Reactive Liquid Siloxane‐Enabled PSi Cooperative Flame Retardancy in Rigid Polyurethane Foam via Condensed‐Phase Char Engineering

A low‐molecular‐weight, hydroxyl‐functional liquid siloxane derived from dimethylsilanediol (DMSD) was developed as a reactive silicon‐containing component for improving the flame retardancy of rigid polyurethane foam (RPUF). Its individual and combined effects with tris(1‐chloro‐2‐propyl) phosphate (TCPP) were evaluated using PUF‐R, PUF‐Si, PUF‐P, and PUF‐P‐Si formulations. The peak heat release rate (pHRR) progressively decreased from 371.9 W/g for PUF‐R to 313.5, 228, and 197 W/g for PUF‐Si, PUF‐P, and PUF‐P‐Si, respectively. Compared with PUF‐P, PUF‐P‐Si reduced pHRR and total heat release (THR) by approximately 13.6% and 21.0%, respectively, while increasing the residual char yield at 800°C from 12.58 to 18.2 wt% and the limiting oxygen index (LOI) from 23.1% to 26.3%. Raman analysis showed a decrease in the I D /I G ratio from 4.16 to 2.09, indicating modification of the phosphorus‐containing char toward a more structurally ordered state. Scanning electron microscopy–energy dispersive X‐ray spectroscopy (SEM–EDS) revealed a compact residual char containing both phosphorus‐ and silicon‐derived species. These results support a cooperative P–Si condensed‐phase mechanism in which phosphorus promotes carbonization while siloxane‐derived Si‐containing structures reinforce and stabilize the resulting char, thereby enhancing the flame retardancy of RPUF.