Silane-grafted black phosphorus nanosheets via ball milling: Simultaneously enhanced flame retardancy and mechanical properties in polyurea composites

Conventional strategies for enhancing polymer flame retardancy face an intractable trade-off: high-loading additives inevitably sacrifice mechanical integrity and processability. To address this challenge, this study employed a novel silane coupling agent, (3-Isocyanatopropyl) trimethoxy silane (3IT), synthesizing modified black phosphorus nanosheets (M-BP) through a low-pollution mechanochemical approach using ball-milling grafting. Covalent surface engineering established covalent bonds and hydrogen bonds, enhancing interfacial bonding in BP/PUA composites. M-BP exhibited a markedly higher char yield (34.7 % at 700 °C) than neat BP, confirming that 3IT modification effectively improved BP's thermal stability and char-forming ability. As a result, when incorporated into the PUA matrix, the M-BP/PUA composite achieved approximately 9.5 times higher char residue than neat PUA, accompanied by reductions of 17.9 % in peak heat release rate (PHRR) and 8.5 % in total heat release (THR). The resultant M-BP/PUA composite preserved BP's excellent flame retardancy while exhibiting an 18.2 % increase in tensile strength, 6.6 % increase in impact strength, a 78.8 % increase in maximum tear force, and 94.7 % improvement in puncture resistance compared to unmodified BP composites. This research provides an innovative way for designing multifunctional flame retardants and creating high-performance polymers with high thermal stability, low flammability, and excellent mechanical properties, holding significant potential for practical applications.