Armed With Science

Naval Research Lab Chemists Search for PFAS-Free Firefighting Foam

Nov. 15, 2019 | BY David Vergun

The U.S. military uses aqueous film-forming foams to rapidly extinguish fuel fires, particularly those involving aircraft. 

The key ingredient that makes the foams so effective is a fluorocarbon surfactant, said Katherine Hinnant, a chemical engineer at the Naval Research Laboratory in Washington. 

The problem with fluorocarbons is that they don't degrade once they're used. And that's not good for humans, she said.

A chemist holds a stopwatch as he times an experiment of a small fire in a lab.
Timed Test
Naval Research Lab Research Chemist Spencer Giles prepares to test an experimental aqueous film-forming foam at the NRL in Washington, Sept. 23, 2019. NRL scientists are conducting research to support the Defense Department’s effort to replace firefighting foams containing fluorine.
Photo By: Navy Petty Officer 3rd Class Manuel Najera, DOD
VIRIN: 190923-D-KT024-0078C

AFFF contains fluorocarbons in the form of per- and polyfluoroalkyl substances, or PFAS. The Environmental Protection Agency has issued a lifetime drinking water health advisor for two substances in the PFAS family: perfluorooctane sulfonate, or PFOS, and perfluorooctanoic acid, or PFOA. This year, the Navy updated the Military Specification for AFFF to set limits for PFOS and PFOA at the lowest detectable levels and removed the fluorine requirement.  

Hinnant said the Naval Research Laboratory is trying to find a replacement for AFFF that is just as effective at putting out fuel fires but does not contain any PFAS.

Although the EPA has identified PFOS and PFOA as potentially harmful in their health advisory, Hinnant said, other PFAS might be deemed harmful in the future. So, chemists at the Naval Research Laboratory are looking for a fluorine-free foam, or F3, replacement that is not harmful to health and that can rapidly extinguish fuel fires, she said.

The line of research has included testing 27 commercial F3 foams, but none of those products was able to extinguish fuel fires in less than 30 seconds, a requirement of military specifications, Hinnant said.

A chemist in a lab begins filling a plastic container with foam.
Experimental Foam
Naval Research Lab Chemical Engineer Katherine Hinnant tests an experimental aqueous film-forming foam at the NRL in Washington, Sept. 23, 2019. NRL scientists are conducting research to support the Defense Department’s effort to replace firefighting foams containing fluorine.
Photo By: Navy Petty Officer 3rd Class Manuel Najera, DOD
VIRIN: 190923-D-KT024-0143C
A chemist in a lab begins filling a metal container with foam.
Test Prep
Naval Research Lab Chemical Engineer Gopal Ananth prepares to test an experimental aqueous film-forming foam at the NRL in Washington, Sept. 23, 2019. NRL scientists are conducting research to support the Defense Department’s effort to replace firefighting foams containing fluorine.
Photo By: Navy Petty Officer 3rd Class Manuel Najera, DOD
VIRIN: 190923-D-KT024-0066C

The specifications, written by officials at Naval Sea Systems Command, require large fuel fires to be put out quickly, saving lives and protecting property, and are especially important for the military where fuel often is near munitions, particularly aboard ships, said Arthur Snow, a Naval Research Laboratory research chemist.

The military started using AFFF after a 1967 fire on the aircraft carrier USS Forrestal resulted in the deaths of 134 sailors, he noted.

Snow said silicone surfactants and hydrocarbon surfactants, of which a great variety exists, are being tested at the lab using a small burn test area.

"You can mix hydrocarbon surfactant with silicone surfactant and you'll get a property that exceeds each," he said, explaining what is called a synergism. "We want to make them work in our favor."

Hinnant explained how surfactants extinguish a fuel fire. The surfactant forms a foam barrier to the fuel pool that stops vapors from rising into the air and combusting. Fluorocarbon surfactant does this quickly and effectively.

The researchers are measuring how quickly fuel vapors travel through the foam layer and how long it takes to put the fire out. Finally, foam degradation is measured — the longer it takes the foam to degrade, the more effective it will be. 

A chemist pours foam on a small fire as another chemist holds a stopwatch.
Expert Experiment
Naval Research Lab Chemical Engineer Gopal Ananth and Research Chemist Spencer Giles test an experimental aqueous film-forming foam at the NRL in Washington, Sept. 23, 2019. NRL scientists are conducting research to support the Defense Department’s effort to replace firefighting foams containing fluorine.
Photo By: Navy Petty Officer 3rd Class Manuel Najera, DOD
VIRIN: 190923-D-KT024-0094C

Xiaohong Zhuang, a postdoctoral chemical engineer at the Naval Research Laboratory, uses computer simulations as an approach that's complementary to the burn testing. She said data analysis is used to simulate the dynamics of the atoms as surfactants interact with the fuel and water or air mixture. Computer modeling saves time, she explained, because it informs researchers so they can better narrow down which materials to synthesize.

Spencer Giles, a Naval Research Laboratory research chemist based in Washington, said that if a substance shows promise, it is delivered to a Navy lab in Maryland, where large-scale burn testing takes place.