Efficient synthesis of phosphorus containing melamine salt for enhancing flame retardancy of poly(lactic acid)/polyamide 11 blends

A tetrakis (melamine)-1-hydroxyethylidene-1,1-diphosphonic acid salt (EA-MEL) was synthesized by a straightforward acid–base neutralization reaction between 1-hydroxyethylidene-1,1-diphosphonic acid (EA) and melamine (MEL). This compound, which contains phosphorus and nitrogen elements, exhibits a 40% carbon residue at a temperature of 800°C. When combined with diethyl aluminium hypophosphite (OP1230) as a flame-retardant composite in polylactic acid (PLA)/polyamide 11 blends through melt blending, a synergistic effect between the two flame retardants is observed. Consequently, the limiting oxygen index value significantly increases to 34.3, showcasing a remarkable 64.6% improvement. Additionally, the material achieves a V-0 rating according to UL-94 standards, and the peak heat release rate drops to 269 kW∙m⁻² with the addition of just 5 wt% EA–MEL. These results demonstrate that EA-MEL and OP1230 contribute to the generation of phosphoric acid derivatives and the formation of a crosslinked ( P N )_n or ( P O N )n network. This network enables the formation of a complete, compact, and expanded char residue, effectively isolating heat and oxygen.