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.

Marker materials and spectroscopic methods for sorting plastic waste (NanoTrust-Dossier No60en - June 2023)

“…Figure 1 shows a possi ble decision tree for the selection of potential mark ers, which was developed in more detail within the research project "PLASTMARK". 26,27 Detection methods for TBS Spectroscopic detection methods have long been used to identify and separate different materials. Mechanical recycling facilities for used plastics primarily use NIR sensors.…”

“…54,55 Figure 1: Proposed decision tree for the selection of marker materials that would be considered for specific plastic streams. 26…”

Correlating Near-Infrared Spectra to Bulk Properties in Polyolefins

“…38,40,42−44 However, in their current states, both Raman and MIR are impractical for independent use in large-scale sorting operations. 45 A combination of NIR and factor analysis has demonstrated some ability to distinguish between various classes of POs, including HDPE and LDPE blends. 41,46,47 Additionally, both Kim et al and Sato et al have demonstrated that partial least-squares regression (PLSR) of NIR spectra may be able to predict properties such as densities, crystallinity, and melting points of PEs made by a single manufacturer.…”

“…Even hyperspectral imaging, an extremely data-rich technique, struggles to distinguish the subtle shifts between subclasses of PE . The use of multiple spectroscopic methods, such as NIR or Fourier transform infrared (FTIR) and Raman measurements, coupled with multivariate data analysis has been shown to identify key properties such as density and crystallinity in POs. ,, FTIR/mid-IR (MIR) can use peak ratios to provide a quantitative value for the branching content, which can then be used to infer trends in crystallinity and density. ,,− However, in their current states, both Raman and MIR are impractical for independent use in large-scale sorting operations . A combination of NIR and factor analysis has demonstrated some ability to distinguish between various classes of POs, including HDPE and LDPE blends. ,, Additionally, both Kim et al and Sato et al have demonstrated that partial least-squares regression (PLSR) of NIR spectra may be able to predict properties such as densities, crystallinity, and melting points of PEs made by a single manufacturer. , However, in both papers, the authors appear to use manufacturer-provided properties and assume they remain constant despite thermal processing.…”

Tracing the origin of VOCs in post-consumer plastic film bales

“…TBS can be done in several ways. One option is applying fluorescent tracer materials and using a corresponding detection unit ( Olscher et al, 2022 ). Another option is the use of digital watermarks, i.e., codes that are integrated into the design of the packaging and can be detected by cameras on high-speed sorting lines ( De Tandt et al, 2021 ; Lange, 2021 ).…”