AbstractIn this article, a nitrogen–phosphorus composite flame retardant, melamine phosphate (MP), was synthesized in a solution of phosphoric acid/macromolecular charring agent, polyamide 6 (PA6). With the progress of the phosphoric acid–melamine reaction producing MP, the acidity of this system decreased and the originally dissolved PA6 resin precipitated on the surface of MP particles to obtain the MP in situ encapsulated by PA6. This encapsulated MP was incorporated with a small molecular charring agent, pentaerythritol (PER), composed of an intumescent system to flame retard polypropylene (PP). As a result of the isolation effects of PA6 resin, the esterfication reaction between MP and PER was effectively suppressed at the processing temperature of PP, thus remarkably improving the thermal stability of such an intumescent system while maintaining the mechanical performance of the flame retardant PP. Additionally, encapsulated MP/PER system showed better charring properties when compared with MP/PER and therefore possessed a higher efficiency in flame retarding PP. The corresponding charring mechanisms of the former, including either a macromolecular charring agent or small molecular one, were investigated. POLYM. COMPOS., 28:163–167, 2007. © 2007 Society of Plastics Engineers