Effect of gas expansion on charging behavior of piezoelectric cellular PP

Cellular polymers showing interesting quasi-piezoelectric properties, upon charging under electrical field, are being developed for a few years. They are manufactured in a way to generate a high density of internal cavities, which play a major role in the charging process. Therefore, investigation of charging mechanisms is fundamental to reach optimization of electret and quasi-piezoelectric properties. This paper investigates efficiency of contact charging of cellular polypropylene (PP) as a function of a manufacturing treatment, i.e. gas expansion, that increases the mean size of internal cavities. Testing techniques are based on partial discharge and space charge measurements. A model for the charging process is presented, with the purpose of providing explanations to the different behavior of cellular and cellular expanded PP.