Large-area approach to evaluate DC electro-thermal ageing behavior of BOPP thin films for capacitor insulation systems

A large-area electro-thermal ageing test setup was developed and utilized to age several laboratory-scale biaxially oriented polypropylene (BOPP)-hydrophobic silica nanocomposite films. The films were aged in test capacitors with self-healing metallized film electrodes, which enabled the ageing test to continue beyond the first breakdowns. Eight different films were aged for 1000 hours under 100 V/µm DC stress at 75 °C, and large-area DC breakdown measurements, dielectric spectroscopy, gel permeation chromatography (GPC), differential scanning calorimetry (DSC) and dielectric spectroscopy were used to detect localized and bulk degradation after the ageing period. The effects of antioxidant contents, different PP grades and compounder screw speed were evaluated. Material characterization indicates no bulk degradation had occurred during ageing, which was associated with moderate temperature stress and inert nitrogen atmosphere. On the other hand, low-field breakdowns (weak points) were observed in all but two of the aged materials, indicating that ageing was dominated by localized degradation which may have introduced new breakdown mechanisms. Weak points were also measured in a similarly aged commercial capacitor-grade BOPP film aged at a lower field, supporting this conclusion. The importance of long-term characterization in material development is demonstrated, and it is shown that long-term properties of the evaluated nanocomposites were at least on the same level compared to neat BOPP films.