Sorting plastics waste for a circular economy: Perspectives for lanthanide luminescent markers

Improved sorting of plastic waste will be needed to increase the circularity of the plastics industry, irrespective of innovations in recycling technologies. The variety of recycling processes available in the future will not alleviate the need for sorting. Rather, nuanced sorting will be necessary to distribute waste streams into tailored feedstocks within an ecosystem comprising of various mechanical and chemical processes. A system for marking plastics with codes that are invisible to consumers, but readily apparent to sorting machines is of interest to enable such nuanced sorting. Luminescent lanthanide markers offer a promising solution, adding unique spectral signatures in the visible and near-infrared (NIR) to allow innovative new sorting options. Currently, 21 codes can be created using mixtures of three upconversion markers when excited with a 978 nm laser and measured with >99% accuracy. This range can be expanded with further NIR emitting markers, whose emission can be obtained at the same time as characterizing the NIR reflectance of plastics.