Flame-Retardant Rigid Polyurethane Foam Composites Based on Piperazine Pyrophosphate/Steel Slag: A New Strategy for Utilizing Metallurgical Solid Waste

In this work, piperazine pyrophosphate (PAPP) and metallurgical solid steel slag (SS) are used to fabricate flame-retardant rigid polyurethane foam (RPUF) composites through a one-step all-water foaming technology. The thermal stability, combustion properties, and flame retardancy of the PAPP/SS composite were investigated by thermogravimetric (TG) analysis, cone calorimetry, limiting oxygen index testing (LOI), and UL-94 vertical burn testing. RPUF-3 showed a char residue of 28.6 wt% at 750 degrees C compared with 24.9 wt% of the pure sample, indicating better thermal stability of the FRPRUF composites. RPUF-3 possessed an LOI of 21.5 vol% and achieved a V-0 level in the UL-94 test. Cone calorimetry displayed the peak heat release rate and fire growth rate index of RPUF-3 were decreased by 12.66% and 41.6%, respectively, compared with those of pure RPUF. PAPP/SS incorporation led to the formation of compact char layer structures during combustion. Pyrophosphoric acid, generated from the decomposition of PAPP, promotes the formation of esters, ethers, and alcohols, whereas metal oxides in SS enhance the compactness of the char layer. This enhanced structural integrity obstructs mass and heat transmission in the combustion zone, effectively improving condensed-phase flame retardancy. This approach offers a novel strategy for the fabrication of high-performance RPUF composites and the high-value utilization of SS.