Aqueous film-forming foam (AFFF) is a type of foam extinguishing agent capable of forming a water film on the surface of combustible liquids. It is mainly composed of film-forming agent, foaming agent, foam homogenizer, water retention agent and cosolvent. Different types of AFFF can be prepared by adding varying dosages of amino acid-based fluorocarbon surfactants into the basic formula. Fire extinguishing time and burn resistance time can be measured through laboratory tests.

1. Spreading Mechanism
The fundamental working principle relies on the spreading of low-concentration fluorocarbon surfactant aqueous solution over combustible liquids. Chemical thermodynamics indicates that the spreading coefficient must be positive to achieve effective spreading.

Spreading performance is mainly determined by the surface tension of oil and water, as well as oil-water interfacial tension. The surface tension of oil generally ranges from 20 to 24 mN/m. Aqueous solution blended with fluorosurfactants features extremely low surface tension, sometimes below 15 mN/m. The oil-water interfacial tension rises along with the lipophobicity of fluorocarbon chains. When the surface tension of aqueous solution drops to around 15 mN/m, it can offset the influence of interfacial tension and enable the solution to spread freely on oil surfaces.

Fluorocarbon surfactant serves as a core component of AFFF. Its molecular hydrocarbon chains are partially or fully substituted by fluorine atoms, delivering superior surface activity. The electron-withdrawing property of fluorocarbon chains prevents precipitation when mixed with oppositely charged surfactants. Strong electrostatic attraction between anions and cations boosts surface adsorption and micelle formation, improving the viscosity and mechanical strength of surface film. Accordingly, foam is less prone to rupture, liquid drainage slows down, air permeability declines, and foam durability and burn resistance get enhanced.

Hydrocarbon surfactants contain both hydrophilic and lipophilic groups and adsorb at oil-water interfaces. Mixed use with fluorocarbon surfactants effectively reduces interfacial tension and meets spreading requirements.

Combined application of the two surfactants ensures excellent fire suppression performance. Reduced surface tension allows stable thin water film to spread and float on hydrocarbon liquid surfaces, effectively preventing flashback. Fast spreading speed and excellent sealing property greatly improve practical fire-fighting performance.

2. Fire Extinguishing Mechanism
AFFF extinguishes fires through three major effects: cooling, smothering and blocking.

2.1 Cooling Effect
The foam structure acts as a heat dissipator. Over 94% of foam composition is water. Foam adheres more firmly to combustible surfaces, especially vertical surfaces. It wets burning materials, absorbs combustion heat and dissipates heat via water evaporation.

2.2 Smothering Effect
With low relative density, AFFF foam flows rapidly and floats on liquid combustibles or adheres to solid surfaces, forming a dense covering layer that isolates burning substances from ambient air.

2.3 Blocking Effect
The formed water film fully covers fuel surfaces. The foam layer shields combustibles from thermal radiation, slows down liquid evaporation and solid thermal decomposition, restricts volatile combustible gas release, weakens flame damage to foam, and strengthens sealing and anti-reignition capacity.