Joint Project Proposal on: Development, Characterization and Applications of Cellular Fluoropolymer Films with Ferroelectret Properties Subproject on: Development of Cellular Fluoropolymer Films

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This joint project of the Universities of Erlangen, Potsdam, Darmstadt, and Linz (the latter as a foreign partner) is concerned with the development of a new group of fluoropolymer films with cellular structure and improved thermal and temporal charge stability for electromechanical transducer applications. So far, most investigations on cellular polymer film electrets have been performed using PP. However, the thermal and temporal electret stability of PP electrets are not sufficient for most applications. Higher stabilities were found for PTFE and amorphous fluoropolymer films, but the manufacturing of these films is technically difficult and not suitable for large-scale production. In this investigation it will be attempted to adapt the proven processing of cellular polypropylene films to fluoropolymers so that the optimised void structure known from polypropylene can be combined with the superior stability of the fluoropolymers. For that aim thermoplastic fluoropolymers based an fluorinated ethylene propylene copolymers (FEP) will be used. These fluoropolymers can more easily be manufactured than PTFE, as they show a thermoplastic behaviour. The development and optimisation of suitable processes for the preparation of thin cellular films from fluoropolymers will be performed according to two routes. The one is the extrusion of films filled with inorganic fillers followed by biaxial stretching for the generation of cells preferably occurring at the particles. The other route is the preparation of foamed sheets using physical blowing agents and an in-line stretching of the foamed sheets by a Rheotens. The produced films will be analysed with respect to their cell structure. In cooperation with our project partners in Potsdam, Linz and Darmstadt their cellular structure will be optimised for different applications as electrets. Besides providing the material base for new fluoropolymer electrets this project will yield widespread results an the development of cellular fluoropolymers in general which could have an impact an other applications, too.