A novel strategy for simultaneously improving the fire safety, water resistance and compatibility of thermoplastic polyurethane composites through the construction of biomimetic hydrophobic structure of intumescent flame retardant synergistic system

The water resistance, compatibility and anti-dripping performance of flame retardant thermoplastic polyurethane (TPU) composites have still been a challenge in the fire science. Inspired by the hydrophobic surface structure of biomaterials, the hydrophobic charring agent N-methyl triazine-piperazine copolymer (MTPC) and modified magnesium oxide (MgO) were assembled on ammonium polyphosphate (APP) surface to prepare the hydrophobic intumescent flame retardant (HIFR) system. The constructed HIFR system presented excellent hydrophobic performance with the water contact angle of 139°. The HIFR was incorporated into TPU matrix. The tensile strength and elongation at break of TPU/10 wt% HIFR composites increased by 48 and 106% compared with that of TPU/10 wt% APP. Meanwhile, the specimens of TPU/10 wt% HIFR composites before and after water resistance tests achieved UL-94 V-0 rating without drippings and the limiting oxygen index value was 27.5 and 27.3%. TPU/HIFR composites still maintained excellent flame retardancy and mechanical properties after water treatment. Cone calorimeter tests revealed that the peak of heat release and smoke production, and CO yield of TPU/10 wt% HIFR composites were significantly decreased with the reduction of 87.0, 63.2 and 28.6% compared with that of neat TPU. The incorporation of HIFR effectively enhanced the fire safety for TPU composites. The investigation of flame retardant mechanism for TPU/HIFR demonstrated that HIFR catalyzed the TPU matrix charring in advance and the generated compact, homogeneous and partial graphitized char layer exerted barrier effect in condensed phase, and the released inert gases evoked dilution effect in gas phase. This presented work provided a novel promising approach for preparing TPU materials with excellent comprehensive performance as well as water resistance and highly flame retardant efficiency.