All-In-One 2D Intumescent Flame Retardants [Fe(OH)₂]⁺ Anchored Melamine Trimetaphosphate (Fe@MAP) Enable Enhanced Flame Retardancy and Smoke Suppression of Epoxy Resins

To investigate the influence of metal ions on the application performance of melamine trimetaphosphate (MAP) composites, a series of [Fe(OH)₂]⁺ anchored melamine trimetaphosphates were synthesized. Their EP composites were prepared and studied. The Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA) were used to investigate the chemical composition, morphology, structure, and thermal stability, respectively. The filler dispersion, flame retardancy, and mechanical properties of EP/Fe@MAP composites were analyzed using scanning electron microscopy, limiting oxygen Index (LOI) test, vertical burning test (UL-94), cone calorimeter test (CONE), stress–strain test, Raman spectroscopy test, coke analysis, and pyrolysis gas analysis, respectively. The Fe@MAP was composed of MAP molecules and [Fe(OH)₂]⁺ particles. The [Fe(OH)₂]⁺ was formed by Fe³⁺ in the reaction process and anchored on the 2D sheets of MAP through the ionic interaction between [Fe(OH)₂]⁺ and trimetaphosphate. It could increase the viscosity of the melt and assist in the carbonization. However, if the content was too high, it would weaken the expansibility of the carbon layer. Compared with EP/MAP, EP/1Fe@MAP and EP/2Fe@MAP exhibited enhanced flame retardancy due to the catalytic carbonization reaction of Fe³⁺, demonstrating a significant synergistic flame-retardant effect between Fe³⁺ and MAP in EP. Introducing Fe³⁺ enhanced the carbonization effect, leading to improved flame retardancy and smoke suppression performance. In this experiment, the sample for optimal flame retardancy and mechanical properties was determined to be 1Fe@MAP. The addition of only 4% resulted in good flame retardancy and mechanical properties for EP/1Fe@MAP, surpassing the EP/MAP composite without Fe³⁺ regulation. EP/1Fe@MAP performed the best in the cone calorimeter test and performed no less than the optimal group in LOI and UL-94 tests. The calculation results of XPS indicated that introducing only about 3.7‰ Fe³⁺ into the EP/MAP system could significantly enhance the flame retardancy of composites in multiple dimensions such as PHRR, PSPR, PCOP, LOI, UL-94, and char residue. At the same time, 1Fe@MAP had the least mechanical damage to composites. The most prominent advantage of 1Fe@MAP was its ability to form a higher-quality carbon layer with increased graphitization. Fe³⁺ as an additional acid source participated in the catalytic carbonization reaction, showcasing a clear synergistic flame retardant effect with MAP.