Aqueous film-forming foams (AFFF) are used to combat Class B liquid pool fires. The low density of the foam allows the foam to float on the pool surface, blocking the transport of fuel vapors to the flame above. Fluorocarbon surfactants in AFFF quickly block fuel vapor transport resulting in rapid fire suppression. The superior performance of AFFF has led to their wide-scale adoption and use worldwide since its original patent in 1966. Though effective at fighting fires, as a result of health-based assessment of fluorinated surfactants in the environment, the decision was made to eliminate fluorinated surfactants in AFFF formulations in shore-based applications. This work focuses on the development of alternative polymeric surfactants derived from polyethylene oxide (PEO).

This project incorporated synthesis and characterization of polyethylene oxide-based polymers for the purposes of developing a fluorine-free AFFF formulation suitable as a drop-in replacement for existing AFFF. The work was broken down into three primary task areas: 1) Synthesis and characterization of PEO-based AFFF polymers, 2) Laboratory-based evaluation of synthesized polymers, and 3) MIL-PRF-32725 testing of PEO formulations at the Naval Research Laboratory Chesapeake Bay Detachment fire testing facility. Specifically, the traditionally hydrophilic PEO were chemically modified to incorporate hydrophobic/oleophilic groups in order to make these polymers more suitable for use in AFFF formulations. A dynamic foam analyzer was used to determine foam properties such as drainage rate and bubble size, followed by mechanistic testing associated with foam fire suppression. Foam degradation and fuel transport – how long the foam maintains coverage on a pool and how fuel transports through the foam layer – were evaluated. These laboratory-based experiments conclude with 19cm pool fire testing. The most appropriate PEO formulations were evaluated using parameters described in MIL-PRF-32725 including 28 ft² pool fire testing against JetA fuel along with a number of select chemical and physical property tests. 

This project has resulted in the development of a number of synthetic routes to functionalized PEO-based polymeric surfactants using reversible-addition-fragmentation-chain-transfer-mediated synthesis. Addition of hydrophobic functionality to the hydrophilic PEO produces surfactant-like properties in the polymer. Methods to attenuate the degree to which hydrophobic functionality was added and the total molecular weight were established. Bench-scale testing of the PEO polymers allowed for refinement in the synthesis process and identification of species best suited for inclusion in fluorine-free foam formulations.

The development of a drop-in fluorine-free replacement for existing AFFF formulations is highly desirable. PEO-based formulations have the potential to provide a low toxicity drop in replacement that is made from commonly available polymer material which provides easier scale up at reduced costs. The demonstration of large-scale synthesis and improved foam stability upon addition to existing formulations shows that PEO-based polymer surfactants may be a viable avenue to improving formulation performance. Of particular interest is the improved hydrolytic stability of siloxane-based surfactants in the presence of PEO polymers and the potential for the synthesis of tri-block polymers. Tri-block polymers have the potential to support foam stability in two ways where the polymer interacts to support the stability of the foam bubble, but also fills the interstitial space between the foam bubbles to prevent vapor transport through the foam blanket. It is anticipated that research focused on the relationship between bubble lamella size and polymer length will be critical in realizing simultaneous foam stability improvements along with vapor transport inhibition.