Economical and environment-friendly synthesis of a novel hyperbranched poly(aminomethylphosphine oxide-amine) as co-curing agent for simultaneous improvement of fire safety, glass transition temperature and toughness of epoxy resins

Epoxy resins inherently suffer flammability and brittleness. Various approaches have been used to improve their fire safety or toughness, however, glass transition temperature (T_g) is always deteriorated concomitantly. Herein, a novel hyperbranched poly(aminomethylphosphine oxide-amine) (HPAPOA) was synthesized from α-aminomethylation reaction of trihydroxymethylphosphine (THP) with piperazine before oxidation. THP was prepared from tetrakis(hydroxymethyl)phosphonium sulfate and barium hydroxide to avoid complex purification in the synthesized process. When HPAPOA was utilized as co-curing agent for epoxy resins, T_g and impact strength of epoxy thermosets are improved due to the increased cross-linking density and the multiple toughening mechanisms. Simultaneously, fire safety include flame retardancy, smoke suppression and toxicity reduction is raised (except 1.0 wt% added). The condensed phase flame retardant mechanism is verified as increased char yield and protective and barrier effect of the formed char layer and the gas phase for release of phosphorus-containing species of HPAPOA. With as low as 3.0 wt% HPAPOA incorporated, epoxy thermoset achieved vertical burning V-0 rating with a limited oxygen index of 30.7%. The blowing-out effect for upgradation of vertical burning rating is elucidated. Moreover, the curing reactivity of epoxy systems is slightly changed and the reaction activation energy is elevated. The thermal decomposition temperature of epoxy thermoset diminishes. With 2.0 wt% HPAPOA added, tensile and flexural properties are improved and those with other contents are slightly depressed. The practical HPAPOA modified epoxy thermosets with obvious disadvantage conquered and higher T_g can be used in industries for high-performance applications.