A medical N95 respirator consists of multiple layers of nonwoven fabric, often made from polypropylene. The two outward protective layers of fabric, covering the inside and outside of the mask, are created using spun bonding. Spun bonding uses nozzles blowing melted threads of a thermoplastic polymer (often polypropylene) to layer threads between 15-35 micrometers on a conveyor belt, which build up into cloth as the belt continues down the line. Fibers are then bonded using thermal, mechanical, or chemical techniques. The two outer layers of the respirator, between 20 and 50 g/m2 in density, act as protection against the outside environment as well as a barrier to anything in the wearer’s exhalations.
Between the spun bond layers there’s a pre-filtration layer, which can be as dense as 250 g/m2, and the filtration layer. The pre-filtration layer is usually a needled nonwoven. Nonwoven material is needle punched to increase its cohesiveness, which is accomplished by sending barbed needles repeatedly through the fabric to hook fibers together. The pre-filtration layer is then run through a hot calendaring process, in which plastic fibers are thermally bonded by running them through high pressure heated rolls. This makes the pre-filtration layer thicker and stiffer, so it can be molded to form the desired shape and stay in that shape as the mask is used.
The last layer is a high efficiency melt-blown electret (or polarized) nonwoven material, which determines the filtration efficiency. Meltblowing is a process similar to spun bonding, in which multiple machine nozzles use air to spray threads of melted synthetic polymers onto a conveyor. However, these fibers are much smaller, as less than a micron wide. As the conveyor continues, the threads build up and bond by themselves as they cool, creating the fabric. However, sometimes melt-blown fabric is also thermally bonded to add strength and abrasion resistance, although the material then begins to lose some of its fabric characteristics.