An Efficient Ethanol-Vacuum Method for the Decontamination and Restoration of Polypropylene Microfiber Medical Masks & Respirators

Albert I. Nazeeri; Isaac A. Hilburn; Daw-An Wu; Kabir A. Mohammed; D. Yovan Badal; Moses H.W. Chan; Joseph L. Kirschvink

Previous studies have assumed that the observed loss of filtering efficiency is due to the neutralization of surface charges on the polypropylene microfibers 3,4,7 . Our data point to a mechanism based on surface wetting, which is reversible by drying. It is well known that in order to remove water molecules that are trapped or adsorbed on solid surfaces the pressure of the vacuum chamber must be reduced to a value that is below the saturation vapor pressure of water; lower pressures will thin the adsorbed water layer. We note that the saturated vapor pressure of ethanol and water at 20˚C are 58 and 23 mBar respectively. When water molecules are adsorbed to surfaces in vacuum chambers, pressures as low as 1-20 mBar are needed to remove them 9 . Hence, our data suggest that water bound to the surface of the fibers is responsible for the loss in filtering efficiency, as evidenced by our observation of the same effect in a respirator soaked in DI water. This surface wetting mechanism for performance degradation is consistent with the nature of the mask materials. The active material of the 3M TM N95 masks is melt-blown, fibrous polypropylene modified by corona discharge to contain embedded charges 4, 10 . SEM indicates that the diameters of the fibrous strands are on the order of ~1 μm 11 , giving the filter a huge specific surface area. When the fibers are cleaned and rinsed with the ethanol solution, thin liquid films are introduced onto the material. The surface tension of the liquid film causes the fibers to wick together and form bundles, thereby opening gaps. In addition, the clumping of individual fibers significantly reduces the surface area available for . CC-BY-NC-ND 4.0 International license It is made available under a author/funder, who has granted medRxiv a license to display the preprint in perpetuity.