Tuning the properties of bio-based epoxy resins by varying the structural unit rigidity in oligomers and curing procedures

Bio-based epoxy resins, which will replace petroleum-based ones, are gaining attention. In this work, three magnolol-based epoxy oligomers with varied structural unit rigidities (biphenyl, monophenyl, and aliphatic chains) were synthesized. Both the processability and the reactive activation of curing systems improved as structure unit rigidity decreased. “First-time curing resins” had only an epoxy-amine curing network, while “second-time curing resins” had dual curing networks. The structural unit rigidity regulated the thermal properties of first-time curing resins, which decreased with decreasing rigidity. However, P-MHQ-EP-n/DDM-1 had better mechanical properties than DGEBA/DDM, e.g., its impact strength (6.2 kJ/m 2 vs. 4.5 kJ/m 2 ) increased by 37.8%, indicating that moderate structural unit rigidity allows for good deformation and absorbs impact energy due to chain mobility. The second-time curing resins with increased crosslinking sites had better heat resistance (with a T g increase of over 71.6%), thermal stability, and char residue in comparison to the first-time curing resins. More impressively, P-MBP-EP-n/DDM-2 and P-MHQ-EP-n/DDM-2 obtain a LOI values greater than 39.0% and UL 94V-0 ratings because they have high-rigidity structural units and high crosslinking densities. In conclusion, well-balanced biobased epoxy resins could be made by regulating the structure-unit rigidity of epoxy oligomers and curing procedures.