New study on the chemical recycling of plastics

Published in the prestigious journal Progress in Energy and Combustion Science (PECS)


A study on the chemical recycling of plastics, involving researchers from the Department of Chemistry, Materials and Chemical Engineering "Giulio Natta" at the Politecnico di Milano, was recently published in the prestigious journal Progress in Energy and Combustion Science (PECS).

Currently, in the European Union, 30% of plastic waste intercepted by collection systems is recycled with mechanical processes.

Various problems such as the degradation of the properties of the recycled polymers, the costs of the separation process of the different polymers contained in the solid plastic waste (polypropylene, polyethylene, polystyrene, PET, nylon, PVC, etc.) and the impossibility of mechanical recycling the large quantity of multi-layer materials (i.e. food packaging), strongly limit the potential and effectiveness of these processes in dealing with the problems of pollution of seas, soil and air. A further complication derives from the geographical and seasonal variability of the quantity and quality of waste, in addition to the heterogeneity of the life cycle of different materials (multilayer food packaging vs PVC window frames).

In this context, 40% of waste is converted into energy through waste-to-energy and the remaining 30% is disposed of in landfills, with the related risks of soil and sea pollution. Not to mention the waste not intercepted by collection which, unfortunately, ends up directly and uncontrollably in the soil and in the sea.

At this level, chemical recycling takes place, through pyrolysis processes, i.e. decomposition of materials by means of heat, and gasification: more mature processes from a technological point of view, flexible and independent of the raw material, and economically advantageous, both in terms of investment, and because the final product consists of hydrocarbons (i.e. fuels, chemicals). Taken to the extreme, one could think of pyrolysing polyethylene, to make ethylene, from which to remake polyethylene. From a circular perspective, from oil it is possible to obtain plastics, use them, recycle them and remake further products similar to those obtainable from the primary fossil source with which the original plastic was produced, with enormous advantages from the point of view of reducing greenhouse gas emissions.

Conceptually it is very simple: providing energy to convert polymers into hydrocarbon products of various kinds. However, from the fundamental chemical-physical point of view, pyrolysis and gasification of plastics are extremely complex processes to study, because purely chemical phenomena (complex networks of reactions and chemical species) are combined with mass and heat transport ones. This interaction affects the design of chemical reactors suitable for processing large quantities of waste.

The current level of knowledge about these phenomena at a molecular level limits the transition of chemical recycling processes from the laboratory scale to the industrial level. The study seeks to fill these gaps by collecting, reinterpreting and rationalizing state-of-the-art literature, and analyzing and proposing future orientations and challenges.

For more information
O. Dogu, M. Pelucchi, R. Van de Vijver, P. H.M. Van Steenberge, D. R. D'hooge, A. Cuoci, M. Mehl, A. Frassoldati, T. Faravelli, K. M. Van Geem
The chemistry of chemical recycling of solid plastic waste via pyrolysis and gasification: State-of-the-art, challenges, and future directions
Progress in Energy and Combustion Science, Volume 84, 2021, 100901
Read the study