Characterisation and optimisation of the d ₃₃ coefficient of cellular PP films

Abstract

Piezoelectric polymer cellular films with silver electrodes are made from a PP film activated by corona discharge. This cellular polymer is characterized by its high piezoelectric d33 coefficients in comparison with piezoelectric ceramics and other polymers like PVDF. One of the advantages is their malleability and shape ability in different kinds of forms. Our work presents the optimization of the activation process and the results of films characterization. All the samples are made from available PP film modified by a thermal biaxial stretching. d33 constant was measured by a quasi-static method and laser interferometry giving similar results. Transducer coefficients of about 700 pC/N were obtained. An approximation of the surface charge density in the polymer voids was obtained with a mathematical model. Thermal stability was checked by thermally stimulated depolarization currents (TSDC) showing a relationship between TSDC and d33 constant. Time stability was determined with more than one year old samples. Finally, the resonance frequency was obtained from interferometry measurement.

Relationship between surface potential and d 33 constant in cellular piezoelectric polymers

“…The polymer film was thermally modified through biaxial stretching in two steps producing a 90% decreased in its thickness (see figure 1). This biaxial stretching increases the piezoelectric properties of the material compared to non-stretched samples: the d 33 constant of non-stretched samples was about 10pC/N compared to 600pC/N for stretched samples [9]. Another effect of biaxial stretching was a reduction of porosity down to 60%, and in increase of density up to 400kg/m³.…”

“…The grid voltage was limited to 3kV due to the surface potential probe limit. Of course, higher d 33 constants could be obtained with higher grid voltages [9] but then it was not possible for our equipment to measure the surface potential. The d 33 piezoelectric constant was measured with an own design setup (figure 3) made of a force actuator, a force sensor, a power controller system that converts the signal from signal generator to the force actuator and the equipment to register the data.…”

“…If this is true, there has to be a relationship with the d 33 constant values. The initial sample of 61x61mm was divided into 9 square smaller samples numbered from (1) to (9). d 33 constants were measured for each new sample and then the values were overlapped with surface potential curves.…”

“…Where ε r is the relative permittivity of PP (a value of 2,2 was measured by dielectric spectroscopy [9]) and Y the Young's modulus of the material (the experimental average value is 0,25MPa [9]). With the previous values, one obtains d 33 = 55,3 pC/N which is in reasonable agreement with the measured value.…”

Charging of Piezoelectric Cellular Polypropylene Film by Means of a Series Dielectric Layer

“…The foamed polypropylene film used in this work is produced by the company Sekisui Alveo B.V. (Roermond, the Netherlands). The film was thermally modified by biaxial stretching, reducing its thickness by 90% from 1 mm to 100 µm [21]. This biaxial stretching increases the piezoelectric properties of the material compared to the non-stretched samples.…”