Synthesis of a novel boron-containing hyperbranched benzoxazine and its flame-retardant properties in copolymer with epoxy resin

A novel boron-containing hyperbranched benzoxazine (HB-B) was synthesized from the reaction of triethanolamine borate (BAE), bisphenol-A, and paraformaldehyde via Mannich condensation. The chemical structures of BAE and HB-B were characterized by Fourier transform infrared (FT-IR) and hydrogen nuclear magnetic resonance ( 1 H NMR). Afterward, the boron-containing hyperbranched benzoxazine/epoxy resin copolymers (E51/HB-B) were prepared. The curing behavior of -B and E51/HB-B was studied by FT-IR and differential scanning calorimeter (DSC). The thermal stability and flame retardancy properties of the cured HB-B and E51/HB-B were evaluated by thermogravimetric analyzer (TGA), limited oxygen index (LOI), UL94 rating test, and microscale combustibility calorimeter experiments (MCC). The initial thermal decomposition temperature and glass transition temperature steadily dropped with increasing HB-B content, while the char yield kept rising. The LOI value of the E51/HB-B-12 wt% was 31.8%, meeting the UL94 V-0 specification. The result of heat release capacity (HRC) decreased by 20.4%. Moreover, the mechanical properties were also improved, and the tensile strength of E51/HB-B copolymer was increased by 88.5%. Finally, the fracture morphology of E51/HB-B was characterized by SEM.