Phosphonitrile decorating expandable graphite as a high-efficient flame retardant for rigid polyurethane foams

Researchers have widely focused on the high flammability and releasing toxic gases of rigid polyurethane foam (RPUF) under fire attack. In this work, the hybrid expandable graphite (EG) intumescent flame retardant is designed and fabricated for the fire safety of RPUF. Hexachlorocyclotriphosphazene (HCCP) is self-assembled on the surface and interlamination of EG (HCCP@EG) via water bath method and incorporated into RPUF matrix to improve the flame retardant efficiency of EG and the interfacial adhesion between EG particles and RPUF matrix. The structural morphological characterization indicates that HCCP@EG is successfully synthesized. HCCP@EG also exhibits good thermal stability and high char residues. Compared with neat RPUF, RPUF composites with 15% HCCP@EG show excellent flame-retardant properties and achieve a UL-94 V-0 level and a limiting index higher than 27.0%, which is due to the construction of a network structure of HCCP in the graphite lamellas and its synergistic and barrier effect. In addition, compared with EG, the presence of HCCP enhances interface compatibility between EG and RPUF, resulting in improving the mechanical property of RPUF composites. In summary, this work provides a facile approach to manufacturing high-performance RPUF products with outstanding fire safety.