Optimization of piezoelectric properties of cellular polypropylene films by repeated expansion

The piezoelectric d₃₃-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 d₃₃-coefficients that can be achieved by choosing expansion pressures of 2 MPa and a range of expansion temperatures up to 120°C for the two expansion processes are discussed. For example, a temperature of 90 to 100°C for the first expansion and 60°C for the second expansion, and exposure times of the order of 1 h result in stable quasistatic d₃₃-coefficients of more than 1000 pC/N.