Optimization of piezoelectric properties of cellular polypropylene films by repeated expansion

The piezoelectric d/sub 33/-coefficients of customary cellular polypropylene (PP) films can be significantly improved by a double-expansion process. In the present paper, the experimental procedure is described and experimental results of d/sub 33/-coefficients that can be achieved by choosing expansion pressures of 2 MPa and a range of expansion temperatures up to 120/spl deg/C for the two expansion processes are discussed. For example, a temperature of 90 to 100/spl deg/C for the first expansion and 60/spl deg/C for the second expansion, and exposure times of the order of 1 h result in stable quasistatic d/sub 33/-coefficients of more than 1000 pC/N.