A study on the modification of polypropylene by a star-shaped intumescent flame retardant containing phosphorus and nitrogen

•A star-shaped phosphorus-nitrogen intumescent flame retardant (BTETP) based on triazine was synthesized.•BTETP was combined with ammonium polyphosphate (APP) to form intumescent flame retardants systems (IFRs) to flame-retard polypropylene.•Flame retardancy and thermal stability improved significantly by the combination of BTETP and APP.•Excellent flame retardancy can be achieved owning to condensed and gas phase synergistic flame-retardant mechanism. In this paper, a star-shaped char forming agent BTETP containing phosphorus and nitrogen is synthesized and characterized in the paper. It is compounded with ammonium polyphosphate (APP) to form an intumescent flame-retardant system. The stable star-shaped structure is constructed, and the insulating char layer is formed on the surface of the polymer matrix by the synergistic effect between phosphorus and nitrogen, thus, the high-performance flame retardant PP is obtained. Its flame retardancy and thermal stability are studied by limiting oxygen index (LOI), vertical combustion test (UL-94), cone calorimetry (CCT), and thermogravimetric analysis (TGA). With only a 20 wt% addition of BTETP/APP (1:2), the PP composite can reach the UL-94 v-0 level, while the peak heat release rate (PHRR), total heat release rate (THR), and average mass loss rate (AV-MLR) are significantly reduced compared with pure PP, showing higher fire safety. Moreover, the residue at 800 °C can reach as high as 9%, indicating that the loading of BTETP/APP is conducive to promoting carbonization of the degradation products of PP to generate more char. In addition, the synergistic effect and the flame retardant mechanism are studied and discussed. This study provides a program for the synthesis of flame retardants with a high-performance and effectively avoids the problem existing in traditional flame retarding approaches.