Molecular Structure-Dependent Flame Retardancy of Phosphonamides in Thermoplastic Polyurethane Elastomer: Aliphatic Chains vs Aromatic Rings

In this study, aliphatic (PHDA) and aromatic (PPDA) phosphonamide flame retardants were designed, and thermoplastic polyurethane elastomer (TPU) blends were prepared through melt blending. Results showed that both flame retardants significantly enhanced TPU’s fire safety: at 3 wt % loading, TPU/PHDA-3 and TPU/PPDA-3 both achieved UL 94 V-0 rating with limiting oxygen index values of 30 and 27%, respectively. PPDA can work as an effective disincentive to the dripping behavior of TPU, while PHDA cannot achieve such an effect even up to 12 wt % addition. At 9 wt % loading, TPU/PPDA-9 attained UL 94 V-0 rating without dripping, reducing peak heat release rate and total heat release by 74 and 40% compared to pure TPU, outperforming the PHDA system (61 and 34%). Compared with TPU/PHDA-9, TPU/PPDA-9 had more char residue, and the percentage of the C–C structure and the degree of graphitization in the residual carbon were higher, indicating that the benzene ring structure in phosphonamidite flame retardants is more favorable for the formation of an effective protective carbon layer compared with aliphatic chains. Moreover, the TPU blends with PPDA exhibited less impact on tensile strength compared with those with PHDA. In particular, TPU/PPDA-3 showed almost the same tensile test results as pure TPU. This study provides an experimental basis for the design and regulation of phosphonamide molecules.