the aramid nanofibers widely applicable in high-performance composite reinforcing materials, thermally conductive and insulating interface materials, special papermaking and advanced coatings, effectively improving the mechanical properties, thermal stability and functionality of materials.

[0025] (2) The method for preparing aramid nanofibers from waste aramid paper provided by this invention achieves the transformation of waste aramid paper into high-value nanofibers through the synergistic effect of four steps: "surface purification → chemical treatment → physical treatment → homogenization and nanofiberization". It has the advantages of high efficiency and low energy consumption. The preparation process causes little damage to the properties of the fiber itself and the fiber has good dispersibility. The four steps provided by this invention constitute an inseparable whole. Step S1 adopts different purification methods for different types of waste aramid paper scraps, which creates the basis for the mild chemical treatment dissociation conditions in step S2. On this basis, the physical and mechanical treatment in step S3 can preserve the integrity of the aramid fiber to the greatest extent, so as to minimize the loss of its properties, and provide the best raw materials for step S4, so as to achieve nanofiberization with optimal efficiency. This complementary synergy ultimately successfully completes the value reshaping of "industrial waste" into "high-end nanomaterials" without sacrificing the inherent excellent mechanical properties and thermal stability of aramid fibers. The product can be widely used in cutting-edge fields such as high-performance composite materials and nanofiltration. Attached Figure Description

[0026] Figure 1 Here is a SEM image of the aramid nanofibers obtained in Example 1;

[0027] Figure 2 This is a SEM image of the aramid nanofibers obtained in Example 5. Detailed Implementation

[0028] The specific embodiments of the present invention will be described in detail below. The present invention can be implemented in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed.

[0029] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used is for describing particular embodiments only and is not intended to limit the invention.

[0030] A method for preparing aramid nanofibers from waste aramid paper, the method comprising the following steps:

[0031] S1. Soak waste aramid paper scraps in a mixed solvent;

[0032] S2. The waste aramid paper scraps processed in step S1 are chemically treated in a mixed solution of KOH and DMAC, and then centrifuged and washed to obtain waste aramid paper pulp.

[0033] S3. Mechanically beat the waste aramid paper pulp from step S2 to obtain fine waste aramid paper pulp.

[0034] S4. The waste aramid paper pulp from step S3 is subjected to strong mechanical treatment to obtain aramid nanofibers.

[0035] Specifically, the waste aramid paper scraps include aramid chopped fibers and aramid precipitated fibers.

[0036] Specifically, in step S1, the waste aramid paper scraps are scraps obtained after wet papermaking, and their surfaces are not coated with resin or a coating:

[0037] In step S1, the waste aramid paper scraps are cut into paper pieces of (10~20) mm × (10~20) mm before soaking;

[0038] In step S1, the mass ratio of waste aramid paper scraps to mixed solvent is 1:(10~20).

[0039] In step S1, the mixed solvent is ethanol and deionized water, wherein the volume ratio of ethanol to deionized water is 1:(1~3); the soaking time is 6~12h;

[0040] In step S2, the mass ratio of the waste aramid paper scraps processed in S1 to the mixed solution is 1:(80~120); the mass ratio of KOH to DMAC is 1:200~500.

[0041] In step S2, the chemical treatment temperature is 60~80℃ and the chemical treatment time is 1~2h.

[0042] Specifically, in step S1, the waste aramid paper scraps are waste aramid paper with a resin or coating on the surface, and the coating thickness is 30~100μm:

[0043] In the embodiments and comparative examples of the present invention, the resin or coating on the surface of waste aramid paper scraps is epoxy resin, but this is not a limitation of the technology of the present invention, and it can also be phenolic resin or other adhesives.

[0044] In step S1, the waste aramid paper scraps are cut into strips of (8~12) mm × (8~12) cm before soaking; the surface of the soaked waste aramid paper scraps is scraped and cleaned.

[0045] In step S1, the mass ratio of waste aramid paper scraps to mixed solvent is 1:(10~20).

[0046] In step S1, the mixed solvent is methanol and dichloromethane, wherein dichloromethane accounts for 60% to 80% of the total volume of the mixed solvent, and methanol accounts for 20% to 40% of the total volume of the mixed solvent, preferably dichloromethane accounts for 80% of the total volume of the mixed solvent and methanol accounts for 20% of the total volume of the mixed solvent;

[0047] In step S1, the soaking time is determined based on the coating thickness of the surface-coated resin or coating layer, and the ratio between soaking time and coating thickness is (0.1~0.2) h: 1 μm;

[0048] In step S2, the mass ratio of the waste aramid paper scraps processed in S1 to the mixed solution is 1:(80~120); the mass ratio of KOH to DMAC is 1:200~500.

