A method for preparing para-aramid nanofiber membrane by airflow-assisted drafting coaxial spinning

Abstract

The present invention discloses a method for preparing para-aramid nanofiber membranes by airflow-assisted coaxial spinning, comprising: ultrasonically drying short PPTA fibers in anhydrous ethanol, and dissolving them in a deprotonated system to obtain an ANF suspension; dissolving PEO powder in N,N-dimethylformamide to obtain a PEO solution, respectively injecting the solution into the corresponding pipes of the coaxial spinning needles, performing airflow-assisted coaxial spinning, depositing the fibers on a water bath receiving net to form a fiber membrane, and then washing and drying the resulting pure ANF. The ANFs obtained by airflow-assisted coaxial spinning retain the excellent properties of macroscopic PPTA fibers while exhibiting the unique characteristics of ANFs, such as small fiber diameter and high membrane porosity. They can be applied to lithium-ion battery separators, thermal insulation, filtration, and other fields.

Description

Technical Field

[0001] The invention belongs to the field of nanofiber preparation, and particularly relates to a method for preparing a para-aramid nanofiber membrane by airflow-assisted drafting coaxial spinning. Background Art

[0002] Para-aramid fiber, also known as poly(p-phenylene terephthalamide) (PPTA) fiber, is a pseudo-network cross-linked polymer with excellent properties such as high strength, high modulus, high temperature resistance, and corrosion resistance. It can be used to prepare high-strength, lightweight honeycomb structural materials, high-temperature resistant insulation materials, and high-performance electronic communication equipment materials. Therefore, it is widely used in aerospace, transportation, electricity, national defense and military fields, and occupies an important position in modern industry.

[0003] With the continuous innovation and development of nanotechnology, aramid nanofibers (ANF) have gradually become the focus of attention. When the material is reduced to the nanoscale, due to the changes in its physical and chemical properties, it exhibits small size effects, surface effects, quantum size effects and macroscopic quantum tunneling effects. Nanofibers usually exhibit unique properties that are not present in the original material while maintaining the original macroscopic material due to changes in size, morphology and structure. However, due to the high orientation and crystallinity of the molecular chains on the surface of PPTA chopped fibers, the large steric hindrance of the benzene ring makes it difficult for the hydrogen on the amide group to react with other atoms. In addition, the fiber surface is smooth, the specific surface area is small, the lack of chemically active groups makes it chemically inert, and there are no physical meshing points with the matrix. As a result, PPTA fibers are not easily dissolved by ordinary organic solvents, and are even more difficult to be cracked into aramid nanofibers, which greatly affects its application in the field of composite materials.

[0004] Para-aramid nanofiber membranes (ANFM) are formed by intra- and intermolecular bonding of ANFs. Their unique properties and broad application prospects have attracted active research by researchers. Currently, the preparation methods of ANFM mainly focus on layer-by-layer self-assembly and electrospinning. Patent document CN106170878A composites ANF with polyethylene oxide (PEO) to obtain a nanocomposite film with high ion flux and good thermal stability through layer-by-layer self-assembly (LBL); patent document CN108084468A composites ANF with polydiallyldichloromethylammonium (PDDA) to prepare a film with excellent mechanical properties through vacuum-assisted LBL; patent document CN110373814A adds PEO powder to an ANF dispersion to prepare a spinning solution, and prepares ANFs / PEO nanofiber membranes through electrospinning technology, which have a wide range of applications.

[0005] Currently, the layer-by-layer self-assembly method typically requires at least 200-300 repetitions, which is extremely time-consuming. Furthermore, the prepared nanocomposite membranes sometimes require supercritical drying or freeze-drying to replace the liquid in the wet gel with gas, which has the disadvantage of high energy consumption and environmental impact. Furthermore, the electrospinning method itself has a number of drawbacks, including the high viscosity and poor conductivity of the polymer melt, the requirement for a high electric field strength, and the risk of electric field breakdown. The fibers produced are mostly micron-sized, and the apparatus is complex, requiring additional high-temperature heating equipment, which can easily cause electrostatic interference with high-voltage equipment. Furthermore, since the ANF dispersion is blended with PEO, some PEO is doped into the fibers, resulting in reduced thermal stability of the resulting composite nanofiber membranes. Reducing the energy consumption and shortening the preparation cycle of ANFM would greatly expand its applications. However, due to the high rigidity of the PPTA molecular chain, it is difficult to form fibers, resulting in poor spinnability. Furthermore, pure ANF is difficult to form into membranes, and even if it does form, the membranes are very fragile, significantly limiting its application. Summary of the Invention

