Aqueous aramid nanofiber composite dispersion, composite film and method of making

Abstract

The application relates to a preparation method of a water-dispersed aramid nanofiber composite dispersion liquid, which comprises the following steps: stirring and dispersing para-aramid fibers in a KOH / DMSO system to obtain an aramid nanofiber dispersion liquid; swelling chitin raw materials, adding an anhydride to perform esterification modification to obtain a modified intermediate, stirring and dispersing the modified intermediate in DMSO to obtain a chitin nanofiber dispersion liquid; uniformly mixing the chitin nanofiber dispersion liquid and the aramid nanofiber dispersion liquid, adding deionized water to perform centrifugal operation, adding a centrifugal precipitate into a NaOH solution to perform mechanical treatment, and obtaining the water-dispersed aramid nanofiber composite dispersion liquid. The water-dispersed aramid nanofiber composite dispersion liquid has excellent water dispersibility, and the preparation method is simple, and flocculation does not occur after being placed for more than 3 months.

Description

Technical Field

[0001] This invention relates to the field of polymer nanomaterials, and more particularly to a water-dispersible aramid nanofiber composite dispersion, a composite membrane, and a method for preparing the same. Background Technology

[0002] Aramid fiber, officially known as poly(p-phenylene terephthalamide) (PPTA), possesses excellent properties such as ultra-high strength, high modulus, high temperature resistance, acid and alkali resistance, and light weight. It is widely used in aerospace, defense, electronics and communications, and petrochemical industries. Its rigid chain structure formed by π-π conjugation within the molecular chain, intermolecular hydrogen bonds, and van der Waals forces make it difficult for molecular chain segments to undergo internal rotation, resulting in a linear, rigid chain structure. This structural characteristic leads to defects such as a smooth surface, few active groups, and strong chemical inertness, resulting in poor interfacial bonding between aramid fibers and matrices such as resins. This limits its application in high-performance composite reinforcing materials, nanomaterials, filter materials, and biomedicine.

[0003] Aramid nanofibers were first prepared using the DMSO / KOH system proposed by Professor Kotov in the United States (Dispersions of Aramid Nanofibers: A New Nanoscale Building Block, M. Yang, KQCao, L. Sui, et al. Acs Nano. 2011, 5, 6945-6954). A homogeneous and stable ANFs / DMSO dispersion was obtained by mixing aramid fibers with KOH and DMSO and continuously stirring at room temperature for 7-10 days. The strong base system of KOH / DMSO can directionally and effectively disrupt the hydrogen bonding cross-linking between macroscopic aramid fiber molecular chains, while simultaneously causing the NH bonds on the amide bonds to break and deprotonate, forming nitrogen anions. The negatively charged molecular chains are dispersed and stabilized under the influence of electrostatic repulsion, π-π conjugation forces generated by the benzene ring and amide bonds, and intermolecular van der Waals forces, exhibiting nanoscale size, high aspect ratio, and high specific surface area. In recent years, aramid nanofibers have become a novel type of functional polymer fiber, comparable to carbon nanotubes and cellulose nanofibers. However, the literature reports a method using dimethyl sulfoxide (DMSO) as a solvent and potassium hydroxide to remove hydrogen atoms from the amide bonds of aramid fibers, dissolving macroscopic aramid fibers into nanoscale fibers. The prepared nano-aramid fibers can only exist in a strongly alkaline DMSO dispersion system. When the dispersion system contains a proton donor, the PPTA polyanion immediately acquires hydrogen atoms and becomes protonated, leading to the regeneration of hydrogen bonds between ANF molecular chains and fiber re-aggregation. Especially when the dispersion system is water, a large number of aramid nanofibers protonate and aggregate to form agglomerated precipitates. Therefore, the PPTA polyanion in the ANFs / DMSO solution is highly sensitive to moisture in the air; the dispersion system easily induces ANFs protonation and forms a thin fragment film, making it unsuitable for long-term storage and requiring sealed preservation.

[0004] Therefore, the aramid nanofiber solutions prepared by the methods described in the literature suffer from problems such as being only dispersed in KOH / DMSO systems, being unable to be uniformly and stably dispersed in aqueous systems, and being unable to be stored for long periods. These limitations greatly restrict their application in water-soluble polymers and hydrophilic materials, preventing the high-performance aramid nanofibers from achieving significant reinforcing effects and thus limiting their application areas. Secondly, the membranes made from aramid nanofibers in the DMSO system have low mechanical strength (approximately 100 MPa), which may be related to the significantly reduced crystallinity of the aramid nanofibers in the DMSO system.

