Joint Project Proposal on: Development, Characterization and Applications of Cellular Fluoropolymer Films with Ferroelectret Properties Subproject on: Development of Cellular Fluoropolymer Films
Final Report Abstract
This joint project of the Universities of Erlangen, Potsdam, Darmstadt (first two years) and Linz (the latter as a foreign partner) was concerned with the development and characterization of electrically charged fluoropolymer films with cellular structure for ferroelectret applications. So far, most of the ferroelectrets are based on polypropylene (PP). However, their thermal charge stability is not sufficient for a range of applications, as for example in the automobile interior. The higher thermal stability along with the excellent dielectric properties of fluoropolymers would fit many application requirements. As the processing of the fluoropolymer polytetrafluoroethylene (PTFE) already investigated for ferroelectret applications is very complex, thermoplastic fluoropolymers were chosen for the development of thermally stable ferroelectrets. Therefore, the focus was on fluorinated ethylene propylene copolymers (FEP). However, due to the elevated melting temperature and the tendency to release hydrofluoric acid, the processing of these materials was more difficult than for PP. Moreover, the standard method for manufacturing ferroelectrets by stretching filler containing films was not applicable for FEP. Attempts to improve this processing behaviour of FEP by using long-chain branched materials or plasticizing the polymer by adding a low molar mass component did not lead to a successful generation of cellular structures. A partially successful way found for generating cellular structures from FEP was its chemical foaming. It resulted, however, in films with very low charge stability due to a discharge effect caused by the solid residues of the blowing agent added to the polymer. According to these insights, the physical foaming of FEP was the preferred method for the further development of cellular structures. Finally, the foaming of FEP using supercritical gas was found to be a successful process for the generation of thin cellular films. A two-step process, first saturating the sample with supercritical CO2 and then triggering the foaming by heating, was found to be not satisfying as it resulted in the formation of cellular structures with very low porosities. This finding could be explained by the too fast diffusion of the absorbed CO2 out of the films. Therefore, a one-stage process was developed which prevented the loss of gas before foaming. FEP films with porosities of up to 70% and homogeneous cellular structures with cell sizes of a few to a few ten micrometers could be generated by this method. The influence of the diverse processing and material parameters was investigated to tailor the cellular structures to the requirements for ferroelectrets. As the piezoelectric activity of the foamed films was very low they were biaxially stretched to flatten and widen the cellular structure in order to get larger charge storing surfaces inside the films as well as lower elastic moduli due to the structure deformation. As a result of the biaxial stretching of the foamed films a pronounced reduction of the elastic modulus could be achieved, which would be beneficial for their piezoelectric coefficient. With the films prepared this way the expected higher thermal charge stability of the voided FEP films in comparison to PP ferroelectrets could be proven. However, mainly due to charging problems of the foamed and biaxially stretched films, which are not completely understood up to now, no piezoelectricity of values appropriate for ferroelectrets could be achieved forthe cellular FEP films generated. The processing of foamed FEP films using supercritical CO2 was successful for generating samples with rather low piezoelectric coefficients. There are two factors determining the piezoelectric activity of cellular films. One is the elastic modulus of the films, which is decisive for the force required to compress the foam and, therefore, to achieve a charge displacement. This film stiffness can be altered by changing the geometry of the cellular structure, e.g. from a round shape to a lens-like one, like it was done in this project. Another way to decrease the elastic modulus would be to enlarge the size of the bubbles as big cells can more easily be compressed. However, this aim cannot be achieved using the foaming by means of supercritical CO2 that was investigated within this project. Therefore, another process for the generation of such cellular structures will have to be developed which could be based on layer patterning for example. The other factor is the storage of charges inside the bubbles of the foam. A stronger piezoelectric effect can principally be achieved for cellular structures exhibiting higher charge densities. Due to unknown charging problems for the foamed and biaxially stretched FEP films, onty few charges were stored inside the films. Therefore, further improvements will be necessary on the charging and storing of charges of FEP films with cellular structures and the reasons for the present difficulties have to be investigated.
Publications
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Polymer ferroelectrets - Promising Materials for Applications in Mechatronics. in: 7th International Congress on Molded Interconnect Devices. 2006. Fürth
L. Zirkel, H. Münstedt, and M. Wegener
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Cellular polyethylenenaphthalate ferroelectrets: Foaming in supercritical carbon dioxide, structural and electrical preparation, and resulting piezoelectricity. Applied Physics Letters (2007), 90 (19), 192908/1-192908/3
P. Fang, M. Wegener, W. Wirges, R. Gerhard, L. Zirkel
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Influence of different process and material parameters on chemical foaming of fluorinated ethylene propylene copolymers. Polymer Engineering and Science (2007), 47 (11), 1740 - 1749
L. Zirkel and H. Münstedt
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Optimized Preparation of Elastically Soft, Highly Piezoelectric, Cellular Ferroelectrets from Nonvoided Poly(ethylene Terephthalate) Films. Advanced Functional Materials (2007), 17(2), 324 -329
W. Wirges, M. Wegener, O. Voronina, L. Zirkel, R. Gerhard-Multhaupt
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Cellular polyethylenenaphthalate films for ferroelectret applications: foaming, inflation and stretching, assessment of electromechanically relevant structural features. e-Polymers (2008), no. 043
P. Fang, W. Wirges, M. Wegener, L. Zirkel, R. Gerhard
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Foaming of thin films of fluorinated ethylene propylene coplolymers (FEP) using supercritical CO2. in: Proceedings of the Polymer Processing Society 24th Annual Meeting. 2008. Salerno (Italy)
L. Zirkel, M. Jakob, and H. Münstedt
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Physical foaming of fluorinated ethylene-propylene (FEP) copolymers in supercritical carbon dioxide: single-film fluoropolymer piezoelectrets. Applied Physics A: Materials Science & Processing (2008), 90 (4), 615-618
O. Voronina, M. Wegener, W. Wirges, R. Gerhard, L. Zirkel, H. Muenstedt
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Foaming of thin films of a fluorinated ethylene propylene copolymer using supercritical carbon dioxide. Journal of Supercritical Fluids (2009), 49 (1), 103 - 110
L. Zirkel, M. Jakob, and H. Münstedt
