Synthesis of phosphorous-containing bio-based curing agent for flame retardant epoxy resin system

A phosphorous containing primary amine-based curing agent, 4-(((3-aminopropyl) imino) methyl)-2-methoxyphenyl(4-(((3-aminopropyl) imino) methyl)-2-methoxyphenyl) phenylphosphonate (VDTS2), was successfully synthesized. The chemical structure of the intermediate and the curing agent was characterized by Fourier transform infrared (FTIR) and nuclear magnetic resonance spectroscopy. An epoxy resin was then cured with the phosphorous-based curing agent used in different ratios and with a petroleum-based curing agent. The flame-retardant (FR) properties and thermal decomposition of the respective films were investigated by the limiting oxygen index (LOI), UL-94 burning test, differential scanning calorimetry, thermogravimetric analysis (TGA) and FTIR spectroscopy. Scanning electron microscopy (SEM) analysis was also conducted to study the characteristics of the char obtained. The results of the FR tests showed that the epoxy films cured with the novel amine VDTS2 were able to achieve flame retardancy with an LOI value of 35% and a UL-94 rating of V-0. The TGA analysis showed that the residue of the flame retardant epoxy systems was almost double that of the epoxy resin cured with the petroleum-based amine. The respective films’ mechanical properties were also investigated, and it was observed that the phosphorous-containing amine-based curing agent showed potential to replace the petroleum-based amine for crosslinking epoxy resins.