[0049] In step S2, the chemical treatment temperature is 60~80℃, and the chemical treatment time is (0.2~0.5) times the soaking time in step S1.

[0050] Specifically, in step S2, the mixed solution of KOH and DMAC needs to be stirred during the mixing process for 0.5 to 2 hours. The stirring time is not a limitation of the present invention, as long as the KOH is completely dissolved.

[0051] Specifically, in step S2, the chemically treated slurry is centrifuged and washed until neutral, and the washing is performed three to five times using deionized water as the washing solution.

[0052] Specifically, in step S2, the waste aramid pulp obtained after cleaning and adjustment is 5~8wt%; the average length of the mixed fibers in the pulp is 0.5~1mm, and the average diameter is 20~30μm.

[0053] Specifically, the mechanical beating in step S3 is carried out in a PFI disc mill with a mechanical beating speed of 20,000 to 80,000 r and a beating time of 15 to 30 min. The freeness of the waste aramid paper pulp is 30 to 60°SR. The concentration of the obtained waste aramid paper fine pulp is adjusted to 0.5 to 0.8 wt%. The average length of the mixed fibers in the waste aramid paper fine pulp is 10 to 100 μm and the average diameter is 5 to 10 μm.

[0054] Specifically, in step S4, the strong mechanical treatment is a mechanical homogenization treatment, with a homogenization pressure of 1000~1200 bar and a homogenization time of 1~2 h; the resulting aramid nanofibers have an average length of 1~10 μm and an average diameter of 50~300 nm.

[0055] More specifically, the waste aramid paper scraps are scraps obtained after wet papermaking, and their surfaces are not coated with resin or coating. The preparation of aramid nanofibers includes the following detailed steps:

[0056] S1. Cut waste aramid paper scraps into paper pieces of (10~20) mm × (10~20) mm; place them in a mixed solvent of ethanol and deionized water and soak for 6~12 hours;

[0057] In this embodiment of the invention, the specific operation steps of the chemical treatment in step S2 are divided into two parts: S2-1 and S2-2.

[0058] S2-1. Place KOH powder in a dried reaction flask, add an appropriate amount of anhydrous DMAC to the reaction flask, immediately seal the flask, place the reaction flask on a magnetic stirrer, and stir magnetically at room temperature until KOH is completely dissolved and the solution color changes from colorless and transparent to pale yellow, thus obtaining a KOH / DMAC mixed solution.

[0059] S2-2. Place the waste aramid paper scraps treated in step S1 into a mixed solution of KOH / DMAC, heat in a water bath at 60-80℃ for chemical treatment for 1-2 hours, and centrifuge and wash to obtain waste aramid paper pulp. Centrifuge and wash the chemically treated pulp until the pH of the pulp is neutral to obtain waste aramid paper pulp, and adjust the mass concentration of the waste aramid paper pulp to 5-8 wt%. The average length of the mixed fibers in the pulp is 0.5-1 mm, and the average diameter is 20-30 μm.

[0060] S3. Place the waste aramid pulp from step S2 into a PFI disc mill for mechanical beating. The mechanical beating speed is 20,000 to 80,000 r, the beating time is 15 to 30 min, and the freeness of the waste aramid pulp is 30 to 60°SR. The concentration of the obtained waste aramid fine pulp is adjusted to 0.5 to 0.8 wt%. The average length of the mixed fibers in the waste aramid fine pulp is 10 to 100 μm, and the average diameter is 5 to 10 μm.

[0061] S4. The waste aramid paper pulp from step S3 is subjected to mechanical homogenization. The homogenization pressure is 1000~1200 bar and the homogenization time is 1~2 h to obtain aramid nanofibers. The average length of the obtained aramid nanofibers is 1~10 μm and the average diameter is 50~300 nm.

[0062] More specifically, the waste aramid paper scraps are waste aramid paper with a resin or coating on the surface, and the coating thickness is 30~100μm. The preparation of aramid nanofibers includes the following detailed steps:

[0063] S1. Cut waste aramid paper scraps into strips of (8~12) mm × (8~12) cm; place them in a mixed solvent of methanol and dichloromethane, and the soaking time depends on the coating thickness of the surface resin or coating. The ratio between soaking time and coating thickness is (0.1~0.2) h: 1 μm. After soaking, scrape and clean the surface of the waste aramid paper scraps.

