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°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°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.
Novel amphiphilic block copolymer modifiers based on chain‐extended polyester for improved toughness of epoxy resins were synthesized by attaching polyols having different structures and compatibilities with a dianhydride chain‐extender. Used polyols in this research were polytetrahydrofuran as miscible and hydroxyl‐terminated polybutadiene as immiscible segment. Generated carboxyl groups will be prepolymerized with an excess of epoxy resin to exclusively form epoxy groups between the polyol spacer. Resulting morphologies in the prepared diglycidyl ether of bisphenol‐A‐based epoxy thermosets cured with dicyandiamide and urea were strongly dependent on the initial parameters of the block copolymerization process. Changing the dianhydride concentration as well as relation between miscible and immiscible polyol spacer showed various particle sizes including unimodal and bimodal distributions. The best mechanical performance in terms of fracture toughness (K1C) could be achieved with bimodal particle size distributions indicating synergistic effects between the different particle sizes in the range of 0.1–7 μm. POLYM. ENG. SCI., 59:E216–E223, 2019. © 2018 Society of Plastics Engineers
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