The biggest challenge when developing filtering face pieces (FFP) is to guarantee the required level of protection while keeping breathing resistance as low as possible. For instance, the FFP2 classification according to the DIN EN 149 standard requires that at an air flow rate of 95 l/min, particle penetration must not exceed 6% and breathing resistance must not exceed 2.4 mbar [1].

The aim of the research project SULA (“Sicherer und leichter atmen”, engl.: ‘safer and easier breathing’) is to significantly reduce both penetration and breathing resistance by improving the manufacture and processing of the nonwoven fabric. Expertise from industry (Reifenhäuser Reicofil, IMSTec) and application-oriented research (Fraunhofer ITWM) is being pooled to investigate the interaction of the individual processes and their effects on the product.

For a meltblown nonwoven to meet the specifications, the fibers are electrostatically charged. In this project, the focus is on the hydrocharging technology, which is applied already during fiber production and thus promises an even distribution of the electric charge in the nonwoven. The corresponding experimental investigations and the associated modeling and simulations are presented in Part 1 of the contribution [2].

This second part is dedicated to the characterization of the electret media produced and the model-based prediction of their protective effect and breathing resistance. The characterization includes basic properties such as thickness and surface weight. In addition, a workflow is being developed for the automatic detection of the nonwoven’s fiber diameter distribution, based on suitable sets of SEM images. In terms of performance, air permeability measurements as well as standard tests for the filter efficiency are performed for both flat sheet samples and mask prototypes. These experimental investigations are carried out for different aerosols (paraffin oil, sodium chloride) and volumetric air flow rates. The influence of the charging is studied by comparing the filtration efficiency for discharged samples of the filter materials with the charged counterparts. To quantify the effectiveness of the charging process, electrostatic fieldmeter measurements are performed on both sides of the flat sheet samples.

These data form the basis for the identification and calibration of models for the prediction of air flow resistance and fractional efficiency of the materials. The modeling allows for