Microcavity formation in engineering polymers exposed to hot water

Microcavity formation in engineering polymers exposed to boiling water has recently been noted in the technical literature. This study describes this observation and presents photomicrographs of the microcavity morphology. The microcavities are lens‐shaped cracks emitting from a nucleation site with regular concentric ridges observed at regular intervals with characteristics similar to fatigue cracking. Cyclic exposure [intervals of hot (96deg;C) followed by cold (23°C) water immersion] was found to significantly increase the microcavity formation in specific polymers. Polymers studied under these conditions included polycarbonate, polysulfone, poly(ether sulfone), and polyetherimide. Only polycarbonate and polyetherimide exhibited internal crack formation. The microcavities of polyetherimide were quite different than those of polycarbonate as the cracks were concentrated in regions of highest molded‐in stress. When polysulfone was purposely spiked with 0.1 wt % NaCl inclusions, microcavity formation was observed but at a magnitude significantly lower than that of polycarbonate (unmodified). A hypothesis is presented to explain this failure mechanism in polycarbonate. Localized regions, (nucleation sites) of higher water solubility can result in higher internal pressure and stress‐induced hydrolysis causing microcavity formation. Additional internal pressure (cyclic conditions) can result from water phase separation yielding further crack propagation. Polycarbonate exhibits a larger difference in equilibrium water solubility between 23°C and 96°C than do the other plymers studied, thus yielding a greater potential for internal pressure resulting from phase separation.