All film forming foams are required to demonstrate their film forming ability during the periodical testing. Film forming foams include aqueous film forming foams (AFFF), alcohol resistant AFFF (AR-AFFF), film forming fluoroproteins (FFFP), and alcohol resistant FFFP (AR-FFFP). The film described by their names is an aqueous film that forms on the surface of the flammable liquid. This is possible because these foam concentrates contain surfactants. Surfactants are chemicals that alter the surface properties of water, which can, depending on the surfactant(s) used, allow for the thin layer of film to be formed over the fuel. This film isolates the fuel from the oxygen and ignition source even when the foam blanket is removed. Foams such as regular proteins or high expansion foams are not designed to have this film feature and rely solely on their foam blanket.

What is the film formation test and how is the spreading coefficient result related?

At Dyne, we start by physically distributing all film forming foams onto a fuel (cyclohexane) and seeing if the film forms on top of the fuel’s surface. This result is recorded as the amount of time it took for the film to fully cover the fuel. Foams that do not cover the fuel or only partially cover the fuel and ignite when an ignition source is introduced do not necessarily fail the film requirement, however. NFPA 11 has adopted the following definition of a film formation under Chapter 3 Definitions:

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.

Therefore, all foams that do not fully cover the fuel on the film formation test are subjected to the spreading coefficient test. For foams that pass the film formation test, they demonstrate a positive spreading coefficient so the exact value does not need to be determined – they meet the film formation requirement.
The spreading coefficient is calculated by measuring the surface tension of the foam solution and the interfacial tension of the foam solution and fuel. These values are important because they are needed to calculate the theoretical spreading coefficient value for the foam solution 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

In theory, a positive spreading coefficient value means the foam solution can form a film on the surface of the fuel. A negative spreading coefficient value means the foam solution cannot form a film on the surface of the fuel. The reason a foam solution can fail the film formation test but still have a positive spreading coefficient is due to the variables in the film formation test itself. The size of the test, amount of foam applied, and amount of time allowed for the film to form are some variables not well defined in any standard so NFPA has chosen to use the well-defined spreading coefficient test to minimize these variables.

What does it mean when a foam fails the spreading coefficient test?

Foams that do not form a film during the film formation test and do not have a positive spreading coefficient fail the film formation requirement for periodical testing. This means that the foam is no longer able to form that film as it did when it was first manufactured. This could be due to a variety of reasons. The surfactant responsible for allowing this film formation may be compromised due to contamination (water dilution, mineral oil, mixing of incompatible foams, etc.) or storage conditions. It is also important to ensure the product specified is indeed a film forming foam. There are foams designed without this film formation technology and should not be required to show this ability during periodical testing.

For an explanation on all physical properties and performance properties tested on firefighting foam samples, refer to Foam Testing Explanation LBTR-4054 on the Dyne Technologies website. If you have any questions, our chemists are always available to answer them. Call Dyne at (800) 632-2304 or email,, for assistance with understanding your test results.

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