Design and Flame‐Retardant Synergism of a Green Phosphorus‐Containing Heterocyclic Compound in Rigid Polyurethane Foam

ABSTRACT Rigid polyurethane foam (RPUF) is highly flammable, which severely restricts its practical applications. To address this issue, a phosphorus‐based flame retardant, BDMP (bis(5‐ethyl‐2‐methyl‐2‐oxo‐1,3,2‐dioxaphosphinan‐5‐yl)methyl phosphonic acid methyl ester), was synthesized using the ionic liquid BmimBF 4 as a green catalyst. During synthesis, the ionic liquid BmimBF 4 functioned as a green catalyst and exhibited higher selectivity than γ‐Al 2 O 3 , affording BDMP in a yield of 52.23% within 18 h. BDMP was incorporated into RPUF at loadings ranging from 5 to 25 wt%, with 15 wt% identified as the optimal loading. At this loading, the limiting oxygen index (LOI) increased from 19.0% to 25.1%, and a UL‐94 V‐0 rating was achieved, while the peak heat release rate (PHRR) and total heat release (THR) were reduced by 36.07% and 35.85%, respectively. Scanning electron microscopy further confirmed the good compatibility between BDMP and the RPUF matrix. The enhanced flame retardancy was mainly attributed to the catalytic charring effect of BDMP, which promoted the formation of a stable and protective char layer. Overall, BDMP demonstrates significant potential as a highly efficient and sustainable phosphorus‐based flame retardant for RPUF.