Breaking the trade-off between mechanical properties and fire safety of epoxy resins based on phosphaphenanthrene derivatives by covalent crosslinking

The high brittleness and flammability issues of epoxy resins (EPs) seriously limit their high-performance applications in industries. However, most of the current research has been failing to simultaneously enhance both toughness and flame retardancy of EP by a facile method. In this work, three phosphaphenanthrene derivatives (DOPO, ODOPM and DOPA) were applied to prepare intrinsic flame-retardant epoxy resins by using diphenylmethane-4,4′-diisocyanate (MDI) as a bridge. The as-prepared EP/MDI/DOPO, EP/MDI/ODOPM, and EP/MDI/DOPA exhibited obviously enhanced flame retardancy and mechanical performance relative to EP. As the P content reached 1 wt%, the limiting oxygen indexes (LOIs) and vertical combustion (UL-94) levels of EP/MDI/DOPO, EP/MDI/ODOPM and EP/MDI/DOPA reached 28.9 %, 31.2 %, and 33.4 %, and V-1, V-0, and V-0. The impact strengths of EP/MDI/ODOPM and EP/MDI/DOPA containing 0.75 wt % of phosphorus were higher than that of EP by 209.3 % and 212.4 %, respectively, due to the covalent linkage between the phosphaphenanthrene-derived groups, MDI, and polymer network. Thereby, a simple method is revealed in this work to improve both fire retardance and toughness of EP, which facilitates its high-performance development.