Less than 20 percent of post-consumer plastic waste is currently recycled, primarily for energy recovery or through mechanical recycling into lower value materials. This article describes state-of-the-art methods for chemical upcycling of plastic waste into new polymers, molecules, or materials. It suggests complementary approaches to conventional recycling methodologies to guide academic and industrial research, and it proposes uncommon routes to reveal new feedstock potentials of plastic waste. Advantages and challenges with each recycling approach are critically discussed considering how they can contribute to realizing a sustainable plastics economy.
Coralie Jehanno, POLYMAT, University of the Basque Country UPV/EHU, Donostia–San Sebastian, Spain, and POLYKEY, Donostia-San Sebastian, Spain; Jill W. Alty, Department of Chemistry, University of North Carolina at Chapel Hill, USA; Martijn Roosen, Laboratory for Circular Process Engineering, Ghent University, Belgium; Steven De Meester, Laboratory for Circular Process Engineering, Ghent University, Belgium; Andrew P. Dove, School of Chemistry, University of Birmingham, UK; Eugene Y.-X. Chen, Department of Chemistry, Colorado State University, Fort Collins, CO, USA; Frank A. Leibfarth, Department of Chemistry, University of North Carolina at Chapel Hill, USA; Haritz Sardon, POLYMAT, University of the Basque Country UPV/EHU, Donostia–San Sebastian, Spain
“The multifaceted challenges that need to be solved to advance a sustainable plastics economy are daunting in their scale and complexity”
Advances in end-of-life treatments for plastics have not kept pace with the growth rate in plastics production. To decrease the amount of plastic pollution as well as reducing the greenhouse gas emissions from the manufacturing of virgin plastics, there is a pressing scientific and societal challenge to reduce, reuse and recycle plastic waste.
The majority of plastic waste is currently landfilled or incinerated, although large-scale recycling of plastics has been practiced over the past 30 years in Western countries. The potential in sorting of plastic waste is limited due to additives, contaminants, and multilayer products, which leads to mis-sorted polymers and substantially decreased material qualities for mechanically recycled products – so called ‘down-cycling’.
As an alternative to mechanical compounding, chemical recycling techniques are emerging, which break down polymers into high-purity monomers. However, only a subset of plastic waste can currently be chemically recycled in an energy-efficient and cost-effective manner.
The inherent challenge to both mechanical and chemical recycling arises in that these processes are more expensive or energy intensive than the simpler production process based on petroleum. New concepts are emerging to overcome this challenge by targeting high-value markets in the circular economy for plastics, also called ‘up-cycling’, where chemical recycling results in new polymers, molecules, and materials of similar or higher quality or functionality than its feedstock.
The article describes three main categories of upcycling: 1) Polymer-to-polymer, 2) Polymer-to-molecule, and 3) Polymer to materials.
“Chemical upcycling of polymers holds promise for a paradigm shift from traditional ways of treating waste plastics by transforming and repurposing them into feedstocks for higher-value products.”
The authors compare the environmental footprint of upcycling alternatives to that of substituting plastic products made from virgin materials through a life-cycle perspective. Three case examples were evaluated, where the carbon footprint of each case is compared with a benchmark scenario in which the plastic materials would be incinerated with energy recovery and the applications that are substituted would be fulfilled with virgin materials.
The three case examples are:
Assumptions were made to compensate for the scale and optimization in the relatively more mature virgin production value chains of the substituted materials. The comparison resulted in that a net benefit, in terms of lower carbon footprint, is feasible in each of the three cases.
Case 1 had the drawback of requiring many resources but had the benefit of low energy consumption and multiple recycling cycles.
Case 2 had the drawback of high energy demand but was still showing high CO2 savings thanks to avoiding the use of virgin resources.
Case 3 had low process yields, but in turn showed the benefits of low energy consumption.
However, the authors point out that assessing circular economy developments through upcycling is complex, and the limitations to life-cycle assessments must be recognized. Future developments in the life-cycle assessment of upcycling are pointed out, including the aspects like how quality affects the factor of substituting virgin materials, the lifetime of products and the recyclability of the substituted products.
Substitution of virgin plastic materials are suggested to be guided by three criteria:
The authors applied these principles to recent research to come to three broad conclusions about the current state of polymer upcycling:
Firstly, polymer upcycling is in its infancy and faces considerable challenges before it will be suited for widescale industrial application. Future research is required to understand and control the chemical mechanisms that lead to selective polymer deconstruction at a molecular level. More improvements are also needed to enhance the energy efficiency of upcycling processes, to enhance upcycling capability to produce targeted products, as well as to improve robustness to handle mixed and contaminated waste streams.
Secondly, incorporating aspects of material performance, sustainability metrics, material flow analysis and market volumes of the upcycled product earlier in the research pipeline is pointed out by the authors as critical. However, these aspects should not limit fundamental research in terms of re-imagining what is possible and to push the boundaries for future developments. As an example, small-scale upcycling may provide an economic benefit enough to offset a cost intensive recycling process – which can raise recycling levels overall in an industry where waste streams are generally dwarfing the potential substitution of virgin materials.
Third, the end-of-life of upcycled products need to be considered. Upcycling not only allows for extending the useful lifetime of plastic, but it also shows potential to transform plastic waste into value-added materials with built-in recyclability for closed-loop upcycling.
“The environmental impact of any of the discussed upcycling methods should ideally be minimized and evaluated by means of LCA or similar holistic methods and circular economy indicators.”