All film forming foams, such as aqueous film forming foams (AFFFs) and film forming fluoroproteins, are required to form a film during the annual quality testing. The film is an extra barrier of protection in addition to the foam blanket that is designed to form when using these products. It involves a fluorosurfactant aqueous layer forming on top of the fuel to separate the fuel from the ignition source (videos of this phenomenon can be seen at dyneusa.com/videos). In contrast, non-film forming foams, such as standard proteins, solely rely on their foam blanket to smother the fire. Since film forming foam was designed, tested, and listed with this film formation in mind, it is required to demonstrate this same level of performance during the periodical testing.
Before NFPA 11, the Standard for Low-, Medium-, and High- Expansion Foam, came out with the 2016 edition, film formation wasn’t well defined. The definition in the previous 2010 Edition of NFPA 11 stated the following in the Chapter 3: Definitions under 126.96.36.199:
Film-Forming Foam. A concentrate that when mixed at its nominal use concentration will form an aqueous film on hydrocarbon fuels. The hydrocarbon fuel typically used as a minimum benchmark for film formation is cyclohexane.
This definition gave no clear requirements on how to determine film formation other than cyclohexane most likely being used as the fuel. For years, Dyne chose to check the film formation on film forming foams by applying foam solution onto cyclohexane and then holding an ignition source near the fuel to check for fuel ignition. Those foams that passed were able to cover the test surface in a defined amount of time. Those that failed did not cover the testing surface, and the cyclohexane ignited. While this test was able to separate out varying degrees of film formation, there were debates in the foam testing industry as to how big the testing surface should be and how much foam solution should be supplied among various other specifics.
NFPA responded to these debates. The current 2016 edition of NFPA 11 does offer very clear guidelines on what classifies a foam as film forming. Again in Chapter 3: Definitions but this time in the 2016 Edition, NFPA 11 states the following in 3.3.7:
Film Formation. A property of aqueous film-forming foams and film-forming fluoroproteins characterized by a positive (>0.0 dynes/cm) spreading coefficient when measured according to ASTM D 1331 using cyclohexane as the hydrocarbon substrate and distilled water to make the foam solution.
The spreading coefficient is a property of the foam that theoretically tells you if the foam will be able to form a film or not by comparing surface and interfacial tensions of the two liquids (foam solution and fuel in this case). It is calculated by the following equation:
Foam Solution Spreading Coefficient Value = Surface Tension of the Fuel (Cyclohexane) – Surface Tension of the Foam Solution – Interfacial Tension between the Fuel and the Foam Solution
Dyne adopted this new definition during the document’s approval phase back in June of 2015 at no additional cost to the customer despite the added test. At that time, Dyne began performing the necessary tests to determine the spreading coefficient values on all foams that failed the previous film formation test. The spreading coefficient test is not conducted on foams that form a film under the old test as they must have a positive spreading coefficient to do so. However, foams that did not readily form a film on cyclohexane under the old method were now subject to an additional test before determining the overall film formation result. Foams with positive spreading coefficients still received a pass as defined by NFPA 11: 2016 Edition. Foams with a negative spreading coefficient failed the film forming requirement.
Previously, how this would show up on a Dyne report was a bit confusing to some customers and AHJs. Some samples had film formation results and some had spreading coefficient results. Similar products appeared to get different tests even though this was not the case. This is because the previous process removed the film formation data in favor of the spreading coefficient data when needed. Here are some examples of the previous reports:
Figure 1: A passing film formation result in the old reporting format.
Figure 2: A failing film formation with passing spreading coefficient result in the old reporting format.
This is no longer the case. Dyne reports will now show both the film formation and spreading coefficient results. Note the film formation result is now reported as “Info Only” as the determining factor in film formation is ultimately defined in terms of spreading coefficient. When a foam forms a film under our old film formation test, it must have a positive spreading coefficient and therefore passes the spreading coefficient requirement. The results will show the spreading coefficient as >0 as the exact value need not be determined. However, when a foam fails the film formation, the spreading coefficient will still be measured and reported. To make room for all this, the surface tension and interfacial tension values were removed from the reporting process. Here are some examples of the current film formation reporting format at Dyne:
Figure 3: A passing film formation result in the new reporting format.
Figure 4: A failing film formation with passing spreading coefficient result in the new reporting format.
If you have questions with this reporting change or the how the results relate to foam performance, feel free to contact a chemist at Dyne. We would be happy to have a discussion with you about your questions or concerns. Rest assured, Dyne is still following NFPA 11 guidelines and is still holding foams to the same accountability as before. We hope this new reporting format, however, makes interpreting the results easier.
By Grant Lobdell, Chemist & Lab Manager
© 2017 Dyne Technologies, LLC