Fire retardancy of epoxy composites: A comparative investigation on the influence of porous structure and transition metal of metal-organic framework

Metal-organic frameworks (MOFs) are crystalline porous materials constructed by metal nodes and organic linkers. A series of key features such as high surface area, catalytic performance provide a platform for preparing MOF-based fire retardants (FRs). However, understanding the role of the porous structure and catalytic metal species remains a key issue towards the fire retardant mechanism of MOFs. This work systematically studied the difference between the performance of a zirconium based MOF(UiO-66), its derived porous zirconium oxide (U-ZrO2), and commercial ZrO2 (C-ZrO2), in imparting fire retardancy, suppressing smoke and charring property towards epoxy. With the presence of 3 wt% porous U-ZrO2, EP/3U-ZrO2 sample showed better performance in suppressing heat (10 % reduction) and toxic carbon monoxide (14 % reduction) than that of EP/3C-ZrO2 due to the "tortuous path" effect and formation of a compact char. Moreover, a greater number of exposed catalytic sites on MOF compared with thermally treated metal oxide significantly reduced total smoke production (TSP) of the EP/MOF sample by 38 %. Catalytic carbonization attributed to the great number of metal sites on MOF is crucial in providing compact char residue, thereby suppressing smoke for EP. In perspective, this work opens a window for understanding the fire retardant mechanism of MOF-based FR towards polymers.