Résumé: mmsized High-Impact Polystyrene (HIPS) granules forming uniform layers at the surface of grounded plate electrodes. The samples were charged using a triode-type electrode arrangement, consisting in: (i) a metallic plate connected to the ground; (ii) a wire-type corona electrode parallel to the grounded plate and energized from an adjustable dc power supply; (iii) a metallic grid of well-defined potential. The decay of the potential at the surface of the samples was measured with the capacitive probe of an electrostatic voltmeter (TREK, model 370). The experimental design methodology was employed for quantifying the effects of three variables that influence the process of potential decay at the surface of a corona-charged layer of insulating granules: (i) the initial potential, imposed by that of the grid electrode (range ±1000 to ±2800 V); (ii) the granule size (range 0.3 to 1.5 mm); (iii) the number of compact granular layers (1 to 3 layers). It was found that the surface potential decay is accelerated by the grid potential, but is slower for coarser, multi-layered granular samples. These results are useful for the design of the electrostatic separation processes involving this class of insulating materials.

Résumé: The initial potential at the surface of the sample, as well as the temperature and the relative humidity of the ambient air are known to influence the surface-potential decay characteristics of corona-charged thin insulating films. The aim of the present work is to demonstrate the effectiveness of the Experimental Design methodology for evaluating the effects of these factors. Thus, a full factorial experimental design was carried out on a thin film of polyethylene terephthalate (thickness: 0.5 mm; surface: 50 mm  50 mm). A negative corona discharge produced in a needle–grid–plate electrode system was employed to charge the surface of the film samples. The variation domains for the three factors were respectively: 1000 V to 1800 V; 25 to 55 C; 50% to 80%. The surface-potential decay process was characterized by two output variables: the time needed for the potential to reduce to respectively 50% and 10% of the initial value. It was found that the former is more affected by the temperature, while the latter is more sensitive to the variation of the relative humidity.