How mask filter fleece is made

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. 

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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. 

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"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. 

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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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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.

"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."

Electrostatic attraction

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."

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.