[0005] The above problems are currently being addressed primarily through the following three methods:

[0006] (1) High pressure injection: For example, Chinese invention patent with patent application number 201810142388.7 proposed "a method for preparing water-dispersible aramid nanofibers and aramid nanopaper". The patent discloses that colloidal aramid nanofibers are obtained by high pressure injection of deionized water and washing with anhydrous ethanol and deionized water. The aramid nanofibers are dispersed in deionized water under stirring, wet-formed and pressed dry to obtain aramid nanopaper with a tensile strength of only 97 MPa.

[0007] (2) High-pressure injection and composite: For example, Chinese invention patent with patent application number 201810141732.0 proposed "an aramid nanofiber film composite aramid paper and its preparation method". The patent discloses that colloidal aramid nanofibers are obtained by high-pressure injection of deionized water and washing with anhydrous ethanol and deionized water. The nanofibers are dispersed in deionized water under stirring. The wet-formed aramid nanofiber film is used as the top and bottom layers. Para-aramid short-cut fibers and para-aramid pulp / precipitated fiber slurry suspension are used as the core layer. After wet lamination, hot pressing is performed to obtain composite aramid paper with a tensile strength of only 73.6 MPa.

[0008] (3) Modification: For example, Chinese invention patent with patent application number 201410833596.3 proposed "a method for preparing water-dispersible aramid nanofibers and its application". This patent discloses that aramid fibers, catalysts and bases are placed in an organic solvent (such as DMF, DMSO, NMP, DMAC) and heated and stirred to disperse evenly. Then, small organic molecules with active functional groups that can undergo nucleophilic reactions with nitrogen anions (bromopropyne, chloropropyne, 5-chloro-1-pentyne and 1-bromo-2-butyne) are added to react and obtain an aqueous solution of aramid nanofibers. However, obtaining water-soluble small molecules or functional groups through modification will lead to a decrease in its tensile strength, modulus and heat resistance. Moreover, the reaction not only has many steps, but also requires a catalyst, and the preparation process is complicated. To obtain aramid nanofibers, freeze drying is required, which consumes a lot of energy.

[0009] Therefore, it is necessary to develop an aramid nanofiber with excellent properties such as tensile strength, which can be uniformly dispersed in water. Summary of the Invention

[0010] Therefore, the present invention provides a water-dispersible aramid nanofiber composite dispersion, a composite membrane and a method for preparing the same, which are high-strength and can be uniformly dispersed in water.

[0011] To achieve the above objectives, the inventors provide a method for preparing a water-dispersible aramid nanofiber composite dispersion, which includes the following steps:

[0012] Step 1: Stir and disperse para-aramid fibers in a KOH / DMSO system to obtain an aramid nanofiber dispersion with a mass concentration of 1‰-4‰;

[0013] Step 2: After swelling the chitin raw material, add acid anhydride for esterification modification to obtain a modified intermediate. The molar ratio of acid anhydride to chitin raw material is 0.1-5:1. Stir and disperse the modified intermediate in DMSO to obtain a chitin nanofiber dispersion with a mass concentration of 0.5‰-1%.

[0014] Step 3: Mix the chitin nanofiber dispersion obtained in Step 2 and the aramid nanofiber dispersion obtained in Step 1 to obtain a mixed solution. The mass concentration of the chitin nanofiber dispersion in the mixed solution is above 5%. After mixing evenly, add deionized water and centrifuge. Take the precipitate obtained by centrifugation and add the precipitate to NaOH solution for mechanical treatment to disperse it in NaOH solution, thereby obtaining a chitin nanofiber / aramid nanofiber composite dispersion with a mass concentration of 0.1‰-2‰, which is the water-dispersed aramid nanofiber composite dispersion.

[0015] The present invention employs the above-described preparation method. In the dispersion treatment of step 1, the para-aramid fibers are pyrolyzed in DMSO and dispersed into a deep red, clear, and transparent dispersion. The aramid nanofiber dispersion flocculates upon contact with water.

[0016] The steps 1 and 2 described above in this invention are not in any particular order.

[0017] In step 2, chitin nanofibers are modified by acid anhydride esterification. The modified chitin nanofibers have amphiphilic properties and superior mechanical properties. The modified chitin nanofibers contain hydrophilic carboxyl groups and lipophilic acid anhydrides, which can be used as surfactants. They have amphiphilic properties and can be well dispersed in NaOH solution and DMSO.

[0018] In step 3, the chitin nanofiber dispersion and the aramid nanofiber dispersion can be uniformly mixed. Upon adding deionized water, the aramid nanofibers precipitate upon contact with water, carrying the chitin nanofibers with them. Simultaneously, because the chitin nanofibers are uniformly distributed, the aramid nanofibers do not tightly bind together. When mechanically treated with NaOH solution, the modified chitin nanofibers, possessing hydrophilic carboxyl groups, tend to disperse in the NaOH solution. Therefore, the aramid nanofibers can be simultaneously dispersed in the NaOH solution along with the modified chitin nanofibers under mechanical treatment.

