Enabling the Polymer Circular Economy: Innovations in Photoluminescent Labeling of Plastic Waste for Enhanced Sorting

Abstract

It is widely accepted that moving from a linear to circular economy for plastics will be beneficial to reduce plastic pollution in our environment and to prevent loss of material value. However, challenges within the sorting of plastic waste often lead to contaminated waste streams that can devalue recyclates and hinder reprocessing. Therefore, the improvement of the sorting of plastic waste can lead to dramatic improvements in recyclate quality and enable circularity for plastics. Here, we discuss current sorting methods for plastic waste and review labeling techniques to enable enhanced sorting of plastic recyclates. Photoluminescent-based labeling is discussed in detail, including UV-vis organic and inorganic photoluminescent markers, infrared up-conversion, and X-ray fluorescent markers. Methods of incorporating labels within packaging, such as extrusion, surface coatings, and incorporation within external labels are also discussed. Additionally, we highlight some practical models for implementing some of the sorting techniques and provide an outlook for this growing field of research.

Figure 1

Schematic overview of inclusion of photoluminescent markers into plastic articles, highlighting key areas for consideration.

Figure 2

Plastic resin code labeling system.

Figure 3

Chemical structures of organic fluorescent dyes used by (a,b) Arenas-Vivo et al., (c,d) Pilon et al., (e) Langhals et al. and (f,g) Müssig et al. to label commodity plastics discussed in this review.

Figure 4

Fluorescent spectra demonstrating the impact of (a) thermal, (b) photochemical, and (c) hygrothermal degradation on HDPE embedded with the V-Quin dye (top row), R6G dye (middle row), and R6G dye along with a montmorillonite clay. For all V-Quin spectra, λ_ex = 350 nm, and for all R6G spectra, λ_ex = 510 nm. Reproduced with permission from ref (28). Copyright 2017 Elsevier.

Figure 5

(a) SEM image of the magnetic fluorescent PS supraparticles with (b) a TEM image of the constituent iron oxide nanoparticles. The colored fluorescent emission of each dyed supraparticles is visible when viewing in (c) daylight versus (d) UV light. (e,f) Color variations are also shown using different supraparticle blends. Reproduced from ref (34) under the Creative Commons CC BY license. Copyright 2022 The Authors.

Figure 6

Two-dimensional photoluminescent emission spectra of LDPE labeled with the terbium complex (1b), erbium complex (2b), and two coumarin dyes (3a,b). Reproduced from ref (30). Copyright Bayer AG.

Figure 7

Digital photographs of PE samples under (a) daylight and under (b) a 367 nm light source. The sample shapes relate to pure PE (rectangles), the crude PE material containing CQDs and SiO₂ (circles), and PE containing only the isolated CQDs (triangles). Reproduced from ref (36). Copyright 2021 American Chemical Society.

Figure 8

Schematic demonstrating the mechanism for the photoluminescent up-conversion process.

Figure 9

Main UC electronic transitions for Ln³⁺ ions, shown on the UV–vis–IR spectrum, to indicate the energy of the emitted UC photon. Reproduced with permission from ref (62). Copyright 2017 Wiley.

Figure 10

Up-conversion photoluminescent (UC PL) spectra of different semitransparent colored HDPE labeled with various quantities of the Yb³⁺/Er³⁺-based marker. The HDPE colors were (top left) yellow, (top right) green, (bottom left) red, and (bottom right) black. Light source: λ_ex = 980, 10 W/cm². Reproduced with permission from ref (31). Copyright 2020 Elsevier.

Figure 11

XRF spectra for PP samples mixed with all seven rare earth metal oxides at concentrations of 0.145 wt % (blue), 0.1 wt % (red), 0.025 wt % (green) and 0 wt % (purple). Reproduced with permission from ref (32). Copyright 2010 Elsevier.

Figure 12

Two-dimensional characterization of various polymers on the basis of their biexponential fluorescent decay constants, τ₁ and τ₂. Symbols relate to different types of PE (filled circles), PET (squares), silicone dehesives (diamonds), the silicone elastomer Tectosil (triangles), and other commodity and technical polymers (unfilled circles). Reproduced from ref (38) under the Creative Commons CC BY license. Copyright 2015 The Authors.

Figure 13

SEM micrographs of PP extruded with an inorganic photoluminescent marker at screw speeds of (a) 100 rpm and (b) 800 rpm. Reproduced from ref (29) under the Creative Commons CC BY license. Copyright 2015 ABPol.

Figure 14

Schematic illustration of how surface coated bottles may be recycled.

Figure 15

Plastic bottles labeled with Nextek’s PolyPRISM external labels viewed under UV light. Reproduced with permission from Nextek. Copyright 2020 Nextek Ltd.

Figure 16

Annotated photograph of a laboratory scale apparatus for sorting photoluminescent-labeled bottles. Reproduced with permission from ref (55). Copyright 2004 Taylor & Francis.

Figure 17

Annotated photograph of a laboratory-scale apparatus for sorting photoluminescent-labeled plastic flakes. Reproduced with permission from ref (57). Copyright 2015 Elsevier.