Multifunctional phosphorus-containing imidazoliums endowing one-component epoxy resins with superior thermal latency, heat resistance, mechanical properties, and fire safety

Epoxy resin (EP) possesses a high flammability risk, but the flame retardants applied often reduce other properties. For the sake of large-scale industrial manufacturing, EP and curing agent are increasingly being premixed as one-component system rather than blended immediately prior to application. However, current designs fail to break the trade-off between flame retardancy and mechanical/thermal/optical properties of one-component EPs. Herein, we developed a series of phosphorus-containing imidazolium compounds, named BADM-X (X = 1, 2, 3, and 4), which served as multifunctional latent hardeners for one-component EPs. BADM-1, BADM-2, BADM-3, and BADM-4 were synthesized in ethanol through the neutralization between bisphenol A diphosphate (BAD) and 2-ethyl-4-methylimidazole (EMI) at molar ratios of 1:1, 1:2, 1:3, and 1:4, respectively, which were easy to industrialize. Varying the imidazole content in BADM-X allowed for adjusting the curing reactivity and thermal latency of EP/BADM-X, making it adaptable for various applications. EP/BADM-2, EP/BADM-3 and EP/BADM-4 not only retained excellent optical properties, but also demonstrated increased glass transition temperatures (Tgs) and mechanical performances. The Tg of EP/BADM-2 reached 162.7 ℃, and its tensile and impact strengths increased by 34.8 % and 61.4 % relative to EP/EMI. EP/BADM-X exhibited enhanced fire safety, and the limiting oxygen index (LOI) and vertical burning (UL-94) classification of EP/BADM-2 reached 28.2 % and V-0, respectively. Besides, EP/BADM-2 demonstrated enduring flame-retardant and mechanical performances, thus showing great application potential. Overall, this study introduces a new class of multifunctional latent curing agents for the fabrication of durable fire-safe, high-performance, one-component EPs.