[0064] In this embodiment of the invention, the specific operation steps of the chemical treatment in step S2 are divided into two parts: S2-1 and S2-2.

[0065] S2-1. Place KOH powder in a dried reaction flask, add an appropriate amount of anhydrous DMAC to the reaction flask, immediately seal the flask, place the reaction flask on a magnetic stirrer, and stir magnetically at room temperature until KOH is completely dissolved and the solution color changes from colorless and transparent to pale yellow, thus obtaining a KOH / DMAC mixed solution.

[0066] S2-2. Place the waste aramid paper scraps treated in step S1 into a mixed solution of KOH / DMAC, heat in a water bath, and chemically treat at a temperature controlled at 60~80℃. The chemical treatment time is (0.2~0.4) times the treatment time in step S1. After centrifugation and washing, waste aramid paper pulp is obtained. The chemically treated pulp is centrifuged and washed until the pH of the pulp is neutral to obtain waste aramid paper pulp. The mass concentration of the waste aramid paper pulp is adjusted to 5~8wt%. The average length of the mixed fibers in the pulp is 0.5~1mm and the average diameter is 20~30μm.

[0067] S3. Place the waste aramid pulp from step S2 into a PFI disc mill for mechanical beating. The mechanical beating speed is 20,000 to 80,000 r, the beating time is 15 to 30 min, and the freeness of the waste aramid pulp is 30 to 60°SR. The concentration of the obtained waste aramid fine pulp is adjusted to 0.5 to 0.8 wt%. The average length of the mixed fibers in the waste aramid fine pulp is 10 to 100 μm, and the average diameter is 5 to 10 μm.

[0068] S4. The waste aramid paper pulp from step S3 is subjected to mechanical homogenization. The homogenization pressure is 1000~1200 bar and the homogenization time is 1~2 h to obtain aramid nanofibers. The average length of the obtained aramid nanofibers is 1~10 μm and the average diameter is 50~300 nm.

[0069] Example 1

[0070] A method for preparing aramid nanofibers from waste aramid paper, wherein the waste aramid paper scraps are scraps obtained after wet papermaking and are not coated with resin or coating. The specific preparation method includes the following steps:

[0071] S1. Cut 10g of waste aramid paper scraps into paper pieces of (10~20)mm × (10~20)mm; place them in a mixed solvent of 50mL ethanol and 100mL deionized water and soak for 6h;

[0072] S2-1. Place 1g of KOH powder into a dried reaction flask, add 300g of anhydrous DMAC into the reaction flask, immediately seal the flask, place the reaction flask on a magnetic stirrer, and stir magnetically at room temperature until KOH is completely dissolved and the solution color changes from colorless and transparent to pale yellow, thus obtaining a KOH / DMAC mixed solution.

[0073] S2-2. Place 2.5g of waste aramid paper scraps from step S1 into a mixed solution of KOH / DMAC, heat in a water bath at 70℃ for 1.5h, centrifuge and wash to obtain waste aramid paper pulp; centrifuge and wash the chemically treated pulp until the pH is neutral to obtain waste aramid paper pulp, and adjust the mass concentration of the waste aramid paper pulp to 5wt%; the average length of the mixed fibers in the pulp is 0.5~0.7mm, and the average diameter is 20~25μm;

[0074] S3. The waste aramid pulp from step S2 is placed in a PFI disc mill for mechanical beating. The mechanical beating speed is 60,000 r, the beating time is 25 min, and the freeness of the waste aramid pulp is 40°SR. The concentration of the resulting waste aramid fine pulp is adjusted to 0.5 wt%. The average length of the mixed fibers in the waste aramid fine pulp is 50-60 μm, and the average diameter is 5-8 μm.

[0075] S4. The waste aramid paper pulp from step S3 is mechanically homogenized at a pressure of 1000 bar for 1 hour to obtain aramid nanofibers. The average length of the aramid nanofibers is 4.2 μm and the average diameter is 250 nm. The scanning electron microscope (SEM) image of the aramid nanofibers is shown below. Figure 1 As shown.

[0076] Example 2

[0077] A method for preparing aramid nanofibers from waste aramid paper is described in Example 2. The method used in Example 1 is the same as that used in Example 1, except that in step S1, the mixed solvent is 50 mL of ethanol and 50 mL of deionized water, and the soaking time is 6 h.

