Molecular Design of a Novel Phosphorus–Nitrogen–Boron Synergistic Reactive Flame Retardant for Highly Fire‐Safe and Smoke‐Suppressive Polyurethane Elastomers

ABSTRACT

Polyurethane (PU) elastomers are essential in modern engineering but suffer from inherent flammability and toxic smoke production. To address these challenges, we designed a novel reactive phosphorus–nitrogen–boron (P‐N‐B) synergistic flame retardant, designated as PBX, via a condensation process involving a boroxine intermediate (BX) and dimethyl methylphosphonate (TMP). PBX was covalently incorporated into the PU matrix to ensure long‐term durability and compatibility. The fire safety of the PU/PBX composites was systematically evaluated. Notably, with a loading of only 9 wt.%, the PU/PBX9 sample achieved a V‐0 rating without drippings in UL‐94 tests. The limiting oxygen index (LOI) for PU/PBX9 sample was enhanced to 33.2%. Cone calorimetry results demonstrated that PU/PBX9 reduced the peak heat release rate (pHRR) to 602 kW/m ² and total heat release (THR) by 44.1% compared to that of pure PU, while simultaneously exhibiting smoke suppression performance. The reason was PBX mainly promoted condensed‐phase flame retardancy. The mechanical properties and durability for PU/PBX composite also indicated that the reactive PBX increased the elongation at break of the PU and exhibited good durability in PU. This work provided a high‐efficiency strategy for developing intrinsically fire‐safe PU elastomers without compromising mechanical integrity.