[0019] In step 3, the mass concentration of the chitin nano-dispersion is controlled to be above 5%, because if the content of chitin nanofibers is too low, it will not be able to prevent the self-aggregation of aramid nanofibers after they come into contact with water.

[0020] Preferably, in step 1, the para-aramid fiber is one of the following: para-aramid yarn aramid, para-aramid chopped fiber, para-aramid pulp fiber, or para-aramid precipitated fiber.

[0021] Preferably, in step 2, the acid anhydride is one of the following: biphenyl anhydride, hexadecyl succinic anhydride, dodecyl succinic anhydride, maleic anhydride, glutaric anhydride, phthalic anhydride, octyl succinic anhydride, succinic anhydride, 1,2,4-phenyltriacrylic anhydride, succinic anhydride, phenylmaleic anhydride, 2,3-naphthalenedicarboxylic anhydride, hexahydrophthalic anhydride, and butyl succinic anhydride.

[0022] Preferably, in step 2, the chitin raw material swelling treatment is performed by adding the purified chitin raw material into a KOH / DMSO system and stirring and swelling at room temperature for 4-72 hours, thereby facilitating further modification of the chitin.

[0023] Preferably, step 2 specifically involves: swelling the chitin raw material and adding acid anhydride, continuing to stir and react at room temperature for 0.5-2 hours to obtain a modified intermediate, dispersing the modified intermediate in DMSO, stirring for 1-4 hours and then centrifuging to obtain a DMSO-dispersed chitin nanofiber dispersion.

[0024] Preferably, in step 3, the mechanical processing equipment is one of the following: a cell disruptor, a juicer, a high-pressure homogenizer, or a microfluidic homogenizer.

[0025] Preferably, in step 3, the pH of the NaOH solution is 10-12.

[0026] The present invention also discloses a water-dispersible aramid nanofiber composite dispersion, which is prepared by the above-described preparation method.

[0027] This invention also discloses a method for preparing a water-dispersible aramid nanofiber composite membrane, which includes the following steps:

[0028] The water-dispersed aramid nanofiber composite dispersion is first centrifuged, then vacuum filtered to form a film and hot-pressed to obtain the water-dispersed aramid nanofiber composite membrane.

[0029] Preferably, the filter membrane used in the vacuum filtration is a nylon filter membrane with a pore size of 0.1μm-0.5μm.

[0030] Preferably, the hot-pressing conditions are: hot-pressing at 60-100℃ for 20-40 minutes, thereby improving the strength of the composite film.

[0031] The present invention also discloses a water-dispersible aramid nanofiber composite membrane, which is prepared by the above-described preparation method.

[0032] In this invention, the performance of the final composite membrane is controlled by adjusting the dosage ratio of chitosan nanofiber dispersion to aramid nanofiber dispersion. Because this invention allows for controlling the mass concentration of aramid nanofibers in the composite membrane to up to 95%, the strength of the composite membrane is significantly improved.

[0033] The above technical solution has the following advantages, unlike existing technologies:

[0034] 1. The water-dispersible aramid nanofiber composite dispersion prepared by this invention has excellent water dispersibility, and the preparation method is simple. It remains stable at a rotation speed of 9800 rpm and does not exhibit flocculation after being left at room temperature for more than 3 months.

[0035] 2. The water-dispersible aramid nanofiber composite dispersion prepared by the present invention is uniformly composed of chitin nanofibers and aramid nanofibers, with high crystallinity and well maintained crystal structure. As a result, the water-dispersible aramid nanofiber composite film prepared has a maximum tensile strength of 344 MPa. Attached Figure Description

[0036] Figure 1 Image of the water-dispersed aramid nanofiber composite dispersion prepared in Example 1;

[0037] Figure 2 The UV transmittance of the water-dispersed aramid nanofiber composite dispersion prepared in Example 1 at different storage times;

[0038] Figure 3 An atomic force microscope image of the water-dispersed aramid nanofiber composite dispersion prepared in step 3 of Example 1;

[0039] Figure 4 Field emission scanning electron microscope (FESEM) image of the cross section of the water-dispersible aramid nanofiber composite membrane prepared in Example 1. Detailed Implementation

[0040] To explain in detail the technical content, structural features, objectives, and effects of the technical solution, the following description is provided in conjunction with specific embodiments and accompanying drawings.

[0041] In all embodiments of the present invention, the chitin raw material is pretreated to obtain chitin. The treatment operation can be carried out by the following method: purifying the chitin raw material by soaking in an acid solution to remove protein, then soaking in an alkaline solution to remove calcium carbonate, and finally soaking in a bleaching solution to remove pigment.

