Melt spun filtration media for respiratory devices and face masks

1. A nanofibrous web comprising polymeric fibers that are intimately comingled and entangled in a single layer, stand-alone network, and wherein:

(a) the fibers comprise at least 70% nanofibers, 5%-25% microfibers, and up to 5% coarse fibers by number percentage;

(b) a number average diameter of all the fibers is less than 1000 nm and a median diameter of all the fibers is less than 500 nm; and

(c) the nanofibrous web has an apparent density of 0.01 to 0.05 g/cm 3 , an electrostatic charge of at least 12 kV as measured at a distance of 25 mm, and an effective quality factor (eQF) of greater than about 2.5 (Pa·g/cm 3 ) −1 .

2. The nanofibrous web of claim 1 , wherein the fibers are composed of a polyolefin.

3. The nanofibrous web of claim 2 , wherein the polyolefin comprises a polypropylene or a blend of multiple, different polypropylenes.

4. The nanofibrous web of claim 2 , wherein the polyolefin comprises a charging promoting agent.

5. The nanofibrous web of claim 1 , wherein the number average diameter of all the fibers ranges from 600 nm to 1000 nm and the median diameter of all the fibers ranges from 300 nm to 500 nm.

6. The nanofibrous web of claim 1 , wherein the nanofibers have a number average fiber diameter ranging from 330 nm to 450 nm and a median fiber diameter ranging from 260 nm to 420 nm.

7. The nanofibrous web of claim 1 , wherein (a) a mass percentage of the microfibers ranges from 15% to 30%; and (b) a mass percentage of the coarse fibers ranges from 50% to 70%.

8. The nanofibrous web of claim 1 , wherein the percentage of the specific surface area of the nanofibers is greater than 90%.

9. The nanofibrous web of claim 1 , wherein the relative surface charge density of the nanofibers is about 10 times greater than the relative surface charge density of the microfibers.

10. The nanofibrous web of claim 1 , wherein the nanofibrous web has a porosity of at least about 94%.

11. The nanofibrous web of claim 1 , wherein a mean flow pore size ranges from 9 μm to 15 μm.

12. The nanofibrous web of claim 1 , wherein a Frazier air permeability is greater than about 10 (m 3 /min)/m 2 as measured at a pressure of 125 N/m 2 .

13. The nanofibrous web of claim 1 , wherein the nanofibrous web has a basis weight from 10 g/m 2 to 40 g/m 2 .

14. The nanofibrous web of claim 1 , wherein the electrostatic charge ranges from +14 kV to +20 kV as measured at a distance of 25 mm.

15. The nanofibrous web of claim 1 , wherein the nanofibrous web is made by a centrifugal melt spinning process.

16. A respiratory device comprising a filtration element comprising the nanofibrous web of claim 1 .

17. The respiratory device of claim 16 , wherein the respiratory device is a face mask.

18. A process for producing a nanofibrous web, comprising:

(i) supplying a molten polymeric spinning melt to a surface of a rotating member having a discharge edge;

(ii) rotating the rotating member at a rotational speed sufficient to form a film of the melt on the member, the film having an instability characterized by a wavy, non-uniform film thickness having regions of undulating thickness that extend radially outward from the center of the rotating member to its edge;

(iii) discharging from the discharge edge a plurality of discrete, continuous filaments derived from the film melt into a stretching zone;

(iv) attenuating the discharged discrete filaments in the stretching zone by centrifugal force to form continuous fibers;

(v) electrostatically charging the filaments in the stretching zone; and

(vi) collecting the attenuated continuous fibers on a collection surface to form the nanofibrous web as a single layer, stand-alone network,

and wherein the fibers comprise at least 70% nanofibers, 5%-25% microfibers, and up to 5% coarse fibers by number percentage, and a number average diameter of all the fibers is less than 1000 nm and a median diameter of all the fibers is less than 500 nm.