Increase of Piezoelectric Constant and Thermal Durability of Polypropylene Electret by Introducing SiO2 and Kaolin Filler and Creating a Cellular Structure

This article presents a method for preparing and testing the piezoelectric properties and stability of cellular electret based on polypropylene (PP). Introducing 5% mineral filler as a mixture of crystalline silica, colloidal silica, and kaolin to isotactic polypropylene followed by a film stretching process resulted in the formation of a composite cellular structure. To manufacture electrets, the films were polarized at a constant electric field in the range from 100 V/mu m to 125 V/mu m, in a climatic chamber heated up to 80A degrees C. The durability of the electrets was determined using thermostimulated discharge currents and approximate calculations of depolarization process activation energy. For electrets made of cellular films, the depolarization temperature T (m) at which the density of the discharge current assumes the highest value was similar to 108A degrees C and the activation energy was 6.25 eV. The response of the polarized composite film to mechanical stress expressed as the piezoelectric constant d (33) was about 3 times higher than for a-PP film of the prevailing atactic phase and poly(vinylidene fluoride) film without a cellular structure. In the range of stress of 1 kPa to 120 kPa it was 135 pC/N for lower stresses and 60 pC/N for higher stresses.