Sensors identify plastics for recycling

Tom Shelley reports on a sensing breakthrough that enables reliable sorting of all plastics for recycling.

While infra-red sensors have been in use for some time to enable fast sorting of two or three plastic streams for recycling, the latest developments allow the rapid recognition and reliable separation of six different plastics at the same time using near infra red laser diodes and detectors.This enables the practicable and cost-effective recycling of mixed plastic waste streams. The need arises because different polymers do not usually mix in the molten state, and even when they do, the mechanical properties of mixtures are greatly inferior to those of the pure materials. European law requires that different plastics be identified in products to assist separation and recycling, but practical recycling of plastic waste from municipal sources requires automated separation. At the Bywaters recycling plant at Lea Riverside in East London, a near Infra-Red system made by Norwegian company Titech, identifies and separates PET – polyethyleneterephthalate, as used in plastic bottles, and HDPE – high density polyethylene. Furthermore, there have for some time been machines that can sort clear and coloured plastic flakes by using visual sensors and air jets to separate material into different bins, such as the Sortex Z+, made in East London. A new, five-diode laser sensing system has been developed by the Japanese company IDEC, and has been demonstrated on an industrial robot equipped recycling system developed by Mitsubishi Electric and the Osaka University Photonics Research Center. The Center comments that in Japan: "Nowadays, only PET and PS are recycled but their production rates are low; 5 and 7.6% of the total plastic production, respectively. PE and PP have not been recycled as they are not distinguishable by eye, though their production rates are the highest; 23 and 22%, respectively." The information available is limited, but a paper published by Koji Inada and other IDEC researchers reveal that the sensing system depends on Indium gallium arsenic phosphorous laser diodes (LDs) and receptors, producing a system that is a small fraction of the size and cost, and much faster than any of the types of infra red spectrometer conventionally used for identifying plastics and other organic materials. The paper describes using the ratio of absorptions at 1722nm, produced by the LD, and visible light from a LED at 644nm, and from another LD at 673nm, to distinguish PET from PVC – polyvinyl chloride. It also notes, however that it is possible to change the wavelength produced by the infra red laser diode by varying its composition and fabrication, and that by using a combination of LDs operating at different wavelengths, it is possible to distinguish between HDPE, Low Density Polyethylene (LDPE), Polycarbonate (PC), PET, polypropylene (PP), polystyrene (PS) and PVC. According to the paper, the cost of a simple LD based system should be about YEN 500,000 =£3,800 and it would measure 50mm x 100mm x 100mm. Source lifetime is estimated at 100,000h, response time, 0.05ms and dissipation power 5W.