Hyperbranched flame retardant to simultaneously improve the fire-safety, toughness and glass transition temperature of epoxy resin

It is highly desired yet very challenging to overcome the flammability and brittleness of epoxy resin without compromising its glass transition temperature (T_g). Herein, an epoxy-terminated hyperbranched flame retardant (EHBFR) was designed and synthesized from 9, 10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO), and renewable protocatechualdehyde as well as guaiacol. When the synthesized EHBFR was taken to modify diglycidyl ether of bisphenol A (DGEBA), it is found that the incorporation of EHBFR remarkably improved its fire safety, endowing it with a UL-94 V-0 rating and a limiting oxygen index (LOI) of 33.0 vol%. Meanwhile, the notched impact strength of modified DGEBA at 25 and −196 °C (quenched by liquid nitrogen) exhibited significant increment by 135% (from 3.39 to 7.97 KJ/m²) and 114% (from 2.34 to 5.01 KJ/m²), respectively. Moreover, the T_g of modified epoxy resin did not show any decrease, and even increased from 172 to 194 °C when the content of EHBFR was 10 wt%, due to the synergistic effect of rigid DOPO-containing groups and high crosslink density. This work offers an efficient strategy for constructing high-performance epoxy thermosets with excellent flame retardancy, superior toughness and strength, as well as elevated T_g.