Measuring the distribution of oxidative damages (OH-groups) by the method of Fourier transform infrared spectroscopy (FTIR) attenuated total reflection (ATR) in thin polypropylene films

Space charges inside the polypropylene film of a high voltage capacitor of the film-foil type changes the homogeneous electrical field. For determining the maximum electrical field stress it is necessary to know the distribution of the space charges depending on the depth of the film. A reason for the build-up of space charges is the existence of oxidative damages by OH-groups to the polypropylene chains where dipole moments can arise and be established. The OH-groups can be detected by Fourier Transform Infrared Spectroscopy (FTIR). A special kind of directing the infrared beam by the method of Attenuated Total Reflection (ATR) allows by changing the angle of incidence a different penetration depth of the beam. In this way an integral value of different film depths can be measured and a distribution of OH-groups can be determined. Measurements are realized on 17,8 μm thin polypropylene films used for industrial application in HV film foil power capacitors. For investigations the film is treated by molecular oxygen at increased temperature for some weeks. This treatment accelerates the oxidation which happens under normal conditions on manufacturing, storing and handling polypropylene films. The results of the measurements with FTIR ATR show that the intensity of oxidation (OH-groups) is the highest at the surface layers of the film but it also exists inside of the polypropylene film. So it can be assumed that charges are also established in the volume of thin polypropylene films and not only on their surface. For dimensioning capacitor dielectric it is important to take care of the space charge distribution underneath the surface of polypropylene films