Influence of the Stoichiometric Ratio on the Curing Kinetics and Mechanical Properties of Epoxy Resin Cured with a Rosin-Based Anhydride

Bio-based alternatives for petroleum-based epoxy resin curing agents, such as maleopimaric acid (MPA), are indispensable for sustainable fiber reinforced polymer composites with thermosetting matrices. However, previous investigations disregarded the importance of choosing the right stoichiometric ratio R between the anhydride groups in the rosin-based curing agent and the epoxy groups in the resin. Therefore, the influence of R on the curing kinetics and mechanical properties of an epoxy resin cured with a rosin-based anhydride is studied. Here, Fourier-transform infrared spectroscopy (FT–IR) indicates that for R ? 0.9 unreacted anhydride groups are present in the thermoset. Consequently, the network density decreases and the glass transition temperature T_g drops by about 40 °C. On the other hand, the steric hindrance of unreacted functional groups for R ? 0.9, increases the flexural modulus and the reduced network density improves fracture toughness. The results indicate that the best R for overall high mechanical performance and good processability is preferably low (R ? 0.7). Here, a low R results in a high T_g and good processability due to a low viscosity. However, the latency of the mixtures is low and therefore, the mixtures are not fit for processing via prepreg technology.