Influence of fuel on foam degradation for fluorinated and fluorine-free foams

Abstract

We performed experiments to quantify fuel-induced foam degradation by applying foams onto liquid fuels and water (for comparison) and measuring foam thickness over time. Our investigation included two firefighting foams, one fluorine-free (RF6-ICAO) and the other fluorinated (AFFF), and a foam made with a common surfactant, SDS. We applied a roughly 2cm thick foam layer onto three liquid fuels (n-heptane, methylcyclohexane, and isooctane) at room and elevated temperatures. Foam lifetime was reduced by 50 and 75% for AFFF and RF6 respectively for foams on fuels compared to foams on water at room temperature. For all experiments, the fluorine-free foams (RF6 and SDS) degraded much faster than AFFF. Further, the effect of fuel temperature was significant when the foams were placed over hot fuel: the lifetime of the firefighting foams decreased by 1–2 orders of magnitude between experiments conducted with fuel at room temperature and 50°C. Prior to the onset of foam degradation over fuels, the firefighting foams experienced a preliminary expansion (by up to 50% in volume). Video recordings of degradation show that expansion results primarily from bubbles near the interface increasing in size with accelerated coarsening by coalescence. We propose and discuss a mechanism for fuel-induced foam degradation based on our observations. Our results show that fluorine-free RF6 degrades faster than AFFF (by a factor of 3 at room temperature and 12 at elevated temperatures over fuel), which may contribute to differences in their firefighting performance.