Microstructure engineering of core-shell flame retardants: SiO₂ and MXene encapsulated ammonium polyphosphate for fire-safe thermoplastic polyurethane

Microstructure manipulation represents a fundamental strategy for enhancing material properties. Herein, a new-type flame-resistant material with core-shell architecture (APP@PEI@SiO₂@MXene) is prepared by iterative deposition of self-assembled monolayers, enabling microstructural control of ammonium polyphosphate (APP) via sequential coating with polyethyleneimine (PEI), SiO₂, and MXene. Then the flame retardants were incorporated into thermoplastic polyurethane (TPU) at a loading of 15 wt.%, this meticulously designed microstructure enables the composite to exhibit significantly improved fire safety. The composite exhibits a markedly increased high-temperature char residue under both air and nitrogen atmospheres, indicating improved thermal resilience and char formation capacity. Thermal stability analysis of the material reveals that the coated APP enables TPU composites to engender a carbonaceous stratum of heightened tenacity. Cone calorimetry tests indicate that, compared with pure TPU, TPU/APP-1BL composites exhibit sharp declines of 87.90% in total heat release (THR) and 76.79% in the peak heat release rate (pHRR), with concurrent effective control of smoke/toxic gas emissions. Moreover, the flame-retardant effects of multi-layer coated APP are notably better than those of single-layer coated APP. Mechanistic studies reveal that the flame-retardant performance chiefly originates from the synergistic interplay of solid and vapor-stage pathways. This work highlights the critical role of microstructure design in the creation of high-performance fire-resistant polymers.