Solidifying process and flame retardancy of epoxy resin cured with boron-containing phenolic resin

Highlights: • Boron-containing phenolic resin as a flame retardant curing agent applied to the epoxy resin curing system. • Boron-containing phenolic resin showed much higher curing reactivity with epoxy group compared with conventional phenolic resin. • Compared with the phenolic resin cured epoxy resin, the curing system of boron-containing phenolic resin has higher glass transition temperature and lower heat release. - Abstract: For the sake of improving the charring performance and flame retardancy of epoxy resin (EP), boron-containing phenolic resin (BPR) instead of a conventional curing agent, linear phenolic resin (LPR) was employed to cure EP. Of several possible chemical structures for BPR, the existence of benzyl hydroxy groups in BPR chains has been confirmed using ¹H nuclear magnetic resonance spectroscopy. The resonance of these groups may reasonably explain the higher curing reactivity of BPR-cured EP than that of LPR-cured EP. Thermogravimetric analysis, observation of the morphologies of the char residues and X-ray photoelectron spectroscopic were performed to characterize the charring process. Due to the presence of B₂O₃ produced on the char surface from decomposition of phenyl borates and the facile high self-crosslinking reaction of BPR, a more continuous and stronger char barrier was formed for BPR-cured EP compared to that for the LPR-cured EP system. Therefore the former exhibited much better flame retardancy. In addition, BPR-cured EP also displayed better dynamic mechanical properties, than those observed for LPR-cured EP. It is not subject to the significant lowering the glass transition temperature of the polymer which accompanies curing with LPR. This suggests that BPR cured resin may meet the requirement for utilization at high temperature.