Toughening of vinyl ester systems

Vinyl esters (VE) represent a major class of resin systems for composite applications where low-cost processing methods such as Vacuum Assisted Resin Transfer Molding (VARTM) are preferred. The reason for this is that vinyl ester resins possess low viscosity at ambient temperature necessary for facile preform impregnation and also possess adequate material properties. However, one of the major deficiencies of VE resins is low fracture toughness when compared to prepreg epoxy resin systems. Toughening vinyl esters is complicated by the need to maintain low viscosity. In this paper, we present results of our efforts to toughen vinyl ester resins while maintaining adequate processing viscosity and minimal reduction in Tg and modulus. Several techniques will be discussed including (a) modification of network structure via manipulation of vinyl ester molecular weight and styrene content, (b) use of reactive and non-reactive rubber modifiers designed to phase separate upon cure, and (c) use of nanostructured thermoplastic fiber mats for interlaminar toughening.