Decay of the electric potential at the surface of coronacharged uniform layers of HIPS granules

The aim of the present paper is to evaluate the factors that affect the discharge of mm-sized 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.