Reinforced Char Networks for Slag/Phosphorus-modified Polyurethane Foams

Rigid polyurethane foams (RPUFs) are widely used as thermal insulation materials but are inherently flammable. This study presents a synergistic flame-retardant strategy based on reinforced char network formation through the combined use of steelmaking slag (S-slag) and a phosphorus-based flame retardant. The hybrid system fundamentally alters the degradation pathway of RPUF by stabilizing the condensed phase, overcoming the drawback of phosphorus-induced early degradation via hybrid charring. Enhanced thermal stability is evidenced by a delayed main degradation region and increased char residue. Correspondingly, all slag/phosphorus hybrid foams achieve the highest HF-1 rating (UL-94 test), outperforming phosphorus-only and slag-only systems. The superior performance originates from the formation of an oxide–phosphate hybrid char network formed by interactions between slag-derived metal oxides and phosphorus species. This work demonstrates a paradigm shift from conventional waste treatment to waste valorization for high-performance material design. By repurposing S-slag as an active, functional condensed-phase component rather than an inert filler, this study offers a novel route to sustainable and synergistic flame-retardant RPUF composites. • Steelmaking slag actively reinforces condensed-phase char in RPUFs. • Overcomes the drawback of phosphorus-induced early degradation via hybrid charring • Slag induces a rightward shift of degradation and increases char residue. • Slag-phosphate hybrid char networks in RPUFs underpin UL-94 performance.