Abstract:

Objective Air pollution has become a serious threat to the environment and human health, and in particular PM_0.3 and virus can hardly to be captured or intercepted by conventional filtering products. The nanofiber membranes with small diameters have high specific surface area, controllable porosity and morphology, and can effectively filter fine particles. However, most research in documents only deals with problems in the ideal state, without considering the long-lasting performance of the filter material. Finding filter materials that can achieve high efficiency and low pressure drop with long-lasting filter performance in practice is essential and necessary.

Method Polyacrylonitrile (PAN) nanofiber has good thermal stability, mechanical properties and solvent resistance, and the strong polar group —CN in PAN can enhance the mutual attraction between nanofiber and particulate matter. Ag nanoparticles (NPs) is widely used as functional additives in the fields of antibacterial materials, biology, and photocatalytic catalysts. In this research, the Ag NPs with antibacterial properties were mixed with PAN solution, and PAN/Ag composite nanofiber membranes were prepared by electrospinning technology. The morphology and structure of nanofibers were observed by field-emission scanning electron microscopy and Fourier transform infrared (FT-IR) spectrometry. In addition, the permeability, humidity performance, wettability and filtering performance of nanofiber membrane were characterized.

Results The addition of Ag NPs changed the microstructure of nanofiber, and the standard deviation of PAN/Ag composite nanofiber membrane was significantly improved. The smallest diameter (180±38) nm was achieved when Ag NPs mass fraction was 0.5%(PAN/Ag-5), and the diameter of PAN/Ag composite nanofiber increased with the Ag NPs mass fraction went higher (Fig. 1). The FT-IR spectra of PAN and PAN/Ag expressed the characteristics of PAN typical absorption peaks (Fig. 3), suggesting the addition of Ag NPs did not change the chemical structure of PAN. The air permeability and moisture permeability of PAN/Ag composite nanofiber membrane decreased with the increase of Ag NPs mass fraction (Fig. 4). The smallest air permeability value was 456.9 mm/s when Ag NPs mass fraction was 0.5% and spinning time was 40 min which attribute to the smallest diameters and thickness of deposit. The smallest moisture permeability value was 1 586.32 g/(m²·d)when Ag NPs mass fraction was 1.5% and spinning time was 40 min. The results of water contact angle test showed that PAN nanofiber membrane had hydrophilicity because of the cyano group, on account of the smallest of diameter and standard deviation of PAN/Ag-5, the water contact angle is 112.5°, showing hydrophobicity. The trend of filtration efficiency and pressure drop curve were basically the same as the diameter of nanofiber (Fig. 5) because the both are closely related to the diameter and standard deviation of nanofiber. The filtration efficiency and pressure drop of PAN/Ag composite nanofiber membrane when Ag NPs mass fraction was 0.5% and spinning time was 40 min reached the highest value 99.935% and 99.381 Pa. In order to test the feasibility of PAN/Ag composite nanofiber membranes in practical applications, the nanofiber membranes were stored statically in the dust cover for 365 d before testing, and found that the gas flow rate remained between 32 and 85 L/min, indicating that the nanofiber membranes maintained excellent filtration performance (Fig. 6). The filtration efficiency and pressure drop of the nanofiber membranes after standing for 365 d decreased partly, on account of the electret effect was reduced because of some shallow trap and even the charge of the deep trap will escape to neutralize the water molecules on the surface of the membrane, the effect of electrostatic adsorption was weakened. The nanofiber membranes prepared with the best experimental parameters were fitted on the filter element of the air conditioner (Fig. 8), and the results showed that the nanofiber membrane could effectively filter the particles produced by burning sandalwood(simulate severe air pollution) in the air to avoid secondary pollution.

Conclusion In general, the best experimental parameters selected from the aspects of filter performance was mass fraction of Ag NPs 0.5%, and the spinning time is 30 min. The obtained results implied that nanofiber membrane can effectively filter the particles in the air to avoid secondary pollution with high efficiency, low pressure drop as well as long-lasting filter performance in practice. The research findings broaden the application scope of nanofiber filters in practical life, and are expected to open up efficient, sustainable, and new implementation approach in the field of precise filtering.

Key words: filtration efficiency, pressure drop, polypropylene, Ag nanoparticle, nanofiber, high efficiency low resistance, electrospinning, air filtration