Durability of polymers for solar thermal applications: Crack propagation in degraded polymers

All polymeric systems have been proposed for solar domestic hot water. In these applications, the polymeric components can degrade due to UV exposure or oxidation (when exposed to chlorinated water). Of particular concern is mechanical failure of polymeric components that have degraded. Mechanical failure occurs when the degraded polymer becomes brittle and the applied stress causes microcracks in the component to grow. In this work, an experimental technique is described for evaluating crack growth in polymers as a function of the extent of degradation. Thin film polyethylene samples (0.3mm thick) are immersed in an 80°C chlorinated (8ppm) water bath for up to 35 days. The extent of degradation is nondestructively characterized by FTIR: samples exposed for over 20 days show significant oxidation/degradation. An initial crack is placed in a degraded sample and crack growth under uniaxial loading is observed through video analysis. Cracks are observed to grow through a stepped mechanism: a process zone is created at the crack tip; voids form in the process zone; as the process zone deforms the voids stretch to form fibrils; and the fibrils break, leading to crack growth. The crack growth mechanism observed in these samples closely replicates the failure mechanism that has been observed in the brittle fracture of polymer pipes.