The current global plastics economy is highly linear, with the exceptional performance and low carbon footprint of polymeric materials at odds with dramatic increases in plastic waste. Transitioning to a circular economy that retains plastic in its highest value condition is essential to reduce environmental impacts, promoting reduction, reuse, and recycling. Mechanical recycling is an essential tool in an environmentally and economically sustainable economy of plastics, but current mechanical recycling processes are limited by cost, degradation of mechanical properties, and inconsistent quality products. This review covers the current methods and challenges for the mechanical recycling of the five main packaging plastics: poly(ethylene terephthalate), polyethylene, polypropylene, polystyrene, and poly(vinyl chloride) through the lens of a circular economy. Their reprocessing induced degradation mechanisms are introduced and strategies to improve their recycling are discussed. Additionally, this review briefly examines approaches to improve polymer blending in mixed plastic waste streams and applications of lower quality recyclate.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/marc.202000415

Mechanical Recycling of Packaging Plastics: A Review.

The current scale of plastics production and the accompanying waste disposal problems represent a largely untapped opportunity for chemical upcycling. Tandem catalytic conversion by platinum supported on γ-alumina converts various polyethylene grades in high yields (up to 80 weight percent) to low-molecular-weight liquid/wax products, in the absence of added solvent or molecular hydrogen, with little production of light gases. The major components are valuable long-chain alkylaromatics and alkylnaphthenes (average ~C30, dispersity Ð = 1.1). Coupling exothermic hydrogenolysis with endothermic aromatization renders the overall transformation thermodynamically accessible despite the moderate reaction temperature of 280°C. This approach demonstrates how waste polyolefins can be a viable feedstock for the generation of molecular hydrocarbon products.

Polyethylene upcycling to long-chain alkylaromatics by tandem hydrogenolysis/aromatization

Plastics are key components of almost any technology today. Although their production consumes substantial feedstock resources, plastics are largely disposed of after their service life. In terms of a circular economy1–8, reuse of post-consumer sorted polymers (‘mechanical recycling’) is hampered by deterioration of materials performance9,10. Chemical recycling1,11 via depolymerization to monomer offers an alternative that retains high-performance properties. The linear hydrocarbon chains of polyethylene12 enable crystalline packing and provide excellent materials properties13. Their inert nature hinders chemical recycling, however, necessitating temperatures above 600 degrees Celsius and recovering ethylene with a yield of less than 10 per cent3,11,14. Here we show that renewable polycarbonates and polyesters with a low density of in-chain functional groups as break points in a polyethylene chain can be recycled chemically by solvolysis with a recovery rate of more than 96 per cent. At the same time, the break points do not disturb the crystalline polyethylene structure, and the desirable materials properties (like those of high-density polyethylene) are fully retained upon recycling. Processing can be performed by common injection moulding and the materials are well-suited for additive manufacturing, such as 3D printing. Selective removal from model polymer waste streams is possible. In our approach, the initial polymers result from polycondensation of long-chain building blocks, derived by state-of-the-art catalytic schemes from common plant oil feedstocks, or microalgae oils15. This allows closed-loop recycling of polyethylene-like materials. Polycarbonates and polyesters with materials properties like those of high-density polyethylene can be recycled chemically by depolymerization to their constituent monomers, re-polymerization yielding material with uncompromised processing and materials properties.

/pdf/closed-loop-recycling-of-polyethylene-like-materials-595ogdlce4.pdf

Closed-loop recycling of polyethylene-like materials.

Critical advances and future opportunities in upcycling commodity polymers

Research efforts to find more sustainable pathways for the synthesis of adipic acid have led to the introduction of new catalytic processes for producing this commodity chemical from alternative resources. With a focus on the performance of oxygen and hydrogen peroxide as preferred oxidants, this minireview summarizes recent advances made in the selective oxidation of cyclohexene, cyclohexane, cyclohexanone and n-hexane to adipic acid. Special attention is paid to the exploration of catalytic pathways involving lignocellulosic biomass-derived chemicals such as 5-hydroxymethylfurfural, D-glucose, γ-valerolactone and compounds representative of lignin and lignin-derived bio-oils.

Emerging catalytic processes for the production of adipic acid

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.

https://onlinelibrary.wiley.com/doi/epdf/10.1002/anie.201915651

Beyond Mechanical Recycling: Giving New Life to Plastic Waste

Das Vergrosern des Recyclingstroms erfordert einen neuen Ansatz, der uber das Schmelzen und Umformen hinausgeht. Es gibt einige Techniken des chemischen Recyclings mit dem Potential, bestehende Recyclingmoglichkeiten zu erganzen. In diesem Aufsatz sind die Methoden des chemischen Recyclings dargestellt und anhand einer Okobilanz bewertet, erganzt durch eine Aufzahlung von Prozessen und Firmen in diesem Bereich. Wir zeigen, dass bestimmte Techniken besonders fur spezifische Mullstrome geeignet sind und dass nur eine Kombination aus den vorhandenen Methoden geeignet ist, das Kunststoffmullproblem zu losen. Aktuelle Forschung sollte realistischeren und weniger reinen Mischstromen grosere Aufmerksamkeit widmen, wahrend Trenn- und Sortierprozesse z.B. durch effektivere Regularien verbessert werden mussen. Dieser Aufsatz soll zur Entwicklung von Verfahren inspirieren, mit denen hochwertige Produkte hergestellt werden konnen, die die Kreislaufwirtschaft antreiben, indem sie notige Wirtschaftsanreize und Erleichterungen fur die Umwelt bieten.

Jos T. F. Keurentjes

Papers

Beyond Mechanical Recycling: Giving New Life to Plastic Waste

Die nächste Generation des Recyclings – neues Leben für Kunststoffmüll