Preparation and properties of flame retardant rigid polyurethane foam with phosphorus–nitrogen intumescent flame retardant

Ethanolamine spirocyclic pentaerythritol bisphosphonate (EMSPB), a novel intumescent flame retardant, was synthesized and used to improve the flame retardancy of rigid polyurethane foam (RPUF). The effects of EMSPB on the flammability, thermal stability, and mechanical properties of RPUF were discussed, respectively. Scanning electron microscopy (SEM) and compression strength tests showed that the EMSPB had favorable compatibility with the RPUF matrix and did not deteriorate the mechanical properties of the RPUF. Flammability of RPUF systems containing various contents of EMSPB was investigated by vertical burning test (UL-94) and limiting oxygen index (LOI) test. Results indicated that when the content of EMSPB was 25 wt%, the LOI of flame retardant RPUF could reach 27.5%, and a UL-94 V-0 rating was achieved. Thermogravimetric analysis showed that RPUF-containing EMSPB had a high yield of residual char at high temperatures, indicating that EMSPB was an effective charring agent. From the SEM observations of the residues of the flame retardant systems burned, the compact charred layers could be seen, which form protective shields to protect effectively the internal structure and inhibited the transmission of heat and heat diffusion during contacting fire.