Protective masks – worn properly - have become the key to wither away the COVID-19 pandemic. Nowadays, the vast majority of these masks are made of nonwoven fabrics. High-quality products have mainly melt-blown filtering layers of nano/microfiber. Melt blowing produces very fine synthetic nonwovens from a wide range of polymers and allows a fair control of the fiber structure and morphology that makes it ideal for filtration purposes. Melt blowing has a high throughput, and the low price of the filter makes these products widely available for civil use. Although melt-blown fiber applications were rapidly growing in the last three decades, we still have limited knowledge on the processing parameters. In this regard, we detailed the melt blowing parameters to obtain a filter media with high particle capturing efficiency and a low-pressure drop. We summarized the melt-blown fiber mat characteristics with specific attention to the pore size, the porosity, the fiber diameter, the fiber packing density and the air permeability desired for highly efficient filtration. Even though we cannot estimate the future social effects and the trauma caused by the current pandemic, and protective masks might remain a part of everyday life for a long while. That also implies that near-future investments in wider manufacturing capacities seem inevitable. This paper also aims to facilitate masks' production with improved filtration efficiency by reviewing the recent developments in melt blowing, the related applications, the effects of processing parameters on the structure and performance of the nonwoven products focusing on the filtration efficiency via knowledge.

In this study, we presented systematic and comparative investigations on the structure of polypropylene fibers generated using different processing conditions via melt blowing. Increasing air pressure, die-to-collector distance, and air temperature reduced the average fiber diameter nearly 3-fold, 2-fold, and 1.75-fold, respectively.An average of 1.4 μm Feret-diameter was observed as the smallest pore size, while the fiber mat solidities ranged from 8 to 13%. Differential scanning calorimetry results showed single melting peaks in the 1st heating cycle and double melting peaks in the second one, due to the phase transition at melt blowing. The fiber crystallinities varied between 43 and 52% by changing the processing conditions. The X-ray diffraction study revealed that the MB fibers exhibited α and mesomorphic crystals depending on the processing parameters. The reference polypropylene sheet made of the same fiber grade polypropylene resin, on the other hand, exhibited a mix of α and γ crystalline forms. The polypropylene fiber mat tensile strength improved by 48% and by 13% with increasing air pressure and air temperature, respectively.Decreasing fiber collection distances resulted in 2.5-fold higher tensile strength while strain at break reduced eightfold. A new factor, mat consolidation coefficient, was introduced and used to efficiently summarize melt-blown fiber mats' process-property-structure relationships. This study details how to control the melt blowing parameters to tailor the polypropylene fiber mat features for the respective application field. It also presents an insight into fiber formation mechanisms during melt blowing for generating self-bonded, defect-free, fine fiber mats.

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The technique of electrospinning has been researched for several decades. Almost all parameters have been investigated in the past years, e.g., solution parameters, process parameters, and environmental conditions. Among the solution parameters, the viscosity of the polymer solution is an extremely important factor for fiber formation and morphology. In general, however, viscosity of the polymer solution is mostly controlled by the solution concentration or by the molecular weight of the polymer in electrospinning field. Herein, we described the reason of a wellknown but not completely explained conclusion that the needle diameter can have an influence on the fiber morphology. In this study, polyethylene-oxide (PEO) with a molecular weight of 400,000 g/mol was dissolved in a mixture of ethanol and water with a proportion of 1:3. The relationship between the viscosity of the polymer solution and shear rate was characterized by a plate-plate rheometer. A shear flow model was discussed, while polymer material was flowing through a needle, which presented that different deformation rates were imposed on materials due to variable needle diameters. Combining the rheological experiments and analysis of the shear flow model, the viscosity of polymer solution flowing out the needle was predicted by needle diameter. Through observing the obtained fibers' morphology by scanning electron microscopy and measuring their diameters by image processing software, it was found that the fiber diameter increased with the increase of needle diameter, as expected, which agreed with the relationship of fiber diameter and polymer viscosity.

This study introduces systematic and comparative investigations of various PLA fine fiber mats prepared by melt blowing. A series of PLLA and PDLA melt-blown fibers from various L and D enantiomers blends were produced. Their morphological, mechanical, and thermal properties were studied, and their decomposition in water and compost was investigated. It was found that the 1:1 ratio blend with stereocomplex crystals had an 80% lower average fiber diameter, 60% higher specific strength and better thermal stability than the PLLA and PDLA fiber mats. In the case of composting, the crystalline peak melting temperature, crystallinity, and thermogravimetric decomposition temperatures marginally decreased after 14 days. The high surface of the fine fiber mats played a crucial role in fast decomposition, as they entirely disintegrated in less than only 40 days. In the case of water, the homocrystalline domains were more susceptible to hydrolysis than the stereocomplex ones. All the PLA fiber mats underwent decomposition and extensive disintegration for 70 days in water. Hydrolysis reduced the amorphous and crystalline fraction of the fibers via surface and bulk erosion, while the decomposition of stereocomplex-crystalline-rich domains mainly exhibited surface erosion. Findings revealed that high porosity and the high surface area of PLA melt-blown fine fiber mats undergo fast decomposition in compost and in water. Graphical Abstract