The effect of urea bond on structure and properties of toughened epoxy resins.

In this article, modified polyoxypropylene diamines were synthesized and used as a new flexible curing agent for epoxy resins. The purpose of modification is to introduce urea group into epoxy resins. The reaction rate, mechanical properties, glass transition temperature Tg, and fracture surface morphology of these toughened epoxy resins were investigated. Because of urea groups, the reactivity between polyoxypropylene diamines and epoxy resins was significantly enhanced. At the same time, the urea groups resulted in strong intersegmental hydrogen bonding between modified polyoxypropylene chain, which reduced the compatibility of polyoxypropylene with epoxy resins and resulted in higher Tgof toughened epoxy. The modified sample had tensile strength of 15.8 MPa and ultimate elongation of 118 at room temperature, whereas the unmodified sample only had 6.2 MPa and 70. The scanning electron microscope analysis showed that the modified system displayed tough fracture feature, whereas the unmodified system showed typical brittle fracture. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010 [ABSTRACT FROM AUTHOR]