Abstract:

Objective The epidemic of COVID-19 and its variants is endangering human health. Wearing protective masks can effectively reduce the infection risk by resisting the inhalation of the polluted air containing the coronavirus. Electrospun polyamide nanofibers can be used as the core layer of protective masks and have lately received growing attention because of their high filtration performance and robust mechanical properties. However, existing electrospun polyamide nanofiber filters are usually prepared from toxic solvents which could cause severe environmental pollution and endanger workers' health, hence, their practical application should be restricted. Therefore, it is imperative to seek and develop green-solvent-based polyamide nanofiber filters.
Method Innovative polyamide nanofiber filters were developed by direct electrospinning technique based on green solvents (Fig.1). Ethanol as the solvent and water as the nonsolvent were adopted to prepare the green-solvent-based polyamide (GSPA) nanofibers by designing spinning solutions with different ethanol/water mass ratios (i.e., 10:0, 9:1, 8:2, 7:3, and 6:4). During electrospinning process, the working voltage, tip-to-collector distance, and solution extrusion speed were set as 30 kV, 15 cm and 1 mL/h, respectively. The nanofibers prepared with the different ethanol/water ratios were denoted as GSPA-0, GSPA-1, GSPA-2, GSPA-3, and GSPA-4, respectively.
Results It was found that water content had a great influence on the morphological structures of polyamide nanofibers (Fig.2). After introducing a small amount of water, the obtained GSPA-1 nanofibers featuring thinner diameter of 332 nm were compared to the GSPA-0 nanofibers (499 nm). The enhanced conductivity (10.5 μS/cm) of waterborne spinning solutions (Fig.3) stimulated more charges on spinning jets and led to larger electrostatic force, thus greatly elongating the jets and thinning the fiber diameter. However, with the further increment of water concentrations from 20% to 40%, the obtained fibers exhibited an increased average diameter ranging from 443 to 1 553 nm, which was mainly attributed to the larger viscosity of spinning solutions. Although water cannot dissolve polyamide, homogenous waterborne polyamide/ethanol solutions can still be obtained with different ethanol/water mass ratios within a broad area in the stable region (Fig.3). The average pore size of GSPA-1 membranes decreased by 55% compared with that of GSPA-0 membranes, contributing to high filtration efficiency. Moreover, with different concentrations (10%, 20%, 30%) of water, the fluffy structure of GSPA nanofibers were achieved with a high porosity (>80%), which would offer more passageways to transmit air rapidly. As the water concentration increased, the breaking strength of membranes increased at first and then decreased (Fig.5), and the GSPA-1 membranes exhibited the highest breaking strength of 5.6 MPa, which was believed to be related to the enhanced entanglements and contacts among the adjacent fibers because of the small fiber diameter. The GSPA-1 membranes displayed the highest filtration efficiency (99.02%) for the most penetration particles (PM_0.3) by virtue of the small fiber diameter but suffered from poor permeability with a pressure drop of 158 Pa. Moreover, the GSPA-1 membranes possessed the highest quality factor of 0.029 3 Pa^-1, suggesting the optimal filtration performance among different GSPA membranes. A high PM_0.3 removal efficiency (>95%) was achieved for GSPA-1 filters under various airflow velocities ranging from 10 to 90 L/min (Fig.7). Compared with conventional melt-blown fibers, the GSPA nanofibers featured a smaller diameter and higher Knudsen number (Fig.8), and PM_0.3 were captured mainly on the surfaces of green polyamide nanofibers (Fig.9), demonstrating the higher adsorption ability benefiting from the larger specific surface area.
Conclusion A cleaner production of polyamide nanofibers for air filtration was proposed by direct electrospinning based on green and sustainable binary solvents of water and ethanol. For the first time, the structure including fiber diameter, porosity, and pore size of electrospun polyamide nanofibers were precisely tailored by manipulating water concentration in spinning solutions. The prepared environmentally friendly polyamide nanofiber filters feature the interconnected porous structure with the nanoscale 1D building blocks (332 nm), mean pore size (0.7 μm), and porosity (84%), thus achieving efficient PM_0.3 capture performance with the filtration efficiency of 99.02% and pressure drop of 158 Pa, which could be comparable to previous toxic-solvent-processed nanofibers. Moreover, the GSPA nanofibers exhibit robust mechanical properties with an impressive breaking strength (5.6 MPa) and elongation (163.9%), contributing to withstanding the external forces and deformation in the practical assembly and usage of resultant filters. It is envisaged that the green-solvent-based polyamide nanofibers could be used as promising candidates for next-generation air filters, and the proposed waterborne spinning strategy can provide valuable insights for cleaner production of advanced polyamide textiles.

Key words: green solvent, electrospinning, polyamide, nanofiber membrane, air filtration, filter material