Lab tests for Spunbond Polypropylene masks

Fourth set of particle tests I have participated in (Apr 2021) [6th], [5th], [3rd], [2nd], [1st]

I mailed three washable masks to a lab in North America for testing.  We've got some amazing results back!

Note: Aaron Collins has previously tested three similar masks from the same batch of Smart Fab (Spunbond Polypropylene - SBPP). Details here.  Aaron's masks were 4, 8 and 12 layers of 38 grams-per-square-meter (GSM) SBPP with and outer layer of poly-cotton.

The masks to this new lab were 8, 12 and 16 layers of the same SBPP (304, 456 and 608 GSM) and no cotton outer. That's a far better strategy for these masks if high filtration is what you're aiming at. 

The actual masks sent to the lab.  Since this set, I make the same mask 

in black SBPP and with higher GSM spunbond fabric.

Testing Equipment

The lab has a TSI 8130A machine for mask testing. See the guide for using that machine. You glue the edges of the mask to a metal plate with a hole in it, and then stream your test particles into it measuring how much the mask filters out.  You also measure the pressure changes so that breathability can be determined. There's no mannequins or humans involved.  Edge leaks can't be detected with this setup but it is world class for actual material filtration testing.  

Flow Rate

Two flow rates were tested 30.1 liters per minute and 85. 85 LPM is for N95 respirators. That is what you'd wear if you were doing manual work in highly dusty situations (over simplified). 30.1 LPM (or thereabouts) is what you'd wear if you were walking inside a building and exposed to things - but not doing manual labor that would cause you to breathe at a higher rate (again over simplified).  All fabrics performing at a certain filtration efficiency at one flow right will perform less well at a significantly higher flow way.  The difference depends on the fabric, no doubt.  Melt blown PP as used in high end respirators might only dip 1% for when the flow rate is dialed up.

Results

I transcribed the results from emails into a table, then in GoogleSheets made a graph with layers of Smart Fab SBPP on the bottom - 38, 76, 114 would be 1, 2 & 3 layers of it.

The test result data points are the extent of the lab’s contribution. Any good or bad determination, or should or should not is not implied. At this moment the lab doesn't want it's name or staff detailed. Just in case that changes, credit for the team/lab is represented by this SHA256: b7f78406f4713321d6bb7a619554ce9b792efc3c7ee3abf9b8223a028aead43c (will decode as they publish later)

More on the three different thicknesses masks

The 12 and the 16 mask were glued together for one more 85 liter per minute test to see how close you could get to the N95 standard - not quite. That is now 28 layers of SBPP and a GSM of over 1Kg. Regardless of the result, the mask would fail the N95 standard for a number of other reasons (no rubber seal, etc). At the same time as the lab making the 28 layer mask, I made one for myself - sewing the two halves together. I can wear it and it is still much more breathable than many cotton masks I made

The masks tested all look like the middle mask from this blog entry  The pattern was RagMask Max and it really suits people with larger (boney) noses and short hair - like me (no gaps, no mask slips). Also people with longer hair that can be put up (and boney noses).

I have been working so far using filtration efficiency data from a MakerMask.org study on three layers of 70 GSM SBPP. That single Filtration Efficiency with a layering formula applied can be extended to predict higher filtration performance for greater GSM than the 210 GSM MakerMask tested. The tests from my North American lab broadly confirm the predictions above the 210 GSM of the MakerMask.org study.

Merging similar SBPP results into one chart

New data for 30.1 flow rate and the prior MakerMask.org test data for their similar 28.3 liters per min flow and Aaron's real-world test rate on one graph:

My masks were made with 38 GSM and Maker mask tested 70 GSM, Both were Smart Fab. The red line's only real data point was the 66.4% @210 GSM. All others are calculated from the layering formula as detailed in the other blog entry I linked to.  The masks I made at higher GSMs don't perform at the place they'd be predicted to. Most likely this is because the needle holes and the seams are paths that air would rather take. A smaller needle in the sewing machine and a tighter stitch might fix that.

Aarons masks were using the same 38 GSM Smart Fab (same batch) but also a layer of poly-cotton. His line is yellow here. It is not like-for-like but I have overlaid it. You can see that his real-world tests incorporate fit as a factor (leaks), given it's not trending above 86%. He also noted that the cotton caused the mask to collapse at higher thicknesses.  He didn't blame the cotton, but I've made masks that are higher than 600 GSM and they don't collapse as your breathe in at all.

More Pics

My 28 layer mask - identical to the one tested, if a bit beaten up

The type of masks I make since these tests

Which SBPP fabric source for you?

I myself have stopped using Smart Fab brand - I'm now using 80 GSM SBPP which is easier to sew to higher GSM and thus easier to get better filtration.  It is also much cheaper and more robust as you wash and dry it. Mine is black. It might be better to have white that'd reflect sunlight better if you were intending to be out in the sun in these for cumulative weeks (UV breaks down the fabric over time).

We also suspect that there is some variance for GSM of SBPP as well as manufacturer.  Most likely a layer of 150 GSM SBPP will perform better than three layers of 50 GSM. We hope that gets tested some day.

I am happy I have dropped the cotton outer layer. It lowered the breathability too much for any addition in filtration it contributed to. I am pretty sure any masks for any pattern I make from now on are 100% SBPP.

Which pattern?

Any that fits your face well. I make a range of patterns, but would be excited to see what others are doing with 100% SBPP masks as my mask patterns are insignificant compared to the breakthrough with the materials.

Conclusion

I'm very grateful to the lab for doing this work. Ideally, they'd have published a study themselves but there was not enough data for that, and they were swamped with other work and this work for me was free.

I feel high filtration masks can be sewn on regular sewing machines using SBPP and that countries that need to survive delta-variant and worse ahead of receiving vaccines. Distributing as many of these as they can be made could buy time for vaccine supply to ramp up.  One US dollar of materials (retail prices) can make three masks (thicker SBPP I've switched to since the study)  Each can be washed many times and last many months. SBPP is abundant worldwide and is an order cheaper than melt blown PP to manufacture.

If other variants make Covid19 worse and the word needs to attempt eradication, then SBPP masks are tool to be used.

Disposable N95, KF94 and FFP2/3 grade respirators are preferable, of course. Only rich countries can afford these though., and even then not for the whole population. We should also remember that commercial disposable respirators often contain SBPP layers that sandwich the melt blown PP material, so we already have many years experience with the materials deployed for protecting our lungs from pathogens. See: COVID-19 airway protection PPE overview (McKinsey & Company, May 2020) - Thanks to Aaron Collins for this link.