Designing high-performance polypropylene <i>via</i> synergistic free radical scavenging-intumescent flame retardancy: excellent mechanical performance and flame retardancy

Polypropylene (PP) faces application limitations due to its inherent flammability. Although halogen-free intumescent flame retardants (IFRs) are environmentally friendly, they often suffer from low flame-retardant efficiency, poor compatibility, and mechanical performance deterioration. Herein, based on a two-phase synergistic flame-retardant mechanism, a highly reactive flame-retardant synergist-1,3-bis(2-acrylamidoethylamino) propane (BAAEP) was designed, and combined with the IFR system including ammonium polyphosphate (APP) and triazine-based char-forming agent (CFA) to achieve synergistic flame retardancy in PP. The multiple active groups in BAAEP (amide groups, C[double bond, length as m-dash]C double bonds, and amino groups) catalyzed crosslinking and char formation in the PP matrix while simultaneously scavenging gaseous free radicals. Furthermore, combined with the intumescent charring effect of IFR, a physical and chemical synergistic flame-retardancy mechanism was established, achieving the integration of high-efficiency flame retardancy and smoke suppression. At the same time, compared to pure PP, there was no significant decrease in thermal or mechanical performance. The optimal flame-retardancy effect was achieved when the addition amount of BAAEP was 4‰. The peak heat release rate (PHRR) of the composite material decreased by 74.3%, total smoke release (TSR) decreased by 83.9%, the vertical burning test (UL-94) rating reached V-0, and the limiting oxygen index (LOI) increased to 32.8%. This work presents a novel strategy for developing high-performance PP with integrated flame retardancy, smoke suppression, and mechanical performance.