Char structure and combustion kinetics of a phenolic-impregnated honeycomb material

The combustion behavior of a honeycomb material used for the core in sandwich panels, consisting of aramid fiber sheets (Nomex) dipped in a phenolic resin, is studied. Char, produced from pyrolysis of thick samples at 950 K, preserves the honeycomb structure, but the total volume is highly reduced in spite of an increase in the wall thickness. Following softening, the more internal fibers lose their shape, giving rise to a highly porous swelled char. Instead, the more external ones, trapped by the resin char, still show their original shape but with evident shrinkage. Thermogravimetric curves in air are used to formulate a combustion mechanism. It consists of three reactions for the stage of oxidative decomposition of the honeycomb material and one reaction for combustion of the resulting char. Activation energies for the various steps vary in a narrow range of high values (190-256 kJ/mol), testifying low reactivity.