Construction and mechanism of efficient flame retardant system for alkali lignin‐enhanced rigid polyurethane foam

Abstract The enhancement of flame resistance and mechanical strength is critical for the utilization of rigid polyurethane foam (RPUF) composites. In this study, the efficient synergistic flame‐retardant system, incorporating dimethyl methylphosphonate (DMMP) and expandable graphite (EG), with the addition of refined alkali lignin (A‐lignin) were aimed to superior fire resistance and mechanical integrity for RPUF. Furthermore, the flame retardancy and charring ability of obtained RPUF were obviously enhanced. Notably, an RPUF mixture containing A‐lignin, DMMP, and EG demonstrated substantial charring at elevated temperatures, improved limiting oxygen index, and met the UL‐94 V‐0 rating. Cone calorimeter test indicated that this formulation produces less heat and smoke than pure RPUF. The flame‐retardant efficacy of obtained RPUF was attributed to the formation of a protective char layer coupled with the free radical neutralization properties of DMMP, collectively enhancing the flame retardancy obtained RPUF. Additionally, this RPUF variant showed significant advancements in both mechanical and thermal performance. This study outlines a successful strategy for developing RPUF composites endowed with enhanced flame retardancy and mechanical strengths.