Design of an all-in-one intumescent flame retardant with DFT Verification: simultaneously enhancing flame retardancy and tensile strength of polylactic acid

While ammonium polyphosphate (APP) has been widely investigated for enhancing the flame retardancy of polylactic acid (PLA), its practical application remains constrained by inherent limitations, such as poor compatibility with the polymer matrix, limited flame-retardant efficiency at lower loadings, and the tendency to aggravate deterioration of mechanical properties. To address these issues, we synthesized a novel modified APP (MAPP) via reaction with piperazine, cyanuric chloride, and Tris reagent, resulting in an all-in-one intumescent flame retardant (IFR) that integrates acid, gas, and carbon sources within a single structure. PLA/MAPP composites achieved self-extinguishing behavior in the UL-94 test, with remarkably improved flame retardant efficiency and limiting oxygen index (LOI) compared to state-of-the-art studies. TGA analysis revealed a char residue of 8.9% at 800 °C, exceeding both the theoretical value and the char yields of conventional PLA composites. The flame-retardant mechanism indicated that MAPP mainly endowed PLA with excellent flame retardancy through the solid phase. Notably, density functional theory (DFT) calculations combined with experimental characterizations validated MAPP's synthesis and enabled in-depth elucidation of the flame-retardant mechanism. Through DFT simulations, MAPP was rationally designed as an innovative flame retardant that simultaneously provided an acid source, gas source, and carbon source. These calculations further elucidated its key properties and validated the enhanced thermal stability of the overall system, which aligned with the core theme of this research. Meanwhile, PLA/MAPP exhibited outstanding tensile strength among all samples, exceeding that of PLA by 20.8% and PLA/APP by 63.3%. This work provides a viable strategy for developing high-performance PLA composites with well-elucidated flame-retardant mechanisms, holding promising implications for advanced flame-retardant polymer materials.