Studies on the properties of epoxy resins modified with chain-extended ureas

Two kinds of reactive toughening accelerators for epoxy resin, amine-terminated chain-extended urea (ATU) and imidazole-terminated chain-extended urea (ITU) were synthesized from polyurethane prepolymer. Compared with the unmodified system, the curing activity, dynamic mechanical behavior, impact property and fracture surface morphology of the modified systems were systematically investigated. Results show that the curing activity of modified epoxy resin E-51/dicyandiamide (dicy) systems is so greatly enhanced that the apparent activation energy of curing reaction decreases from 130.2 kJ/mol for the unmodified system to 75-85 kJ/mol for the modified systems. The curing reaction mechanism of E-51/dicy system accelerated by ITU is different from that of the system accelerated by ATU, and a little different from that of the system accelerated by imidazole. Furthermore, the impact strength of the cured systems modified with ITU are 2-3 times higher than that of the unmodified system, while the glass transition temperatures are little altered, and the fracture surfaces of all modified systems display tough fracture feature. (C) 2001 Elsevier Science Ltd. All rights reserved.