Beyond Mechanical Recycling: Giving New Life to Plastic Waste

Abstract

Increasing the stream of recycled plastic necessitates an approach beyond the traditional recycling via melting and re-extrusion. Various chemical recycling processes have great potential to enhance recycling rates. In this Review, a summary of the various chemical recycling routes and assessment via life-cycle analysis is complemented by an extensive list of processes developed by companies active in chemical recycling. We show that each of the currently available processes is applicable for specific plastic waste streams. Thus, only a combination of different technologies can address the plastic waste problem. Research should focus on more realistic, more contaminated and mixed waste streams, while collection and sorting infrastructure will need to be improved, that is, by stricter regulation. This Review aims to inspire both science and innovation for the production of higher value and quality products from plastic recycling suitable for reuse or valorization to create the necessary economic and environmental push for a circular economy.

Keywords: catalysis; chemical recycling; circularity; plastic waste; solvolysis.

Figure 1

Illustration of an envisioned plastics value‐chain that could enhance the transition to circularity. Currently most plastic is incinerated or landfilled (bottom left), because collection and sorting produce very contaminated and mixed plastic‐waste streams. Better techniques for collection and sorting lead to streams of plastic waste that can be recycled by the various chemical recycling methods. These routes are still going to be complemented by traditional mechanical recycling for the purest steams of a single polymer. The shares that each of the techniques corresponded to in 2016 bottom left of the waste processing method and prediction for 2030 are shown at the bottom right of the waste processing method. These values are based on the McKinsey report.2 The plastic objects are sold to the consumer and after its life cycle collected again for sorting to undergo another recycle.

Figure 2

Top: CO₂‐equivalent emissions of different EoL treatment technologies applied for several plastic‐waste streams, in relative emissions indexed to incineration (100 wt %). Bottom: CO₂‐equivalent emissions of different EoL treatment technologies in absolute emissions in ton CO₂/ton waste by life ‐cycle stage.

Figure 3

Microwave heating (b), plasma reactors (c) and supercritical fluids (d) can address some of the problems encountered in conventional solvolysis and pyrolysis (a).

Figure 4

Products obtained through the different solvolysis pathways of PET, PU, and PA and how these products can be used to recycle back to the polymer or to obtain valuable products.

Figure 5

The plastic is dissolved and undissolved fragments, such as pigments, are removed by filtration. The top route describes the dissolution/precipitation technique using a single solvent, which is removed by evaporation, crystallizing the polymer for recovery. For the bottom route an anti‐solvent is used to precipitate the polymer, which can be recovered by filtration. Both routes require a solvent removal step, which can be time and energy consuming unless a supercritical anti‐solvent is employed.

Figure 6

With an increase in the number of functional groups and heteroatoms in the backbone of the polymer (top), the distribution of products and the pyrolytic mechanisms become less complicated. Process parameters provide a higher degree of control over the product distribution for polyolefins, in this example HDPE, although the ultimate monomer yield is lower than for PS and PMMA. A common theme of all pyrolysis is that an excessively high temperature leads to coke formation given that the residence time is long enough. Although, most reaction steps occur at lower temperatures, similar trends are observed for catalytic processes. BTX denotes benzene, toluene, and xylene.