Study of electrets stored at pressures lower than atmospheric

20 mu m thick polypropylene electrets obtained in a corona discharge were studied. Before charging, the samples were cleaned in an ultrasonic bath with alcohol for 4 min, rinsed with distilled water and dried under room conditions. After charging, the electret surface potential was measured by the vibrating electrode method with compensation. The measurement system was located in a vacuum chamber, with a gauge to measure the pressure. The surface potential vs storage time relationships for electrets stored at various pressure (20, 50, 100, 150, 200 and 300 Torr) were measured. The dependence of the electret surface potential on the time of storage at room conditions, after the samples had been taken out of the vacuum chamber, and the second evacuated effect on the electret surface potential were also studied. A desorption from the electrets is suggested to explain the results obtained. The dependence of the surface potential on time is satisfactorily described by an equation that is analogous to the equation of desorption. The results also show that if the electrets are initially held at a pressure much lower than 200 Torr for a time, their subsequent time stability under room conditions increases. (C) 2005 Elsevier B.V. All rights reserved.