Effects of Stoichiometric Variations in L-Arginine-Cured Epoxy Resins

For the purpose of reducing environmental and health risks in the production of fibre-reinforced polymers, biomolecules are increasingly examined as alternative resources. For example, amino acids can serve as curing agents for epoxy resins. However, their particular appearance and possible reactions differ from those of conventional hardeners. To find a performance-optimised mixing ratio, it is relevant to know how deviations in the mixing ratio affect the reactions that take place and the resulting thermo-mechanical properties. Consequently, in this study, eleven mixing ratios of L-arginine-cured DGEBA without a catalyst or accelerator were investigated optically, thermo-mechanically, and via FTIR analysis. Based on the theoretical stoichiometric ratio, a wide range of good thermo-mechanical properties between stoichiometric ratios of R = 0.8 and R = 1.0 could be determined. However, this study led to an extension of a possible reaction mechanism for the curing of epoxides with amino acids, particularly L-arginine, postulating the thermo-induced deprotonation of α-NH₃⁺ groups, etherification as part of successful crosslinking, and the unfavourable reactivity of the guanidium group in the case of L-arginine, shifting the optimal to slightly sub-stoichiometric configurations.