Electrostatic action induced interfacial accumulation of layered double hydroxides towards highly efficient flame retardance and mechanical enhancement of thermoplastic polyurethane/ammonium polyphosphate

In the past work, few study considers an effect of nanoparticle as a synergistic flame retardant on the interfacial interaction between flame retardants and polymer matrix, as well as further influence on the flame retardancy and mechanical properties of flame-retarding polymers. In this work, the electrostatic action was used to prepare a novel ammonium polyphosphate@layered double hydroxide (APP@LDH) in which the LDH nanoparticles accumulated at the surface of APP to achieve the highly-efficient synergistic action of LDH in thermoplastic polyurethane/APP (TPU/APP) and reduce the deterioration of APP to mechanical properties of TPU. Scanning electron microscope, etc., demonstrated that the APP@LDH was prepared successfully. Combustion tests results showed that the APP@LDH had much higher flame retarding efficiency than the APP/LDH which was prepared through a simple physical blending process. Only 1.0 wt% LDH made the TPU pass the V-0 rating with no dripping in the UL-94 test and a limiting oxygen index (LOI) of 29.2% in the case of 7.0 wt% APP@LDH. However, the TPU/APP/LDH with 1.0 wt% LDH did not pass the V-0 rating, and a dripping behavior also existed at 7.0 wt% APP/LDH. In cone calorimeter test, the heat release and smoke production of TPU/APP@LDH were also lower than the corresponding values of TPU/APP/LDH under equal amount of LDH. The analysis of flame-retardant mechanism proved that an enhanced condensed action induced by the interfacial LDH dominated the better flame retardance of APP@LDH system than that of APP/LDH system. Another important aspect is that the mechanical properties of APP@LDH system showed remarkable improvements in comparison with those of APP/LDH system. The tensile strength of TPU with 7.0 wt% APP@LDH was 32.5% higher than that of TPU with 7.0 wt% APP/LDH under equal 1.0 wt% LDH. Meanwhile, the elongation at break for the former was maintained at 863.0%, almost equal to that of TPU. This work illustrates that the interfacial accumulation of LDH may enhance its synergistic flame-retarding efficiency and meanwhile achieve the mechanical enhancement for TPU/APP. (C) 2019 Elsevier Ltd. All rights reserved.