Materials that react with water are not combustible themselves, but they produce sufficient heat to ignite nearby combustibles or flammable gases produced by the reaction. The temperature of the material is not an important factor in this case because the reaction is sufficiently fast even at room temperature. Reactions that cause spontaneous ignition include polymerization reactions and those between two highly reactive materials. Examples are given later in this Digest. In general, the actual mechanism of self-heating is not simple because two or more reactions often occur, either simultaneously or consecutively.
Water plays a number of important roles in self-heating reactions.
As a reactant in certain exothermic chemical reactions and an influence in the growth of microorganisms in biological reactions its action has already been mentioned. Water may also suppress self-heating reactions of a chemical nature by absorbing heat by vaporization and by assisting heat dissipation. One reason is that a material in a finely divided state has a lower thermal conductivity than one in the solid state so that heat is accumulated more easily. Another is that any decrease in particle size results in increased surface area. An oxidation reaction takes place on the surface exposed to the air.