The use of electrostatic techniques for the identification and sorting of waste packaging materials

Abstract

Techniques have been developed which enable items of domestic waste packaging to be identified and sorted into material groups prior to recycling. Plastic packaging items can be separated from items of similar size and shape constructed from cardboard, wood or paper. This is achieved by applying a controlled level of electrostatic charge and comparing the rate of charge relaxation using a non-contacting electric field meter. Plastic packaging items such as bottles, food tubs and trays are subsequently streamed into their polymer groups using a series of tribo-electric probes. The probes are brought into contact with the packaging materials to be identified and comprise a rotating head or drum, which generates an electrostatic charge due to the friction between the head and the item to be identified. Since the magnitude and polarity of the generated charge depends on the polymers comprising both the head material and the packaging material, a degree of identification can be obtained. Using this technique has resulted in: (a) the successful separation of plastics from non-plastics and (b) plastics separated into polypropylene (PP), polyethylene terephthalate (PET)/polystyrene (PS), polyvinyl chloride (PVC) and high-density polyethylene (HDPE) streams. The technique ideally lends itself to incorporation on an automated recycling line.