A deeper dive into molecular recycling
Research supported by Greyson Assa
Advanced recycling, also known as chemical recycling, or more accurately, molecular recycling, is a developing technology meant to address the plastic crisis our planet is facing. Molecular recycling seeks to increase the amount and variety of plastics able to be processed in the recycling system, pushing the system towards greater circularity. At present, there are roughly 60 molecular recycling technology providers (Closed Loop Partners (CLP) 2021 (1)). Some major providers include Eastman, Chevron Phillips Chemical, ExxonMobil, LyondellBasell, Nexus, SABIC, and BASF (CLP 2021 (2)).
In 2021, the United States recycling rate, including all forms of recycling, was around 5% (Beyond Plastic 2022 & The Department of Energy 2022 (3&4)). Currently, mechanical recycling processes the majority of plastic inputs from the system. Molecular recycling will expand the system to incorporate more waste plastic inputs (RIC #3-7, in addition to RIC #1-2) to produce a wider range of economically viable plastic outputs. The hopeful outcome is that our newly produced and accumulating plastic waste can be input back into the recycling system. This would reduce the amount of virgin resins and newly extracted resources needed for plastic production, alongside reducing energy, water and other resource consumption. Presently, the technologys’ capacities are not at a stage developed enough nor at a scale large enough to address this crisis (Plastics Today 2020 (5)).
What Processes Make Up Molecular Recycling (CLP 2021 (1))
Molecular recycling encompasses dozens of transformational technologies that use solvents, heat, and enzymes to purify or break down plastic waste to create the basic building blocks of resins – polymers, monomers, oligomers – and hydrocarbon products. The three overarching technologies are purification, depolymerization/decomposition, and conversion. These technologies add to mechanical recycling to create this ecosystem:
Closed Loop Partners Plastic Supply Chains
Conversion involves the breaking of polymer chains of the mixed plastic feedstock into simple molecules called monomers. This thermal process produces diverse hydrocarbon products with a relatively large range of molecular weights like naphtha, paraffin waxes, other petrochemical products, and fuels. A subset of conversion technologies completely break down the polymer to form syngas or elemental carbon products like methanol and hydrogen. These raw materials may enter different supply chains, such as fuels for energy production or for the production of plastic resins.
Depolymerization or decomposition (D/D). D/D is similar to conversion in that it involves breaking down the polymer chains to produce monomers or oligomers. Monomers are precursors to polymers and can be synthesized (i.e. “repolymerized”) to produce a plastic resin. Oligomers are longer chained monomers and include products like polypropylene wax. This process can be biological, chemical, or thermal, and in some cases, a combination of two or three of these methods.
Purification is a physical process that involves dissolving single-polymer or mixed plastic feedstock in a solvent, then separating and purifying the mixture to extract additives and dyes to ultimately obtain a “purified” plastic (e.g. clear resins). The purification process does not change the polymer on a molecular level. These processes guarantee a plastic-to-plastic, circular outcome.
The molecular recycling process chosen is dependent upon qualities of the feedstock (plastic inputs). If the feedstock is mixed resin, then conversion and purification can be used. If the feedstock is single resin, then depolymerization is an option. It is important to note that, with any plastics recycling process, the feedstock needs to be within a specific cleanliness and contamination threshold, based on material recovery facility (MRF) tolerances.
Comparisons of Processes
Looking at these technologies through a developmental, economic and environmental lens leads to delineations in each’s feasibility. Developmentally, conversion leads the technologies in terms of commercial availability, followed by purification and D/D. Conversion also allows for the largest potential input of waste plastics due to its process method. Inversely, when considering environmental impact and intensity of the technologies the order flips: purification, D/D, conversion. A general rule is the less a plastic is broken down the more favorable it is environmentally and economically (less mass/plastic loss). Purification, which doesn’t involve the molecular deconstruction of plastic resin, involves less environmentally harmful processes i.e. less chemical addition and energy consumption. Mechanical recycling is atop of all these rankings.
Overall, the ranking is purification, D/D and then conversion. Consideration for which technology to incorporate into a recycling system involves understanding the plastic wasteshed i.e. the inputs into a recycling system. However, the greatest results would involve a mixed-technology approach. Understanding that the recycling ecosystem is complex and involves all steps yields the highest potential benefit and solutions to the problems. Depending on the inputs in the system, a particular recycling process will be more or less favorable and efficient. Therefore, the solution is manyfold: private investment, policy changes, consumer habits and economic desirability all play a role in the adoption and advancement of these technologies.
Future Considerations on Molecular Recycling
As these technologies are extremely new, criticisms exist over their feasibility in the current economic system. Furthermore, with regards to reducing the plastic issue and looming climate crisis, doubt of the technologies’ actual benefit and fears of an “all talk” publicity model make understanding the efficacy of these technologies difficult. It doesn’t help that private companies, majorly petrochemical companies, are spearheading research and development. As such, a system of accountability and transparency needs to be put in place.
Pact’s Position
One of Pact’s goals is to research and utilize the highest and best use cases for beauty packaging waste. As such, molecular recycling’s ability to recycle mixed plastics, a shortfall of current recycling technologies, is an opportunity to further reduce unusable waste. Pact sees molecular recycling, in its current development, as a next-best option. First is mechanical recycling - upcycling, recycling, and downcycling - because of the maturity and ideal environmental footprint of this technology. In the future, molecular recycling may become an ideal process for recycling plastic waste that is otherwise rejected by more mature technologies. To explore this possibility, Pact works closely with Pact member Eastman, a company that uses both depolymerization and conversion to recycle plastics. Through working with Eastman, Pact may be able to get beauty waste back into future beauty packaging.
Greyson Assa is a recent masters graduate from Stanford University. Studying Sustainability Science and Practice, he is now a director for the creative engineering consultancy ENTITY design studio, pushing sustainability forward through research and design.