The U.S. Air Force has been using aqueous film forming foam (AFFF) as a firefighting agent for the suppression of combustible and flammable liquid fuel fires, particularly in the area of aircraft rescue and firefighting. Legacy C8 and C6 AFFF, however, contains per- and polyfluoroalkyl substances (PFAS)—primarily perfluorooctanoic acid (PFOA), perfluorooctyl sulfonate (PFOS) perfluorohexanesulfonate, and perfluorohexanoic acid—fluorinated surfactant compounds that persist in the environment and are not readily degraded by natural processes. Additionally, PFOA and PFOS are considered chemicals-of-concern because they do not have established regulatory standards, but research and historical studies have identified an associated risk for exposure. However, fluorinated surfactants do provide inherent advantages to AFFF fighting fires particularly in extinguishment times and long burn back times. The objective of this project was to design a PFAS-free fire suppression agent (PFF) by using siloxane-based surfactants that are currently approved for use in medicine and cosmetics blended with other environmentally friendly ingredients. The findings will also be used to further expand the body of knowledge as to how to design PFF.

Two commercial PFFs were blended with three novel poly dimethyl siloxane surfactants synthesized here at Tyndall Air Force Base. The list of additives grew to include a siloxane super-spreader (Silwet L-77), a fuel-thickening agent (aluminum 2-ethylhexanoate), solid nanoparticles (silica and several forms of boehmite), foam stabilizer (triethanolamine stearate), and a hydrogel biopolymer (alginic acid). These additives were tested with the PFFs by MIL-PRF-24385F standard for drain time, surface tension, interfacial tension, refractive index, pH, biodegradability and fire testing. The fire testing was done on a small-scale fire test apparatus with compressed air foam. This allowed the project team to compare extinguishment time and burn back performance of the PFF samples to determine the effect of the additives.

The team tested four amino phosphate siloxane surfactants. DMS-N12 produced an insoluble solid gel. SIA0604.5 did not produce a foam. MCR-A11 and DMS-A11 produced a foam that hydrolyzed over the course of six months. The siloxane surfactants were revisited in a second fire test. They were tested at a lower concentration (1% versus 10%) and they were tested within two weeks of mixing with the base PFF. They still did not enhance the ability of the commercial off the shelf foams to extinguish a fire. In retrospect, this issue should have been addressed sooner since there is supporting literature to indicate that hydrolysis of siloxanes could pose a problem during storage, especially at pH higher or lower than 7 or in the presence of water.

The siloxane surfactants did not enhance the fire suppressing performance. The siloxane super spreader, solid nanoparticles, and the foam stabilizer did not either. However, the hydrogel biopolymer and the fuel-thickening agent showed a reduction in extinguishment time for one of the PFFs. These materials may warrant further study.

There was an issue with the fire test that the project team would like to address before proceeding with any more samples. The fire pan is slid out from under the compressed air foam nozzle immediately after the fire was extinguished. If a candidate foam extinguished the fire quickly, as was the case with the alginic acid samples, then there was less of a foam layer on top of the gasoline during the burn back portion of the test. This puts that sample at a disadvantage when evaluating its burn back capability. This test should be standardized to include 30 seconds of additional time to administer foam to the pan after the fire is extinguished.

The team found that the Avios Green control took 125 seconds to extinguish a fire, however, with additives the time decreased. Specifically, with 1% alginic acid (56 seconds to extinguish) and 1% Al soap (54 seconds to extinguish). These two additives showed promising results during these trials and should continued to be investigated, especially as an additive at higher concentrations. In trials involving BioEx, the BioEx control displayed a 45 second time to extinguish. While most additives did not show a significant increase or decrease in the time to extinguish, future research should be done concerning the additive concentration to determine if a higher concentration will have an effect on the time to extinguish. 

A consideration for the future is to investigate how to bring Avios Green and BioEx closer together in terms of burn back time and time to extinguish. Avios Green has a longer time to extinguish, but a longer burn back time. On the other hand, BioEx shows a shorter time to extinguish and a shorter burn back time. To optimize the capabilities of both Avios Green and BioEx, a shorter time to extinguish should be investigated for Avios Green and a longer burn back time for BioEx.