Thermal stabilities and flame retardancies of nitrogen–phosphorus flame retardants based on bisphosphoramidates

Various nitrogen–phosphorus (P–N) compounds based on phosphoramidate were synthesized as model compounds to investigate the relationships among the chemical structure of linker connecting diphenylphoryl groups between the phosphoramidates, the N content, thermal stability, and flame-retarding ability. The flame-retarding efficiencies were evaluated by the limiting oxygen index (LOI) and UL-94 vertical test methods. It was found that bisphosphoramidates are more thermally stable and produce more charred residues when compared to the corresponding bisphosphate compounds. Aromatic phosphoramidates show fairly good flame retardancy for PC and UL-94 V-0 ratings are achieved with addition of as small amount as 3–5wt%. However, no rating is found for ABS at 30wt% loading of bisphosphoramidate FRs which leave the remarkably high residues at 600°C. The thermogravimetric analysis (TGA) results indicate that these compounds work in condensed phase rather than in gas phase. The effect of chemical structure of linker on the flame retardancy is also discussed.