Author: kmc4cy

Evaluating Foam Degradation and Fuel Transport Rates through Novel Surfactant Firefighting Foams for the Purpose of AFFF Perfluorocarbon Replacement
K. Hinnant et al., Eastern States Meeting of the Combustion Institute, March 2018

 

Abstract: Perfluorocarbon surfactants are used in aqueous film forming foams (AFFF) world-wide to suppress Class B pool fires. We believe the rapid fire suppression capabilities of perfluorocarbon surfactants are related to their ability to generate stable foams and foams that are resistive to fuel transport. Through a research program aimed at replacing perfluorocarbon surfactants in AFFF, we have evaluated foam degradation and fuel transport through foams at elevated temperatures for various classes of surfactants. The surfactants were evaluated individually and in a mixture with a hydrocarbon surfactant (Glucopon) to emulate a previously designed reference AFFF. This data was compared to the performance of a perfluorocarbon surfactant reference AFFF and a commercial AFFF.

Fast fire suppression is dependent on a foams ability to block fuel vapors traveling between the burning fuel pool below and the flame above, and maintain physical coverage over the fuel pool. Foam degradation is effected by heat from the fuel pool, heat from the fire above, and physicochemical interactions between the foam and fuel. Fuel transport through the foam is influenced by diffusion properties between surfactants in the foam and fuel as well as the foam layer thickness which is significantly impacted by increased foam degradation. By measuring foam degradation and fuel transport through the foam at elevated temperatures, we are better able to understand how fuel transport changes with increased foam degradation which mimics some characteristics of a flame environment.

Foam solutions were made by measuring the critical micelle concentration for individual surfactants and surfactants mixed in a 3:2 volumetric mixture with the surfactant and Glucopon, respectively. The foam solutions had surfactant concentrations 6 times greater than the critical micelle concentration. Foams were then generated from the foam solutions to measure foam degradation and fuel transport through the foam. Foam degradation was measured by placing 4 cm of foam above a heated n-heptane pool and monitoring the change in foam height over time. Fuel transport through the foam was measured by placing 4 cm of foam above a heated fuel pool in a specially designed fuel flux apparatus that monitored the concentration of fuel vapors above the foam over time using a nitrogen sparger and an FTIR. Surfactants analyzed included hydrocarbon surfactants, silicone surfactants, and sulfonated surfactants. From the currently evaluated surfactants, none have matched the foam degradation or fuel transport performance of the reference or commercial AFFF. However, certain surfactants performed better than others indicating potential directions for future AFFF surfactant replacement.

Presentation available here: Eastern States CI Presentation 3_2_18-250wawi

 

Measuring Fuel Transport through Fluorocarbon and Fluorine-free Firefighting Foams
K. Hinnant et al., presented at the 12th International Symposium for Fire Safety Science, Fire Safety Journal, 2017, 91, 653-661

Abstract: A flux chamber was designed to measure the transient fuel transport through a foam layer before significant degradation of foam occurred. The fuel transport rate through AFFF (fluorinated foam) was much slower than through RF6 (fluorine-free foam) with break-through times being 820 s and 276 s respectively over n-heptane. The fuel flux through AFFF covering three fuel pools (n-heptane, iso-octane, and methyl-cyclohexane) was also measured. AFFF had the smallest flux over iso-octane with a break-through time over 1900 s and the highest flux over methyl-cyclohexane with a break-through time under 80 s even though the fuels have similar vapor pressures at room temperature. Despite the lack of aqueous film formation on an iso-octane fuel pool, the fuel vapor flux through AFFF was much smaller relative to the methyl-cyclohexane pool, which enables film formation due to its higher surface tension than iso-octane. Our measurements of transient fuel flux show that the foam layer is a significant barrier to fuel vapor transport. The data suggest a transient mechanism based on the suppression of fuel adsorption onto bubble lamellae surfaces due to the oleophobicity of fluorocarbon surfactants, which is consistent with fuel solubility data. This suggests that surfactants that suppress fuel adsorption and solubility into bubble lamellae surfaces may reduce fuel transport through foams.

Influence of fuel on foam degradation for fluorinated and fluorine-free foams
K. Hinnant et al.
Colloids and Surfaces A, 2017, 522, 1-17

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 2 cm 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.

 

Development of an analytical AFFF formulation for the evaluation of alternative surfactants
K. Hinnant et al., presented at the 10th U.S. National Meeting of the Combustion Institute
Full paper available for download here: 10thCombustionMeetingPaper_Final-1spqpee

Abstract: Following criteria set by MIL-F-24385F for firefighting foams, we are developing an analytical aqueous firefighting foam (AFFF) formulation to be used for laboratory testing in place of proprietary commercial AFFF formulation. An aqueous reference foam solution containing 0.15% Capstone (fluorocarbon surfactant), 0.05% Triton X-100 (hydrocarbon surfactant), and 0.95% diethylene glycol butyl ether, by weight and was found to meet MilSpec criteria for aqueous film formation, which was inferred to be essential for improved fire extinction. However, the reference AFFF extinguished the 28 ft² gasoline pool fire in 54 seconds, exceeding the maximum 30 second qualification standard. Despite similar surface tensions and spreading coefficients between the reference and commercial AFFF indicating adequate film formation, the fire extinction time criterion could not be met. However, we found that the reference AFFF and a commercial AFFF degraded in 30 and 45 minutes respectively when the foams were placed over 50°C n-heptane fuel at bench-scale consistent with burn-back Milspec test results. Differences in foam degradation may prove to be more valuable to fire extinction than the role of film formation.