Facile-controllable synthesis of 3D cross-shaped ZIF-67 to achieve highly efficient flame retardancy and smoke suppression for TPU composites

Abstract

Zeolite imidazolium frameworks (ZIFs), as a subcategory of metal-organic frameworks (MOFs), are widely used in many fields. And, the physical morphology, particle size, and shape of ZIFs have an important influence on their application performance, such as flame retardancy for polymer. Here, three types of ZIF-67 were prepared with different physical morphology structures including dodecahedron (ZIF-67), cube (ZIF-67 NC), and cross-shaped (ZIF-67 H), by adjusting the proportion between 2-methylimidazole and cobalt (Ⅱ) nitrate hexahydrate together with the suitable amount of surfactant (CTAB), and they were used to enhance the flame retardancy and smoke suppressing properties of thermoplastic polyurethane (TPU). It has been found the novel ZIF-67 H with 3D cross-shaped superior catalytic and flame-retardant capabilities than the other two during the burning process of TPU composites. Compared with those of pure TPU, the peak heat release rate (pHRR), total smoke production (TSP), and smoke factor (SF) of TPU/2wt%ZIF-67 H were decreased by 45.4%, 50.2%, and 72.9%, respectively. At the same time, both TPU/1wt%ZIF-67 H and TPU/2wt%ZIF-67 H samples showed stronger tensile properties than pure TPU at the same elongation. This work further enriches the study of multi-structured ZIFs for flame retardant TPU.