Flame retardant rigid polyurethane foam composites based on hexaphenoxycyclotriphosphonitrile: flame retardancy, combustion properties and pyrolysis kinetics

The phosphorus–nitrogen flame retardant hexaphenoxycyclotriphosphorus (HPCTP) was used as a flame retardant for rigid polyurethane foam (RPUF) to fabricate a series of RPUF/HPCTP composites by all-water foaming technology. On this basis, the fire retardancy of the composites were investigated by thermogravimetric (TG), thermogravimetric–infrared (TG-FTIR), scanning electron microscopy (SEM), microcalorimetry, and Raman Spectroscopy. The tests showed that the RPUF/HPCTP composites reached the maximum limiting oxygen index (LOI) value of 24.2 vol% and passed UL-94 V-1 rating. It was also observed that RPUF/HPCTP composites exhibited thermal conductivity of 0.035 W m^-1K^-1, suggesting excellent thermal insulation property of the composites. Thermal kinetic investigation confirmed that the activation energy of the initial RPUF is 102.26 kJ·mol^-1. RPUF/HPCTP15 possessed the highest activation energy of 105.24 kJ·mol^-1, indicating the highest thermal stability. TG-FTIR confirmed that HPCTP could decrease the release intensity of CO₂ and isocyanate, indicating enhanced fire safety of RPUF/HPCTP composites. Raman spectra and SEM investigation showed that the graphitization degree and compactness of char residue for RPUF/HPCTP composites were significantly enhanced, which were benefit to fire retarding enhancement for the composites in fire. This work provided a new way for preparation of fire retarded RPUF composites.