Inorganic fluorescent marker materials for identification of post-consumer plastic packaging

•To meet future challenges current polymer sorting technology needs to be improved, supported or replaced by other means for fast, high-throughput sorting.•Inorganic fluorescent tracer materials (oxide crystals doped with ytterbium, Yb3+ sensitizer ions and either erbium (Er3+), holmium (Ho3+) or thulium (Tm3+) activator ions) enable up-conversion fluorescence as a unique means for polymer marking and product identification.•Selected polymer products (film and blow moulded products made from PE-HD) equipped with 10, 100 and 1000 ppm tracer materials were used in optical measurements.•In the optimal case, all markers can be mixed with white or yellow pigments without any restrictions. There are also no limitations to mix the NIR marker with colour pigments, except for black pigments. Current plastic packaging waste management practices in Europe, but also in other countries require improvement due to legal and societal requirements. To meet high recycling rates, significant changes among others in post-consumer packaging waste sorting become necessary. This waste stream is dominated by plastic packaging. Inorganic fluorescent tracer materials (oxide crystals doped with ytterbium Yb3+ sensitizer ions and either erbium (Er3+), holmium (Ho3+) or thulium (Tm3+) activator ions) enable a sorting criterion which is independent of the properties of the packaging materials. The authors propose to use up-conversion fluorescence as a unique mean for polymer marking and product identification. To this end, PE-HD film samples, with 10, 100 and 1000 ppm of marker concentration in different polymer matrix colours (semi-transparent, yellow, green, and black) were irradiated with 980 nm diode laser radiation, with an excitation intensity up to 10 W/cm2. The performance of three different marker types with their maximum emission in green, red, and NIR was measured and assessed both with and without polymer matrix. Moreover, PE-HD sample bottles with tracers were tested, and a tracer regime for specific code generation for improved polymer identification is proposed.