An experimental study was performed to develop bench-scale test methods and analysis techniques for screening the fire extinguishing performance and burnback (i.e., re-ignition) resistance of new per-and polyfluoroalkyl substances (PFAS)-free foam formulations. These tests were intended to provide prediction about the ability of novel foams to pass the MIL-PRF-24385F, Amendment 2 (MIL-SPEC) 28ft², full-strength, fresh-water extinguishment and burnback test. This test is often used as the first evaluator for firefighting performance.
Initial development testing was conducted on nine different firefighting foams (five aqueous film forming foams and four PFAS-free foams) to develop the recommended bench-scale tests and criteria. Bench-scale testing included measurements of the foam quality (expansion and drainage), the fluidity of the foam over a liquid surface, the vapor retention and stability of the foam on fuel when heated, and a reduced scale extinguishing and burnback test. Foam for all tests was generated using the MIL-SPEC 2.0 gallons per minute (GPM) aerated nozzle, as extensive testing determined that the foam quality produced by the MIL-SPEC nozzle was difficult to reproduce exactly using reduced scale foam production nozzles.
The recommended bench-scale MIL-SPEC screening tests identified in this effort include a beaker ignition test of foam over heated fuel and a reduced-scale pool fire extinguishing/burnback test. Distinct performance metrics for the beaker ignition and reduced-scale pool fire tests were developed using machine learning to identify critical parameters and thresholds to predict potential passing foam solutions. A scoring system was also developed to analyze the reduced-scale pool fire results with promise as a predictor of MIL-SPEC 28 ft² (fresh-water, full strength) extinguishing time.
The primary limitation to testing is the use of the 2 GPM MIL-SPEC nozzle. Each test requires a small amount of foam, and simultaneous foam capture was utilized to minimize waste, but this requires several staff members working together. While matching exact foamability of multiple foams is challenging, future demonstration that test performance can be maintained with an alternative, reduced flow nozzle (not MIL-SPEC 2 GPM) could greatly reduce the staffing, apparatus, and foam concentrate consumed during testing.