Charge retention in biaxially-oriented polypropylene films containing various additives

Isotactic Polypropylene (i-PP) films containing additives such as the commercial alpha-nucleation agent NA11 and the anorganic filler particles CaCO3 and Al2O3 were biaxially stretched. As a result, the films assume a cellular morphology with oblong cavities extending in the direction of the film elongation. In the present study, stretched films of 50 pm thickness with additive concentrations of 0.05 to 10 mass percent were charged with a corona method to potentials of 400 V or 500 V. The stability of the charges was tested isothermally at temperatures of 90 degrees C and 120 degrees C and by means of thermally stimulated discharge (TSD) experiments. The isothermal measurements show, for the above additives with concentrations higher than about 0.3 %, a reduction of the charge decay with increasing additive concentrations. Compared to reference films of pure PP, the potential decay of the films containing additive concentrations of 10 % is significantly reduced. Correspondingly, the TSD measurements indicate a shift of the main discharge peak to higher temperatures up to the melting temperature. Generally, the voiding and thus the stability also increases with the stretching ratio. These improvements of the charge stability are attributed to the barrier effect of the cavities. The results are of interest with respect to the various applications of PP electrets, such as ferroelectret devices and air filters.