| An investigation is conducted to study and model the aging of polypropylene, a widely used polymer dielectric in energy storage and transport devices, under the sequential application of thermal and DC electrical stresses. Continuous thermal aging in air was carried out at temperatures of 90;Testing of a sufficiently large number of randomly chosen specimens allows for the analysis of all effects of the stress factors independently and interactively. Statistical analysis of the data included nonparametric and parametric assessment of adequate life distributions and life-stress models (linear and nonlinear) utilizing Least Squares and Maximum Likelihood regression methods. Several diagnostic tests, including Electron Scattering for Chemical Analysis (ESCA), Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM), Fourier Transformed Infrared (FTIR) and Nuclear Magnetic Resonance spectroscopy, Wide Angle X-ray Diffraction (WAXS), plus other mechanical and electrical characterizations including DC breakdown voltage, dielectric constant, dissipation factor, tensile stress and strain and Young's Modulus, for the control and aged samples were carried out. Results were then compared to detect any correlating changes and interpreted in the light of available aging theories.;The obtained results revealed the possible contribution of thermal aging to morphological and mechanical variations in the film. Changes of surface properties of the thermally aged film samples (vs. Control) were more pronounced than bulk variations, which consequently led to altered interfacial (electrode/insulator) characteristics. This, combined with the inherent stochastic spatial and temporal inhomogeneities of the film samples and possible space charge effects under the constant DC field, resulted in the observation of localized interfacial breakdown (possibly electronic) for the thermally aged film samples. On the other hand, a seemingly bulk limited breakdown of a mechanical nature was deduced for the control film samples. Statistically, thermal aging had a significant effect on the DC lifetimes that contributed a multiplicative effect interaction term (between aging time and temperature) to the life-stress model. |
