Bacteria, viruses, or toxic substances spread widely as ultrafine particles such as PM0.3 as carriers at room temperature, posing a threat to public health. In addition, industrial high-temperature exhaust gases cause serious environmental pollution. In order to optimize public health and the environment, it is urgent to construct filtering membranes that can be simultaneously applied to room temperature sources containing bacteria and high-temperature sources. Currently, most filtering membranes face technical bottlenecks such as poor filtering durability and slow industrialization processes. In response to these issues, the research group led by Dr. Liu Xin from the School of Materials Science at Wuhan Textile University used centrifugal spinning technology to allow polyimide to spontaneously acquire a polarized molecular structure during the spinning process without the need for an external electric field. This resulted in the generation of super-strong self-sustaining electrostatic forces on the surface of the fiber mat, and the imparting of antibacterial properties through the in-situ growth of silver nanoparticles, achieving antibacterial and long-lasting filtration. This technology also enabled the large-scale production of filtering membranes. The related work was published in Advanced Functional Materials under the title Heat-Resistant Air Filters Based on Self-Sustained Electrostatic and Antibacterial Polyimide/Silver Fiber Mats. The co-first authors of the paper are Dr. Lv Pei from the School of Textiles at Wuhan Textile University and Ju Zheng, a master's student from the 2023 cohort at Wuhan Textile University. Academician Xu Weilin and Professor Liu Xin from Wuhan Textile University are the corresponding authors of the paper.
The generation of super-strong self-sustaining electrostatic forces on the surface of polyimide fiber mats in this research is mainly attributed to macroscopic friction and microscopic dipole polarization. During the centrifugal spinning process, friction occurs between fibers and air as well as between fibers themselves, creating a strong electrostatic field. This electrostatic field triggers the polarization of polyimide molecules, further strengthening the electrostatic field. Due to the high insulation and excellent dielectric properties of polyimide, its electrostatic loss is small, slowing down the dissipation of surface electrostatic forces. A comparison with polyimide films obtained by casting method showed that this super-strong electrostatic force is only present on the surface of centrifugal spun fiber mats. Molecular simulations further confirmed that the degree of polarization of polyimide molecules obtained by centrifugal spinning and casting methods is different. The hydrogen bond energies of the centrifugal spun fiber mat and cast film were calculated to be 28.54 kJ/mol and 19.50 kJ/mol, respectively, consistent with their thermal stability. Additionally, the absolute polarity parameter of the centrifugal spun fiber mat was higher than that of the cast film, further confirming that the centrifugal spinning process induces the polarization of polyimide molecules, enhancing molecular polarity.
Analysis of the morphology and physicochemical properties of polyimide and its silver nanoparticle composite fiber mat demonstrated that in-situ growth method successfully attached silver nanoparticles to the polyimide fiber mat. Within the temperature range of 30-350 °C, the weight loss of polyimide/silver nanoparticle fiber mat (PI/Ag) did not exceed 5%; thermal resistance tests showed that even after prolonged exposure to 280 °C, the fibers of PI/Ag maintained their continuous form, with no significant change in fiber diameter. The excellent thermal stability of PI/Ag allows air filters based on this material to be used for long periods at environmental temperatures of 200-300 °C.
Performance tests of PI/Ag showed that the filtration efficiency of the fiber mat for PM0.3 with a thickness of 260 µm was 99.1%, while for a thickness of 180 µm, the filtration efficiency was 98.1%, with a pressure drop reduced to 73.67 Pa and an average surface electrostatic voltage of -713 V. In contrast, the surface electrostatic voltage of commercial polyimide fiber mats was only -10 V, with a PM0.3 filtration efficiency of 58.5%. The ultra-high surface electrostatic voltage and the 3D network structure constructed by centrifugal spinning synergistically enhanced the air filtration efficiency of PI/Ag. Even after 330 days, the surface electrostatic voltage of PI/Ag remained above -700 V, and after further exposure to 280 °C for 1 hour, the filtration efficiency for PM0.3 remained above 91.3%. Therefore, PI/Ag can ensure low pressure drop in high-temperature environments while achieving long-lasting filtration. Antibacterial tests showed that PI/Ag exhibited significant antibacterial properties against Escherichia coli and Staphylococcus aureus. Thus, the PI/Ag prepared in this study can be used for air filtration from room temperature bacterial sources as well as for filtering industrial high-temperature flue gases.
In summary, the authors used centrifugal spinning technology to prepare polyimide fibers with super-strong self-sustaining electrostatic forces and introduced silver nanoparticles in-situ to impart antibacterial properties. Due to the action of self-sustaining electrostatic forces, this antibacterial and high-temperature resistant fiber mat maintains high PM0.3 filtration efficiency while ensuring low pressure drop. This work provides a new approach for the large-scale and continuous production of multifunctional and efficient air filtration fiber materials.
