Evaluation of Marker Materials and Spectroscopic Methods for Tracer-Based Sorting of Plastic Wastes

Abstract

Plastics are a ubiquitous material with good mechanical, chemical and thermal properties, and are used in all industrial sectors. Large quantities, widespread use, and insufficient management of plastic wastes lead to low recycling rates. The key challenge in recycling plastic waste is achieving a higher degree of homogeneity between the different polymer material streams. Modern waste sorting plants use automated sensor-based sorting systems capable to sort out commodity plastics, while many engineering plastics, such as polyoxymethylene (POM), will end up in mixed waste streams and are therefore not recycled. A novel approach to increasing recycling rates is tracer-based sorting (TBS), which uses a traceable plastic additive or marker that enables or enhances polymer type identification based on the tracer's unique fingerprint (e.g., fluorescence). With future TBS applications in mind, we have summarized the literature and assessed TBS techniques and spectroscopic detection methods. Furthermore, a comprehensive list of potential tracer substances suitable for thermoplastics was derived from the literature. We also derived a set of criteria to select the most promising tracer candidates (3 out of 80) based on their material properties, toxicity profiles, and detectability that could be applied to enable the circularity of, for example, POM or other thermoplastics.

Keywords: Circular Economy; fluorescent markers; plastic waste recycling; polyoxymethylene; post-consumer plastic waste; sensor-based sorting; spectroscopy; thermoplastics; tracer-based sorting.

Figure 1

Scheme of the workflow to select a suitable substance that qualifies for tracer-based sorting of plastic wastes. The yellow rectangle displays the priority of information, as in best case that the tracer is commercially available, indicated by the CAS number, and safe (not persistent, not bio accumulative, and non-toxic) according to the REACH and CLP regulation summarized in safety data sheets (SDS).

Figure 2

Proposed decision tree for selecting tracers from the compiled excel sheet (Supplementary Material S2). Parameters can be switched or modified according to requirements. Costs of the tracer substances were not considered in this study.

Figure 3

Overview of production, use, and disposal routes for POM plastics in the EU. Recycling efficiency for POM waste may be significantly improved by applying the tracer-based sorting concept.