Photoluminescent Tracer Effects on Thermoplastic Polymer Recycling

Abstract

German packaging law demands an increase in the plastic packaging recycling quota from 36% to 63% by 2022. Application of inorganic crystalline fluorescent substances for tracer-based sorting (TBS) provides an innovative approach to meet this recycling goal. TBS is already applied in industry to separate ground polyvinylchloride (PVC) window profiles with and without fiber reinforcement with a sorting capacity of 0.5–2.5 t/h. In Germany, a TBS recycling system for post consumer packaging is being developed, starting with bottles as a model product. More than 80% of end-of-life plastic bottles from households show colors and transparency in favor of TBS. Selected trivalent lanthanide elements – erbium (Er³⁺), holmium (Ho³⁺), and thulium (Tm³⁺) – serve as effective tracer materials. These markers result in bright emission lines in green, red and near-infrared (NIR, at 800 nm) when excited with 980 nm laser light. This process of upconverting light – such that the energy of the emitted photons is greater than that of incident photons – is a unique anti-Stokes process. Tracer substances as a part of the printing ink used on labels or directly on the packaging were identified to be most efficient for industrial application. Mixtures of these tracer substances were applied on samples with a 100 µg/cm² concentration. In total 15 samples were tested, and experiments demonstrated that 11 marker combinations out of the 15 can be identified, based on the ratio of the individual signals. Using tracers as additives in the ppm range renders products identifiable by optical identification regardless of the polymers’ physical or chemical properties, thus sorting for product properties becomes possible. Examples include distinguishing food from non-food packaging, or identification of hazardous goods, even if identical packaging materials are being used.