EPOXY NETWORKS BASED ON DICYANDIAMIDE - EFFECT OF THE CURE CYCLE ON VISCOELASTIC AND MECHANICAL-PROPERTIES

Benzyldimethylamine-catalysed epoxy networks based on dicyandiamide hardener were prepared using various cure schedules. It was shown using 13C nuclear magnetic resonance and Fourier-transform infra-red spectroscopies that the chemical reactions are temperature-dependent. At low cure temperature the formation of ether linkages was favoured, thus increasing the crosslink density. Consequently, the glass transition temperature of the networks increases with decrease of the cure temperature. The viscoelastic spectra exhibited both α and β relaxations. The amplitude of the α peak increased with increasing cure temperature. Also the rubbery modulus allowed us to calculate M ̄c. The Young's moduli in the glassy state did not depend on the cure schedule. The ability to deform plastically determined both via K parameter and yield stress increases on decreasing the crosslink density, as shown with the high cure temperature. Linear elastic fracture mechanics allowed us to determine KIc and GIc. Both of the fracture parameters increased with decreasing crosslink density. The improvement in the retardation of crack propagation is probably due to a crack blunting mechanism associated with the ability of the material to deform plastically. © 1990.