Non Woven Fabrics

Summary
Date	Summer 2020 - ?
Tags	COVID19, polymer processing, nonwoven fabric, respiratory protective equipment
Partners	Pre. Martine Dubé, Aurelia gloves

We are currently recruiting a MEng and a PhD student to work on this subject. Contact me if you are interested.

This project is partly financed by MITACS.

Context

The current pandemic has pushed us to employ all the means available to slow down the spread of the virus (keep R0 as low as possible) first through (1) quarantining measures, which appears to be the most efficient kind of measure, and then through (2) social distancing and minimizing travel outside of home as much as possible. Several governments and states worldwide are feeling that the spread of the virus is now under control, and quarantining measures are being gradually lifted in some places. However, that does not mean that the virus is gone, the virus is still out there and is still a significant threat for a large portion of the population¹. Now that re-opening the old economy appears to be unavoidable, we must employ all the means at our disposal to ensure that the spread of the virus will remain as low as possible. In addition to social distancing, the usage of (3) Respiratory Protective Equipment (RPE) helps reducing the spread of the virus through aerosols from an infected individual.

Respiratory Protective Equipment (RPE)

Although the usage of RPE appears to have become a social norm for infectious individuals in some parts of the world for dozens of years now, wearing a mask is still seen as a shady practice in most places worldwide. Recently, a strong push to enforce wearing masks in public places due to the pressure of the pandemic was seen worldwide. However, it is important to note that the efficiency of such RPE in various situations is still being studied, and the scientific community is not able to make clear recommendations yet². Before enforcing permanent laws, it is important to ensure that they are based on sound and valid arguments. There is however a certain agreement that covering one’s face might help reduce the spread from the wearer to its close vicinity, it is uncertain if it protects the wearer or how efficient the filtration actually is ³.

Dr Antonio Lazzarino of the Department of Epidemiology and Public Health at University College London, agreed. “That is not a piece or research. That is a non-systematic review of anecdotical and non-clinical studies,” he said. “The evidence we need before we implement public interventions involving billions of people, must come ideally from randomised controlled trials at population level or at least from observational follow-up studies with comparison groups,” said Lazzarino noting that will allow experts to look at the pros and cons of wearing masks.²

Another major issue regarding the global adoption of masks and RPE, is simply the lack of capability to manufacture enough RPE for everybody. We are not able to clearly quantify the impact of masks when used within a community by normal people, however, we are fairly certain that RPEs change everything when used by qualified medical workers that are directly exposed to infectious individuals. The issue is that if RPEs become mandatory, a direct consequence might be that there will be no more masks for those who really need them, health workers. It appears that this has actually been happening since the very beginning of this pandemic⁴. As states and cities are re-opening worldwide, the demand for RPEs is bound to strongly increase during the next years, independently of the actual effectiveness of RPEs within communities, as demand for the healthcare sector alone is steadily increasing. I would be extremely surprised if you were not actually already aware of the lack of RPEs currently happening worldwide.

Different masks

There are different types of masks. The simplest type of mask you can use, that is currently being recommended by more and more cities and states daily, is a fabric mask. A fabric mask is similar to a bandana or scarf that you would somehow hold around your mouth and nose. These kind of masks will fit loosely, and will not provide a tight fit around the wearer’s face. Particles will be able to go around the fabric. In addition, the effectiveness of fabric to actually stop the virus or aerosols from spreading is still doubtful (although it seems like several layers might help though).

The main different types of masks are shown on the left. The morphology of the filter layer (the melt-blown Non-Woven Polypropylene) is presented on the right, that is the filtration material used in masks. (Left ⁵, Right ⁶)

Your best option after that is a surgical mask. These masks are not made of fabric like the one used in clothes, they’re made of Non Woven Fabric (NWF). A NWF is a mat of randomly arranged fibers (presented in figure above). In most medical applications, the NWF used is a Non-Woven Polypropylene (NWP) made of fibers that are around 1-20 \(\mu\text{m}\) in diameter. Although NWPs can filter out particles of very small size, they’re still not quite efficient in the case of a surgical mask as they loosely fit around the nose and mouth. Aerosols can easily be inhaled with unfiltered air around their edges. In addition, although it might not look like it, surgical masks are specialized equipment that must be worn a certain way to be efficient⁷.