[0006] In order to solve the problems of difficulty in preparing ANFM, long preparation cycle and high energy consumption in the prior art, the purpose of the present invention is to provide a method for preparing para-aramid nanofiber membrane by airflow-assisted drawing coaxial spinning, in which PEO solution is used to wrap ANF dispersion and ANFM is prepared by airflow-assisted drawing coaxial spinning. This method is not only conducive to the complete removal of PEO, but also has no effect on the thermal stability of ANFM.

[0007] To achieve the above object, the present invention adopts the following technical solutions:

[0008] The present invention provides a method for preparing a para-aramid nanofiber membrane by airflow-assisted drafting coaxial spinning, comprising the following steps:

[0009] Step 1: PPTA staple fibers were ultrasonically treated in anhydrous ethanol for 2 h and then dried. The treated PPTA staple fibers were dissolved in a deprotonated system to obtain an ANF suspension. PEO powder was dissolved in N,N-dimethylformamide (DMF) to obtain a PEO solution.

[0010] Step 2: The ANF suspension and the spinning aid are respectively injected into the pipes corresponding to the coaxial spinning needle, including: the ANF suspension is injected into the injection pump corresponding to the core layer solution channel, and the PEO solution is injected into the injection pump corresponding to the shell layer solution channel. The liquid outlet of the propeller injection pump is connected to the coaxial spinning needle through the liquid inlet pipe, the propulsion speed is set, and the air tank, pressure regulating valve, and air compressor are connected through the air pipe;

[0011] Step 3: airflow-assisted drafting coaxial spinning, including: turning on the air compressor, air inlet, pressure regulating valve and injection pump, the spinning solution is pushed into the liquid inlet pipe and flows out from the outlet of the coaxial spinning needle, blown by high-speed airflow, and deposited in the water bath net curtain to form a film, thereby forming a fiber membrane;

[0012] Step 4: Take out the deposited fiber membrane, wash it with deionized water to remove PEO, and dry it to obtain pure ANFM.

[0013] Preferably, in step 1, the deprotonation system consists of potassium hydroxide (KOH), dimethyl sulfoxide (DMSO) and anhydrous ethanol.

[0014] Preferably, in step 1, the PPTA staple fiber has a fineness of 200 denier, a concentration of 0.8-1.0 wt%, preferably 1.0 wt%; the PEO molecular weight is 600,000, a concentration of 5-7 wt%, preferably 6.5 wt%.

[0015] Preferably, in step 2, the spinning aid is one of PEO, polyvinyl pyrrolidone (PVP) or polyvinyl alcohol (PVA), more preferably PEO.

[0016] Preferably, in step 2, the propulsion speed of the syringe pump corresponding to the ANF suspension is 0.1-0.3 mL / min, more preferably 0.2 mL / min; the propulsion speed of the syringe pump corresponding to the PEO solution is 0.3 mL / min.

[0017] Preferably, in step 2, the distance between the receiving net and the spinning needle is 20 to 40 cm, preferably 30 cm; the inner diameter of the coaxial spinning needle is 16G+21G or 17G+22G, more preferably 16G+21G.

[0018] Preferably, a method for preparing ANFM comprises:

[0019] (a) PPTA chopped fibers were immersed in anhydrous ethanol and ultrasonicated for 2 h before drying. KOH and the treated PPTA chopped fibers were added to DMSO and anhydrous ethanol at a volume ratio of 1:20. The mixture was stirred in a water bath to obtain an ANF suspension with a concentration of 1.0 wt%;

[0020] (b) PEO powder was added to DMF and stirred in a water bath to obtain a PEO solution with a concentration of 6.5 wt%;

[0021] (c) ANF suspension and PEO solution were injected into a coaxial needle. The propulsion speed ratio of the PEO solution to the ANF suspension was 3:2. Core-shell fibers were obtained by airflow-assisted pulling and stretching. The fibers were randomly deposited on a water bath curtain to form a film. The receiving distance was 30 cm.

[0022] (d) The ANFM was washed with deionized water to completely remove PEO and air-dried at room temperature to obtain pure ANFM.

