Sustainably sourced p-coumaric acid-derived bio-based epoxy resins: Functionalized thermosetting epoxy materials with excellent flame retardancy, ultra-high strength, toughness, and transparency

Commercial epoxy resins are significantly limited in their applications in structural materials, packaging materials, and other fields due to inherent drawbacks such as the non-renewability of synthetic raw materials, high brittleness, and flammability. During the modification process, challenges related to the compatibility of additives and the “see-saw” issue between mechanical performance and flame retardancy persistently arise. This study presents the first synthesis of sustainable flame-retardant epoxy resins (p-CAIE) using biomass-derived p-coumaric acid (p-CA) and isophorone diisocyanate (IPDI) as raw materials. p-CAIE is utilized as a modifier for commercial epoxy resins, resulting in thermosetting materials (p-CAIE-E51/DDM) with excellent thermal stability, superior mechanical properties, and enhanced flame retardancy. The compatibility between the components is significantly improved, and the introduction of extensive hydrogen bonding within the crosslinked network leads to remarkable enhancements in performance. Specifically, the tensile strength and elongation at break of the 15 % p-CAIE-E51/DDM cured resin system increased by 119 % and 360 %, respectively, while the impact strength improved by 151 % compared to pure E51/DDM. The limiting oxygen index (LOI) reached 29.5 %, and the UL-94 test achieved a V-0 rating. Additionally, the peak heat release rate (PHRR), total heat release (THR), total smoke production (TSP), and carbon monoxide production (COP) were reduced by 54.2 %, 55.5 %, 53.6 %, and 40.5 %, respectively, effectively suppressing thermal and smoke release. The material also exhibited high transparency within the visible light range (400 nm-760 nm). Thus, this research provides an environmentally friendly strategy for the multifunctional manufacturing of bio-based epoxy resins, offering a promising solution for simultaneously enhancing the flame retardancy, toughness, and transparency of thermosetting epoxy materials.