Electric strength of μm-polymer-films exemplified by micromilled PMMA

The electric strength of mum-polymer-films has been analyzed for thicknesses s = 5 ... 85 mum by different authors. The maximum of the electric strength is found in the range of 10 ... 20 mum by all authors. In the experiments presented in this paper the thickness range is expanded down to 1 mum and partial discharges before breakdown are avoided. This is achieved with special test items, produced with diamond micro milling, a precision high voltage source and a voltage metering with very high accuracy. The experimental results in this paper verify the former works. The values of the electric strength though, are found to be significantly higher, which may have been caused by the missing of partial discharges. For film thicknesses below the range that has been analyzed, the electric strength does not decrease towards zero, as expected. At approximately s = 4 mum a local minimum of the electric strength occurs. For decreasing film thickness the electric strength increases again. This anomaly can possibly be understood as the loss of the macroscopic properties of the polymer.