[0023] Compared with the prior art, the method for preparing para-aramid nanofiber membrane by airflow-assisted coaxial spinning provided by the present invention has the following beneficial effects:

[0024] 1. The present invention uses airflow-assisted traction and stretching coaxial spinning to obtain ANFM, which not only retains the excellent properties of macroscopic PPTA fibers, but also exhibits the unique characteristics of ANFs such as small fiber diameter and high membrane porosity. It has a wide range of applications in lithium-ion battery separators, thermal insulation, filtration and other fields.

[0025] 2. The preparation method provided by the present invention not only eliminates the supercritical drying or freeze-drying process required in the preparation of ANFM by vacuum-assisted layer-by-layer self-assembly, the preparation process is relatively environmentally friendly and the preparation process is shorter, but also completely removes PEO in ANFM to obtain pure ANFM.

[0026] 3. The preparation method provided by the present invention is simple and can be operated continuously, is green and environmentally friendly, has moderate cost, and is conducive to scale-up for industrial production. The obtained ANFM has the characteristics of light weight, high strength, high flexibility, high chemical stability and high thermal stability, and can be used in extreme environments or special environments, further expanding the application of this type of material. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 Schematic diagram of the airflow-assisted drafting coaxial spinning process in the embodiment.

[0028] Figure 2 This is a digital photo of the para-aramid nanofiber membrane obtained in Example 1.

[0029] Figure 3 This is a scanning electron microscope photograph of the para-aramid nanofiber membrane obtained in Example 1.

[0030] Figure 4 This is the pore size distribution diagram of the para-aramid nanofiber membrane obtained in Example 1.

[0031] Figure 5 This is the tensile stress-strain curve of the para-aramid nanofiber membrane obtained in Example 1. DETAILED DESCRIPTION

[0032] The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0033] like Figure 1 As shown, the following embodiment provides a method for preparing a para-aramid nanofiber membrane by airflow-assisted coaxial spinning, the steps are as follows:

[0034] (1) Preparation of spinning solution: PPTA staple fibers were ultrasonically treated in anhydrous ethanol for 2 hours and then dried. The treated PPTA staple fibers were dissolved in a deprotonating system to obtain an ANF suspension, which consisted of KOH, DMSO, and anhydrous ethanol. PEO powder was dissolved in DMF to obtain a PEO solution. By adjusting the weights of the PPTA fibers and PEO powder, different concentrations of ANF suspensions and PEO solutions were prepared to prepare nanofiber membranes with different properties. The PPTA fiber fineness was 200 denier, the concentration was 0.8-1.0 wt%, and the preferred concentration was 1.0 wt%. The PEO molecular weight was 600,000, the concentration was 5-7 wt%, and the preferred concentration was 6.5 wt%.

[0035] (2) Spinning preparation: Inject the ANF suspension into the syringe pump 2 corresponding to the core layer solution channel, and inject the PEO solution into the syringe pump 3 corresponding to the shell layer solution channel. Connect the liquid outlet of the propeller syringe pump to the coaxial spinning needle 8 through the liquid inlet pipes 6 and 7. Set the propulsion speed, and connect the air tank 14, pressure regulating valve 5, and air compressor 1 through the air pipe 4. The propulsion speed of the ANF suspension syringe pump is set to 0.1-0.3 mL / min, preferably 0.2 mL / min; the propulsion speed of the PEO solution syringe pump is fixed at 0.3 mL / min.

[0036] (3) Spinning process: The air compressor 1, the air inlet 9, the pressure regulating valve 5 and the injection pumps 12 and 13 are turned on. The spinning solution is pushed into the liquid inlet pipes 6 and 7 and flows out from the outlet of the coaxial spinning needle 8. Then, the outlet is stretched by the air flow and deposited on the water bath net curtain 10 to form a fiber membrane 11. During this process, the distance between the receiving net and the spinning needle, the pressure of the pressure regulating valve, and the coaxial spinning needles of different apertures are adjusted to obtain fiber membranes 11 with different structures. The distance between the receiving net and the spinning needle is set to 20 to 40 cm, preferably 30 cm; the inner diameter of the coaxial spinning needle is set to 16G+21G or 17G+22G, preferably 16G+21G.

[0037] (4) End of spinning: The deposited fiber membrane was taken out and washed in deionized water for 30 min to remove PEO in the membrane, and then dried in an oven at 100 °C to obtain pure ANFM.

