Novel heat durable electromechanical film processing: preparations for electromechanical and electret applications

New heat durable electromechanical films from cyclo-olefin polymers have been developed. Electromechanical film can act as a sensor and as an actuator. As a sensor dynamic force on the surface of the film generates electric charge and contrary as an actuator voltage is applied to the film and the electric field drives the film in an air gap producing acoustic sound (or vibration). Electromechanical operation and electret operation are possible for both application types. Electromechanical operation is based on the changes of the thickness of the film and electret operation is based on the movement of the film between the air gap. The application properties strongly depend on the preparations made to the film. The film preparations consist of stretching (void formation), gas diffusion expansion, electrostatic charging, fabrication of metal electrodes, making electrical connections and tensioning on a frame in electret operation. Cast film needs to be stretched to form voided structure to the film. Processing window for the stretching of the heat durable electromechanical films is very narrow compared to the PP-based films. Optimal voids in the film microstructure are flat and lens like. Gas diffusion expansion increases the film softness and thus the electromechanical sensitivity by further opening the cellular microstructure. Maximum film thickness is about 100 /spl mu/m prior to the electrostatic charging. Charges applied to the film trap into the plastic-gas interfaces and form permanent space charges in the film. Partial discharges are being controlled by a constant current charging method. Metal electrodes are applied to the film and further connected by leads to operating systems and amplifiers. The sensitivity of the electromechanically operating sensors and actuators can be increased by stacking multiple layers of the film together. Applications based on a flat loudspeaker need to be tuned by tensioning the film on a support frame. Correct tensioning is essential in reducing distortion levels. In general lower tensioning leads to higher sound pressure levels due to the larger film movement but on the other hand higher tensioning reduces the distortion levels.