In situ formation of phosphorus-containing COFs on boron nitride surfaces: improvement of the fire resistance, smoke suppression and mechanical properties of epoxy resins

Abstract

Since adding flame retardants typically results in the degradation of mechanical properties, maintaining mechanical performance while improving the flame retardancy of EP remains a crucial research focus. To attain this goal, a phosphorus/nitrogen/boron-containing COF-based hybrid flame retardant (PA-COF@BN) was synthesized via a solvothermal method using a covalent organic framework (COF), boron nitride (BN), and phytic acid (PA) as precursors. With the incorporation of 4 wt% PA-COF@BN, the EP/PA-COF@BN composite exhibited significant improvements in fire safety performance: the LOI (limiting oxygen index) increased to 26.3%, the PHRR (peak heat release rate) decreased by 31.58%, the THR (total heat release) reduced by 39.41%, the SPR (smoke production rate) dropped by 67.68%, the TSP (total smoke production) declined by 62.69%, and the char residue content increased from 10.31% to 26.73%. PA-COF@BN significantly inhibited both the thermal degradation process and the smoke emission behavior of epoxy resin. Remarkably, the EP/4 wt% PA-COF@BN composite exhibited a maximum tensile strength of 78.72 MPa and elongation at break of 4.56%; therefore, after the addition of flame retardants, not only the flame retardancy, smoke suppression, and char formation catalysis of the system are enhanced, but also there is a slight improvement in its mechanical properties. This study designed a P–N–B synergistic COF-based flame retardant, offering a novel approach for developing multifunctional epoxy resins with combined flame retardancy, smoke suppression, and mechanical property retention.