A core-shell-structured APP@COFs hybrid for enhanced flame retardancy and mechanical property of epoxy resin (EP)

Epoxy resin (EP) is a commercially important resin with many important industrial applications but is impeded by its inherent flammability. Ammonium polyphosphate (APP) represents an eco-friendly and effective fire retardant for EP, but its moisture sensitivity and poor interfacial compatibility with EP often give rise to unsatisfactory fire retardance and adverse impacts on mechanical properties of EP. To address these issues, we herein report a core-shell-structured modified APP, APP@COFs, using Schiff base covalent organic frameworks (COFs) as a surface modifier. The results show that the addition of 2 parts per hundreds of resins (phr) APP@COFs effectively enhances the flame retardancy of EP, leading to a self-extinguishing capability and a limiting oxygen index of 27.1%. Compared with virgin EP, the peak heat release rate is decreased by 54.7% due to the modes of action of APP@COFs in both gas and condensed phases. Additionally, because of improved interfacial compatibility, the resulting EP/APP@COFs composites show improved mechanical properties, e.g., a 37% increase in the impact toughness of EP/2 phr APP@COFs relative to that of EP. This work provides an effective method for modifying APP and the development of advanced fire-retardant EP materials for practical applications in industries.