AFFF Foam can extinguish a flammable liquid fire in less than the time of  conventional foams  & keep it extinguished.  The foam spreads over the surface of burning fuel forming a blanket  in the same manner as conventional foams.  More important, an aqueous solution drains from the foam bubbles and forms a vapours sealing film.  This film floats on the fuel surface.  This film is tough and persistent and suppresses volatile vapours.   In addition, it has reforming, self-sealing action which prevents reflash should the foam be disturbed.  QUALITY 'AFFF' also takes the fire hazard out of fuel spills because it seals and secures non-ignited  areas and prevents ignition.  The ability to form vapor seal  over flammable liquids permits AFFF to extend its performance well beyond that of ordinary foam.  This performance is important in situation where there is limited or no access.  The vapor sealing film flows with the fuel and secures as it goes.