Foamed and solid polymer electrets with high piezoelectric constants

Piezoelectric properties of polymer electrets consisting of cellular polypropylene (PP) or of porous polytetrafluoroethylene (PTFE) with thicknesses of 50 to 100 μm are studied. In addition, bilayer or multilayer structures composed of one of these polymers in solid or foamed form plus an additional air layer are also investigated. In particular, the quasistatic and the dynamic piezoelectric d₃₃-constant are determined. Typical constants are of the order of 100 to 350 pC/N for the cellular PP and much lower for the porous PTFE. The bilayer or multilayer structures show considerably higher constants which reach up to 20,000 pC/N for certain combinations. The effect is linear as function of load pressure at relatively low pressures, ranging up to 100 Pa for some air-gap systems and up to several kPa for the cellular PP. A frequency dependence of the d₃₃-constant is generally not observed between 0 and 1000 Hz; however, some multilayer systems do show a frequency dependence which is probably due to resistive air flow within the structures.