Enhancing mechanical properties of flash-spun filaments by pressure-induced phase separation control in supercritical high-density polyethylene solution

Flash-spun nonwoven (FS-NW) is gaining attention in the PPE field due to its excellent barrier and mechanical properties resulting from its non-uniform diameter distribution and unique filament morphology. The unique network structure of flash-spun filaments (FSF) comprising the FS-NW can be controlled by phase separation behavior in the supercritical fluid (SCF) process. This study proposes a simple method to control the microstructure of FSFs by controlling the pressure-induced phase separation (PIPS) process in polymer/SCF solution. This phase separation behavior of an HDPE/SCF solution was confirmed by using a high-pressure view cell. A multistage nozzle allowing for phase-separated pressure to form different phases was also designed. HDPE-FSFs were synthesized by flash-spinning, and their morphology, crystallinity, and mechanical properties were investigated. The results demonstrated that the filaments obtained by PSP control at 220 °C and with an HDPE concentration of 8 wt% showed a network structure composed of strands, wherein the diameters ranged from 1.39 to 40.9 μm. Optimal FSF was obtained at 76 bar, with a crystallinity of 64.0% and a tenacity of 2.88 g/d. The PIPS method can thus effectively control the microstructure more feasibly than temperature- or solvent-induced techniques and can allow the effective synthesis of various products.

The safety and wellness of people in modern society are vulnerable to factors that threaten the human body, such as severe air pollution, pathogens, and viruses. The novel coronavirus disease (COVID-19) is a striking example of this phenomenon as it has caused a global pandemic since it was first observed in 2019 and continues to exact a significant human toll. Viruses are typically known to spread through small aerosols (usually defined as < 5 µm), or larger respiratory droplets expelled when coughing, sneezing, or breathing. Therefore, the development of personal protective equipment (PPE) to prevent the spread of infection, and to protect both patients and medical workers from dangerous exposure is gaining increasing importance.

Generally, PPE is worn to minimize exposure to hazards that can cause serious workplace injury and illness, and may include items from gloves and safety glasses to shoes, earplugs, hard hats, respirators, and full-body suits. PPE material requisites certain characteristics such as considerable mechanical/structural strength that can stand for strenuous activity, barrier properties against the external environment, and filtration of pollutants. Among the materials that are used to construct PPE, micro/nanofiber nonwoven is currently very popular as an essential constituent of respiratory or full-body protective equipment. Micro/nanofiber nonwovens have a high filtration efficiency owing to several advantageous properties such as small fiber diameter, large surface area to volume ratio, high porosity, and good internal connectivity. These nonwovens are generally obtained via widely practiced spun-bond or melt-blown processes that allow for excellent air permeability and filtration efficiency. However, it is challenging to obtain products with mechanical strength capable of handling vigorous human activity via these methods.

Flash-spun nonwoven (FS-NW) fabric is attracting attention as a promising PPE material owing to its excellent functional traits such as high tensile and tear strength and moisture-permeable waterproof properties. FS-NW fabric consists of microfibers with a diameter distribution ranging from tens of micrometers to hundreds of nanometers, resulting in higher tensile and tear strength than typical spun-bond nonwoven fabric with a fiber diameter of ≥ 10 μm, and barrier properties comparable to those of polymer membranes. The network filament morphology, attributed to the flash-spinning process, allows for these unique properties of FS-NW. Flash-spinning is a high-end process for the production melt-spun nonwoven fabric, utilizing a supercritical fluid (SCF) process. SCFs can be used as highly effective media in polymer processing as they exhibit liquid-like density and solubility while also possessing gas-like transport properties. Additionally, the phase behavior of their solutions can easily and conveniently be controlled by changes in temperature and pressure17. In the flash-spinning process, a polymer is dissolved in a high-pressure and temperature (HPT) SCF and then spun via instantaneous ejection at normal pressure and temperature (NPT). Prepared by spontaneous pressure while heating the polymer–solvent mixture, this single-phase polymer/SCF solution separates by a decrease in pressure and subsequently ejects through an orifice into a substantially lower pressure and temperature (usually NPT) region to form the FSF. Phase separation in the SCF mixture during this procedure can lead to profound structural changes in the flash-spinning filaments (FSF), the extents of which depend on the process parameters, such as temperature, pressure, and concentration. Although studies on phase separation behavior in polymer/SCF solutions are being conducted, it is difficult to apply the research approach to the actual flash-spinning process, so systematic studies on the effect of phase behavior on the material properties of the resulting product are insufficient.

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