[0078] In step S2-1, 200g of anhydrous DMAC is added to the reaction flask;

[0079] In step S2-2, the temperature is controlled at 60℃ for chemical treatment, and the treatment time is 2h; waste aramid pulp is obtained, and the mass concentration of waste aramid pulp is adjusted to 6wt%; the average length of the mixed fibers in the pulp is 0.6~0.8mm, and the average diameter is 25~30μm;

[0080] In step S3, the mechanical beating speed is 20,000 r, the beating time is 30 min, and the freeness of the waste aramid paper pulp is 60°SR; the average length of the mixed fibers in the waste aramid paper fine pulp is 65~70 μm, and the average diameter is 8~10 μm.

[0081] In step S4, the homogenization pressure is 1000 bar and the homogenization time is 1 h to obtain aramid nanofibers with an average length of 5.1 μm and an average diameter of 300 nm.

[0082] Example 3

[0083] A method for preparing aramid nanofibers from waste aramid paper is described in Example 3. The method used in Example 1 is the same as that used in Example 3, except that in step S1, the mixed solvent is 50 mL of ethanol and 150 mL of deionized water, and the soaking time is 6 h.

[0084] In step S2-1, 500g of anhydrous DMAC is added to the reaction flask;

[0085] In step S2-2, 5g of waste aramid paper scraps from step S1 are placed in a KOH / DMAC mixed solution and chemically treated at 80℃ for 1 hour to obtain waste aramid paper pulp. The mass concentration of the waste aramid paper pulp is adjusted to 8wt%. The average length of the mixed fibers in the pulp is 0.3~0.5mm and the average diameter is 15~20μm.

[0086] In step S3, the mechanical beating speed is 80,000 r, the beating time is 15 min, and the freeness of the waste aramid paper pulp is 30°SR; the average length of the mixed fibers in the waste aramid paper fine pulp is 40~45 μm, and the average diameter is 3~5 μm.

[0087] In step S4, the homogenization pressure is 1000 bar and the homogenization time is 2 h to obtain aramid nanofibers with an average length of 3.7 μm and an average diameter of 160 nm.

[0088] Example 4

[0089] A method for preparing aramid nanofibers from waste aramid paper is described in Example 4. The method is the same as that in Example 1, except that in step S4, the homogenization pressure is 1200 bar and the homogenization time is 1 h, resulting in aramid nanofibers with an average length of 4.0 μm and an average diameter of 200 nm.

[0090] Example 5

[0091] A method for preparing aramid nanofibers from waste aramid paper, wherein the waste aramid paper scraps are waste aramid paper with a resin or coating layer on the surface, and the coating thickness is 50 μm, and the specific preparation method includes the following steps:

[0092] S1. Cut 10g of waste aramid paper scraps into strips of (8~12)mm × (8~12)cm; place them in a mixed solvent of 40mL methanol and 60mL dichloromethane and soak for 7.5h. After soaking, scrape and clean the surface of the waste aramid paper scraps.

[0093] S2-1. Place 1g of KOH powder into a dried reaction flask, add 300g of anhydrous DMAC into the reaction flask, immediately seal the flask, place the reaction flask on a magnetic stirrer, and stir magnetically at room temperature until KOH is completely dissolved and the solution color changes from colorless and transparent to pale yellow, thus obtaining a KOH / DMAC mixed solution.

[0094] S2-2. Place 2.5g of waste aramid paper scraps from step S1 into a mixed solution of KOH / DMAC, heat in a water bath at 70℃ for 2.25h, centrifuge and wash to obtain waste aramid paper pulp; centrifuge and wash the chemically treated pulp until the pH is neutral to obtain waste aramid paper pulp, and adjust the mass concentration of the waste aramid paper pulp to 5wt%; the average length of the mixed fibers in the pulp is 1~1.1mm, and the average diameter is 40~45μm;

[0095] S3. The waste aramid pulp from step S2 is placed in a PFI disc mill for mechanical beating. The mechanical beating speed is 60,000 r, the beating time is 25 min, and the freeness of the waste aramid pulp is 30~60°SR. The concentration of the resulting waste aramid fine pulp is adjusted to 0.5 wt%. The average length of the mixed fibers in the waste aramid fine pulp is 75~80 μm, and the average diameter is 30~35 μm.

[0096] S4. The waste aramid paper pulp from step S3 is mechanically homogenized at a pressure of 1000 bar for 1 hour to obtain aramid nanofibers. The average length of the aramid nanofibers is 9.1 μm and the average diameter is 270 nm. The scanning electron microscope (SEM) image of the aramid nanofibers is shown below. Figure 2 As shown.

