Development of cellular films for electret applications

Cellular polymer films are suitable for electret and piezoelectric applications. Considerable attention is currently focused on thermoplastic polymer foams such as isotactic polypropylene (i-PP). The cellular morphology leads to barrier effects effectively improving the charge storage stability. However, the application of neat cellular polypropylene electrets is limited due to their relatively low working temperature. Therefore, new research activities are mainly focused on processing of cellular high temperature polymers such as polyetherimide (PEI). A promising approach is a physical batch foaming process using carbon dioxide as a blowing agent, which allows the manufacturing of thin cellular films. The control of the cellular morphology plays an important role with respect to the charge storage behavior and is influenced by the processing parameters such as blowing agent concentration or foaming temperature. Investigations of the cellular morphologies are achieved by scanning electron microscopy (SEM). Electret properties are characterised by potential decay measurements at 90°C after corona charging to surface potentials of about 400 V. In general, such cellular films show improved charge storage behavior in comparison to the compact materials.