Thami Zeghloul

Laboratory bench for the characterization of triboelectric properties of polymers

The use of polymers as materials for sliding machine components is due to their low cost, ease of manufacturing, as well as appropriate mechanical and thermal properties. The aim of this paper is to present the experimental bench designed for the study of the triboelectric charge generated in sliding conformal contacts between flat polymer materials. The experiments were performed with 4-mm-thick samples of polystyrene and 5-mm-thick samples of poly-vinyl-chloride.The normal contact force can be adjusted using an appropriate control system and measured by a force sensor (± 50 N). The translational back-and-forth motion of the samples is produced by a crank-shaft system that generates a sinusoidal translational speed profile, with amplitudes between 12 and 50 mm/s, for strokes of 36 to 60 mm. The distribution of charge at the surface of the samples is measured by the capacitive probe of an electrostatic voltmeter (± 10 kV). The experiments pointed out that this bench enables the evaluation of the non-uniformity of the electric charge accumulated on the sliding bodies and the study of the correlations that might exists between this charge and the external forces applied to the contact.

Sliding Conformal Contact Tribocharging of Polystyrene and Polyvinyl Chloride

The electric double layer is known to affect thin-film lubrication. Other researchers have shown that tribocharging affects the breaking torque in a metal shaft-oil-lip seal system and that an external electric field may be employed to control this phenomenon. One main issue in all these studies is the characterization of the charging state of the bodies in contact. This is quite a difficult challenge when the contact occurs between two insulating materials. The ultimate aim of this paper is to validate an experimental procedure appropriate for ensuring the reproducibility of the tribocharging conditions in sliding contacts between polymeric materials. The experiments were performed with samples of polystyrene and polyvinyl chloride. Surface potential measurements were done to evaluate the tribocharging effects. The experiments pointed out that the amount of charge accumulated on the sliding bodies depends on the external force applied to the contact and the number of tribocharging cycles (i.e., back-and-forth relative motions). However, this charge is nonuniformly distributed on the surface of the samples. This means that it will be difficult to use the tribocharging effect for controlling the charging state and, hence, the lubrication of the contact between two insulating bodies.

Sorting of finely-grinded granular mixtures using a belt-type corona-electrostatic separator

The commercial roll-type corona-electrostatic separators, which are currently employed for the recovery of metals and plastics from mm-size granular mixtures, are inappropriate for the processing of finely-grinded wastes. The aim of the present work is to demonstrate that a belt-type corona-electrostatic separator could be an appropriate solution for the selective sorting of conductive and non-conductive products contained in micronized wastes. The experiments are carried out on a laboratory-scale multi-functional electrostatic separator designed by the authors. The corona discharge is generated between a wire-type dual electrode and the surface of the metal belt conveyor. The distance between the wire and the belt and the applied voltage are adjusted to values that permit particles charging without having an electric wind that puts them into motion on the surface of the belt. The separation is performed in the electric field generated between a high-voltage roll-type electrode (diameter 30 mm) and the grounded belt electrode. The study is conducted according to experimental design methodology, to enable the evaluation of the effects of the various factors that affect the efficiency of the separation: position of the roll-type electrode and applied high-voltage. The conclusions of this study will serve at the optimum design of an industrial belt-type corona-electrostatic separator for the recycling of metals and plastics from waste electric and electronic equipment.

Influence of surface roughness on the tribo-electric process for a sliding contact between polymeric plate materials

Polymers are used in many industrial applications due to their good mechanical and thermal properties. Studies have shown that electrical insulators, such as the polymers, are generating electrostatic charge by frictional contact. This tribo-charging effect influences de sliding conditions, and the level of charge depends of several factors. The aim of this paper is to validate two paradigms: (I) the electrostatic charge increases with contact pressure; (II) the level of charge is influenced by the number of contact points on the surface. The tribocharging experiments were carried out with samples cut from two polymers: Acrylonitrile-Butadiene-Styrene, 5 mm x 15mm x100 mm, roughness: Ra = 0.4 µm; and Polyvinyl Chloride, 5 mm x 50 mm x 180 mm, with various roughness values: Ra = 3-12 µm and RSm = 150-350 µm. The modification of asperity density was accompanied by changes in the electric potential measured at the surface of the PVC samples. Thus, its average value V increased from -0.33 to -0.85 kV, and its maximum value Vmax, from -1.56 to -4.83 kV.

Factors That Influence the Efficiency of a Propeller-Type Tribocharging Device for Granular Plastics

This work is focused on the experimental study of a new propeller-type tribocharging device specifically designed to operate in conjunction with an electrostatic separator for mixtures of granular insulating materials. The device consists of two coaxial aluminum propellers rotating in a polyvinyl chloride (PVC) pipe; it has two control variables: the material feed-rate and the rotation speed of the propellers. Virtual instrumentation using LABVIEW software is used for measuring the mass and the electric charge of the tribocharged samples of four types of polymers: polypropylene (PP); high impact polystyrene (HIPS); high-density polyethylene (HDPE); and PVC. In accordance with the sign and the magnitude of the charge/mass ratio, the four polymers are arranged in a triboelectric series, starting from the negative polarity: PVC—HDPE—PP—HIPS. The effectiveness of the tribocharging process is validated by using a free fall electrostatic separator. The best results (purities and recoveries higher than 90%) are obtained in the case of PVC/HIPS granular mixtures.