[0038] Example 1

[0039] The preparation method of ANFM in this embodiment includes the following steps: 1.2g PPTA fiber and 0.6g KOH are weighed respectively and added to 100mL DMSO and 5mL anhydrous ethanol, and then stirred at 350rpm / min in a 60°C water bath for 2h to obtain a dark red ANF suspension with a concentration of 1.0wt%; PEO powder is then added to DMF, and stirred at 350rpm / min in a 60°C water bath for 3h to obtain a PEO solution with a concentration of 6.5wt%; the ANF suspension and the PEO solution are then injected into the inner and outer channels of a coaxial needle, the specifications of the coaxial needle are 21G and 16G, respectively, and then subjected to airflow-assisted traction and stretching to assist fiberization, and randomly deposited on a water bath mesh curtain to form a gel film, and then washed in deionized water for 30min to remove PEO, and dried in a 100°C oven to obtain ANFM.

[0040] like Figures 2 to 5 As shown in the figure, the average pore size of the ANFM prepared in this example is 0.2 μm, the average diameter of the ANFs is 20-30 nm, and the density is 0.12 g / cm 3 , porosity is 62%, tensile strength is 50MPa, and elongation at break is 9.2%.

[0041] The above description of the embodiments is intended to facilitate understanding and use of the present invention by those skilled in the art. Those skilled in the art will readily be able to make various modifications to these embodiments and apply the general principles described herein to other embodiments without resorting to creative effort. Therefore, the present invention is not limited to the above-described embodiments. Any improvements or modifications made by those skilled in the art based on the principles of the present invention that do not depart from the scope of the present invention should be considered within the scope of protection of the present invention.

Claims

1. A method for preparing para-aramid nanofiber membrane by airflow-assisted drafting coaxial spinning, characterized in that: The following steps are involved: Step 1: PPTA staple fibers are ultrasonically treated in anhydrous ethanol for 2 hours and then dried. The treated PPTA staple fibers are dissolved in a deprotonated system to obtain an ANF suspension; PEO powder is dissolved in N,N-dimethylformamide to obtain a PEO solution; The deprotonation system consists of potassium hydroxide, dimethyl sulfoxide and anhydrous ethanol; The PPTA staple fiber has a fineness of 200 denier and a concentration of 0.8-1.0 wt %; the PEO has a molecular weight of 600,000 and a concentration of 5-7 wt %; Step 2: The ANF suspension and the PEO solution are respectively injected into the pipes corresponding to the coaxial spinning needle, including: the ANF suspension is injected into the injection pump corresponding to the core layer solution channel, and the PEO solution is injected into the injection pump corresponding to the shell layer solution channel. The liquid outlet of the propeller injection pump is connected to the coaxial spinning needle through the liquid inlet pipe, the propulsion speed is set, and the air tank, pressure regulating valve, and air compressor are connected through the air pipe; The injection pump propulsion speed corresponding to the ANF suspension is 0.1~0.3mL / min; The injection pump propulsion speed corresponding to the PEO solution is 0.3 mL / min; The distance between the receiving net and the spinning needle is 20-40 cm, and the inner diameter of the coaxial spinning needle is selected from 16G+21G or 17G+22G; Step 3: airflow-assisted drafting coaxial spinning, including: turning on the air compressor, air inlet, pressure regulating valve and syringe pump, pushing the spinning solution into the liquid inlet pipe and out of the coaxial spinning needle outlet, airflow drafting, and depositing on the water bath receiving net to form a fiber film; Step 4: Take out the deposited fiber membrane, wash it with deionized water to remove PEO, and dry it to obtain pure ANFM.

2. The method for preparing para-aramid nanofiber membrane by airflow-assisted coaxial spinning according to claim 1, characterized in that: In step 1, the concentration of the PPTA staple fibers is 1.0 wt %, and the concentration of PEO is 6.5 wt %.

3. The method for preparing para-aramid nanofiber membrane by airflow-assisted coaxial spinning according to claim 1, characterized in that: In step 2, the propulsion speed of the syringe pump corresponding to the ANF suspension is 0.2 mL / min, and the propulsion speed of the syringe pump corresponding to the PEO solution is 0.3 mL / min.

4. The method for preparing para-aramid nanofiber membrane by airflow-assisted drafting coaxial spinning according to claim 1, characterized in that: In step 2, the distance between the receiving net and the spinning needle is 30 cm, and the inner diameter of the coaxial spinning needle is 16G+21G.

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