[0042] The process of removing protein by soaking in an acidic solution, removing calcium carbonate by soaking in an alkaline solution, and finally removing pigment by soaking in a bleaching solution is as follows: Calcium carbonate is removed by soaking in a 2 mol / L hydrochloric acid solution for 48 hours; protein is removed by soaking in a 4% aqueous hydroxide solution for 48 hours; pigment is removed by soaking in a bleaching solution at 80°C for 3 hours; then the chitin is washed with deionized water and dried to obtain the purified chitin. The percentage content of each component in the bleaching solution is: 0.38% sodium hypochlorite, 0.55% sodium hydroxide, 1.58% acetic acid, and the remaining component is water.

[0043] Example 1

[0044] Step 1: Mix 1g of para-aramid yarn aramid raw material with 1.5g KOH and 500mL (550g) DMSO, and stir at room temperature for 7 days to disperse the mixture, so as to obtain an aramid nanofiber dispersion with a mass concentration of 2‰.

[0045] Step 2: Mix 80 mg of purified chitin with 20 mg KOH and 20 mL (22 g) DMSO, and stir and swell at room temperature for 72 h. Then add 88.4 mg of biphenyl anhydride (the molar ratio of biphenyl anhydride to chitin structural units is 1:1), and continue stirring at room temperature for 1 h. Add 20 mL of DMSO to the product and stir and dilute for 2 h to obtain a chitin nanofiber dispersion with a DMSO dispersion concentration of 2‰.

[0046] Step 3: Take 40 mL of aramid nanofiber dispersion in DMSO system and mix it with 10 mL of chitin nanofiber dispersion in DMSO system. Add deionized water, centrifuge at 9800 rpm for 15 min, and take the precipitate. Add the precipitate to 100 mL of pH 11 NaOH solution, mechanically treat it with a cell disruptor for 30 min, and then take it out to obtain a 1‰ water-dispersed aramid nanofiber composite dispersion.

[0047] Step 4: Centrifuge the water-dispersed aramid nanofiber composite dispersion at 9800 rpm for 15 min, and then vacuum filter it through a 0.22 μm pore size filter membrane to obtain a composite membrane. Place the composite membrane in a hot press and hot press it at 0.04 MPa and 80℃ for 30 min to obtain the water-dispersed aramid nanofiber composite membrane.

[0048] In this embodiment, water-dispersed aramid nanofiber composite dispersion is prepared in steps 1-3, and water-dispersed aramid nanofiber composite membrane is obtained through further operation in step 4.

[0049] The density of the water-dispersed aramid nanofiber composite membrane was measured to be 1.02 g / m³. 3It has a light transmittance of 68%, a tensile strength of 350 MPa, and a toughness of 23.2 MJ / m. 3 .

[0050] Figure 1 This is a photograph of the water-dispersed aramid nanofiber composite dispersion prepared in step 3 of this embodiment after bottling. Figure 1 It can be seen that the dispersion does not exhibit flocculation or other phenomena, and its dispersibility is very good.

[0051] After the water-dispersed aramid nanofiber composite dispersion prepared in step 3 was bottled, it was left to stand for 90 days. The UV transmittance of the dispersion was observed at different standing days (0 days, 30 days, 50 days, 70 days, and 90 days). The test results are shown in [Figure number missing]. Figure 2 .Depend on Figure 2 It can be seen that after 90 days of storage, the permeability of the water-dispersed aramid nanofiber composite dispersion is still very high, indicating that the dispersion has good dispersibility and no flocculation or other phenomena.

[0052] The aqueous dispersion of aramid nanofiber composite prepared in step 3 was photographed using an atomic force microscope. See the attached image for details. Figure 3 As shown. From Figure 3 As can be seen, the water-dispersible aramid nanofiber composite dispersion prepared in step 3 shows uniform fiber dispersion without agglomeration under an atomic force microscope, indicating that the dispersion has good dispersibility.

[0053] The cross-section of the water-dispersed aramid nanofiber composite membrane prepared in step 4 of this embodiment was scanned by field emission scanning electron microscopy. The scanning results are shown in the figure. Figure 4 .Depend on Figure 4 It can be seen that the cross-section of the water-dispersed aramid nanofiber composite membrane has a regular layered structure, and the composite membrane has high strength.

[0054] Example 2

[0055] Step 1: Mix 80 mg of purified chitin with 20 mg KOH and 20 mL (22 g) DMSO, and stir and swell at room temperature for 72 h. Then add 88.4 mg of biphenyl anhydride (the molar ratio of biphenyl anhydride to chitin structural units is 1:1), and continue stirring at room temperature for 1 h. Add 20 mL of DMSO to the product and stir and dilute for 2 h to obtain a chitin nanofiber dispersion with a DMSO dispersion mass concentration of 2‰.

[0056] Step 2: Mix 1g of para-aramid yarn aramid raw material with 1.5g KOH and 500mL (550g) DMSO, and stir at room temperature for 7 days to obtain an aramid nanofiber dispersion with a mass concentration of 2‰.

