Synergistic flame retardancy of piperazine pyrophosphate/magnesium hydroxide/fly ash cenospheres-doped rigid polyurethane foams

In order to explore rigid polyurethane foams (RPUFs) with excellent flame retardancy and mechanical properties, this study incorporates the ternary flame-retardant system of piperazine pyrophosphate (PAPP)/magnesium hydroxide (MH)/fly ash cenospheres (FAC) into RPUFs. The results show that doping 6 wt% FAC significantly improves the flame retardancy of the PAPP/MH/FAC-doped RPUFs. The peak heat release rate (p-HRR) decreases from 236.7 kW·m−2 to 127.2 kW·m−2, while the fire resistance index (FRI) increases from 1.00 to 5.34, and passes a vertical burning (UL-94) test V-0 rating. Meanwhile, the RPUFs possess an intact, compact, and robust non-flammable shielding layer due to the formation of the interpenetrating network structure and the physical accumulation of silicon carbon phosphorus residues. Moreover, the pyrolysis kinetics is modeled using the G(α) with reaction order n = 2, PAPP/MH/FAC-doped RPUFs make the pyrolysis activation energy Eα climb from 78.97 to 131.79 kJ·mol−1 at 237–340 °C, corresponding to the decomposition of PAPP and the excellent thermal stability of FAC. Furthermore, PAPP/MH/FAC-doped RPUFs enhance compressive strength while improving flame retardancy and exhibit excellent formaldehyde adsorption performance (adsorption capacity is 64 %). Therefore, this study explores a novel PAPP/MH/FAC-doped RPUFs, promoting the recycling of industrial coal-fired waste and the development of high-performance building materials.