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

摘要

Space charges inside the polypropylene film of a high voltagecapacitor of the film-foil type changes the homogeneous electricalfield. For determining the maximum electrical field stress it isnecessary to know the distribution of the space charges depending on thedepth of the film. A reason for the build-up of space charges is theexistence of oxidative damages by OH-groups to the polypropylene chainswhere dipole moments can arise and be established. The OH-groups can bedetected by Fourier Transform Infrared Spectroscopy (FTIR). A specialkind of directing the infrared beam by the method of Attenuated TotalReflection (ATR) allows by changing the angle of incidence a differentpenetration depth of the beam. In this way an integral value ofdifferent film depths can be measured and a distribution of OH-groupscan be determined. Measurements are realized on 17,8 μm thinpolypropylene films used for industrial application in HV film foilpower capacitors. For investigations the film is treated by molecularoxygen at increased temperature for some weeks. This treatmentaccelerates the oxidation which happens under normal conditions onmanufacturing, storing and handling polypropylene films. The results ofthe measurements with FTIR ATR show that the intensity of oxidation(OH-groups) is the highest at the surface layers of the film but it alsoexists inside of the polypropylene film. So it can be assumed thatcharges are also established in the volume of thin polypropylene filmsand not only on their surface. For dimensioning capacitor dielectric itis important to take care of the space charge distribution underneaththe surface of polypropylene films