High performance polymers: The ORNL study offers new insights into the COVID-19 filter efficiency of N95

When COVID-19 was declared a pandemic in March 2020, Parans Paranthaman of Oak Ridge National Laboratory was suddenly working from home like millions of others.

As a Corporate Fellow in the laboratory's chemical sciences division, he quickly realized that his background in solid state chemistry and materials could benefit the healthcare industry when they needed equipment that could filter out the nanometer-sized particles of COVID-19.

“Merlin Theodore, who leads the research effort at the Carbon Fiber Technology Facility, called me and said, 'I need to understand which material on our production line is best for making N95 mask filter media, and I need to know that yesterday. & # 39; Paranthaman remembered. "And she asked if we could use neutrons and nanoscience facilities to prove it."

Parans Paranthaman, a researcher in the Department of Chemical Sciences at ORNL, coordinated the research effort to study the filter efficiency of the N95 material. His published results represent one of the first studies of polypropylene in relation to COVID-19. Photo credit: ORNL / USA. Energy department

Theodore is part of a team led by ORNL Corporate Fellow Lonnie Love who coordinated a COVID-19 research response on manufacturing within the Department of Energy's National Virtual Biotechnology Laboratory. The team also consulted Peter Tsai, a retired professor at the University of Tennessee who invented the electrostatic charging process used to make N95 filter media, to learn how to incorporate the capabilities of the CFTF.

"We had never tried anything like this in this period," said Paranthaman. "We've stepped up research that should have taken a year or more in a couple of weeks to be used by industry by the summer."

"But there is no challenge that I have not yet faced. The purpose of my research is to find solutions."

Polypropylene focus

Paranthaman's research Results on the N95 filter media recently published in ACS Applied Polymer Materials, outline the science that led to ORNL's successful production of material on the CFTF's forerunner production line. The technology was later transferred to two industrial partners under user agreements – Cummins and DemeTECH – For commercial use, resulting in the delivery of millions of masks in the US and the creation of thousands of jobs.

The N95 filter material – made from polypropylene – was manufactured on the melt-spinning precursor line in DOE's Carbon Fiber Technology Facility at ORNL. Using neutrons and microscopy, Paranthaman analyzed three different mixtures of materials to determine the properties needed to improve filter efficiency. Caption: ORNL / U.S. Energy department

As one of the earliest studies of polypropylene, also known as PP, in relation to COVID-19, Paranthaman's article serves as a guide to understanding how a novel virus reacts to polymer-based materials. PP has long been the industry standard material for filtration. However, understanding which commercial compounds or precursors of the material are best suited for mass production usually requires time consuming experimentation and error.

“We had a unique situation with COVID-19. First, it is a novel virus that not much is known about. Second, it is small, ranging from 60 to 140 nanometers, which means that the particles are able to penetrate the smallest of openings. And thirdly, we didn't have time to make mistakes, ”explained Paranthaman. “We had to have a material that could filter out more than 95 percent of these submicron particles. It had to be practically impermeable, but at the same time it had to be breathable. "

The N95 mask is made of two-ply PP, a non-woven material that is permanently electrostatically charged with millions of microfibers layered on top of each other to form a film. Theodore's team at CFTF used meltblowing, the process of extruding a polymer resin through a nozzle at high air velocity, into a fabric microfibers to make three samples of commercial PP for paranthaman for evaluation.

"We at ORNL used various characterization methods to better understand the filtering efficiency of PP and capitalized on the strengths of user facilities such as the Center for Nanophase Materials Science and the Spallation Neutron Source," said Paranthaman.

Characterization methods included differential scanning calorimetry to measure the amount of energy transmitted between the meltblown fibers; X-ray diffraction to understand the crystal orientation or texture of the fibers; and neutron scattering to study molecular vibration. Scanning electron microscopy has been used to understand the arrangement of meltblown fibers, their microstructure, and to characterize their diameters.

"It's important to understand how much particles the filter is stopping," Paranthaman said.

The team used sodium chloride aerosol particles that mimicked the size of COVID-19 to enter the filter, and then measured the particles as they hit the PP. Two layers of the meltblown fiber were stacked together for testing at an air flow rate of 50 liters per minute.

Crystal clear results

Paranthaman's investigations showed that, although the composition of the starting materials was almost identical, they performed very differently when charged. The most notable difference was in the crystallization, or how the material solidified atoms and molecules into a structured shape.

“We compared charged and uncharged PP material with an additive and without an additive,” explained Paranthaman. “The crystallization had a significant influence on the filtering ability of the material in each example. A larger number of crystallites form a stronger electrical charge, which leads to more effective filtration. "

The research also found that material with higher crystallization temperatures, slower crystallization, and a greater number of smaller microscopic crystallites is more effective at filtration. Paranthaman's examination of the PP samples indicated which material was likely to meet the filtration goal in terms of fabric weight, efficiency, resistance, fiber diameter size, and percentage of static electricity.

In late April, the CFTF was producing material that filtered 99% of the virus. By May, the technology was transferred to industry.

The research team won the ORNL Director's Award for Mission Support for the rapid development of the N95 filter media and technology transfer. According to Paranthaman, however, the scientific work on N95 filter media is only just beginning.

"This paper offered a three-dimensional view of the materials so we could see all the changes in the charged fiber versus the uncharged," Paranthaman said. “We knew that charging would, for example, reduce the fiber diameter, but it would also change the porosity, and that is critical to the performance of the material. Our follow-up paper will clearly outline the differences between charged and uncharged and provide an even better insight into N95 filter media. "

The title of the ACS Applied Polymer Materials The article reads "Polymers, additives and processing effects on the N95 filter performance". In addition to Paranthaman and Theodore, the researchers of the article were Gregory Larsen, Yongqiang Cheng, Luke Daemen, Tej Lamichhane, Dale Hensley, Kunlun Hong, Harry Meyer, Emma Betters, Kim Sitzlar, Jesse Heineman, Justin West, Peter Lloyd, Vlastimil Kunc and Lonnie Liebe .

ORNL's research was carried out in coordination with the U.S. Department of Health, and was supported by the DOE Office of Science through the National Virtual Biotechnology Laboratory, a consortium of national DOE laboratories focused on responding to COVID-19 and funded by the coronavirus CARES Act. CFTF research is also funded by the DOE's Energy Efficiency and Renewable Energy Bureau. The Center for Nanophase Materials Science and the Spallation Neutron Source are user facilities of the DOE Office of Science.

Source: ORNL

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