Liquid phosphorus-based bis-imidazole compounds as latent curing agents for enhancing thermal, mechanical, and flame-retardant performances of single-component epoxy resins

The escalating need for advanced, fire-resistant, single-component epoxy resin (EP) is fueled by its practicality and economic benefits, highlighting the necessity for the development of multifunctional flame-retardant latent curing agents. Herein, two liquid phosphorus-containing bis-imidazole compounds, PPDM and DPCMI, were synthesized as latent curing agents for EP, demonstrating multiple effects in improving latency, thermal stability, mechanical properties, and fire safety. EP/PPDM and EP/DPCMI showcased superior storage stability and rapid curing at moderate temperatures, with EP/PPDM standing out for its long shelf life of 7 d and being gelled within 18 min at 100 degrees C. The resulting thermosets presented increased glass transition temperatures (189.5 and 178.9 degrees C), due to the enhanced crosslinking densities. The presence of bis-imidazole groups in PPDM and DPCMI enabled EPs to form denser crosslinked networks, leading to improved mechanical strength and toughness. The limiting oxygen index (LOI) values of EP/PPDM and EP/DPCMI reached 29.5% and 29.0%, respectively. Compared to the control EP, EP/PPDM and EP/DPCMI showed 23.1% and 18.8% reductions in total heat release and 22.0% and 23.11% decreases in total smoke production. These results confirm the enhanced flame retardancy and smoke suppression of EP/PPDM and EP/DPCMI because of introducing phosphorus-containing groups. Even though the curing time was halved to 2.5 hours, EP/PPDM systems maintained satisfactory overall performances. Therefore, this work offers a scalable strategy for the fabrication of single-component EP systems combining rapid curing, satisfactory flame retardancy, and enhanced thermal stability and mechanical properties, aligning with the needs of industrial applications.