The Effect of Annealing on the Structure and Electric Performance of Polypropylene Films

Biaxially oriented polypropylene (BOPP) and uniaxially oriented polypropylene (UOPP) films were annealed. The effect of annealing temperature (Ta) on dielectric strength was studied. The electric breakdown strength (Eb) of BOPP and UOPP films changes in a quite different trend with the annealing process. Eb of BOPP films decreases with the increase in Ta, whereas Eb of UOPP films increases first and then decreases with Ta. The structural changes during annealing were investigated. The crystallinity rises with Ta, while the orientation degree and Eb show a similar trend with Ta. Although the crystallinity and crystal structure can affect Eb of polypropylene films, the orientation of chain segments has a much larger correlation with Eb. Our results indicate that the deterioration of the metallized BOPP film capacitor may originate from the orientation degree decrease of chain segments after experiencing high temperature.