[0057] Step 3: Take 30 mL of aramid nanofiber dispersion in DMSO system and mix it with 20 mL of chitin nanofiber dispersion in DMSO system. Add deionized water, centrifuge at 9800 rpm for 15 min, and take the precipitate. Add the precipitate to 100 mL of pH 11 NaOH solution, mechanically treat with a cell disruptor for 30 min, and then take it out to obtain a 1‰ water-dispersed aramid nanofiber composite dispersion.

[0058] Step 4: Centrifuge the water-dispersed aramid nanofiber composite dispersion at 9800 rpm for 15 min, and then vacuum filter it through a 0.22 μm pore size filter membrane to obtain a composite membrane. Place the composite membrane in a hot press and hot press it at 0.04 MPa and 80℃ for 30 min to obtain the water-dispersed aramid nanofiber composite membrane.

[0059] Example 3

[0060] Step 1: Mix 80 mg of purified chitin with 20 mg KOH and 20 mL (22 g) DMSO, and stir and swell at room temperature for 72 h. Then add 88.4 mg of biphenyl anhydride (the molar ratio of biphenyl anhydride to chitin structural units is 1:1), and continue stirring at room temperature for 1 h. Add 20 mL of DMSO to the product and stir and dilute for 2 h to obtain a chitin nanofiber dispersion with a DMSO dispersion mass concentration of 2‰.

[0061] Step 2: Mix 1g of para-aramid yarn aramid raw material with 1.5g KOH and 500mL (550g) DMSO, and stir at room temperature for 7 days to obtain an aramid nanofiber dispersion with a mass concentration of 2‰.

[0062] Step 3: Take 20 mL of aramid nanofibers dispersed in DMSO system and mix them with 30 mL of chitin nanofibers dispersed in DMSO system. Add deionized water, centrifuge at 9800 rpm for 15 min, and take the precipitate. Add the precipitate to 100 mL of pH 11 NaOH solution, mechanically treat with a cell disruptor for 30 min, and then take out the dispersion to obtain a 1‰ water-dispersed aramid nanofiber composite dispersion.

[0063] Step 4: Centrifuge the water-dispersed aramid nanofiber composite dispersion at 9800 rpm for 15 min, and then vacuum filter it through a 0.22 μm pore size filter membrane to obtain a composite membrane. Place the composite membrane in a hot press and hot press it at 0.04 MPa and 80℃ for 30 min to obtain the water-dispersed aramid nanofiber composite membrane.

[0064] Example 4

[0065] Step 1: Mix 80 mg of purified chitin with 20 mg KOH and 20 mL (22 g) DMSO, and stir and swell at room temperature for 72 h. Then add 88.4 mg of biphenyl anhydride (the molar ratio of biphenyl anhydride to chitin structural units is 1:1), and continue stirring at room temperature for 1 h. Add 20 mL of DMSO to the product and stir and dilute for 2 h to obtain a chitin nanofiber dispersion with a DMSO dispersion mass concentration of 2‰.

[0066] Step 2: Mix 1g of para-aramid yarn aramid raw material with 1.5g KOH and 500mL (550g) DMSO, and stir at room temperature for 7 days to obtain an aramid nanofiber dispersion with a mass concentration of 2‰.

[0067] Step 3: Mix aramid nanofibers dispersed in 10 mL of DMSO system with chitin nanofibers dispersed in 40 mL of DMSO system, add deionized water, centrifuge at 9800 rpm for 15 min, collect the precipitate, add the precipitate to 100 mL of pH 11 NaOH solution, mechanically treat with a cell disruptor for 30 min, and then collect the dispersion to obtain a 1‰ water-dispersed aramid nanofiber composite dispersion.

[0068] Step 4: Centrifuge the water-dispersed aramid nanofiber composite dispersion at 9800 rpm for 15 min, and then vacuum filter it through a 0.22 μm pore size filter membrane to obtain a composite membrane. Place the composite membrane into a hot press and hot press it at 0.04 MPa and 80℃ for 30 min to obtain the water-dispersed aramid nanofiber composite membrane.

[0069] Example 5

[0070] Step 1: Mix 80 mg of purified chitin with 20 mg KOH and 20 mL (22 g) DMSO, and stir and swell at room temperature for 48 h. Then add 83.6 mg of octyl succinic anhydride (the molar ratio of octyl succinic anhydride to chitin structural units is 1:1), and continue stirring at room temperature for 30 min. Add 20 mL of DMSO to the product and stir and dilute for 4 h to obtain a chitin nanofiber dispersion with a DMSO dispersion concentration of 2‰.

[0071] Step 2: Mix 1g of para-aramid yarn aramid raw material with 1.5g KOH and 500mL (550g) DMSO, and stir at room temperature for 7 days to obtain an aramid nanofiber dispersion with a mass concentration of 2‰.

