Fraunhofer Institute for Industrial Mathematics ITWM Hall 7 / M15

Stainless steel wire meshes offer high mechanical strength, corrosion resistance, thermal stability, and versatility through various mesh sizes, weave types, and wire cross-sections. These characteristics make them well suited for filtering highly viscous fluids such as polymer melts, which typically show non-Newtonian behavior. Simulations for flow through wire mesh filters must therefore incorporate this effect to ensure an accurate determination of the fabric’s permeability and predictions of the pressure drop and filter efficiency.

The geometric properties of such fabrics can be parametrized very well, facilitating the performance of microscale computational fluid dynamics (CFD) studies. The microgeometry is reconstructed by modeling individual metal wires and their contact regions using an in-house development from Fraunhofer ITWM (TexMath) [1]. Polypropylene (PP) rheology models then capture the non-Newtonian behavior of the melt. Together, these enable accurate CFD simulations to calculate the expected flow field (and differential pressure) depending on the fluid under consideration (density and viscosity law) and the inflow velocity with another in-house tool (FLUID) [2]. This, in turn, reduces the need for physical prototypes and the associated experimental effort to test the weave design.

The flow simulation is coupled with particle tracking to simulate the separation performance of the fabric. Various rheology models are compared with each other in terms of differential pressure and filter efficiency. In the talk, we present the different models and discuss the corresponding results.

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 SULA (“Safer and Easier Breathing”) research project was to significantly reduce both permeability and breathing resistance compared to the FFP2 classification by improving the production and processing of the nonwoven fabric and selecting the optimal (combination of) nonwoven media for the manufacture of the filter mask. Expertise from industry and application-oriented research was pooled to investigate the interaction of the individual process steps and their effects on the final product. The company Reifenhäuser Reicofil was responsible for producing nonwoven fabric samples and conducting experimental investigations into process design for the meltblown and hydrocharging processes.

IMSTec carried out material characterization of the samples and measured their filter performance, produced masks from the nonwoven fabrics, and tested them for compliance with the specifications. The main tasks for Fraunhofer ITWM were to create models and perform simulations for the most important steps in nonwoven fabric production, as well as to optimize the mask material. in such a way that the protection level is higher than required by the FFP2 standard and

For a meltblown nonwoven to meet the specifications, the fibers are electrostatically charged. In this project, the focus was on the hydrocharging technology, which is applied already during fiber production and thus promises an even distribution of the electric charge in the nonwoven. Extensive test runs on the nonwoven fabric line in conjunction with systematic simulation studies led to a significant improvement in the filter efficiency of the nonwoven fabrics without significantly increasing breathing resistance, as subsequent analysis of the filter performance showed.

The data on flow resistance and separation efficiency served as the basis for identifying models to predict filter performance. Linking these models to the manufacturing process made it possible to produce better nonwovens for personal protection. The models could also be used to find optimal combinations of the new nonwoven fabrics so that the final mask would have the desired improved properties.

In this presentation, we will introduce the project and the key aspects of the research and development work carried out and discuss the results. We will also address the insights gained and provide an outlook for the future.