Polyester ferroelectret

The optimized PETP and PENP ferroelectrets exhibit high piezoeleetrie J33 coefficient of 500 and 140 pC/N, respectively. The piezoeleetrie sensitivity of polyester ferroelectrets is stable at least up to 80 °C, slightly higher than that of cellular PP ferroelectrets. In particular, PENP ferroelectrets are still piezoelectrically active even after storage at 100 °C for 5 days. 

Unlike the homopolymer PTFE, FEP is a meltprocessable thermoplastic copolymer. The above described technique of foaming with SCCO2 was also adopted to make cellular FEP foams (Veronina et al. 2008). The procedures for foaming of the material are similar to the case of polyester polymers. Usually, only one big void across the film thickness is generated in the FEP foams. After charging and electrode evaporation, the cellular FEP ferroelectrets show 33 coefficients up to 50 pC/N. 

As was also discussed in this chapter, very significant progress has been made in the development of ferroelectrets. A large number of polymer electrets, including polyolefins (PP and PE), polyesters (PETP and PENP), COC, PC, and fiuoropolymers, were adopted to prepare ferroelectrets using a variety of techniques. By now, ferroelectrets with excellent electromechanical properties and with well-controlled cavity structures are becoming available. 

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