Cellular polypropylene : optimizing structure, charging and energy harvesting conditions

Abstract

Many studies have been conducted in the last decades on Cellular Polypropylene (Cell-PP) films, to be used as a lightweight cheap alternative to expensive conventional piezoelectric materials. There is a limited number of studies considering the non-linear dependence of the piezoelectric properties on the mechanical load applied. This work investigates the influence of morphological and charging parameters on the electromechanical coupling coefficient k233 within the non-linear region, with the aim to increase it. Void morphological parameters were extracted out of four types of films, (Treofan GmbH), treated under Gas Diffusion Expansion (GDE). The effect of these parameters to the stiffness were analyzed. Samples were charged with a corona triode and the direct/inverse piezoelectric effect as well as the film ageing were investigated. The mechanical and piezoelectric responses were obtained simultaneously under compressive tests and the electromechanical coupling coefficient k233 was derived. The optimum harvesting conditions were also investigated. A 64 runs design of experiments was built and statistical analysis was done on the responses. Finite Element Analysis (FEA) was done on 2D cross-sections of 3D modelled films for comparison of the mechanical response with the real films. A universal method of defining the morphological distributions of Cell-PP films is presented and a high correlation is revealed between the void morphology and the mechanical response. Stiffer materials revealed higher k233 while further increment is achieved when harvesting within the optimum strain region. With optimum charging, an increment in thermal stability and charge density was achieved, while ageing was reduced and obtained charges were increased. As indicated by this study, Cell-PP is a high charge/density material able to compete with conventional materials for Energy Harvesting (EH), when charged and used under optimal conditions. Furthermore, EH within the optimum region is a good solution for applications where only small displacements are available.