Cloth filtration experiment
|Kona||Kona cotton||A typical quilting fabric widely used in the sewing community and also tested by Lustig et al.||147.5 gsm
|P40||Pellon 40 sew-in midweight stabilizer||A non-woven 85% polyester, 15% viscose interfacing, which Pellon incorrectly described as "100% polyester" on its webpage (!) and so I used it for a while before realizing the error.|
|930||Pellon 930 sew-in midweight||A non-woven 100% polyester interfacing.||41.1 gsm
0.511m x 2m piece weighs 42g
|OlyFun||OlyFun||A non-woven polypropylene intended for crafting projects. Some sewists believe this makes an excellent filter; see makermask.org.||65 gsm
|Flannel||Robert Kaufman cotton flannel||169.5 gsm
5 oz per sq yard
|Sateen||Spoonflower Organic Cotton Sateen||100% organic cotton, weave has smaller gaps than Kona.||130 gsm
(Specified: 3.8 oz per square yard)
142 gsm (Measured)
Microscope photos of these (and other) fabrics are at: ofb.net/~ania/fabrics/
Many studies of cloth masks use submicron particle sizes to test filtration by cloth fabrics and cloth masks, consistent with the 0.3um particle size standard for N95 masks. However, Marr 2020 notes that NaCl and proteins are usually part of the viral droplet, so even dried out particles floating the the air could be expected to be much larger than a single virus. However, Marr does not quantify what the overall particle size range may be.
So... Test with 0.3um or 3.0um particles?
Following is my list of samples for NaCl submicron filtration and 3.0um BFE (Bacterial Filtration Efficiency). Total of 9 samples for NaCl, 4 for BFE.
|Sample ID||Test @0.3um||Test @3.0um||Outer layer||Middle layer||Face layer||Washed?||Description|
|3a||Yes||-||Kona||P40||Flannel||No||My most common construction.|
|4a||Yes||-||Sateen||OlyFun||Flannel||No||My present best guess at the best combination|
|5a||Yes||-||Kona||-||Kona||No||Community minimum: This is common in the sewing community.|
|6a||Yes||-||Kona||OlyFun||Kona||No||Non-woven polypropylene is believed by some sewists to be superior to non-woven polyester for filtration. This sample will allow a comparison, with 6a vs 7a comparing polypropylene vs polyester. Also, while some studies show non-woven polypropylene as filtering well, I haven't seen assessment of filtration after washing.|
|7a||Yes||-||Kona||930||Kona||No||A standard construction in the sewing community.|
|8a||Yes||-||Sateen||930||Sateen||No||Kona cotton has ~130um gaps visible in microscope photographs while this sateen is a similar weight, thinner yarns, and without such gaps. 7a vs 8a will allow comparison of Kona (quilting woven cotton) vs sateen (a woven cotton with fewer gaps between yarns)|
|10a||Yes||-||OlyFun||OlyFun||-||No||Compare similar weight of different non-wovens.
Added circa March 31; results expected mid/late April.
|11a||Yes||-||SmartFab Double-Thick||SmartFab Double-Thick||-||No|
Figure: The lines on the left correspond the submicron NaCl filtration, with solid lines being for unwashed and dashed lines for washed samples. The large X's correspond to BFE data for each plate, with the ranges for each plate annoated at the top. The bars near the middle of the graph show BFE average over all particle sizes, which is the typically reported result from this test.
Observations on combined submicron NaCl and BFE results:
Observations on submicron NaCl results:
|Results of submicron NaCl testing|
|Sample ID||Layers||Washed?||Filtration at most penetrating particle size||Resistance (mmH2O)|
MakerMask.org has a nice summary of this new standard for face coverings. Masks are rated for filtration of 0.3um particles, like N95 standards and unlike the European standard for community face coverings (CWA 17553) which tests with 3.0um particles. Masks are also rated for breathability.
|ASTM F3502||My samples|
|Filtration||Level 1||≥20%||All my samples filtered ≥20% @ 0.3um except the two layers of Kona cotton.|
|Level 2||≥50%||None of my samples filtered ≥50% @ 0.3um, though the sateen/OlyFun/flannel was close. The data shows a surprisingly large dip right at 0.3um. If this dip is measurement noise, and we interpolate betewen the 0.2um and 0.4um points, sateen/OlyFun/flannel would pass.|
|Breathability||Level 1||≤ 15 mm H2O||All my samples were consistent with Level 1|
|Level 2||≤ 6 mm H2O||Several samples were marginal, suggesting that a larger batch (as required by the standard) would be unlikely to pass due to normal manufacturing variation, but that masks sewn with the proposed materials would not be grossly less comfortable than a Level 2 mask.|
We can extract the filtration improvement due to the addition of layers by comparing samples that differ in only that one layer. Assume for simplicity that: PTOT = P1 * P2, where PTOT is the total penetration of a multi-layer fabric combination, and P1 and P2 are the penetration amounts for the constituent single layers.
|For 0.8μm, assuming PTOT = P1 * P2|
|Calculated Filtration Efficiency of Single Layer||Source data (measured)||Comments|
|OlyFun||47.78%||Kona/OlyFun/Kona, Ptot=39.2%||Kona/Kona, P1=73.6%|
|Pellon 930||27.65%||Kona/930/Kona, Ptot=53.5%||Kona/Kona, P1=73.6%||Pellon 930 is 41.1gsm (measured) while OlyFun is 65gsm (specified). If we scale up the estimated Pellon 930 efficiency by 65/41.1, we get 43.77%, which is very close to OlyFun. This suggests that a similar weight of Pellon 930 (a non-woven polyester) may give similar filtration as a similar weight of OlyFun (a non-woven polypropylene).|
|PTOT = 73.6% = PKona2|
Admittedly, we know from that the first layer seems to contribute more than successive layers of the same type. Zangmeister 2020