Dual‐Phase Flame‐Retardant Action of Bio‐Based <scp>DOPO</scp> ‐ <scp>ITA</scp> : Improved Fire Safety, Thermal Stability and Mechanical Properties of Rigid Polyurethane Foam

ABSTRACT Rigid polyurethane foam (RPUF) was widely applied in thermal insulation and lightweight structures, but was flammable with heavy smoke. And high‐dose fillers in current flame retardancy compromised its structure and mechanical properties. In this study, a reactive bio‐based phosphorus‐containing flame retardant (DOPO‐ITA) was synthesized via the reaction between 9,10‐dihydro‐9‐oxa‐10‐phosphaphenanthrene‐10‐oxide (DOPO) and bio‐based itaconic acid (ITA). The resultant DOPO‐ITA was covalently incorporated into the cross‐linked network of RPUF. The effects of DOPO‐ITA addition on the heat release characteristics, smoke generation behavior, char layer structure evolution, and mechanical properties of the modified RPUF were systematically investigated. Cone calorimeter tests showed that when the loading of DOPO‐ITA reached 5 wt%, the peak heat release rate (PHRR) and total smoke release (TSR) of the modified foam decreased by 40.74% and 37.88%, respectively, compared with the unmodified sample. The limiting oxygen index (LOI) value of the corresponding sample increased to 21.5%, and the sample passed the UL‐94 V‐1 rating, exhibiting superior self‐extinguishing performance. Scanning electron microscopy (SEM) and Raman spectroscopy of the char residues demonstrated that DOPO‐ITA facilitated the formation of a continuous, low‐defect phosphorus‐rich char layer. The I D / I G ratio of the char layer decreased from 4.58 to 3.01, revealing a significant improvement in the ordering degree of the char layer. RPUF‐2 with 5 wt% DOPO‐ITA achieved the optimal comprehensive performance, synergistically improving the flame retardancy, smoke suppression, mechanical properties, and thermal stability of the RPUF. This bio‐based reactive flame retardant solved the performance degradation issue of traditional flame‐retardant modifications and provided an eco‐friendly solution for fire‐sensitive applications.