[0097] Example 6

[0098] A method for preparing aramid nanofibers from waste aramid paper is described in Example 6, which uses the same method as Example 5, except that the soaking time in step S1 is 5 hours and the ratio between soaking time and coating thickness is 0.1 hours: 1 μm.

[0099] The chemical treatment time in step S2-2 is 2.5 hours, which is 0.5 times the treatment time in step S1. The average length of the mixed fibers in the slurry is 1.2~1.3 mm and the average diameter is 45~50 μm.

[0100] In step S3, the mechanical beating speed is 80,000 r, the beating time is 30 min, and the beating degree of the waste aramid paper pulp is 30°SR; the average length of the mixed fibers in the waste aramid paper fine pulp is 60~65 μm, and the average diameter is 40~45 μm.

[0101] In step S4, the homogenization pressure is 1000 bar and the homogenization time is 2 h to obtain aramid nanofibers with an average length of 8.6 μm and an average diameter of 282 nm.

[0102] Example 7

[0103] A method for preparing aramid nanofibers from waste aramid paper is described in Example 7, which uses the same method as Example 5, except that the soaking time in step S1 is 10 hours and the ratio between soaking time and coating thickness is 0.2 hours: 1 μm.

[0104] The chemical treatment time in step S2-2 is 2 hours, which is 0.2 times the treatment time in step S1. The average length of the mixed fibers in the slurry is 0.8~0.85 mm and the average diameter is 35~40 μm.

[0105] In step S3, the mechanical beating speed is 20,000 r, the beating time is 30 min, and the beating degree of the waste aramid paper pulp is 60°SR; the average length of the mixed fibers in the waste aramid paper fine pulp is 100~110 μm and the average diameter is 30~35 μm.

[0106] In step S4, the homogenization pressure is 1200 bar and the homogenization time is 1 h to obtain aramid nanofibers with an average length of 10.1 μm and an average diameter of 264 nm.

[0107] Example 8

[0108] A method for preparing aramid nanofibers from waste aramid paper is described in Example 8, which uses the same method as Example 5, except that the coating thickness is 30 μm.

[0109] In step S1, the soaking time is 4.5 h, and the ratio between soaking time and coating thickness is 0.15 h: 1 μm.

[0110] In step S2-2, the chemical treatment time is 0.9h, which is 0.2 times the treatment time in step S1. The average length of the mixed fibers in the slurry is 0.8~0.9mm and the average diameter is 35~40μm.

[0111] In step S3, the average length of the mixed fibers in the waste aramid paper fine pulp is 70~75μm and the average diameter is 30~32μm.

[0112] In step S4, the obtained aramid nanofibers have an average length of 8.2 μm and an average diameter of 270 nm.

[0113] Example 9

[0114] A method for preparing aramid nanofibers from waste aramid paper is described in Example 9, which uses the same method as Example 5, except that the coating thickness is 100 μm.

[0115] In step S2-2, the average length of the mixed fibers in the slurry is 1.2~1.3 mm and the average diameter is 50~53 μm.

[0116] In step S3, the average length of the mixed fibers in the waste aramid paper fine pulp is 70~75μm and the average diameter is 45~50μm.

[0117] In step S4, the obtained aramid nanofibers have an average length of 10.3 μm and an average diameter of 293 nm.

[0118] As can be seen from the experimental results of Examples 1-9, the present invention uses waste aramid paper scraps as raw materials and follows the preparation method for preparing aramid nanofibers from waste aramid paper provided by the present invention. The final aramid nanofibers obtained have an average length of 1-10 μm and an average diameter of 50-300 nm. The waste aramid paper scraps are of two types: one type is scraps obtained after wet papermaking with no resin or coating on the surface; the other type is waste aramid paper with a resin or coating on the surface, with a coating thickness of 30-100 μm. The present invention proposes different preparation methods for different types of waste aramid paper scraps. In Examples 1-4, the waste aramid paper scraps are scraps obtained after wet papermaking with no resin or coating on the surface; in Examples 5-9, the waste aramid paper scraps are waste aramid paper with a resin or coating on the surface, with a coating thickness of 30-100 μm.

