With the development of society, air pollution poses a serious threat to human health and the ecological environment. Electrospun nanofibers have advantages such as small diameter, high specific surface area, and high porosity, showing potential applications in air filtration. However, existing electrospun membranes (ENMs) mostly face a trade-off between removal efficiency and pressure drop, as well as issues of non-degradability or non-recyclability. Therefore, developing environmentally friendly chemicals using natural or biodegradable synthetic polymers is of significant scientific and technological importance for reducing environmental pollution and establishing a sustainable society.
Based on this, Professor Meng Hong’s team at Beijing University of Chemical Technology developed bead-on-string PLA electrospun nanofibrous membranes using green solvents, ensuring high filtration efficiency and low pressure drop. The mixed solvent of ethyl acetate (EA)/N,N-dimethylformamide (DMF) was chosen as a relatively green non-halogenated solvent, which not only improved the environmental friendliness of the ENMs preparation process but also endowed the ENMs with a bead-on-string structure.This paper studies for the first time the eco-friendliness and solubility behavior of polylactic acid in the EA/DMF mixed solvent. Then, by changing the volume ratio of EA/DMF and the concentration of PLA, the evolution of the bead-on-string structure of PLA ENMs was studied in depth. Finally, the effects on membrane filtration performance were systematically studied, including filtration efficiency, pressure drop, long-term performance, and actual performance. From the perspective of basic research and industrial application, it is believed that this work can provide a good reference for the next generation of green ENMs to efficiently and sustainably address air pollution. The related research results were published in the journal Nano Energy under the title “Green-solvent-processable biodegradable poly(lactic acid) nanofibrous membranes with bead-on-string structure for effective air filtration: ‘Kill two birds with one stone’.”
Figure 1: Schematic diagram of the electrospinning experimental device.
Since the use of solvents is crucial to the electrospinning process, exploring relatively environmentally friendly or green solvents is a focus of the electrospinning process from the perspective of environmental protection and green production. Therefore, in this work, the researchers conducted a comprehensive and quantitative comparison of the EA/DMF system and other solvents for preparing PLA ENMs using three indicators. As shown in Table 1, the values of EA and DMF are both smaller than those of DCM and chloroform. The values of EA and DMF are 1.40×10-1 and 1.52×10-2, respectively, at least one order of magnitude lower than DCM and chloroform, indicating that the EA/DMF system used in this work poses less harm to human or environmental health and can serve as a more environmentally friendly alternative to DCM and chloroform.
In addition to these three indicators, a green analysis was also conducted using the GSK solvent sustainability guidelines to further assess the environmental impact of the above solvents. The results indicate that the use of the EA/DMF mixed solvent as a PLA dissolving agent can significantly reduce environmental pollution, making the electrospinning process more environmentally friendly.
According to the latest standards for air filters (EN779: 2012), the aerosol filtration performance of PLA bead-on-string ENMs was studied at a flow rate of 32 L/min. With the increase of the EA volume fraction in the EA/DMF mixed solvent, the filtration efficiency of PLA bead-on-string ENMs increased from 43.23% to 94.13%, and then decreased from 94.13% to 78.51%. The filtration efficiency (43.23%) and pressure drop (2.0 Pa) of the E1D9 filter membrane were low, possibly due to the discontinuity of the nanofibers and the wide distribution of particles (Figure 4), which favored permeability but could not capture small aerosol particles. In addition, increasing the particle size could increase the collision area with aerosol particles, enhancing the inertial collision effect.
The trend of filtration efficiency changes is closely related to the trends of microbead diameter and membrane pore size (Figures 4 and 5(b)). In other words, the larger the particle diameter and the narrower the pore size, the better the filtration efficiency of the membrane. The increase in pressure drop is due to the formation of dense structures caused by increased bulk density and decreased porosity, as well as the increase in membrane thickness..
On the day of the experiment, the air quality index (AIQ) was 200-300 (heavy pollution), with concentrations of PM2.5 and PM10 reaching 200 μg/m3 and 300 μg/m3, respectively. After filtering with E7D3-10 ENM, the concentrations of PM2.5 and PM10 were reduced to 30 μg/m3 and 50 μg/m3, respectively, meeting excellent air quality standards.
On this basis, the capture mechanism of PLA bead-on-string nanofibers was studied. As shown in Figure 7(h), on one hand, the presence of microbeads can not only reduce the pore size of the membrane but also increase the collision area with PM, thereby enhancing the multiple effects of interception, diffusion, and inertial collision. On the other hand, the PLA bead-on-string ENMs prepared during the electrospinning process randomly stack on the substrate, resulting in curved air paths within the ENMs. Therefore, PM takes a long time to pass through the membrane, leading to high efficiency. Furthermore, due to the larger volume and weight of PM10, the role of interception and gravity effects is more pronounced during the capture of PM10. Finally, the numerous particles within the nanofibers can increase the space between adjacent nanofibers, reducing the friction between airflow and nanofibers, thereby alleviating filtration resistance and lowering pressure drop.
In summary, this paper adopted a relatively green EA/DMF mixed solvent to prepare a biodegradable and environmentally friendly PLA bead-on-string ENM. The results indicate that the solvent composition has a significant impact on the morphology and structure of the ENMs. By optimizing the solvent composition and PLA concentration, the ENMs can achieve a removal rate of over 98% for aerosol particles, with a pressure drop of only 29.3 Pa.
The excellent removal efficiency is attributed to the presence of beads in the nanofibers, while the lower pressure drop is due to the open interconnected airflow channels caused by the beads. More importantly, compared to commercial masks, the prepared PLA bead-on-string ENM (E7D3-10) exhibits higher filtration efficiency in real smog environments (PM2.5-92.6% and PM10-95.4%), resulting in a PM index value (less than 50 μg/m3) that is relatively safe. Therefore, this study proposes a novel green and environmentally friendly method for preparing PLA bead-on-string ENMs, and the researchers believe that this ENM will play a significant role in applications such as respiratory protection, window glass, and indoor air purification.
Paper link: https://doi.org/10.1016/j.nanoen.2022.107237
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