Low-Viscosity Eutectic Hardeners for More Sustainable Cross-Linking Conditions

Preparation of very-low-viscosity eutectic mixtures was proposed for incorporating high-melting-temperature citric acid (CA) as a reactive cross-linker for curing of thermoset resins. By preparing the 1:1 molar ratio of CA with either diethyl malonate (DEM), diethyl succinate (DES), diethyl glutarate (DEG), diethyl oxalate (DEO), or triethyl citrate (TEC), the citric acid melting temperature (153 °C) decreased under −50 °C, which is more than 200 °C lower than the pure CA melting temperature. Furthermore, a significant depression in the mixture viscosity to few pascals (1.5 to 5.3 Pa•s) was observed for CADEG, CDAES, CADEM, and CADEO leading to optimal mixing with the resin. The eutectic mixtures were then used to cure the epoxidized linseed oil (ELO) in two steps. The highest glass transition of 40 ± 1 °C was obtained for CADEM-ELO, followed by a significantly higher Young's modulus of 101 ± 7 MPa, a tensile strength of 45 ± 3.7, and a stress at break of 10 ± 0.7 compared to other thermosets using EHs. Considering the thermal and mechanical properties, the superior performance of CADEM-ELO over other systems was confirmed by its lowest swelling ratio (78 ± 10) and highest gel fraction (93 ± 2).