The cellular polypropylene electret material. Electromechanical properties: Dissertation

The cellular polypropylene (PP) film is soft and elastic and it encloses multiple polymer-gas interfaces between thin non-cellular surface layers. With a high enough corona voltage it is possible to achieve a high space charge density on the internal interfaces. The cellular charged structure forms an effective electret material that has an electromechanical transducer coefficient, d33, typically around 200 pCN-1 for the 70 µm thick film. The sensor and actuator operations in thickness mode are reciprocal according to electromechanical models for simplified multiple and single layer structures. The direct d33 coefficient related to sensor operation in units of CN-1 equals the indirect d33 coefficient related to the actuator operation in units of mV-1. Dynamical measurements seem to give a lower transducer coefficient compared to quasi static measurements. Corona charging in increased electric fields by using ambient gas with higher electrical strength is a way to increase the initial value of the d33 coefficient. Introducing nitrogen gas inside the cellular structure by consecutive low and high-pressure treatments before corona charging increased the initial d33 value up to 790 pCN-1. Part of the enhanced electromechanical effect is probably related to the removal of moisture from the cellular structure during the gas exchange procedure. The d33 coefficient increases as a function of temperature due to the softening of the PP especially above the glass transition, where temperature dependence for d33 is about 2.2 pCN-1K-1. The charge stability of the film is mostly related to the typical thermal properties of PP. According to the thermally stimulated current (TSC) measurements, discharge current is already detected at 320 K. Ageing tests show a 50% decrease in the d33 coefficient when storing the cellular film at 323 K for 8 days.