[0119] The method for preparing aramid nanofibers provided by this invention, when the raw material is waste aramid paper scraps coated with resin or a coating with a coating thickness of 30-100 μm, in Examples 5-9, through the synergistic effect of four steps—"surface purification → chemical treatment → physical treatment → homogenization and nanofiberization"—the waste aramid paper coated with resin or a coating is successfully converted into high-value-added aramid nanofibers. In step S1, a methanol / dichloromethane mixed solvent combined with scraping and cleaning can efficiently remove the 30-100 μm thick resin or coating, "clearing obstacles" for the chemical treatment in S2, allowing the KOH / DMAC solution to act uniformly and fully on the aramid fiber body, avoiding local degradation caused by uneven reaction; at the same time, it ensures the purity of the raw materials in steps S3 and S4, preventing impurities from being mixed in during mechanical pulping and mechanical homogenization and thus entering the final product, thereby ensuring the purity of the aramid nanofibers. In step S2, after KOH / DMAC treatment, the hydrogen bond network and weak amorphous regions between fibers are effectively destroyed, and the internal bonding force of the fibers is greatly reduced. The mild chemical treatment conditions of 60~80℃ avoid complete dissolution of the fibers. This allows the desired degree of freeness (30~60°SR) to be achieved in the pulping process of step S3 with a lower pulping speed (20000~80000r) and a shorter time (15~30min). The chemical treatment in step S2 makes the fibers tend to split rather than be simply cut, which is conducive to forming fine pulp of appropriate length and ensures the uniformity and controllability of its morphology. Step S3 provides the optimal raw material for step S4 through mechanical pulping, precisely controlling the pulp concentration at 0.5~0.8wt% and ensuring that the average fiber length is 10~100μm and the average diameter is 5~10μm. This effectively avoids clogging of the cavity during operation and ensures that the fine pulp can generate uniform and intense shearing under a pressure of 1000~1200bar. This allows the homogeneous force to be concentrated on "tearing" it into nanofibers rather than cutting it, ultimately obtaining aramid nanofibers with an average length of 1~10μm and an average diameter of 50~300nm.

[0120] The method for preparing aramid nanofibers provided by this invention, when the raw material is waste aramid paper scraps obtained after wet papermaking and the surface is not coated with resin or coating, in Examples 1-4, according to the preparation method provided by this invention, in step S1, the raw material is slightly swollen, and after working together with steps S2, S3 and S4, the aramid nanofibers required by this invention are finally obtained.

[0121] Comparative Example 1

[0122] A method for preparing aramid nanofibers from waste aramid paper is described. This Comparative Example 1 uses the same raw materials and method as Example 5, except that the treatment of soaking in the mixed solvent in step S1 is not performed, and the treatment starts directly from step S2, which fails to produce aramid nanofibers.

[0123] This is because, firstly, the impact of resin or coating on purity is obvious; secondly, the resin or coating on the surface of the raw material makes the chemical treatment step difficult to carry out smoothly, as the resin or coating reacts with the strongly alkaline KOH, leading to a decrease in the effective alkali concentration in the system; and thirdly, the resin swells and dissolves in DMAC, forming a viscous system. Since the bonding force between the fibers of waste aramid paper remains intact and very strong, the beating speed provided by this invention in step S3 cannot smoothly break up the fiber clumps. Simultaneously, the resin coating is sheared, broken, and mixed with the fibers, adhering to the grinding disc surface and contaminating the equipment, resulting in uneven beating. Therefore, step S4 was not performed due to concerns about equipment maintenance.

[0124] Comparative Example 2

[0125] A method for preparing aramid nanofibers from waste aramid paper is described in Comparative Example 2, which uses the same raw materials and methods as Example 5. The difference is that after the waste aramid paper scraps are soaked and scraped off in step S1, they are directly subjected to mechanical pulping as described in step S3 without chemical treatment. In step S3, the pulping speed provided by this invention cannot smoothly obtain pulp, and the speed needs to be increased to 200,000 r. The diameter distribution of the obtained aramid nanofibers is uneven, with an average length of 1~3 μm and an average diameter of 200~500 nm.

[0126] This is because the hydrogen bond network between molecules of untreated waste aramid fiber paper is very strong, and the fiber itself is very tough. It requires a lot of energy to break it into pulp, which causes severe fiber damage. The average length of the fiber is far less than 10-100μm, which cannot meet the requirements of step S3 of the present invention, and is further damaged in step S4.

[0127] Comparative Example 3

[0128] A method for preparing aramid nanofibers from waste aramid paper was described. Comparative Example 3 used the same raw materials and methods as Example 5, except that the order of chemical treatment and mechanical pulping was reversed without changing any parameters. That is, after the waste aramid paper scraps were soaked and scraped off in the mixed solvent in step S1, they were first mechanically pulped, then chemically treated, and finally processed according to step S4. It was found that uniform nano-sized aramid fibers could not be achieved.

