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Coronaviruses, like other viruses, are tiny — far too small to get caught in most textiles. To prevent them penetrating a mask filter, engineers have to use various physical tricks.

Whenever several people come together in these coronavirus times, they wear masks. Medical mouth-nose protection masks and and other high-quality protective masks always have a special filter fleece built in. This fleece is produced using the so-called meltblown process.

The Reifenhäuser family company, with its subsidiary Reicofil in Troisdorf near the former German capital of Bonn, is one of the world market leaders for machines capable of manufacturing such special nonwovens. 

Read more: Coronavirus: The German face mask conundrum

It all starts with polypropylene

Detlef Frey is head of research and development at Reicofil. He opens the door for us to the technical center where he and his colleagues are researching the production of so-called nonwoven textiles, i.e., fabrics made of synthetic fibers that do not have to be spun and woven first.

Pilot plant in crisis mode

"We have 2,000 square meters (21,528 square feet) and three production facilities here. Everything that we have built here corresponds to the plants that produce outside at the customers' premises," says Frey. 

Read more: Coronavirus: How well do face masks protect against viruses, droplets and dust?

Head of R&D at Reicofil Detlef Frey. The roll of filter material will be enough for 100.000 masks.

"We actually set up the facilities here so that we could help customers develop products. Our customers can already use on site everything we do here with the manufacturing processes. However, because of the coronavirus pandemic, we have decided that we will now use the laboratory facilities to produce mask filter material." 

And that material is meant to be able to remove all kinds of pollutants from the air people breathe — not just viruses and bacteria, but also abrasives or other dusts, tiny aerosol droplets or asbestos fibers. For this to work, the fleece must have an extremely fine structure. 

Read more: Do fuels made from plastic make eco sense?

During production, the plastic polypropylene (PP) is first melted until it has approximately the consistency of liquid honey. Then it flows through tiny nozzles and forms a wafer-thin thread underneath. But it is still far from being as thin as it will be later. To achieve this final degree of slenderness, the melted thread is blown, in what is thus often called the meltblown process. 

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Nanometer-thin threads, but extremely durable

"Our polypropylene has a melting point of 160 degrees Celsius. The air is about 250 degrees. The hot air and the hot melt meet there under extreme acceleration," Frey says.

The air hits the plastic threads at about 300 meters (980 feet) per second. In a normal atmosphere, that's almost the speed of sound. However, because the air stream hits the plastic threads from two sides and chaotic vortex states occur in a very small area, the relative speed acting on the endless liquid plastic threads is multiplied. 

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Hardly blown out of the nozzle, the cobweb-like polypropylene filaments fall down like snow.

For a short time, they are accelerated to almost 40,000 kilometers per hour (24,855 miles per hour) — faster than the orbital speed of the International Space Station (ISS). This makes the threads — also known as filaments — incredibly thin.

"At the same time, we must prevent the filaments from breaking off," says engineer Frey. "It is fascinating when you think that this plastic can withstand these conditions and that we are able to manufacture such a product with consistent quality."

Quality control in the laboratory and the machines

It's not easy, adds Alexander Klein, who works as a development engineer in the technical center. "You have to adjust the settings in such a way that you get a homogeneous fleece without any disturbances, with continuous filaments and no filament breaks, so that in the end you have a homogeneous product with the fineness of the fibers," he says.

That's why it's important to control the production process. "We use inspection systems for this purpose that optically detect any imperfections in the product," says Klein. "In addition, we regularly take material samples that we test in the laboratory for air permeability and separation rate at the filter so that we meet the specifications of the respective classification."

In addition, sensors also automatically measure the air permeability of the finished filter material. "By testing, we can detect if anything changes that indicates that something in the process is not working as it should," says engineer Klein.

And that can easily happen, because the threads are still almost fluid and quite sticky when they are mixed up by the hot air.

Alexander Klein controls the composite line, which produces meltblown and spunbonded fabric simultaneously.

"These chaotic movements help us to form a tangled web that forms a net, and because the polymer has not yet completely cooled down, it even sticks together on the screen belt. This net has a physical pore size of about 10 micrometers — perhaps a little smaller," explains Frey, the head of research.

