Improving Filtration of Homemade Surgical Masks by Hydrocharging
Real-Life Viability Status: Half-baked
Polypropylene (reusable shopping bag) surgical masks and possible DIY enhancements to filtering ability such as hydrocharging
- UPDATE #2: The middle layer of commercial surgical masks is made of webs of non-woven polypropylene, the same material used to make the vegetable cover cloth used by vegetable farmers to protect plants from frost and insects. Though further research is needed, it appears that vegetable cloth might be useful in making DIY surgical masks with the same protective qualities of commercial surgical masks. Hydrocharging the vegetable cloth couldn’t hurt.
“Both the 1816 and 1818 masks are three layer masks with non-woven polypropylene filter webs”
https://multimedia.3m.com/mws/media/82004O/common-questions-3m-tie-on-surgical-masks.pdf
Though not medical grade, and possibly of different consistency, this is ground cloth / vegetable cloth / tobacco cloth. One of the major brands is Agribon, produced by Berry Plastics.
Berry Global Produces Materials Critical for Coronavirus Defense; Expands Wipes Capacity
http://ir.berryglobal.com/news-releases/news-release-details/berry-global-produces-materials-critical-coronavirus-defense
https://youtu.be/m4O-3wQQjpA?t=60 - UPDATE #1: The below technique applies to “microfiber webs” of melt-blown non-wovn polyproylene, which is a more fibrous material (more like gauze) than shopping bags and is what is often used as the lining of the baby side of disposable diapers. Although I doubt spraying your shopping bag with water can make things worse, they probably won’t have the same properties.
- Untested proposed technique – Make surgical masks (or filter media) out of reusable shopping bags (constructed of non-woven polypropylene). Then treat the mask fabric material to improve filtration performance by spraying both sides with water at 25 PSI (170 kPa) from a pressure washer / power washer / garden hose nozzle. Accelerating the drying time is less of an issue as this is not a mass production process and we are not planning to immediately put the fabric into large rolls. But do not apply heat. I do not know if this method will survive sterilization by boiling in water, so it may not be applicable to healthcare settings. But for your own individual use, it is as good as your own hygiene practices.
- Real-Life Viability Status: Half-baked. Not tested yet.
- A reusable shopping bag in my home had a fabric thickness of 0.36 mm to 0.40 mm, but had a weave pattern stamped into it such that I could compress it to a thickness of 0.24 by squeezing the micrometer calipers with my fingers. The web material in the study had a thickness of 0.1 cm (1mm).
- Non woven polypropylene (NWPP) is commonly used in surgical masks and can be easily sewn into masks that can block the spread of droplets from coughs and sneezes.https://www.project-cloth-masks.com/ – Using non-woven polypropylene
https://www.project-cloth-masks.com/about – Research - Hydrocharging, the use of a high pressure water jet to enhance the filtration capability of non-woven polypropylene (NWPP) is detailed in a 3M patent application (see below). Note that a patent application (even if granted) is not proof that a technique actually works or works well, just some legal assurance that the technique is new, so we should dig up some research on actual efficacy. Always independently test new concepts from strangers on the internet.
“Surprisingly, it has been found that merely by impinging these jets of water or stream of water droplets onto the nonwoven microfiber web, the web develops filtration enhancing electret charge. The charging can be further enhanced by subjecting the web to corona discharge treatment prior to impingement by the water. Preferably, the web is formed from melt blown polypropylene microfibers, poly(4-methyl-l-pentene) microfibers or blends thereof. The term “hydrocharging” will be used herein to describe this method.”
“Each sample passed beneath the spray bar at a rate of 3.5 m/min, and was treated once on each face, vacuum extracted [Alex’s note: this is to speed up drying in mass production] and dried at 70 °C for one hour…hydrocharging (at pressures of at least about 170 kPa [Alex’s note: This is 25 PSI.]) develops useful levels of electret enhanced filtration characteristics”
“FIG. 4 is a perspective view of a pump action sprayer useful in the present invention”
Method of charging electret filter media
Agadjivand, S. A.; Jones, M. E.; Meyer, D. E. PCT International Application WO 9505501, 1995. - “Another method of charging which appears to rely heavily on triboelectric effects is hydrocharging (10). Hydrocharging involves the use of a high pressure water jet directed through a nonwoven filter media in order to impart charge. It is reported that quite good filtration media can be made using the hydrocharging technique (10). A drawback of the hydrocharging technique is the drying of the charged filters. If the drying is done at an elevated temperature, thermal charge carries are liberated within the dielectric leading to charge dissipation. If the web is insufficiently dried, the residual moisture can lead to fungal or bacterial contamination of the filter media during storage.”
Electret Media For HVAC Filtration Applications
By David L. Myers and B. Dean Arnold, Nonwoven Fabrics Research and Development, Kimberly-Clark Corporation, 1400 Holcomb Bridge Rd., Roswell, GA 30076