Spiro-Type Cyclic Phosphazene Derivatives as Dual-Function Additives to Improve Thermal Insulation and Flame Retardancy of Rigid Polyurethane Foams

A significant portion of global energy consumption has been due to the building sector, highlighting the urgent need for highly efficient thermal insulation materials. Among these, polyurethane has gained widespread use due to its excellent insulation performance. However, its inherent flammability remains a major concern, necessitating further enhancement of its flame-retardant as well as thermal insulating properties. The development of novel spiro-type cyclic phosphazene derivatives designed to simultaneously improve both the thermal insulation and flame retardancy of rigid polyurethane foams (RPUFs) has been carried out. The phosphazene derivatives (<b>PN-#s</b>) can be readily produced on a 50 g scale, exhibit high thermal stability, and possess char-promoting capabilities for flame resistance. When incorporated into RPUFs at a low loading of 5 wt %, <b>PN-#s</b> effectively suppressed the time-dependent deterioration of thermal conductivity (<λ = 25 mW/m·K), while promoting a high limiting oxygen index of 28.3%. This demonstrates the potential of this new class of spiro-type cyclic phosphazene-based additives for advanced flame-retardant and thermal insulating materials for building applications.