Influence of the stoichiometric ratio on the in-situ formation of crystals and the mechanical properties of epoxy resin cured with L-tyrosine

One of the most influential ways to alter the network structure of thermosets is to change the stoichiometric ratio (R) between the functional groups of the resin and curing agent. Therefore, the influence of the stoichiometric ratio on the network structure and mechanical properties of diglycidyl ether of bisphenol A (DGEBA) cured with L-tyrosine was investigated with the goal to derive structure-property relationships. Mixtures of DGEBA and L-tyrosine were prepared using three-roll milling, with stoichiometric ratios ranging from 0.9 < R < 1.5. Upon increasing R, a greater abundance of L-tyrosine crystals was observed, serving as reinforcements within the epoxy matrix. As a result, the network structure and glass transition temperature displayed only marginal changes compared to epoxy resins cured with conventional amine curing agents. Notably, the flexural strength and strain at failure exhibited insignificant variations. Interestingly, the addition of additional L-tyrosine to the resin resulted in an augmented fracture toughness, indicating the toughening capabilities of L-tyrosine crystals. As a result, the amino acid acts not only as a curing agent but also as a toughening agent. Consequently, further incorporation of L-tyrosine into the thermoset proved advantageous, enhancing its overall mechanical performance.