Improvement of piezoelectric coefficient of cellular polypropylene films by repeated expansions

Pressure expansion can be used to improve the piezoelectric d(33)-coefficients of customary cellular polypropylene (PP) films. In the present paper, the experimental procedure for a double expansion process is described and experimental results of d(33)-coefficients that can be achieved by choosing expansion pressures from 0.2 to 2 MPa and expansion temperatures from 20 to 120 degrees C for the two processes are discussed. For example, expansion pressures of 2 MPa, a temperature of 90 to 100 degrees C for the first expansion and 60 degrees C for the second expansion, and exposure times of the order of 1 h result in quasistatic d(33)-coefficients of more than 1000 pC/N which are relatively stable at room temperature. (c) 2006 Elsevier B.V. All rights reserved.