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

Abstract

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/μm to 125 V/μm, in a climatic chamber heated up to 80°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 ∼108°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.