• ITA RWTH Aachen

How filters for face masks typically work

Face masks are made out of several textile layers. The combination of the layers is changed depending the use-case. Best known are sugical and K95 (FFP2, KN95, ...) masks. The test standards can be downloaded here free of charge. The textiles used for the mask are so-called spunbonded nonwovens.

For spunbonded nonwovens, continuous fibers (filaments) are first spun from a melt or solution. The filaments are directly drawn off mechanically or aerodynamically and stretched. To produce particularly fine microfibers, the still liquid filaments are torn apart by a hot air stream (meltblown). The fibers thus produced and deposited on a cooling belt produce microfiber fleeces.

Surgical masks (DIN EN 14683:2019-10) are typically worn by doctors during operations to protect the patient from droplets. These masks are made of three layers:

- an inner spunbond layer with hydrophobic properties (20-30 g/m²),

- a meltblown layer (20-40 g/m²),

- and an outer spunbond layer (20-30 g/m²).

Protective masks like K95, K99 or FFP1 - FFP3 (DIN EN 149:2009-08) are worn by professionals who need to protect themselfes of hazardous particles in the air. They typically consist of 3 - 7 layers:

- an inner spunbond layer (20-50 g/m²),

- up to three meltblown layers (20-70 g/m²),

- hot cotton (a dense spunbond material as moisture barrier and filler)

- an outer spunbond layer (20-50 g/m²).

To enhance the filter properties the filter layers are often electro-statically loaded (especially in FFP3). The electrical load of the fibers works like a magnet for hazardous particles. It's the same effect that makes your hair stick to the surface of a balloon after rubbing it agains your polyester t-shirt.