Synthesis and characterization of DOPO-derived curing agent and lignin-epoxy system

In response to the urgent need for sustainable alternatives to petroleum-based resources, particularly in the production of traditional thermosetting epoxy resins, this study introduces a pioneering approach using alkali lignin as a renewable substitute for phenol and glyoxal for formaldehyde. By synthesizing a lignin-based epoxy resin and combining it with a DOPO-diamine compound (DDC), we have developed a novel bio-based epoxy resin that not only aligns with eco-friendly principles but also exhibits intrinsic flame retardancy. The DDC-cured lignin-based epoxy resin (DDC-LER) stands out for its exceptional thermal stability, with a high char yield of 38.78 % in nitrogen at 800 °C, outperforming the DDM-cured counterparts. The glass transition temperature (Tg) of DDC-LER, as measured by DMA, is elevated to 145.2 °C, surpassing the 112.3 °C of DDM-LER, a result corroborated by DSC assessments. The flame retardancy of the DDC-cured materials is further highlighted by their high Limiting Oxygen Index values of 37.5 % and 35.3 %, earning them UL-94  V -0 rating. The DDC-LER system also demonstrates a significant reduction in heat release rate (HRR) and total smoke production (TSP), showcasing its superior safety profile. Through a comprehensive analysis of both condensed and gas phases, the underlying flame retardancy mechanism of DDC-LER is elucidated. In conclusion, the bio-based epoxy materials synthesized in this study represent a significant step towards environmentally responsible materials science, offering a compelling combination of properties that suggest a wide range of potential applications across various sectors.