Abstract Charring polymers may be used as carbonization agents in fire retardant (FR) intumescent additive master batches. This paper presents typical polyamide-6-based master batches which are extensively used in polyethylenic FR formulations. It is shown that a blend of polyamide-6 and an ethylene–vinyl acetate copolymer allows the incorporation of the carbonization catalyst, i.e. ammonium polyphosphate, in the polymeric material. The problem of the migration of the phosphate throughout the polymeric matrix is discussed. Solid state NMR spectroscopy is presented as a tool to predict and to explain the part played by the interfacial agent, i.e. the functionalized polymer.