The filaments that make up the mesh are only half a micrometer thick. With a single 7-gram thread of this diameter, for example, you could span the entire earth. Such a thread would, in turn, be enough for about two to four face masks, depending on the quality of the mask.

Viruses are much, much smaller still

Although it is already very fine, this nonwoven net would still be far from sufficient to screen viruses out of the air, simply because they are so minute. The openings in the filter material are about one hundred times as large as the virus, with its 0.12 micrometers.

So the engineers use physical tricks, such as exploiting the tendency of small particles to attach themselves to surfaces.

"One is diffusion caused by Brownian molecular motion, combined with inertia.  The particle attaches to the surface as it travels through. It hits a filament and gets caught on it because of frictional  or intermolecular forces," explains Frey. "Forces between molecules are important when we think in terms of viruses."

But that still wouldn't be enough to filter viruses out of the air. The engineers benefit from the fact that viruses usually have greasy surfaces. "Polypropylene is lipophilic, which means fat-attracting," he says. "Any medium that is greasy on its surface will attach itself very easily to these substances."

But even that would leave a lot to chance. So Frey and his colleagues do more to help the filtering process: "We have to introduce an additional force that separates the viruses and draws them in. These are electrostatic forces,  which we are currently using extensively."

These forces are used as follows: During production, the finished fleece runs over a grounded roller. On the other side, there are numerous high-voltage electrodes. "A relatively simple technology but very effective," says engineer Frey. "Thirty kilovolts are applied and a small current flows through the ionized air."

The filter fleece runs over a grounded roller. Above it are countless high-voltage electrodes.

Afterwards, the fleece feels like an electrostatically charged mop, like the ones for dusting that can be bought in many drugstores these days.  The filter material works the same way as they do.

"So I put an electrostatic charge into the product, have a certain polarity and can then use it to exert forces — and here comes the limitation — on particles that are somehow conductive. But as long as there's water, they're conductive," Frey says.

This would apply to aerosol droplets that somebody coughs out, for example. As long as the viruses are already attached to those droplets or swimming in them, the filter catches them, even if they are tiny.

This means, however, that viruses floating freely in the air could theoretically still pass through the filter if none of the other physically effective forces hold them back. But in practice, this is probably the exception. And such small amounts of virus do not play a very important role in infections. 

It has not been proven that the face masks seen above can effectively protect you against viral infections. That said, these masks are probably able to catch some germs before they reach your mouth or nose. More importantly, they prevent people from touching their mouth or nose (which most people do instinctually). If you are already sick, such masks may keep you from infecting others.

One of the best ways to protect yourself from the virus is to frequently clean your hands, according to the World Health Organization's (WHO) list of recommendations. The WHO recommends alcohol-based hand rub, like the ones seen here in a hospital.

The simpler day-to-day solution is to use water and soap, if you've got some handy. But make sure to wash your hands thoroughly. Health authorities in the US recommend washing your hands for at least 20 seconds, making sure to pay attention to areas like your fingertips, thumbs and underneath your nails.

So here's what the doctors recommend: When coughing and sneezing, cover your mouth and nose with your flexed elbow. Or use tissue — but then immediately throw that tissue away and wash your hands. With your shirt or sweater, however, no, you don't need to throw them away. Do wash them frequently, though, or take them to the dry cleaner's.

Another recommendation that may not work for everybody: Avoid close contact with anyone who has fever and cough! If you have to tend to sick people, make doubly sure to take additional protective measures.

If you have fever, cough and difficulty breathing, seek medical care early. Avoid public places so you don't infect others. And also, explain to your doctor where you've previously traveled and who you may have come in contact with.

When visiting live markets in areas currently experiencing cases of the novel coronavirus, avoid direct unprotected contact with live animals. That includes any surfaces that are in contact with animals as well.

Cook meat thoroughly. The consumption of raw, or undercooked, animal products should be avoided. Raw meat, milk or animal organs should be handled with care to avoid cross-contamination with uncooked foods. These are good food safety practices and help prevent the spread of illnesses.

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