[0129] This is because, in the preparation method provided by this invention, the main role of chemical treatment is to break the hydrogen bonds and swollen amorphous regions between fibers, preparing for subsequent longitudinal splitting. In this comparative example, mechanical pulping is performed first, but the mechanical rotation speed is not high enough to fully and uniformly mechanically process the waste aramid paper. At the same time, the fibers are forcibly cut, resulting in large and uneven short rod-shaped fibers. The fiber properties are already damaged in this step, resulting in losses due to excessive cutting. Subsequent chemical treatment cannot effectively guide further fiber dissociation; instead, the presence of precipitated fibers causes agglomeration, leading to an uneven system. Consequently, the aramid fibers obtained cannot achieve nano-scale formation and remain at the micron level.

[0130] Comparative Example 4

[0131] A method for preparing aramid nanofibers from waste aramid paper is described. This Comparative Example 4 uses the same raw materials and methods as Example 5, except that in step S4, the homogenization pressure is 500 bar and the homogenization time is 3 h, resulting in recycled aramid fibers with an average length of 5~15 μm and an average diameter of 500~800 nm.

[0132] This is because insufficient homogeneous pressure results in insufficient nano-sizing of aramid fibers.

[0133] Comparative Example 5

[0134] A method for preparing aramid nanofibers from waste aramid paper was described. Comparative Example 5 used the same raw materials and methods as Example 5, except that in step S1, 90 mL of dichloromethane and 10 mL of methanol were added. During the scraping and cleaning process in step S1, the resin or coating on the surface of the waste aramid paper had a high viscosity, resulting in filamentation during scraping. Agglomeration occurred in steps S2 and S3. The final aramid nanofibers had an average length of 20-30 μm and an average diameter of 800-900 nm.

[0135] This is because the excessive amount of DCM at this time increases the viscosity of the resin or coating, making it difficult to clean the surface and resulting in a thinner layer of resin or coating contamination. This leads to agglomeration during subsequent processing, resulting in uneven fiber size.

[0136] Comparative Example 6

[0137] A method for preparing aramid nanofibers from waste aramid paper is described. This comparative example 6 uses the same raw materials and method as example 5, except that in step S1, 50 mL of dichloromethane and 50 mL of methanol are added. After soaking in step S1, the resin or coating on the surface of the waste aramid paper scraps is difficult to scrape off and clean.

[0138] This is because the amount of methanol used at this time is too high, which affects the swelling and dissolving ability of DCM on the resin and coating, thus affecting the smooth progress of step S1.

[0139] Comparative Example 7

[0140] A method for preparing aramid nanofibers from waste aramid paper is described. This comparative example 7 uses the same raw materials and methods as example 5, except that in step S2, when the chemical treatment temperature is 100°C, nanofibers cannot be formed.

[0141] This is because the main chain of aramid molecules is connected by amide bonds. When the temperature is 100°C, it exceeds the temperature range set in this invention. KOH is a strong base, and under this temperature condition, it will greatly accelerate the alkaline hydrolysis reaction of the amide bonds in the fiber.

[0142] Comparative Example 8

[0143] A method for preparing aramid nanofibers from waste aramid paper was described. Comparative Example 8 used the same raw materials and method as Example 5, except that in step S1, the soaking time was 4.5 hours, and the ratio between soaking time and coating thickness was 0.09 hours:1 μm. After soaking, the resin and coating on the surface could not be easily scraped off completely.

[0144] This is because insufficient soaking time affects the dissolution and swelling effect of the mixed solvent on the resin and coating, causing step S1 to fail.

[0145] Comparative Example 9

[0146] A method for preparing aramid nanofibers from waste aramid paper was described. Comparative Example 9 used the same raw materials and method as Example 5, except that in step S2, the chemical treatment time was 5 hours, which is 0.67 times the soaking time in step S1. The resulting mixed fibers in the waste aramid paper pulp had an average length of 0.1~0.2 mm and an average diameter of 10~15 μm.

[0147] This is because the chemical treatment time in step S2 is too long, exceeding the time range set by the present invention. This causes KOH to excessively hydrolyze the amide bonds in the aramid molecular chain, shortening the aramid polymer chain and causing excessive degradation that leads to gelation and the formation of irreversible gel blocks.

[0148] Comparative Example 10

[0149] A method for preparing aramid nanofibers from waste aramid paper was described in Comparative Example 10. This method employed the same procedure as in Example 1, except that the type of waste aramid paper scraps was different. The waste aramid paper scraps were waste aramid paper with a resin or coating layer on the surface, with a coating thickness of 50 μm. In step S1, the resin or coating layer could not be scraped off, and subsequent steps were not performed.

