A review of dendritic hyperbranched polymer as modifiers in epoxy composites

Dendritic hyperbranched polymers have been shown to be able to double the i nterlaminar fracture resistance of epoxy-based composites and to reduce the internal stress level by as much as 80% with only 10 phr of modifier. Thes e property improvements were obtained without affecting the viscosity, and thus the processability, nor the glass transition temperature of the epoxy resin. In the investigation, both fully soluble and phase-separating epoxy- functionalised hyperbranched polymers were used, the latter showing more ef ficient toughening properties. In these blend formulations, however, a clos e control of the phase separation mechanism was required, in order to avoid filtering effects before or during fibre impregnation. In composite plaque s, the phase separation was investigated as a function of fibre surface tre atment. In a few cases, a heterogeneous nucleation of modifier particles oc curred at the fibre surface as a consequence of favoured fibre/particle int eractions. This reduced the fibre/matrix bonding strength and led to adhesi ve failures at the fibre/matrix interface. In using dendritic hyperbranched polymer modifiers, maximum toughness enhancement and internal stress reduc tion, were thus obtained when the modifier nucleated within the matrix phas e and adhesive failure at the fibre/matrix interface was avoided by selecti ng suitable fibre surface treatments. (C) 2001 Elsevier Science Ltd. All ri ghts reserved.