[0072] Step 3: Take 40 mL of aramid nanofibers dispersed in DMSO system and mix them with 10 mL of chitin nanofibers dispersed in DMSO system. Add deionized water, centrifuge at 9800 rpm for 15 min, and take the precipitate. Add the precipitate to 100 mL of pH 11 NaOH solution, and mechanically process it with a juicer for 60 min. Then take out the dispersion to obtain a 1‰ water-dispersed aramid nanofiber composite dispersion.

[0073] Step 4: Centrifuge the water-dispersed aramid nanofiber composite dispersion at 9800 rpm for 15 min, and then vacuum filter it through a 0.22 μm pore size filter membrane to obtain a composite membrane. Place the composite membrane in a hot press and hot press it at 0.04 MPa and 80℃ for 30 min to obtain the water-dispersed aramid nanofiber composite membrane.

[0074] Example 6

[0075] Step 1: Mix 80 mg of purified chitin with 20 mg KOH and 20 mL (22 g) DMSO, and stir and swell at room temperature for 24 h. Then add 5.8 mg of phthalic anhydride (the molar ratio of phthalic anhydride to chitin structural units is 0.1:1), and continue stirring and reacting at room temperature for 30 min. Add 140 mL of DMSO to the product and stir and dilute for 1 h to obtain a chitin nanofiber dispersion with a DMSO dispersion mass concentration of 0.5‰.

[0076] Step 2: Mix 0.5g of para-aramid pulp fiber raw material with 2.5g of KOH and 500mL (550g) of DMSO, and stir at room temperature for 7 days to obtain an aramid nanofiber dispersion with a mass concentration of 1‰.

[0077] Step 3: Take 20 mL of aramid nanofibers dispersed in DMSO system and mix them with 40 mL of chitin nanofibers dispersed in DMSO system. Add deionized water, centrifuge at 9800 rpm for 15 min, and take the precipitate. Add the precipitate to 400 mL of pH 11 NaOH solution, mechanically treat with microfluidic jet for 30 min, and then take out the dispersion to obtain a water-dispersed aramid nanofiber composite dispersion with a concentration of 0.1‰.

[0078] Step 4: Centrifuge the water-dispersed aramid nanofiber composite dispersion at 9800 rpm for 15 min, and then vacuum filter it through a 0.1 μm pore size filter membrane to obtain a composite membrane. Place the composite membrane in a hot press and hot press it at 0.04 MPa and 60℃ for 40 min to obtain the water-dispersed aramid nanofiber composite membrane.

[0079] Example 7

[0080] Step 1: Mix 80 mg of purified chitin with 30 mg KOH and 40 mL (44 g) DMSO, and stir and swell at room temperature for 24 h. Then add 197 mg of succinic anhydride (the molar ratio of succinic anhydride to chitin structural units is 5:1), and continue stirring at room temperature for 2 h. Add 40 mL of DMSO to the product and stir and dilute for 1 h to obtain a chitin nanofiber dispersion with a DMSO dispersion concentration of 1‰.

[0081] Step 2: Mix 1g of para-aramid precipitated fiber raw material with 2.0g KOH and 500mL (550g) DMSO, and stir at room temperature for 7 days to obtain an aramid nanofiber dispersion with a mass concentration of 2‰.

[0082] Step 3: Take 40 mL of aramid nanofibers dispersed in DMSO system and mix them with 20 mL of chitin nanofibers dispersed in DMSO system. Add deionized water, centrifuge at 9800 rpm for 15 min, and take the precipitate. Add the precipitate to 50 mL of pH 12 NaOH solution, and perform mechanical treatment by high pressure homogenization for 30 min. Then take out the dispersion to obtain a 2‰ water-dispersed aramid nanofiber composite dispersion.

[0083] Step 4: Centrifuge the water-dispersed aramid nanofiber composite dispersion at 9800 rpm for 15 min, and then vacuum filter it through a 0.50 μm pore size filter membrane to obtain a composite membrane. Place the composite membrane into a hot press and hot press it at 0.02 MPa and 80℃ for 40 min to obtain the water-dispersed aramid nanofiber composite membrane.

[0084] Example 8

[0085] Step 1: Mix 80 mg of purified chitin with 20 mg KOH and 20 mL (22 g) DMSO, and stir and swell at room temperature for 24 h. Then add 132.6 mg of biphenyl anhydride (the molar ratio of biphenyl anhydride to chitin structural units is 1.5:1), and continue stirring at room temperature for 1 h to obtain a chitin nanofiber dispersion with a mass concentration of 4‰ in DMSO.

[0086] Step 2: Mix 2g of para-aramid short-cut fiber raw material with 1.0g KOH and 500mL (550g) DMSO, and stir at room temperature for 7 days to obtain an aramid nanofiber dispersion with a mass concentration of 4‰.

