Degradation and characterization methods for polyethylene gas pipes after natural and accelerated aging

How to solve the divergence between natural and accelerated aging behavior is the core for the lifetime prediction of polymeric materials, so special attention should be paid to the degradation mechanism and fitness of engineering-based characterization methods of materials in various conditions. In this paper, the degradation of polyethylene (PE) pipes was evaluated by accelerated tests with exposure time up to 10,000 h. Meanwhile, the aging behavior was also evaluated for PE pipes in operation after a long-term service. To characterize the effect of thermo-oxidative aging on PE pipes, commonly used methods were performed such as tensile testing, scanning electron microscopy, differential scanning calorimetry, infrared spectroscopy, as well as a new method- nanoindentation. The results reveal that the degradation of polyethylene PE pipes under oxidative conditions possesses time-dependent properties and spatially heterogeneous oxidation profiles. Due to diffusion-limited oxidation, PE pipes shows more distinct heterogeneous oxidation process in high temperature than that in service condition. In addition, indentation depth is a useful parameter to characterize the long-term aging in samples, especial the heterogeneity. Moreover, the result also elucidates that thermal oxidation stability of PE100 high density polyethylene (HDPE) gas pipes is greater than that of PE80 medium density polyethylene (MDPE) gas pipes. Furthermore, the current engineering-based prediction of PE pipe failure or lifetime lacks consideration of diffusion-limited oxidation and in-depth discussion of degradation mechanism, which indicates it is too early to estimate the lifetime or the remaining lifetime of the PE pipes in operation.