The objective of this project is to develop a per- and polyfluoroalkyl substances (PFAS)-free foam formulation (F3) that satisfies the MilSpec requirements and can serve as a drop-in replacement. This will be accomplished by combining 1) new custom-synthesized silicone surfactants that are formulated based on knowledge of their molecular properties to further lower surface tension with 2) encapsulated foam additives designed to release at specific moments to enhance foam stability while mitigating issues these additives cause. The approach is built on a recognition that no single material can replace PFAS, however, proper combinations of foam ingredients released at the times they are needed to transform the properties of the foam after application, may overcome the challenges of building a fluorine-free firefighting foam.

The aim of this project is to develop F3 that meet military specifications by combining new siloxane-based surfactants with controlled release of additives. The project is divided into four hypothesis driven objectives.

Objective #1: Develop new siloxane-based surfactants and mixtures with the spreading coefficient and spreading speed to meet MilSpec criteria. In this objective, a new family of silicone-based surfactants will be developed based on the tetrasiloxane group tris(trimethylsiloxy)silylmoiety (TRIS) with the goal of obtaining a spreading coefficient and spreading speed to meet MilSpec criteria. Three hypotheses to determine how much better TRIS-based surfactants are than related materials and how close they are to fluorosurfactants in behavior (hypothesis 1), if ionic or non-ionic head group is optimal (hypothesis 2) and what type and how much co-surfactant is necessary (hypothesis 3) will be pursued. At the end of this objective a knowledge base on silicone surfactants along with optimal formulations for F3 will be developed.

Objective #2: Develop the chemistry and methodology to encapsulate foam formulation additives with temperature release capabilities with the goal of minimizing foam degradation. The lifespan of firefighting foams is typically increased via the addition of viscosifiers to reduce foam drainage, but the increase in viscosity can impede foam spreading. The project team aims to solve these issues by encapsulating the viscosifiers into temperature releasing polymer matrices. It is hypothesized that (hypothesis 1) viscosifiers can be successfully encapsulated in temperature-sensitive microcapsules and (hypothesis 2) the viscosifiers can be released during fire-fighting operations increasing foam viscosity and reducing foam degradation without impacting foam generation and spreading. Silicone wax and poly(methyl methacrylate) will be tested as encapsulants with the goal of releasing the polysaccharides in the foams at temperatures ranging from 40 to 65 °C.

Objective #3: Combine the best siloxane surfactant mixtures with encapsulated additives to prepare formulations that satisfy firefighting foam specifications and perform life-cycle analysis on formulae. In this objective the project team will combine the best surfactants developed in objective #1 with additives encapsulated in objective #2 to prepare foam formulations that meet the MilSpec. It is hypothesized that (hypothesis 1) foam formulations that combine new siloxane surfactants with encapsulated additives can have low viscosity, form foams with bubble sizes comparable to aqueous film-forming foams (AFFF) formulations, spread quickly over fuel and show long-term stability and that (hypothesis 2) down-selected foam formulations can extinguish flame and meet MilSpec requirements in a lab-scale fire test.

Objective #4: Assess the relative toxicity of the new foam formulation compared to a PFAS-containing AFFF formulation. In this objective, the project team will conduct standardized toxicity trials to determine the toxicity of the new foam formulation relative to a PFAS-containing AFFF formulation. The hypothesis is that new foam formulation will be less acutely toxic than the PFAS-containing AFFF formulation.

The benefits of this project include the development of a new class of siloxane-based surfactants and co-surfactants along with an innovative approach to the controlled release of additives in foam formulations with the ultimate goal of finding a replacement to fluorosurfactants fire-fighting foam formulations.