Fabrication of a novel P-N flame retardant and its synergistic flame retardancy with expandable graphite on rigid polyurethane foam

An innovative phosphorus-nitrogen flame retardant (ZrMP) is fabricated by reacting a-zirconium phosphate (a-ZrP), phenylphosphinic acid with melamine, and applied to improve the flame retardancy of rigid polyurethane foam/expandable graphite (RPUF/EG) composites. The synergistic effect of ZrMP and EG on the flame retardancy and thermal stability of RPUF was examined by the limiting oxygen index (LOI), UL-94 vertical burning test, and thermogravimetric analyses. Benefiting from the P and N elements of ZrMP and the expanded char layer of EG, the synergistic effect of ZrMP and EG endows FPUF with excellent flame retardant and thermal properties. The RPUF composites containing 12.5% EG and 2.5% ZrMP can achieve a V-0 level and its LOI value increases to 26.7%. RPUF/EG/ZrMP composites exhibit good thermal stability compared with pure RPUF. Accordingly, the flame-retardant mechanism of RPUF composites is proposed by thermogravimetric analyses/infrared, scanning electron microscopy, and X-ray photoelectron spectroscopy. The results indicate a combination of physical barriers in condensed-phase and gaseous-phase dilution activity during combustion. Therefore, this study provides a scalable and promising strategy for constructing flame-retardant RPUF composites.