A novel organophosphinic acid salt for the facile synthesis and simultaneous imparting of flame retardancy, strengthening, and toughening to epoxy resins

Achieving flame retardancy in epoxy resin (EP) while preserving its essential properties, including strength, toughness, and thermal stability, offers considerable challenges in the application of flame retardant EP modifications. In this study, we were successful in synthesizing a novel ionic-based flame retardant, organophosphinic acid salt (DPM), using a simple ionic reaction. DPM possesses a unique structure consisting of rigid groups and flexible chain segments. Fourier transform infrared (FTIR) and nuclear magnetic resonance (NMR) techniques were used to characterise the structure of DPM. Incorporating DPM into the EP yielded remarkable improvements in mechanical properties, reduced fire hazard, enhanced processability, a lower curing temperature, and good thermal stability. The EP/DPM-7.5 composites demonstrated excellent flame retardancy by effortlessly achieving a V-0 rating in vertical combustion (UL-94) testing, while an impressive limited oxygen index (LOI) value of 36.3% was attained, a substantial 42.4% enhancement compared to the EP. Moreover, cone calorimeter (CC) tests revealed that EP/DPM-7.5 exhibited a 41.7% decrease in peak heat release rate (PHRR), a decreased fire growth rate index (FIGRA), and an increased fire performance index (FPI). Additionally, the tensile strength and flexural strength of EP/DPM-7.5 improved by 26.0% and 23.7%, respectively. Comparatively, EP/DPM-5 maintained the glass transition temperature (Tg) and thermal stability of the EP while significantly enhancing the impact strength by 85.4%. The newly developed flame retardant, DPM, exhibited excellent comprehensive performance and fire safety characteristics, thereby demonstrating the significant potential for practical applications in flame retardant EP systems.