The importance of carbon-fiber reinforced epoxy composites is increasing with the current efforts to increase efficiency through weigh-reduction (lightweighting). The brittle nature of the epoxy material that is often used for the matrix is an issue that needs to be addressed. Many different approaches of toughening the epoxy polymer matrix have been established, however these are usually associated with a corresponding decreases in other mechanical properties. This paper discusses an approach to increasing the toughness of carbon-fiber reinforced aromatic epoxy composites by looking at two areas of the composite: first, the fiber/matrix interface; second, the polymer matrix itself. By using a di-functional aliphatic epoxy polymer in these two area, a toughening of the composite should be possible without a reduction in other mechanical properties. The preparation of the fiber surface via a 90 second ozone treatment under UV light has been shown to increase the surface concentration of oxygen, leading to better fiber/matrix adhesion. A 60% increase in interfacial shear strength (IFSS) was seen for the UVO-treated over the as-received fiber. An additional increase of 7% in IFSS was measured when an aromatic epoxy sizing was applied to the UVO treated fiber. By contrast the application of an aliphatic epoxy sizing yielded a substantial increase in IFSS of 32% over the UVO-treated fiber. To address the toughness of the matrix, the application of low concentrations (1wt%) of aliphatic copolymers has been shown to substantially increase (∼60%) the impact toughness of the epoxy matrix material, without reducing other mechanical properties. In cases where the IFSS was at low, the aliphatically toughened matrix increased the IFSS by about 10%. By contrast at high IFSS values a 10% reduction of IFSS was seen. In between, the aliphatically toughened matrix showed similar IFSS to the neat matrix.
