Resorcinol-Based Epoxy Resins Hardened with Limonene and Eugenol Derivatives: From the Synthesis of Renewable Diamines to the Mechanical Properties of Biobased Thermosets

Synthesis of fully eco-friendly thermosets based on renewable resorcinol diglycidyl ether (RE) hardenered with different biobased diamines having aliphatic, cyclic, or aromatic backbones were prepared. Two diamines were successfully synthesized from D-limonene and allyl-eugenol by thiol-ene addition in the presence of cysteamine hydrochloride to obtain cyclic diamine-limonene (DA-LIM) and aromatic diamine-allyl eugenol (DA-AE), respectively. New thermosets RE/HDMA (hexamethylene diamine), RE/DA-LIM, and RE/DA-AE were further obtained by an epoxy-amine curing process. Thermal stability, flammability, and mechanical properties were investigated by thermogravimetric analysis (TGA), pyrolysis combustion flow calorimeter (PCFC), and dynamic mechanical analysis (DMA), respectively. Although the a transition temperature T-alpha of the cyclic and aromatic biobased thermosets were lower than the one based on HDMA (similar to 95 degrees C instead of 110 degrees C), the flexural moduli were comparable (>2 GPa). Interestingly though, it was observed that their fracture toughness behavior (K-IC and G(IC)) is attractive, as the G(IC) of the cyclic biobased resin RE/DA-LIM (208 J/m(2)) is practically two times greater than that of RE/HMDA (90 J/m(2)). Indeed, the fully biobased thermosets containing cyclic or aromatic diamines offer an outstanding compromise between high stiffness and high fracture toughness properties. In terms of thermal stability, RE/DA-AE exhibits a char yield higher than RE/DA-LIM (21% vs 10%) which could lead to a potential material having better flame retardancy. These results suggested that the aromatic biobased epoxy/amine resin would be well suited for reinforced composite materials and could potentially be used as an alternative to current thermosets.