Metal-phosphorus-imidazole synergistic complexes for enhancing latency and fire safety in single-component epoxy resins

Abstract

In response to the increasing need for fire-resistant single-component epoxy resin (EP) systems, this research introduces five metal-based phosphorus/imidazole-containing complexes (M-DA) as latent curing agents. These complexes, synthesized by coordinating Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺, or Zn²⁺ into a phosphorus-containing imidazole derivative (DA), were designed to improve flame retardancy, smoke suppression, and latency of single-component EPs. EP/M-DA mixtures exhibited prolonged shelf life and rapid gel times at moderate temperatures, with latency improvements following the trend: Cu²⁺ > Ni²⁺ > Co²⁺ > Zn²⁺ > Fe³⁺. Notably, EP/Cu-DA achieved a storage life of 43 days. In addition, it demonstrated improved thermal stability as well as superior mechanical strength and toughness. All EP/M-DA thermosets achieved a UL-94 V-0 rating and high limiting oxygen index (LOI) values exceeding 29.0 %, with EP/Cu-DA showing the highest LOI of 37.5 %. EP/Cu-DA also achieved significant reductions (46.5 % and 21.1 %) in peak heat release rate and total smoke production, highlighting its superior flame retardancy and smoke suppression. These improvements were attributed to synergistic effects between transition metal ions and phosphorus, which promote condensed-phase carbonization and gaseous-phase combustion inhibition. Among the single-component EPs, EP/Cu-DA exhibited the best combination of latency, mechanical strength, fire safety, and smoke suppression, providing a promising strategy for developing high-performance single-component EPs.