Toughening of a dicyandiamide-cured epoxy resin: Influence of cure conditions on different rubber modifications

Cure kinetics of modified epoxy resins cured with dicyandiamide are studied. The influence of different heating rates in the curing process, such as curing behavior, morphology, and thermo-mechanical properties, is studied. Additionally, three different post-cure cycles at 180 degrees C are employed. Two butadiene-based toughening agents are used, a carboxyl-terminated polybutadiene-co-acrylonitril (CTBN) prepolymer and a functionalized block copolymer of polytetrahydrofuran and hydroxyl-terminated polybutadiene. The amphiphilic block copolymer enables investigations with a bimodal particle size morphology. All results are contrasted with those of the neat resin and butadiene-free block polymer. Faster curing processes result in smaller average particle sizes and better fracture toughness of the modified epoxy resins. Further improvements are achieved with additional post-cure cycles at 180 degrees C. An increased interfacial adhesion between the particles and the epoxy matrix is considered to be the main mechanism. Optimized lengths of the post-cure process can be determined with the butadiene-based toughening agents indicating a competing thermal degradation. Longer post-cures than 40 min lead to lower fracture toughness in the butadiene-based modified materials. In general, similar influences of the curing and post-curing process on the bimodal and unimodal distributed system can be observed differing in more intense dependencies of the bimodal system.