Corona-charged polypropylene electrets analyzed by XPS

In the present study, we use X-ray photoelectron spectroscopy (XPS) analysis to clarify how different polarities of corona initiate various changes in the surfaces of polypropylene (PP) electrets subject to corona discharge. The samples were charged in three-electrode corona discharge system using positive and negative corona polarities at both −20 and 75 °C temperatures. The tests were divided into four groups. The surface potentials of the electret samples were measured using the vibrating electrode method with compensation. XPS studies were carried out by means of a VG ESCALAB Mk II electronic spectrometer using an Al Kα excitation source (hν=1486.6 eV). The spectra of C1s, O1s and N1s lines for all groups and for untreated samples were recorded and analyzed. The investigations that we carried out show that for negative-corona-charged samples, the oxygen content is approximately 2.4 times higher than that in positive-corona-charged samples. Based on the results we have obtained, we may assume that the changes in oxygen content in samples charged by different polarity coronas lead to the formation of different surface local levels. This assumption agrees well with the experimental measurement made on the electrets.