Osmotic cracking nucleation in hydrothermal-aged polyester matrix

In wet environments, unsaturated polyester based composites perish generally by blistering/cracking due to an osmotic process, characterised by an induction time rind This article focuses on the mechanism of crack initiation in order to understand the relationships between the network structure and its stability expressed by the t(ind) value. The proposed mechanism is based on the demixing of the polyester/small organic molecule system. By simulating the presence of glycols in the network, the dependence of crack induction time on glycol fraction is effectively demonstrated. Water then enters the micropockets induced by the phase separation and crack propagation results from the increase of osmotic pressure in the micropocket. A simple kinetic model for small organic molecule formation is proposed. This allows to explain the difference in behaviour between the two networks under investigation and some previously reported effects of catalyst residues. (C) 1999 Elsevier Science Ltd. All rights reserved.