Triboelectric charging properties of ABS and PS granules with different concentration of Brominated flame retardants

Abstract

— Waste plastics are a significant source of pollution as they contain substances identified as persistent organic pollutants (POPs), such as brominated flame retardants (BFRs). These substances are added to many polymer materials to comply with safety flammability standards, to enhance their fire resistance, delay ignition, slow the spread of fire, or delay the time of flashover. BFRs are released from plastic materials into the environment during use, disposal, and recycling; they require careful management to reduce the risks associated with their dangerous nature for human health. Among the plastics found in WEEE, Acrylonitrile Butadiene Styrene (ABS) and Polystyrene (PS) are the most prevalent, as their excellent insulating and mechanical properties render them essential for the electrical and electronic equipment sector. However, their high flammability makes them particularly targeted for the addition of high levels of BFRs to enhance their fire resistance and comply with safety standards. Moreover, their similar surface properties and close mass density values significantly limit wet separation methods, such as density and flotation methods. In this case, tribo-electrostatic separation might be a low-cost, low-polluting solution that can preserve the original properties of plastic. Tribo-electrostatic separation was chosen over wet methods because it is a dry process that requires no chemical pretreatment, no expensive wetting reagents, and also eliminates wastewater treatment issues. This method exploits the differences in triboelectric properties of ABS and PS, enabling their separation without altering their chemical structure, which is critical for preserving recyclability. The aim of this paper was to investigate the effect of the presence of BFR with different concentrations in ABS and PS materials on their triboelectric charging characteristic and tribo-electrostatic separation process of brominated ABS and PS particles. The tribocharging properties of brominated plastic particles were studied by a fluidized bed tribocharger. The brominated polymers used in the tribocharging experiments were BFR-free ABS, 0.23 % BFR ABS, 4.6 % BFR ABS, BFR-free PS, 0.23 % BFR PS, and 4.6 % BFR PS, produced by CRITT France, using hexabromobenzene as a model of BFR. In the first part of the experiments, this work investigated the influence of the tribocharger wall material, the triboelectric behavior of BFR ABS and PS plastics, their charge density evolution over time, and finally the effect of tribocharging duration. In the second part of the experiments, tribo-electrostatic separation tests were conducted on different mixtures (BFR-free ABS/BFR-free PS, 4.6 % BFR ABS/4.6 % BFR PS, etc.).