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Recently, a research team composed of researchers such as Chen Yang, Dong Xie, Suzhe Li, Luyuan Song, and Zixuan Yuan from South China University of Technology published a research paper titled "Electrospun fine-diameter polyimide nanofiber membranes via metal wire-based needle-free technique for high-efficiency PM0.3 filtration at high-temperature" in the journal Separation and Purification Technology. Through an innovative metal wire-based needle-free electrospinning machine technique, the team successfully prepared polyimide (PI) nanofiber membranes with an average diameter of only 122 nm. These membranes can maintain a PM0.3 filtration efficiency of 99.96% even in high-temperature environments, and also feature a low air flow resistance of 189.18 Pa and excellent hydrophobicity (contact angle of 130°±3°), providing an efficient and reliable solution for industrial high-temperature flue gas particulate filtration.
During the process of industrialization and urbanization, the emissions of fine particulate matter (PM2.5–0.3) have increased. These particles mainly come from combustion activities and natural phenomena. The complex composition of these particles may carry bacteria, causing great harm to human health, such as triggering asthma, cardiovascular diseases, and cancer. Existing high-temperature filtration technologies have poor removal effects on particles smaller than 2.5 microns. Nanofiber filters reduce air flow resistance due to the "slip effect" and exhibit higher filtration efficiency and lower pressure drop. Among them, polyimide (PI) has become a high-performance air filtration material due to its excellent mechanical properties and thermal stability. However, the electrospinning technology for traditional PI nanofiber membranes has problems such as complex preparation processes and long preparation times, which limit its economic viability and scalability in industrial applications.
This study uses the metal wire-based needle-free electrospinning device process to control the diameter of nanofibers by adjusting the voltage from 60 kV to 80 kV, so as to rapidly produce fine-diameter PI nanofiber membranes suitable for high-temperature and high-efficiency filtration within 8 to 12 minutes. This membrane not only has high filtration efficiency and low resistance but also exhibits good thermal stability and hydrophobic properties, providing a new solution for particulate matter (PM) filtration in high-temperature environments.
(1) Preparation of Fine-Diameter Polyimide Nanofiber Membranes by Metal Wire Needle-Free Electrospinning
The research used the metal wire-based needle-free electrospinning technique to process polyimide (PI) materials. The electrospinning machine was set to work under a high voltage of 80 kV (corresponding electric field strength: 333.33 kV/m), and a nanofiber membrane with an average fiber diameter of 122 nm was successfully prepared in just 10 minutes for PM0.3 filtration. This preparation technique avoids the problems of nozzle clogging and wear in traditional needle-based electrospinning, and the fiber diameter can be precisely controlled by adjusting the voltage. (See Figure 1)
Fig. 1. (a)PAA Solution Preparation Process; (b) Metal Wire-based Needle-Free Electrospinning Process and Principles; (c) Thermal Imidization Process of PAA Nanofiber Membrane.
(2) 99.99% PM0.3 Filtration Efficiency, 189.18 Pa Low Resistance Post-Slip Flow Effect
The fine-diameter PI nanofiber membrane shows an extremely high filtration efficiency for PM0.3, reaching 99.99%, while the operating resistance is only 189.18 Pa. Figure 5 intuitively shows that the PI16 - 80 - 10 sample has a PM0.3 filtration efficiency of 99.99% and the comparison of filtration efficiency and resistance with H13 under different wind speeds. When comparing the filtration performance of nanofiber membranes prepared from different concentrations of PAA solutions, different electrospinning times, and voltages, it was found that the PI16 - 80 - 10 sample performed best. While ensuring high-efficiency filtration, it has good air permeability and low energy consumption. In the comparison test with glass fiber air filter paper H13, the filtration efficiency is equivalent under the same conditions, but the filtration resistance is 60% lower than that of H13. (See Figure 2)
Fig. 2. Filtration performance of Fine-diameter PI nanofiber membranes made from PAA solutions of varying concentrations and electrospinning conditions for PM0.3 (a) and PM0.5–2.5 (b); Comparative filtration performance of Fine-diameter PI nanofiber membranes and Glass Fiber air filter paper H13 for PM0.3 (c) and PM0.5–2.5 (d).
