Novel piperazine-containing oligomer as flame retardant and crystallization induction additive for thermoplastics polyurethane

In flame-retarding polymer field, it is very difficult to achieve simultaneous outstanding flame retardancy and excellent mechanical properties, thus causing the serious lack of high-performance flame-retarded polymers. In this work, a novel piperazine-containing additive with phosphorus and alkynyl, named poly (2-butyne-1,4-diol piperazine bisphosphonate) (PPBPP), was designed to fabricate flame-retarding high-performance thermoplastics polyurethane (TPU). Experimental results showed that 3.0 wt% PPBPP endowed TPU with a UL-94 V-0 rating in burning test, and the melt dripping of TPU disappeared at 12.0 wt% PPBPP. More importantly, mechanical properties of flame-retarding TPU/PPBPP remarkably recovered with increasing the PPBPP, and the elongation at break for TPU containing 12 wt% PPBPP reached 812.1%, be almost equivalent to that of pure TPU, which is totally different that occurred in traditional flame-retarding TPU system. The study on mechanical recovery mechanism demonstrated that the PPBPP-induced crystallization of TPU made up for the destruction of PPBPP to hydrogen bonding in the hard segment domain of TPU and the following mechanical loss of matrix, leading to the remarkable recovery in mechanical properties of flame-retarding TPU. The study on the flame-retardant mechanism illustrated that alkynyl in PPBPP underwent crosslinking process during heating, and the formed network structure joined the charring of phosphorus-nitrogen flame retardant and effectively promoted the condensed-phase flame-retardant efficiency of PPBPP and anti-melt dripping performance of TPU/PPBPP. This work proposed an efficient route for fabricating flame-retarding high-performance TPU, namely, introduction of piperazine groups into functional flame retardant compensates for the damage of additive to mechanical properties via the induced crystallization and then achieves simultaneously flame retardation and high performance of TPU.