A flame-retardant and transparent epoxy resin based on phosphorus-containing ionic liquid and its mechanical, thermal and flame-retardant properties

This study introduced the synthesis of a novel phosphorus-containing ionic liquid flame retardant called DOPA-MZ, which was used to develop epoxy resin (EP) composites with transparency, exceptional mechanical properties, and flame retardancy. Results indicate that adding 3% DOPA-MZ to the EP composite increases the Limiting Oxygen Index (LOI) to 30.3%, achieving a UL-94 V-0 rating. Differential Scanning Calorimetry (DSC) tests showed that DOPA-MZ promotes curing reactions, significantly raising the EP composite's glass transition temperature (Tg). Additionally, compared to pure EP, EP/8% DOPA-MZ reduced total smoke production (TSP) and total heat release (THR) by 23.8% and 5.9%, respectively. Thermogravimetric analysis (TGA) and scanning electron microscopy (SEM) were employed to analyze the flame-retardant mechanisms. These analyses revealed that DOPA-MZ primarily inhibits flames in the gas phase and enhanced flame retardation in the condensed phase. Furthermore, DOPA-MZ contains tertiary amine groups, which participated in the curing reactions between epoxy resin and curing agents. This tightens the integration of DOPA-MZ molecules with the cross-linked network formed during the epoxy resin curing process, leading to a 22.0% increase in tensile strength and a 17.0% increase in bending strength in EP/3% DOPA-MZ composite materials.