Model for the inflation process of cellular space-charge electrets

Ferroelectrets are closed-cell polymer foams that behave like ferroelectric materials during and after internally charging the voids of the cellular structure. In order to optimize the electromechanical properties of ferroelectrets, the voids within the cellular polymer are expanded in a controlled manner in a high pressure gas chamber. Thereby, the gas pressure is first increased, subsequently released and finally the expanded structure is stabilized by a heat setting treatment at elevated temperatures. Dielectric dilatometry provides an easy means for the in-situ monitoring of the multi-step foaming process, and therefore allows for the control of the degree of foaming. The mechanical stabilization of the expanded cellular polymer structure is explained in terms of the enhanced crystallinity as a result of the heat setting process. A simple model for the gas exchange process between the gas in the voids and the gas in the high pressure gas chamber is developed. For the model a simplification of the cellular structure is used, where an air-gap is separated from the electrodes by two polymer layers. The gas transport through the polymer is modelled by Fick´s law with permeabilities taken from the solution diffusion gas transport model. Good agreement between model calculations and experiment is obtained for the diffusion expansion of cellular polypropylene with CO₂ and He as blowing agents.