[0150] This is because, in this invention, after coating the surface of waste aramid paper scraps with resin or a coating layer, a mixed solvent of dichloromethane and methanol is used for soaking in step S1. Dichloromethane is the main solvent for dissolving and swelling the resin; it can effectively penetrate into the resin, disrupting the van der Waals forces between the polymer chains, thus causing swelling and dissolution. Methanol plays a supporting role in penetration, regulating polarity and volatility, and works synergistically with dichloromethane. When the mixed solvent is replaced with ethanol and deionized water, the dissolving capacity of ethanol is very limited, only causing slight swelling.

[0151] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are exhaustively listed. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0152] For those skilled in the art, various modifications and improvements can be made without departing from the concept of the present invention, and these modifications and improvements are all within the scope of protection of the present invention. The scope of protection of the present invention is defined by the appended claims.

Claims

1. A method for preparing aramid nanofibers from waste aramid paper, characterized in that, The preparation method includes the following steps: S1. Soak waste aramid paper scraps in a mixed solvent; S2. The waste aramid paper scraps processed in step S1 are chemically treated in a mixed solution of KOH and DMAC. After centrifugation and washing, waste aramid paper pulp is obtained. The chemical treatment temperature is 60~80℃. S3. Mechanically beat the waste aramid paper pulp from step S2. The mechanical beating is carried out in a PFI disc mill. The mechanical beating speed is 20,000 to 80,000 r, the beating time is 15 to 30 min, and the beating degree of the waste aramid paper pulp is 30 to 60°SR, to obtain fine waste aramid paper pulp. S4. The waste aramid paper pulp from step S3 is subjected to strong mechanical treatment, which is mechanical homogenization. The homogenization pressure is 1000~1200 bar and the homogenization time is 1~2 h to obtain aramid nanofibers. The waste aramid paper scraps are scraps obtained after wet papermaking, without any coating on the surface; or the waste aramid paper scraps are waste aramid paper with a coating on the surface, with a coating thickness of 30~100μm. When the waste aramid paper scraps are scraps obtained from wet papermaking and the surface is not coated: In step S1, the mass ratio of waste aramid paper scraps to the mixed solvent is 1:(10~20); the mixed solvent is ethanol and deionized water, wherein the volume ratio of ethanol to deionized water is 1:(1~3); the soaking time is 6~12h. When the waste aramid paper scraps are waste aramid paper with a surface coating and the coating thickness is 30~100μm: the surface coating of the waste aramid paper scraps is epoxy resin or phenolic resin. In step S1, the mass ratio of waste aramid paper scraps to the mixed solvent is 1:(10~20); the mixed solvent is methanol and dichloromethane, wherein dichloromethane accounts for 60%~80% of the total volume of the mixed solvent, and methanol accounts for 20%~40% of the total volume of the mixed solvent; the soaking time is determined according to the coating thickness of the surface coating, and the ratio between soaking time and coating thickness is (0.1~0.2)h:1μm; In step S2, the chemical treatment time is (0.2~0.5) times the soaking time in step S1.

2. The method for preparing aramid nanofibers from waste aramid paper according to claim 1, characterized in that, When the waste aramid paper scraps are scraps obtained after wet papermaking and the surface is not coated, the chemical treatment time in step S2 is 1~2 hours.

3. The method for preparing aramid nanofibers from waste aramid paper according to claim 1, characterized in that, In step S2, the mass ratio of the waste aramid paper scraps after S1 treatment to the mixed solution is 1:(80~120). The mass ratio of KOH to DMAC is 1:200~500.

4. The method for preparing aramid nanofibers from waste aramid paper according to claim 1, characterized in that, In step S2, the mass concentration of the waste aramid pulp obtained is 5~8wt%; the average length of the mixed fibers in the pulp is 0.5~1mm and the average diameter is 20~30μm.

5. The method for preparing aramid nanofibers from waste aramid paper according to claim 1, characterized in that, In step S3, the concentration of the obtained waste aramid paper fine pulp is 0.5~0.8 wt%; In step S3, the average length of the mixed fibers in the waste aramid paper pulp is 10~100μm and the average diameter is 5~10μm.

6. An aramid nanofiber, characterized in that, The aramid nanofibers are prepared according to any one of claims 1-5, wherein the average length of the aramid nanofibers is 1-10 μm and the average diameter is 50-300 nm.

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