Novel Ultrathin Layered Double Hydroxide Nanosheets with In Situ Formed Oxidized Phosphorus as Anions for Simultaneous Fire Resistance and Mechanical Enhancement of Thermoplastic Polyurethane

Thermoplastic polyurethane (TPU) is widely used in wires, cables, etc., facing a big challenge in its fire safety under many cases. Traditional nanoparticles with a flame-retardant feature are showing an application prospect in preparing flame-retarding TPU with excellent overall performance because of their unique structural characteristic. However, almost all of these nanoparticles are not highly efficient in fabricating flame-retarding polymers. In this work, we developed novel ultrathin layered double hydroxide nanosheets with in situ formed oxidized phosphorus as anions (P-LDH) via a simple and versatile ball-milling method to prepare flame-retarding TPU. Compared with nanoparticles which were used in TPU before, the P-LDH showed an obvious advantage in simultaneously enhancing and flame-retarding TPU. At only 5.0 wt % of P-LDH, TPU/P-LDH achieved a UL-94 V-0 rating and a limiting oxygen index of 28.0% during burning tests, obviously superior to a UL-94 V-2 rating and a limiting oxygen index of 23.0% for pure TPU; its total heat release was decreased by 70.9% compared with that of pure TPU in the cone calorimeter test. Meanwhile, both tensile strength and elongation at break of TPU were increased by 9.3 and 6.2% in comparison with the corresponding values of pure TPU at 5.0 wt % of P-LDH. A study on the fire resistance mechanism revealed that the structural change of char residue dominated the improved flame retardance of TPU/P-LDH. For the effect of P-LDH on mechanical properties of TPU, the improved dispersity of ultrathin P-LDH nanosheets played a leading role. All of these experimental results demonstrated that mechanochemical reaction-induced anion substitution and high exfoliation for LDH nanosheets is an efficient method for preparing TPU with simultaneous flame retardance and excellent mechanical properties.