Facile alteration of the molecular structure of phosphonamide for efficient flame retardancy in TPU

In this work, three phosphonamide flame retardants with different chain segment structures (PPP, PPDA, EDP) were designed and synthesized, and efficient flame-retardant TPU systems were obtained by comparing the flame-retardant effects of three phosphonamide flame retardants in TPU. First, it was found that phosphonamide flame retardants containing p-phenylenediamine groups in the chain segment structure had better flame retardant effects than those containing piperazine groups. This was reflected in the experiment that the TPU/9%PPP only achieved UL 94 V-2 rating, but TPU/9%PPDA successfully passed the V-0 test. Then, EDP was obtained by further replacing the phenyl in PPDA with ethoxy and p-phenylenediamine group retained, which had the highest flame-retardant efficiency among all phosphonamide flame retardants. Its limited oxygen index (LOI) value exceeded 30 %, higher than that of TPU/9%PPDA (27.8 %), and achieved anti-melt dripping during UL-94 test. The LOI value of TPU/9%EDP was increased by 36 %, while the peak heat release rate and total heat release were significantly reduced by 77 % and 52 %, respectively, compared with pure TPU. Based on the analysis of char residue, EDP formed a protective char layer at the earliest time, playing a barrier protection role. This work provides a pathway for efficient flame retardant TPU. Also, the design idea of phosphonamide flame retardants is a useful reference for the preparation of multifunctional flame retardants.