"Improving Stability of PFAS-free Firefighting Foams with Polyurea Microcapsules" 

Elizabeth Malek, MSE PhD Candidate 

Advisor(s): Prof. Carlos Martinez & Prof. Jeffrey Youngblood

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ABSTRACT

Aqueous Film Forming Foams, or AFFFs, are the leading fire extinguishing agent for class B firefighting foams targeting flammable fuel fires. AFFFs extinguish fuel fires by deploying a leading film of liquid that triggers initial extinction, while the bulk of the foam contributes to long-term stability by slowing gas diffusion and preventing burn back. The ability of AFFFs to deploy the leading liquid film is due mostly in part to the addition of fluorinated surfactants that reduce surface tension low enough for the film to spread quickly. The use of per- and polyfluoroalkyl substances (PFAS) has been largely criticized in recent years due to toxicity concerns from exposure to humans and the environment. Efforts to remove PFAS from AFFFs have been achieved commercially but increase the solution viscosity by several orders of magnitude from the direct addition of additive stabilizing agents making them incompatible with current infrastructure and lead to longer extinction times. This dissertation aims to combat the current deficiencies of commercial PFAS-free AFFF replacement by introducing polyurea microcapsules into the concentrate that will act as foam stabilization agents while keeping initial formulation viscosity low. The polyurea microcapsules were fabricated via an inverse single emulsion interfacial polycondensation reaction that yielded aqueous cores with polyurea shells suspended in organic solvent. Synthesis optimization tests narrowed down a composition and procedure that reliably reproduced strong, small (~2-10 µm), and uniform capsules via vortex mixing. The microcapsules were characterized for size, shape, shell morphology, and core-release characteristics. Analysis of short and long-term foam behavior using solutions prepared from commercial PFAS-free concentrate combined with varying concentrations of the polyurea microcapsules showed that the use of microcapsules in concentrations at and above 10 vol.% slowed liquid drainage, maintained smaller foam bubble diameter, and formed a particle network within the lamella contributing to improved foam stability compared to concentrate alone. Rheological measurements also showed little change in solution viscosity across varying capsule concentrations from 0-15 vol.% suggesting compatibility with current infrastructure. These contributions provide valuable insights on the array of complexities and benefits of inverse emulsion interfacial polymerization for capsule formation and the use of capsules as carriers to support aqueous foam systems.