N95 Respirator: Melt-Blown, Charged and Welded

The filter of an N95 respirator is nonwoven polypropylene made by melt blowing: molten polymer is pushed through a spinneret and jets of hot air stretch it into very fine fibres that land on a moving belt as a web. The fibres carry an electric charge, an electret, which helps them catch particles; charges can be put on such materials by corona discharge. Typical filter layers weigh 10–100 g per square metre. The filter is layered between cover webs, cut and ultrasonically welded; an aluminium strip is sealed in at the top as a nose clip, and the straps are stapled on.