Finally, the best option you can get that you probably heard about at least once in the media recently is the N95 respirator. It guarantees to filter out at least 95% of microscopic airborne particles (0.3+\(\mu\text{m}\)), including viruses. N95 respirators only work if they fit tightly around the nose and mouth, this means that they cannot be used by children or people with face beard. Respirators contain a layer of permeable acrylic layer (non-filtering) that allows them to be molded to the shape of the face while remaining rigid. Another very important point here is that N95 masks and surgical masks are disposable, they should not be used more than once. Indeed, the NWP used to manufacture surgical masks and N95 respirators is also electrostatic to improve its ability to capture small particles. The fabric will however loose its charge after long usage due to breathing (humidity) and being soiled. The structure of a N95 respirator is shown in the figure above, they are made of several layers of NWP between which is a rigid but air permeable acrylic layer to allow the user to mold the mask to their face’s shape for a better fit.

NWP: melt blown

While Chinese firms make almost three million tons of nonwoven fabric each year, less than 1% of it is melt-blown fabric, according to the China Nonwoven and Industrial Textiles Association. Before the pandemic, half the world’s face masks were made in China and most US companies that have converted the factories to make N95 face masks get their melt-blown fabric from China. NPR.

Non Woven Polypropylene is critical for RPEs, this filtration material is manufactured through a process called melt-blowing. It consists in extruding a polymer-melt (thermoplastic) through nozzles using high speed blowing gas. The polymer-melt quickly cools down due to the high speed and forms micro and nano fibers that are then randomly gathered on a collector.

The melt blown manufacturing process is presented on the left⁸, a line of nozzles blowing polymer melts on a collector is shown on the right⁹.

The process is simple and it does not require solvents or dangerous products. The current issue is that factories worldwide are producing as much NWP as quickly as possible, in addition to manufacturing as many melt blowing machine as quickly as possible, there is still however a significant risk, due to the uncertainty regarding the arrival of unexpected second waves, that the demand will remain higher than the production capabilities for months or even years to come.

Cotton-candy masks

Centrifugal spinning is a technique for manufacturing polymeric fibers to form nonwoven textiles. Inspired by the cotton-candy method, this technique relies on centrifugal force to eject a jet of molten polymer and to stretch it rapidly before collecting the consolidated fibers into an entangled mat. By controlling the rotation speed, the temperature of the molten material and the geometry of the nozzles, this technology allows the production of high quality fibers with micrometric or even nanometric diameters. The high productivity and versatility of the polymers used makes the cotton-candy method very attractive for the biomedical, filtration and high performance textile industries.

The GIF on the left shows a centrifugal melt spinneret while it’s extruding a fiber using and ultra high speed cameraé

We are currently looking for a PhD student to help us adapt this method for boidegradable polymers.

References and notes

1. Study: 5% of Spanish population has had coronavirus ↩

2. Report on face masks’ effectiveness for Covid-19 divides scientists ↩ ↩²

3. This is also strongly dependent on the way a mask is worn, the kind of mask, and other important points that will not be explored in this short introduction to filtration materials for RPE. New results are being published daily. ↩

4. Shortage of personal protective equipment endangering health workers worldwide ↩

5. Virus ‘eminently capable’ of spreading through speech: study ↩

6. Assessment of a respiratory face mask for capturing air pollutants and pathogens including human influenza and rhinoviruses ↩

7. Next time you go outside (for your essential needs), try to pay attention to the nose fit of people wearing these masks. In my case, I noticed that a lot of people do not pinch the metallic or plastic edge to properly fit it to their nose. Some people do not even properly expand the pocket of the mask before wearing it. And I won’t even mention people who, for some reason, decide to wear the mask on their mouth only. But I digress, this is another topic. ↩

8. Melt Blown Process ↩

9. Biax-Fiberfilm meltblown process ↩