Effect of Melamine and its Salts on Combustion and Thermal Decomposition of Polyamide 6

Abstract

Melamine and its salts added to polyamide 6 (PA-6, nylon 6) improve its fire retardancy as measured by oxygen index and UL94 tests. The mechanism of the fire-retardant action of the additives was studied using thermogravimetry, kinetics of thermal decomposition and characterization of solid residues and evolved high-boiling products. It was found that melamine, melamine oxalate, melamine phthalate and melamine cyanurate facilitate thermal decomposition of PA-6 with increasing evolution of oligomeric chain fragments instead of caprolactam, which is the principal product evolved from the nonfire-retarded PA-6. These additives promote non-combustible flow dripping and help extinguishment of the flame. The observed increase in solid residue from the thermal decomposition of the formulations or the endothermic cooling due to melamine evaporation might give an additional but less important contribution to fire retardancy. In the case of dimelamine phosphate and melamine pyrophosphate, PA-6 reacts with liberated phosphoric acids producing phosphoric esters which give char upon further thermal decomposition. The fire-retardant effect of these two salts is mostly attributed to polymer mass retention and intumescent layer protection mechanisms. © 1997 John Wiley & Sons, Ltd.