Fracture behavior of degraded polyethylene thin films for solar thermal applications

Thin polymer films and tubes are under consideration as materials for use in roof top mounted solar thermal collectors. When polymer components are exposed to chlorinated water or UV light, degradation can lead to diminished mechanical performance and premature failure from rupture. In this paper, fracture toughness and crack growth of degraded polyethylene (MDPE) are investigated. Thin film PE samples (0.3mm thick) are degraded through exposure to 80°C chlorinated water (8 ppm chlorine) for up to 45 days. The extent of degradation is quantified by carbonyl index, a nondestructive measure of the chemical byproduct of degradation, such that a high carbonyl index indicates significant degradation. Single edge notched specimens are prepared from the degraded samples and mode I fracture tests are performed. In one set of tests, samples are loaded at a constant rate and the fracture toughness of the film samples is evaluated by the essential work method. In another set of tests, samples are loaded under a constant load and the crack growth rate is measured. The data can be used to inform design of polymeric components for solar applications. For designs that use thin films, the fracture toughness can be used to obtain a conservative estimate of the critical stress σ_c. Hence, an appropriate film thickness can be selected to ensure that stresses do not exceed the design stress value during the lifetime of the component. For designs that use tubes, the rate of crack growth can be used to estimate the time to rupture. Tube thickness can be selected to extend the lifetime.