Fabrication and properties investigation of flame retardant rigid polyurethane composites based on converter OG sludge (a metallurgical by-product): A novel strategy for solid waste utilization

This study proposes a novel strategy for the value-added utilization of converter OG sludge by incorporating it as a flame-retardant additive in rigid polyurethane foam (RPUF). A series of RPUF/OG composites were fabricated via a one-step all-water foaming technique. The effects of OG sludge on the thermal stability, combustion behavior, and flame retardant properties of RPUF were systematically investigated using multiple analytical methods, including limiting oxygen index (LOI) test, vertical burning test (UL-94), thermogravimetric analysis (TG), and cone calorimeter test (CCT). The results show that OG sludge can significantly enhance the fire safety of the composites. While traditional metrics like the limiting oxygen index showed limited improvement, CCT results indicate that the peak heat release rate (pHRR), total heat release (THR), and total smoke production (TSP) of the RPUF/OG50 composite decrease significantly by 16.5%, 24.1%, and 43.6%, respectively. This confirms the effectiveness of OG sludge in suppressing fire intensity and smoke generation. TG reveals that OG sludge improves the thermal stability of RPUF composites, with a residual char yield of up to 21.7 wt% at 800°C. Scanning electron microscopy (SEM) and Raman spectroscopy analyses demonstrate that the complex metal oxides in OG sludge enhance the compactness of the char residue of RPUF composites, forming a stable barrier layer that inhibits the transport of heat and substances during the combustion of the composites. This study provides a new approach for the high-value and sustainable utilization of OG sludge, and offers directions for the development of flame-retardant RPUF composites for building thermal insulation applications.