Synthesis of low phosphorus flame retardant containing benzimidazole and hydroxyl and its application in reducing combustion smoke for epoxy resin

Abstact In this work, an economic and environmentally friendly phosphorus‐nitrogen‐based flame‐retardant containing benzimidazole structure and hydroxyl group (DOPO‐BHM) was successfully synthesized by a simple two‐step procedure. The as‐prepared DOPO‐BHM was characterized by 1 H NMR, 31 P NMR, and FTIR, and incorporated into epoxy matrix as co‐curing agent to remarkably improve the flame retardancy of epoxy resin (EP). When the amount of DOPO‐BHM in EP was 6% and the phosphorus content was as low as 0.408%, the EP/DOPO‐BHM composites achieved a high limiting oxygen index (LOI) of 32.7% and V‐0 vertical firing grade. Furthermore, the cone calorimeter (CC) test results showed that the values of total heat release (THR), heat release rate (HRR), total smoke production (TSP), and smoke production rate (SPR) of EP/6% DOPO‐BHM decreased by 12.9%, 28.5%, 30.0%, and 47.8% relative to EP, respectively, indicating that the modified EP had good performance of reducing heat release and inhibiting smoke release. In addition, FTIR, Raman spectroscopy, X‐ray photoelectron spectroscopy (XPS), and pyrolysis‐gas chromatography/mass spectrometry (Py‐GC/MS) were carried on to understand the flame retardancy mechanism. The characterizations indicated that the flame retardancy effect of DOPO‐BHM was in both condensed and gaseous phase, and gaseous phase played a dominant role. In addition, the mechanical test results showed that the tensile strength, bending strength, and non‐notched impact strength of EP/6% DOPO‐BHM were improved to a certain extent. In summary, DOPO‐BHM could significantly improve the flame retardant effect of EP without loss of mechanical properties and reduce emissions of phosphorus and smoke.