Hyperbranched Phosphorus-Containing Benzoxazine for Epoxy Modification: Flame Retardant and Toughening Agent

To overcome the high flammability and brittleness of epoxy resin (EP) without sacrificing its glass transition temperature (Tg) and mechanical properties, a series of hyperbranched benzoxazines (HB1.0–HB2.5) were synthesized using 6-(bis(4-hydroxyphenyl)methyl)dibenzo[1,2]oxaphosphinine 6-oxide (DOPO-2OH) as the “hard” segment and Jeffamine T403 as the “soft” segment. The property investigation revealed that HB2.0 with the optimized ratio of “hard/soft” segments was the appropriate additive for commercial EP. Furthermore, the comprehensive performance of the EP/HB2.0 system was significantly enhanced compared to the pristine EP, demonstrating a 115% increase in impact strength and 5.7 °C in Tg. Impressively, when the loading of HB2.0 was only 10 phr (phosphorus content is 0.34 wt %), the modified EP passed the UL-94 V-0 rating and achieved a limited oxygen index (LOI) value of 30.8%, mainly resulting from the flame inhibition effect of the gaseous phase. This research provided an appealing technique for simultaneously improving the toughness and flame retardancy of EP.