[0087] Step 3: Take 20 mL of aramid nanofibers dispersed in DMSO system and mix them with 30 mL of chitin nanofibers dispersed in DMSO system. Add deionized water, centrifuge at 9800 rpm for 15 min, and take the precipitate. Add the precipitate to 250 mL of pH 10 NaOH solution, and mechanically process it with a juicer for 60 min. Then take out the dispersion to obtain a water-dispersed aramid nanofiber composite dispersion with a concentration of 0.8‰.

[0088] Step 4: Centrifuge the water-dispersed aramid nanofiber composite dispersion at 9800 rpm for 15 min, and then vacuum filter it through a 0.22 μm pore size filter membrane to obtain a composite membrane. Place the composite membrane into a hot press and hot press it at 0.05 MPa pressure and 100℃ for 20 min to obtain the water-dispersed aramid nanofiber composite membrane.

[0089] Example 9

[0090] Step 1: Mix 80 mg of purified chitin with 20 mg KOH and 20 mL (22 g) DMSO, and stir and swell at room temperature for 24 h. Then add 182.3 mg of hexahydrophthalic anhydride (the molar ratio of hexahydrophthalic anhydride to chitin structural units is 3:1), and continue stirring at room temperature for 1 h to obtain a chitin nanofiber dispersion with a mass concentration of 4‰ in DMSO.

[0091] Step 2: Mix 2g of para-aramid short-cut fiber raw material with 1.0g KOH and 500mL (550g) DMSO, and stir at room temperature for 7 days to obtain an aramid nanofiber dispersion with a mass concentration of 4‰.

[0092] Step 3: Take 20 mL of aramid nanofibers dispersed in DMSO system and mix them with 30 mL of chitin nanofibers dispersed in DMSO system. Add deionized water, centrifuge at 9800 rpm for 15 min, and take the precipitate. Add the precipitate to 133.3 mL of pH 10 NaOH solution, and mechanically process it with a juicer for 60 min. Then take out the dispersion to obtain a 1.5‰ water-dispersed aramid nanofiber composite dispersion.

[0093] Step 4: Centrifuge the water-dispersed aramid nanofiber composite dispersion at 9800 rpm for 15 min, and then vacuum filter it through a 0.22 μm pore size filter membrane to obtain a composite membrane. Place the composite membrane into a hot press and hot press it at 0.05 MPa pressure and 100℃ for 20 min to obtain the water-dispersed aramid nanofiber composite membrane.

[0094] Example 10

[0095] Step 1: Mix 80 mg of purified chitin with 20 mg KOH and 8 mL (8.8 g) DMSO, and stir and swell at room temperature for 24 h. Then add 132.6 mg of biphenyl anhydride (the molar ratio of biphenyl anhydride to chitin structural units is 1.5:1), and continue stirring at room temperature for 1 h to obtain a chitin nanofiber dispersion with a mass concentration of 1% in DMSO.

[0096] Step 2: Mix 2g of para-aramid short-cut fiber raw material with 1.0g KOH and 500mL (550g) DMSO, and stir at room temperature for 7 days to obtain an aramid nanofiber dispersion with a mass concentration of 4‰.

[0097] Step 3: Take 47.5 mL of aramid nanofibers dispersed in DMSO system and mix them with 1 mL of chitin nanofibers dispersed in DMSO system. Add deionized water, centrifuge at 9800 rpm for 15 min, and take the precipitate. Add the precipitate to 100 mL of pH 10 NaOH solution, and mechanically process it with a juicer for 60 min. Then take out the dispersion to obtain a 2‰ water-dispersed aramid nanofiber composite dispersion.

[0098] Step 4: Centrifuge the water-dispersed aramid nanofiber composite dispersion at 9800 rpm for 15 min, and then vacuum filter it through a 0.22 μm pore size filter membrane to obtain a composite membrane. Place the composite membrane into a hot press and hot press it at 0.05 MPa pressure and 100℃ for 20 min to obtain the water-dispersed aramid nanofiber composite membrane.

[0099] The water-dispersed aramid nanofiber composite membranes prepared in Examples 1-10 were subjected to tensile strength, elongation at break, modulus, and toughness tests. The tensile strength and elongation at break were tested using a 68TM-10 high and low temperature testing machine manufactured by Instron Corporation (USA). Before testing, the membranes were cut into 5cm × 1cm rectangles, and the tensile rate was set to 2mm / min. Each sample was tested three times. The modulus and toughness were calculated using Origin software. The test results are shown in Table 1.

