Thermal aging of biaxially oriented polypropylene films part II morphology and dielectric properties

Despite many studies carried out on polypropylene (PP) resins, few data concerning the biaxially-oriented (BO) PP film have been reported (1,2). From a practical point of view, it is important to clarify the fundamental properties of such BO-PP films, in view of their increasing use in high voltage apparatus. The authors reported at this conference results concerning the general aspects of the thermal aging of BO-PP films in various atmospheres (air, vacuum, N2, SF6) (2). Thermally aged BO-PP films exhibit an increase of the crystallinity, a new melting peak on DSC fusion curves, and crystal phase reorganization; furthermore, their surfaces are characterized by the appearance of a droplet-like structure in the early stage of thermal aging. Similar trends were observed with films aged in vacuum, N2 and SF6 with, however, a stronger effect of SF6 on the dimensional changes and crystallinity increase of the material. In this report morphological variations and dielectric behavior of BO-PP films aged in SF6 are discussed.