Environmental stress cracking of polycarbonate in mixed environments

This paper examines the environmental stress cracking (ESC) behaviour of Lexan polycarbonate in a range of mixed environments. The main aim of the work was to establish whether thermodynamic correlations, which predict ESC to some extent with single environments, hold when dealing with mixed environments. The binary solvent mixtures were produced from combinations of water, ethanol, benzyl alcohol, trimethylhexanol, and ethylhexanol. Samples of polycarbonate were subject to three point bend tests while immersed in environments of various composition. From these, the critical strain (ξcrit) required to produce crazing was determined in each case. The use of the Hildebrand solubility parameter (δ) to predict ESC behaviour was examined. The law of mixtures suggests that given a mixture of two environments with δ1 and δ2 either side of δ for the polymer, there should exist a composition at a which the solubility parameter of the polymer and mixture would be equal and solvation occur. Plots of ξcrit versus δ show that this is not necessarily the case. Finally an enthalpic parameter, based on the Flory Huggins theory and utilising partial solubility parameters is applied and shown to give a reasonable correlation with the critical strains obtained.