When heating beyond a critical temperature, intumescent materials begin to swell and then to expand. The result of this process is a foamed cellular charred layer on the surface which protects the underlying material from the action of heat flux or flame. The proposed mechanism is based on the charred layer acting as a physical barrier which slows down heat and mass transfer between gas and condensed phases.
Intumescent paints are composed of three active ingredients: an acid source, a carbon source and a blowing agent linked together by a binder. The mechanism of intumescence is usually described as follows: first, the acid source breaks down to yield a mineral acid, then it takes part in the dehydration of the carbonization agent to yield the carbon char, and finally the blowing agent decomposes to yield gaseous products. The latter causes the char to swell and provides an insulating cellular protective layer. This shield limits at the same time the heat transfer from the heat source to the substrate and the mass transfer from substrate to the heat source resulting in a conservation of the underlying material.