firefighting foam

=chemistry =suggestion =fire =environment =safety


Firefighters usually use water foams to fight fires. Foam is better than plain water because it sticks to stuff instead of going into the ground, and because it can smother fires, which requires much less mass than extinguishing fires by cooling them.


More recently, a few firefighters have started using water gels. Those are effective for preventing ignition of things, but are still relatively uncommon.


Most current firefighting foams use fluorosurfactants. Those are bad. Newer "environmentally friendly" firefighting foam mixes, such as this, use shorter fluorosurfactants, but it's not PFOA or PFOS specifically that are bad; all the fluorosurfactants are bad, and those were just the common ones that got specific bans once negative effects were noticed.



That annoyed me, so I designed a firefighting foam composition, or rather, a combination foam/gel with a specific amount of polymer that improves foaming instead of reducing it. Polymer can smother the fire even after the water has evaporated, and I chose a composition that produces a decent amount of char at higher temperatures.


This is a "class A" foam for fighting forest/structure fires. It's not designed to quickly spread across the surface of oil like "class B" / "AFFF" foams, which is one of the main reasons fluorosurfactants are used in firefighting foam. Here's a video showing the difference. However, fluorosurfactants are still currently used for fighting forest fires in some places, despite class A foams without them already being fairly common. Anyway, here's the composition I came up with:

➯ water
➯ 1/600 (by weight) ammonium carboxymethyl hydroxyethyl cellulose
➯ 1/1000 (by weight) sodium alkyl sulfonate surfactant
➯ 1/4000 (by weight) ammonium sulfate
➯ 1/20000 (by weight) diammonium phosphate


Sodium alkyl sulfonate surfactant can be made from alkenes and sodium bisulfite with a free radical addition. This is added as a foaming agent.

Carboxymethyl hydroxyethyl cellulose is made by reacting cellulose with chloroacetic acid and ethylene oxide. This improves foam strength, and acts as a thin barrier after water has evaporated.

Ammonium sulfate can be made from ammonium carbonate and calcium sulfate. It's added mainly to increase char yield of the carboxymethyl hydroxyethyl cellulose.

Diammonium phosphate has a similar effect to ammonium sulfate, and acts as a pH buffer.



What about replacing "AFFF" foams? Fluorosurfactants are used because the low surface energy and enthalpy of wetting of fluorosurfactants means they don't "stick" to the oil surface and can easily thin out as film spreads. Is there any replacement for fluorocarbons? The answer has often been assumed to be "no" but very branched hydrocarbons can actually give similar results.




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