[0100] Table 1 Performance test data of water-dispersible aramid nanofiber composite membranes prepared in Examples 1-10

[0101]

[0102]

[0103] As shown in Table 1, the tensile strength of the water-dispersed aramid nanofiber composite membrane prepared by this invention is above 170 MPa, reaching a maximum of 344 MPa. In Examples 1-4, with a fixed solution concentration, the amount of chitosan nanofiber dispersion was gradually increased while the amount of aramid nanofiber dispersion was gradually decreased. The test data in Table 1 show that as the amount of aramid nanofiber dispersion decreased, the tensile strength of the final aramid nanofiber composite membrane gradually decreased, and the elongation at break, modulus, and toughness also decreased simultaneously. This is because aramid nanofibers have greater strength than chitosan nanofibers, and a decrease in the proportion of aramid nanofibers has a smaller effect on improving strength.

[0104] It should be noted that although the above embodiments have been described herein, this does not limit the scope of patent protection of the present invention. Therefore, any changes and modifications made to the embodiments described herein based on the innovative concept of the present invention, or equivalent structural or procedural transformations made using the content of the present invention's specification and drawings, directly or indirectly applying the above technical solutions to other related technical fields, are all included within the scope of patent protection of the present invention.

Claims

1. A method for preparing a water-dispersible aramid nanofiber composite dispersion, characterized in that: It includes the following steps: Step 1: Stir and disperse para-aramid fibers in a KOH / DMSO system to obtain an aramid nanofiber dispersion with a mass concentration of 1‰-4‰; Step 2: After swelling the chitin raw material, add acid anhydride for esterification modification to obtain a modified intermediate. The molar ratio of acid anhydride to chitin raw material is 0.1-5:

1. Stir and disperse the modified intermediate in DMSO to obtain a chitin nanofiber dispersion with a mass concentration of 0.5‰-1%. Step 3: Mix the chitin nanofiber dispersion obtained in Step 2 and the aramid nanofiber dispersion obtained in Step 1 to obtain a mixed solution. The mass concentration of the chitin nanofiber dispersion in the mixed solution is above 5%. After mixing evenly, add deionized water and centrifuge. Take the precipitate obtained by centrifugation and add the precipitate to NaOH solution for mechanical treatment to disperse it in NaOH solution, thereby obtaining a chitin nanofiber / aramid nanofiber composite dispersion with a mass concentration of 0.1‰-2‰, which is the water-dispersed aramid nanofiber composite dispersion.

2. The method for preparing the water-dispersible aramid nanofiber composite dispersion according to claim 1, characterized in that: In step 1, the para-aramid fiber is one of the following: para-aramid yarn aramid, para-aramid chopped fiber, para-aramid pulp fiber, or para-aramid precipitated fiber.

3. The method for preparing the water-dispersible aramid nanofiber composite dispersion according to claim 1, characterized in that: In step 2, the acid anhydride is one of the following: biphenyl anhydride, hexadecyl succinic anhydride, dodecyl succinic anhydride, maleic anhydride, glutaric anhydride, phthalic anhydride, octyl succinic anhydride, succinic anhydride, 1,2,4-phenyltriacid anhydride, phenylmaleic anhydride, 2,3-naphthalenedicarboxylic anhydride, hexahydrophthalic anhydride, and butyl succinic anhydride.

4. The method for preparing the water-dispersible aramid nanofiber composite dispersion according to claim 1, characterized in that: In step 2, the chitin raw material swelling treatment is as follows: the purified chitin raw material is added to the KOH / DMSO system and stirred and swollen at room temperature for 4-72 hours.

5. The method for preparing the water-dispersible aramid nanofiber composite dispersion according to claim 1, characterized in that: Step 2 specifically involves: swelling the chitin raw material and adding acid anhydride, then stirring and reacting at room temperature for 0.5-2 hours to obtain a modified intermediate. The modified intermediate is then stirred and dispersed in DMSO, stirred for 1-4 hours, and centrifuged to obtain a DMSO-dispersed chitin nanofiber dispersion.

6. The method for preparing the water-dispersible aramid nanofiber composite dispersion according to claim 1, characterized in that: The mechanical processing equipment in step 3 includes one of the following: cell disruptor, juicer, high-pressure homogenizer, and microfluidic homogenizer.

7. A water-dispersible aramid nanofiber composite dispersion, characterized in that: It is prepared by the preparation method described in any one of claims 1-6.

8. A method for preparing a water-dispersible aramid nanofiber composite membrane, characterized in that: It includes the following steps: The water-dispersed aramid nanofiber composite dispersion of claim 7 is first centrifuged, then vacuum filtered to form a film and hot-pressed to obtain the water-dispersed aramid nanofiber composite membrane.

9. The method for preparing the water-dispersible aramid nanofiber composite membrane according to claim 8, characterized in that: The hot pressing conditions are: hot pressing at 60-100℃ for 20-40 minutes.

10. A water-dispersible aramid nanofiber composite membrane, characterized in that: It is prepared by the preparation method described in claim 8 or 9.

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