Novel core–shell (dendrimer) epoxy tougheners: Processing and hot–wet performance

The effects of novel core–shell (dendrimer) (CSD) particles on the mechanical properties of epoxy are further studied with the focus on addressing associated processing challenges and hot–wet performance. As reported earlier, the particles are produced in a novel grafting reaction between vinyl-containing groups of a monomer of interest (e.g., styrene) and amine-containing polymer (e.g., polyethyleneimine, PEi) in the presence of t-butyl hydroperoxide (TBHP) initiator and with mild heating. Strong interactions between PEi and epoxy occur when the concentrated particle dispersion is mixed with the epoxy/curing agent system, causing a significant increase in the viscosity of the mixture with attendant difficulties in processing. Furthermore, the additional amine groups in the modified epoxy are expected to attract more water molecules, leading to a possible increase in water absorption and subsequent decrease in mechanical properties. Both problems are addressed with the addition of a small amount of styrene monomer to the mixture. First, the viscosity of the curing mixture is reduced dramatically, but not at the expense of subsequent mechanical properties, and SEM micrographs suggest that there are no notable changes in the particle–epoxy interactions. Secondly, while the modulus of the CSD reinforced system obtained under hot–wet conditions is reduced by approximately 20% from values obtained under dry conditions, this reduction shrinks to less than 10% with the addition of styrene monomer.