Filtration Efficiency tests for multi-layer smart-fab spunbond polypropylene (SBPP)

Citizen Scientist, Aaron Collins (@masknerd) tested three more masks of mine. 

• Four layers of spunbond polypropylene (38 gsm Smart-fab), with one outer layer of 180 TPI of poly-cotton
• Same, but with eight layers of the spunbond polypropylene
• Same, but with twelve layers of the SBPP

Arron's test rig uses 65nm particles. 

Pros: super easy to setup and be accurate with. So easy, he can do it live on video and not lose viewers.  

Cons: this particle size is smaller than the of SARS-CoV-2 viron itself.  No matter though, his test is excellent at determining best-case fit for a mask, and of how close the materials are to comparable tests for known 'good' N95 masks which perform great at that particle size. Spoiler: regular cloth masks made with normal fabrics can probably never get close to that performance and still be breathable. E.g. 20 layers of polyester satin will have great filtration, but pass no air at all.

Aaron's results and experience report

Aaron' two hour mask review video bookmarked to moments my masks are being tested:

https://www.youtube.com/watch?v=73JWxBXqIR0&t=2462s

https://www.youtube.com/watch?v=73JWxBXqIR0&t=6904s

Results:

Four layers SBPP (152 gsm) + one of 180 TPI polycotton

Filtration Efficiency:            73.0%

Time to Max Breath @ 0.25" H20:    2.23

Pressure Drop:                     0.13

Quality Factor:                   10.2

Eight layers SBPP (304 gsm) + one of 180 TPI polycotton

Filtration Efficiency:            84.9%

Time to Max Breath @ 0.25" H20:    3.02

Pressure Drop:                     0.17

Quality Factor:                   10.9

Twelve layers SBPP (456 gsm) + one of 180 TPI polycotton

Filtration Efficiency:            85.7%

Time to Max Breath @ 0.25" H20:    3.84

Pressure Drop:                     0.22

Quality Factor:                    8.7

The first two are showing great results. The last one (12 layers of 38 gsm smart fab) are showing that air preferred to come in at the edges than through the mask. Fit is increasingly important at that thickness. Size of mask guides fit. These ones can be made extra-small through extra large. There's a possibility that these would require a rubber seal at the edges like some very high grade masks do. More tests needed.  Maybe too at higher layering the 180 TPI polycotton should be eliminated - it's no longer helping in the quest for higher filtration.

There's enough spunbond NWPP in the world (and it is easy enough to make) that we could put the entire population into these masks. Or have the in back pockets for outbreaks. Eradication is now a possibility. Frankly, it always was, but we can add chap high-filtration masks to the mix.

Why the quest for high-filtration & washable is important

The developing world can't compete for disposable N95,  FFP2 (Europe), and KF94 (South Korea) grade masks. That's a cost thing. Plus supply of those ebbs and flows.  The developed world can barely afford them (hospitals, individuals).

The environment can't afford the cost of the disposables in multiple ways.

Notes

I'm using a RagMask Max design because it's quick to sew (10 mins each), but there are plenty of other designs that would have the same performance if they are tight on the face 4 layers of 38 gsm smart-fab.

Aaron's aging test equipment broke after this mega test run. He's to attempt a repair in the coming weeks. 

US and UK snail-mail systems conspired to make all UK->US shipments of masks take longer that 4 weeks. That was regardless of requiring signature or not. Aarons masks (two shipments) took over a month each. Fedex would have been quicker by multiple weeks. 

Prior Art

Joceyn Songer and the MakerMask comnunity did the pioneering work on spunbond NWPP.  The test they commissions - https://makermask.org/fabric-mask-material-testing-results show that.

My predictions for effectiveness based on their test data:

makermask tests
(28.3 liters per minute)
70 gsm "Smart fab" @ 300 nm
layers	SB.NWPP gsm	Lab Measured filtration effective-ness	calculated filtration effectiveness using layer formula	Incr F.E. pct
1	70	0.31	👈 reverse engineered from 0.66
2	140		52.4%	21.39%
3	210	0.66	67.1%	14.76%
4	280		77.3%	10.18%
5	350		84.4%	7.03%
6	420		89.2%	4.85%
7	490		92.6%	3.35%
8	560		94.9%	2.31%
9	630		96.5%	1.59%

Mask design and fit are key.  Layering beyond 5 layers probably doesn't match predictions because of edge leaks.