The triboelectric pen: an electrostatic method for the identification of plastics in recycling

The automotive industry faces worldwide pressure to help find environmentally friendly ways of disposing of end of life vehicles. The metals in cars are already recycled and therefore the nonmetallic content provides the only opportunity for improvement. Most modern cars contain between 21% and 23% of solid nonmetallic materials. Plastics make up about half of this in the form of fabrics, resin bonded products and moulded components. In order to successfully recycle plastics it is first necessary to identify them and sort them into streams based on polymer type, e.g., polyethylene, ABS, nylon, etc. Failure to do this can have serious consequences on the later stages of the recycling process where contamination by noncompatible polymers can ruin whole batches of recyclate. Identifying plastics for recycling has always been an extremely difficult and messy business. It has usually meant burning the unidentified plastic and looking at the colour of the flame or smelling the smoke it produces. This technique is often inaccurate, time consuming and unhealthy. In a project sponsored by the Ford Motor Company, Wolfson Electrostatics and the Prototype Design Group at the University of Southampton have developed the triboelectric pen. This device can identify different families of plastic literally 'at a stroke'. The principles and use of the triboelectric pen are described.