Surface-Electric-Potential Characteristics of Tribo- and Corona-Charged Polymers: A Comparative Study

The ability of polymers to acquire and retain electrostatic charges is valued in designing new equipment like electrostatic separators, electret filters, and powder coating devices. Several recent studies have pointed out the beneficial effect of electric charges on the lubricated contacts. The aim of this paper is to compare two physical mechanisms of electric charging of polymers: 1) triboelectric effect; and 2) corona discharge. The experiments are performed with 5-mm-thick samples of Acrylonitrile-butadiene-styrene, poly-vinyl-chloride, and polypropylene. The cartography of the electric potential is made using the induction probe of an electrostatic voltmeter. The tribocharging is done by back and forth movement of two polymer plates in conformal contact with each other (speed: 7 to 30 mm/s; constant normal force: 10 N). Uniform corona charging is performed by moving the sample in the space charge zone generated by a wire-type high-voltage electrode used in a triode configuration with the potential of the grid electrode set at 1 and 2 kV. The electric potential at the surface of tribocharged polymers is less uniform, but decays slower than that of corona-charged samples.

Factors that affect tribocharging of PE granules after exposure to a dielectric barrier discharge (DBD)

Triboelectric charging is used in many applications including electricity nanogeneration, safety or electrostatic separation of insulating materials. Insulating granules charge must be the highest in electrostatic separators so that the electric field forces allow their separation. Dielectric Barrier Discharge (DBD), used for ozone production, depollution or surface treatment, is able to change the triboelectric properties of insulating materials. The aim of this study is to improve the triboelectric charging of Polyethylene (PE) particles by exposing them to an atmospheric Dielectric Barrier Discharge. Signal shape, exposure duration, voltage amplitude and frequency, which rule the DBD, are studied. After exposing the particles to DBD, they are charged in a vibratory device during 5 min. Then, particles are put in a Faraday pail connected to an electrostatic voltmeter to measure the acquired charge. Results show that the square waveform has the best effect and that the duration of the DBD treatment should not exceed 10 s, for the amplitude of the applied voltage of 21 kV.

Optimal Operation of a Plate-Type Corona-Electrostatic Separator for the Recovery of Metals and Plastics from Granular Wastes

Plate-type electrostatic separators are commonly employed for the selective sorting of conductive and nonconductive granular materials. Unfortunately, the purity of the conductive products is often compromised by the fact that the trajectories of the nonconductive granules cannot be controlled. The aim of the present work is to validate a relative simple solution to this problem: subjecting the granules to a corona discharge during their sliding down the plate electrode. This discharge does not affect the conductive granules but charges the nonconductive ones, so that the electric force would tend to pin them to the surface of the grounded electrode. In this way, the friction forces between the granules and the plate increase and the inertia forces decrease. Design of experiments methodology is employed to identify the optimum high-voltage, relative position of the electrodes, and configuration of the collector. The conclusions of this study served at the optimum design of an industrial corona-assisted plate-type electrostatic separator for the recycling of metals and plastics from granular electric cable wastes.

Comparison between the surface-electric-potential characteristics of tribo- and corona-charged polymers

The ability of polymers to acquire and retain electrostatic charges is valued in designing new equipment like electrostatic separators, electret filters and powder coating devices. Several recent studies have pointed out the beneficial effect of electric charges on the lubricated contacts. The aim of this paper is to compare two physical mechanisms of electric charging of polymers: triboelectric effect and corona discharge. The experiments are performed with 5-mm-thick samples of polystyrene (PS), poly-vinyl-chloride (PVC) and polypropylene (PP). The cartography of the electric potential is made using the capacitive probe of an electrostatic voltmeter. The tribocharging is done by back and forth movement of two polymer plates in conformal contact with each other (speed: 7 to 30 mm/s; constant normal force: 2 N). Uniform corona charging is performed by moving the sample in the space charge zone generated by a wire-type high-voltage electrode, used in a triode configuration, with the potential of the grid electrode set at 0.5 kV, 1 kV and 2 kV. The electric potential at the surface of tribo-charged polymers is less uniform and decays faster than that of corona-charged samples.

Continuous operation of a fluidized-bed disk-type electrostatic separator for micronized plastic waste

The objective of this work is to highlight the peculiarities of the continuous operating regime of a new tribo-aero-electrostatic separator for the recycling of plastics contained in micronized waste electrical and electronic equipment. The experiments were carried out with an ABS/PS mixture. The particles are charged by triboelectric effect in a fluidized bed in the presence of an electric field created between two disk electrodes connected to high voltage supplies of opposite polarities. The installation described in this paper has several control variables: the high voltage, the mass of the mixture initially introduced into the fluidized bed, the flow of fluidizing air and the rotation speed of the disk electrodes. The influence of each control variable of the machine on the efficiency of the separation was studied by simultaneously and continuously measuring the mass and the electrical charge of the collected products. The quality of the products obtained and the high hourly rates recommend this type of installation for industrial use.