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

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