(3) The Membrane Maintains 99.96% PM0.3 Filtration Efficiency After 5-Hour High-Temperature Treatment
After continuous treatment at a high temperature of 390 °C for 1 hour, the fine-diameter PI nanofiber membrane can still maintain a PM0.3 filtration efficiency of 99.96% and can completely filter PM2.5. Figure 3 (a) shows that the membrane has good thermal stability below 550 °C through thermogravimetric (TG), differential thermogravimetric (DTG), and differential scanning calorimetry (DSC) curves; Figure 3 (b) and (c) show the changes in the membrane's filtration efficiency at different temperatures. Even after treatment at 400 °C for 1 hour, although the filtration efficiency for PM0.3 slightly decreases to 99.77%, the filtration efficiency for PM2.5 still remains 100%.
Fig. 6. (a) Thermogravimetric (TG), Differential Thermogravimetric (DTG), and Differential Scanning Calorimetry (DSC) curves of Fine-diameter PI nanofiber membranes; Comparative filtration performance of Fine-diameter PI nanofiber membranes and Glass fiber air filter paper H13 after treatment at various temperatures for PM0.3 (b) and PM0.5 ～ 2.5 (c); (d) Comparison of water contact angles on the surfaces of Fine-diameter PI nanofiber membranes and Glass fiber air filter paper H13 after treatment at different temperatures.
(4) Hydrophobicity of 130°±3°, Enhancing Filtration in Humid Conditions
The membrane exhibits excellent hydrophobic properties with a static contact angle of 130°±3°. Figure 3 (d) shows the comparison of water contact angles on the surfaces of fine-diameter PI nanofiber membranes and glass fiber air filter paper H13 after treatment at different temperatures, demonstrating the hydrophobic advantage of the membrane. Compared with glass fiber air filter paper H13, the PI nanofiber membrane has better hydrophobic properties, and its hydrophobicity is minimally affected after heat treatment at different temperatures. This is due to the removal of hydrophilic groups during the high-temperature imidization process at 350 °C, and the smaller pore sizes formed by the finer nanofibers increase the tension on water droplets, which helps to maintain the filtration performance in humid environments.
This research successfully developed a fine-diameter polyimide (PI) nanofiber membrane with excellent high-temperature filtration performance through a new strategy. Using the metal wire-based needle-free electrospinning technique, the nanofiber membrane prepared at a voltage of 80 kV (electric field strength: 333.33 kV/m) has an average fiber diameter of 122 nm. The fine-diameter nanofibers significantly improve the interception efficiency for PM0.3, achieving a filtration efficiency of 99.99%, and reduce the filtration resistance to 189.18 Pa through the slip flow effect, outperforming the performance of glass fiber air filter paper H13. However, this comes at the cost of mechanical properties, with a tensile stress of only 6.355 MPa, which still needs to be optimized to enhance mechanical properties.
Below 550 °C, the fine-diameter PI nanofiber membrane exhibits good thermal stability. After 5 hours of high-temperature treatment at 350 °C to 390 °C, it can still maintain a PM0.3 filtration efficiency of 99.96%. Although the filtration efficiency for PM0.3 slightly decreases to 99.77% after treatment at 400 °C for 1 hour, the filtration efficiency for PM2.5 remains at 100%. The research results provide new insights into the performance limits of PI nanofibers in high-temperature filtration applications.
In addition, the fine-diameter PI nanofiber membrane exhibits hydrophobic properties with an initial static contact angle as high as 130°±3°, which not only helps to maintain filtration effectiveness in humid environments but also provides a new strategy for enhancing the surface hydrophobicity of materials. Given its high filtration efficiency, low resistance, good thermal stability, and hydrophobic properties, the fine-diameter PI nanofiber membrane has great potential for application in the field of high-temperature particulate matter filtration. Subsequent research should focus on optimizing the mechanical properties of fine-diameter PI nanofiber membranes to expand their application range.
Electrospinning Nanofibers Article Source: https://doi.org/10.1016/j.seppur.2024.130115
