Recycling plastic has long been a central pillar in creating sustainability across the globe. However, some plastics are challenging to recycle with traditional mechanical methods, leading to the emergence of new chemical plastic recycling technologies.

These methods use heat, chemical reactions, or both to reduce plastics back into materials like polymers, monomers, or even base chemical feedstocks. These raw materials can then be used to create new plastic, fuel, and other chemicals. However, in such a fast-paced industry, what opportunities are there to innovate chemical plastic recycling?

Purification: Improving an imperfect method

Closest to its mechanical recycling predecessor, and often combined with physical purification methods, chemical purification involves the use of solvents to remove additives or contaminants, such as plasticizers, from plastic to be left only with the polymer. In the ideal method, the polymer is unaffected by the purification process and can be reformed into a new plastic product, but this requires careful design and selection of purification solvents.

Solvent-based purification is not currently considered a perpetual method, as with each recycling pass, there is a risk of remaining contaminants and the loss of the polymer’s properties due to solvent damage. This means that there is a limit to the number of times the same plastic can be recycled in this way before the product is lost. There is also concern about the high levels of energy consumption needed to remove solvents after the purification process, which creates time and cost barriers to scaling.

In spite of these challenges, large-scale commercialization of purification is still taking place, creating an opportunity for more efficient and cost-effective methods to meet this demand. Companies, including Unilever with CreaSolv® and P&G with PureCycle, have invested in this form of chemical recycling of plastics. The goal is to use it in combination with physical purification methods as part of their own sustainability measures to create circularity within their polyethylene (PE) and polypropylene (PP) usage.

Depolymerization: Catalysing polymer lysis

With depolymerization, chemical plastic recycling goes a step further than purification and breaks polymers down into constituent parts. The resulting product of either monomers or shorter polymers, known as oligomers, can then be used to create high-quality recycled polymers which are indistinguishable from new polymers.

The main barrier when depolymerizing, often called chemolysis, is that it is currently only possible for condensation polymers like polyethylene terephthalate (PET) and cannot yet be practically applied to addition polymers. This excludes plastics that make up a large proportion of plastic waste, including PP, PE, and polyvinyl chloride (PVC).

However, for the chemical recycling of plastics like PET and other condensation polymers, depolymerization catalysts are an area of intense research. Since the lysis of polymers requires much energy, often in the form of high temperature or pressure, the design of strong catalysts for depolymerization reactions can substantially increase the cost-effectiveness of the recycling process—an attractive prospect for the industry.

Pyrolysis and gasification: Refining conversion reactions

One option for recycling common waste plastics, including PP and PE, is plastic pyrolysis. Here, plastics are heated to high temperatures in the absence of oxygen until they break down into hydrocarbons. The process can be fine-tuned to favor the production of lighter or heavier hydrocarbons, and the products can be used either to build new polymers or as a fuel source. However, the process has received heavy criticism surrounding its sustainability.

More inclusive than pyrolysis, gasification can be used on any kind of plastic, making mixed plastic recycling even easier. The process works similarly to pyrolysis but with the addition of a small amount of oxygen, producing syngas, a mixture of gasses mainly made up of hydrogen and carbon monoxide.

Pyrolysis and gasification are both established for use in mixed plastic recycling, but there are still opportunities for new technologies. The balance of waste conversion reactions can be altered by refining parameters like the temperature and heating rate or introducing novel catalysts.

Hydrothermal treatment: Bringing research to real life

One of the newest chemical recycling technologies is hydrothermal treatment (HTT). The process involves using water to both heat and dissolve mixed plastic recycling under supercritical conditions. Since there is no direct combustion of the plastics, HTT does not produce the same toxic combustion products as pyrolysis, and it produces higher product yields than pyrolysis or gasification.

Though it is considered a more viable chemical recycling solution, as a new technology, HTT requires more parameter tuning than pyrolysis and gasification before it can be commercialized. To create a circular plastic economy, scientists must define the ideal temperature, reaction time, and ratios for successful HTT chemical recycling of plastics. HTT also provides researchers with the opportunity to develop novel catalysts that can accelerate the reaction and reduce energy burdens for this process.

Is chemical recycling of plastics the future?

Chemical plastic recycling presents an opportunity for circularity not afforded by traditional mechanical recycling methods, which could be the key to more practical and economically viable sustainability within the plastics industry. However, these technologies are only just emerging and can be challenging to scale for widespread plastic recycling compliance.

This creates an exciting environment for researchers within the plastics and plastic recycling industries, with the chance to position themselves and their innovations at the heart of future chemical recycling infrastructure.

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