Abstract:

Objective Polypropylene (PP) melt-blown fabric has an electrically charged surface that generates an electrostatic field to adsorb micron-sized particles and is widely used as a filter layer in disposable protective masks. However, the free charge was found to be easily lost in the presence of moisture, leading to rapid failure of the filter layer and mask. Disposing of large numbers of masks also puts enormous pressure on environmental protection efforts. Therefore, there is an urgent need to develop masks with high protection capacity and long service life.

Method A piezoelectric poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) fiber layer was prepared by electrospinning on a PP melt-blown fabric substrate, resulting in a PP melt-blown fabric/P(VDF-TrFE) modified filter layer. The modified filter layers took advantage of the fact that piezoelectric materials can generate a bound charge under external forces, providing the mask with a durable electrostatic adsorption capability. The influence of spinning process on the morphology and structure of piezoelectric fibers was studied, and the research further investigated the changes in static voltage, smoke filtration efficiency and breathability of the modified filter layer after durability treatment such as water washing and steam treatment.

Results An optimized electrostatic spinning process was found to prepare excellent-performance piezoelectric P(VDF-TrFE) fiber membranes. The fibers prepared by spinning were free of beads and uniform diameter, with a distinct piezoelectric crystal phase structure, which ensures the breathability of the fiber membrane while providing the mask with a stable load of charge and a durable electrostatic adsorption capacity (Fig. 2, Fig. 3). The piezoelectric fiber layer showed a 25% higher filtration efficiency for PM_2.5 than the ordinary PP melt-blown fabric in the smoke filtration effect test. In the standard test of medical masks, the filtration efficiency of the modified filter layer was improved by 10.5% and 27.5% at an inlet airflow of 32 and 85 L/min, respectively (Fig. 6). The static voltage on the surface of the ordinary mask and the modified filter layer was reduced after treatment by water washing, alcohol washing, and steam sterilization. Owing to the presence of the piezoelectric fiber membrane in the modified filter layer, the piezoelectric fiber is still able to recover the static surface voltage of the filter layer well after being subjected to the mechanical effects of bending and pressing several times (Fig. 4). After the durability treatment, the modified filter layer maintained a filtration efficiency of 90% at a both inlet airflow of 32 and 85 L/min, respectively, exceeding the filtration efficiency of the melt-blown fabric itself. However, the organic solvent could destroy the structure of the piezoelectric fibers, resulting in a significant reduction in the filtration efficiency of the ethanol-treated modified filter layer(Fig. 7).

Conclusion As fossil energy sources become depleted and environmental pollution increases, it is necessary to develop and use more effective and durable masks to prevent infectious diseases. The innovation of this work is the introduction of piezoelectric polymer material in the production of masks by an electrostatic spinning process, which significantly improves the difficulty of maintaining the electrostatic adsorption capacity of PP melt-blown fabrics over a long period. Experiments have demonstrated that the modified masks can maintain electrostatic adsorption capacity after washing and steam treatment. The application of the piezoelectric polymer material not only improves the filtration performance of the mask but also endues the mask a long-lasting protection capability. The prolonged service life of the masks can effectively solve the problem of poor utilization of materials.

Key words: poly(vinylidene fluoride-trifluoroethylene), polypropylene melt-blown fabric, electrospinning, piezoelectric material, long-lasting filterability, breathing resistance, mask