Synthesis of Metal Complexes with DOPO-Based Moiety: Effects on Flame Retardancy, Thermal Stability and Mechanical Properties of Epoxy Resin
Epoxy (EP) composites holding DOPO-based flame retardant (DD) and its metal complexes (Cu–DD and Zn–DD) were synthesized to improve flame retardancy. The flammability performance was measured using UL-94 and limiting oxygen index (LOI) tests. Pure EP exhibited poor flame resistance with a LOI of 18.2% and no UL-94 rating, whereas EP/Cu–DD and EP/Zn–DD attained LOI values of 29.0 and 29.4%, respectively, and V-0 ratings in UL-94 tests. SEM–EDX analysis confirmed homogeneous distribution of phosphorus, copper, and zinc within the epoxy matrix. Thermogravimetric analysis revealed that the addition of DD and its metal complexes increased char yield significantly from 4.2% in pure EP to 34.9% in modified composites. The enhanced char formation contributed to reduced heat and oxygen diffusion during combustion. SEM images of chars formed at 400°C displayed dense, honeycomb-like structures in metal–DD composites, indicating improved barrier effects. EDX mapping further validated lower C/O ratios, suggesting limited oxidation. The glass transition temperature from 213°C for pure EP decreased to 160–176°C for modified systems attributed to increased chain mobility. However, the storage modulus improved by up to 42.7% demonstrating enhanced crosslinking. Overall, the synergistic effect of phosphorus and metal complexes imparted flame retardancy, thermal stability, and mechanical strength to epoxy composites.