(A) Plastic waste made from polyoxymethylene (POM), which is used for various technical components due to its mechanical strength. (B) POM–containing different concentrations of the marker material yttrium oxide. No discoloration of the POM specimens was observed.
Source: BAM, division Dangerous Goods Packagings
Plastics are used in a wide variety of product groups due to their advantageous mechanical, chemical and thermal properties. The wide range of plastic types (i.e. polymers including fillers / additives), steadily growing waste volumes, and the (partially) inefficient systems for sorting plastic waste according to type are currently the biggest challenges in plastics recycling. In the first recycling step, the separation of the different types of plastics into waste streams with a high grade of purity is very important in order to subsequently ensure a high quality of the recycled products. In addition to the separate collection of, for example, packaging waste in households, plastics are usually sorted in waste treatment plants equipped with automated sensor-based sorting systems. Near-infrared-based detection methods are most commonly used for this purpose, but sensors based on X-ray fluorescence or operating in the visible and ultraviolet range are also used. Such sensor systems can efficiently separate conventional plastics, such as films made of polyethylene, from mixed plastic waste streams.
However, there are also so-called engineering plastics, such as polyoxymethylene (POM), which are used as gears et cetera in many electronic devices or construction materials, and only become waste after years of use. Engineering plastics are not usually separated by type and thus cannot be recycled. Therefore, POM-containing wastes are mostly incinerated or thermally utilized. A promising technical approach to increase recycling rates is so-called marker-based sorting. For this, markers are incorporated as additives to give valuable engineering plastics, such as POM, a unique spectroscopic fingerprint, which in turn enables targeted sorting during recycling. In order to be able to implement such marker-based sorting on a large scale in the future, a suitable marker must already be selected during product design, which must also be environmentally compatible. Our work therefore aimed to develop a selection and evaluation method for plastic markers, focusing on technical feasibility and not yet considering the costs of implementation. From a technical and ecological point of view, markers made of rare earths, such as cerium or yttrium oxide, would be suitable for sorting out POM from mixed plastic waste streams in the future.
Evaluation of marker materials and spectroscopic methods for tracer-based sorting of plastic wastes
Christoph Olscher, Aleksander Jandric, Christian Zafiu, Florian Part
published in Polymers, Vol. 14, Article number 3074, pages 1-20, 2022.
BAM, department Containment Systems for Dangerous Goods, division Dangerous Goods Packagings