Phase separation influence on the performance of CTBN-toughened epoxy adhesives

Epoxy adhesive systems were formulated and tested for their bonding behavio r. Fumed silica and elastomer type and concentration were varied for the pu rpose of investigating their influences on intermediate and final adhesive performance properties. Of special interest was analyzing the role that fle xibilized elastomer, that which does not phase separate during cure, played in these properties. Adhesive systems were tested with a commercial prepre g system using both bonded and cocured geometries. Dynamic mechanical analy sis and optical microscopy showed the phase separation of the rubber was si gnificantly influenced by the silica addition. Elastomer-rich dispersed pha se growth rates were depressed resulting in a lower percentage of toughenin g domains. Mode I and II fracture energies were a strong function of disper sed phase particle size, flexibilized elastomer type/content, and residual stress considerations at the resin-resin interface. There were distinct dif ferences between bonded and cocured fracture results with bonded samples ge nerally outperforming cocured specimens due to changes in the surface rough ness, resin flow properties, and interfacial stress build-up. Collectively, this work illustrates the importance of phase separation behavior on the f inal part morphology and performance for Liquid elastomer toughened epoxy a dhesives.