A series of multifunctional castor oil-based epoxy resins, named methoxy castor oil polyglycidyl ether (MOCOGE), allyloxy castor oil polyglycidyl ether (AOCOGE), and castor oil-based non-glycidyl ether (CONGE), was designed through a ring-opening etherification reaction to broaden the application of E−51, a common type of bisphenol A epoxy resin. Research on its curing properties showed that adding castor-oil-based epoxy resins greatly enhanced the flexibility, toughness, and mechanical strength of E−51. In particular, when the blending amount was 10%, the tensile strength increased by 31.4%, 29.1%, and 34.4% compared to that of pure E−51, reaching 84.13, 82.66, and 86.07 MPa, respectively. The elongation at break increased by 29.2%, 29.8%, and 21.9%, respectively, and the impact strength increased by 59.5%, 39.5%, and 53.1%, respectively. Our results indicate that these castor oil-based epoxy resins improved E−51 properties, expanding the application of castor oil-based epoxy resins.
