Sparks

The new plastic is circular

 By Paola Arpino

From the United States comes a new type of plastic material whose properties resemble Lego. It is colourful and can be broken down and rebuilt in different forms, just like the famous toy bricks, which made “assembling” into a new kind of game back in the 1960s. The material is known by a simple three-letter acronym, PDK, but behind them lies everything we need to transform how we currently recycle…

Three letters, boundless possibilities: that’s because PDK (polydiketoenamine), developed by researchers at the US Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), is a revolutionary plastic that can be recycled an unlimited number of times while retaining its structure and colour.

 

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Scientists from Berkeley Lab have made a next-generation plastic that can be recycled again and again into new materials. Left to right: Peter Christensen, Kathryn Loeffler and Brett Helms (Marilyn Chung, Berkeley Lab)

A lesson in plastic

To understand the importance of the discovery of PDK, we need to remember that plastic – as a substance that is not present in nature, like wood for example – has to be made using a process known in the industry as “polymerisation”.
This process starts with monomers, small molecules derived from refining oil, which are then chemically joined to form larger molecules, or polymers.
The plastic is in a liquid state when first produced and is then poured into suitably shaped moulds. Once cooled, it is ready to use.
Other chemicals, additives and dyes can be added during the production process to make each different type of plastic, such as fillers to make the plastic harder and plasticisers to make it more flexible. These added components determine the differences between the various plastic products, for example between plastic bottles and shopping bags. They therefore have an impact on the quality of the plastic and the ability to recycle it.

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Example of polymeric material, structural material consisting of macromolecules, of natural or synthetic origin (chimicare.com)

The integrated recycling system

Non-Recyclable material is a waste product. Plastic is one of the most common materials in the environment, and if it cannot be recycled then the toxicity of the gas that is released during the decomposition process feeds into the degradation of an ecosystem that is already vulnerable.
The most effective waste disposal solution we have to date is the “integrated disposal system”, which is based on two main strands. The first is the recovery of materials through separated collection, which makes it possible to separate out municipal solid waste (MSW) from household hazardous waste (HHW); the second is the isolation of “non-recoverable” waste and sending it to landfill and incinerators.

An invisible problem

As the old saying goes: “Prevention is better than cure”. While useful, waste disposal systems mitigate the “effects” of a problem whose roots lie in the production phase. However, the discovery of DKP makes it possible to separate non-recyclable substances from the recyclable ones.
During recycling, the plastic that is thrown in the relevant bins ends up being mixed together and broken down into small fragments to create a new material. But each fragment maintains its original plastic form (PE, PVC, PET, etc.). The resulting plastic item will therefore be a mixed product, because it will contain all of these fragments, each of which would have been intended for different use. What we end up with is a new product, but one that will not retain the transparency, permeability or strength properties specific to each of its original components.

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Reversible, dynamic covalent diketoenamine bonds. Diketoenamine bonds form spontaneously from triketones and both aromatic and aliphatic amines (Springer Nature)

The new “circular” plastic

The real innovation of PDK plastic is that “intruder” substances can effectively be removed from the plastic. When immersed in an acid, PDK decomposes into monomers, breaking away from the chemical additives that make other plastic materials only partially recyclable.
Going back to our Lego analogy, we might think of constructions made of individual “bricks” instead of “blocks” of them: the possible combinations would be endless.
In the case of plastic recycling, the monomer is the individual brick. By recovering the original monomers, PDK is 100% recyclable and therefore overcomes the limited number of times that a product can be reused, thus making it a “circular material”.
Brett Helms, the main author of the study that led to the findings, said the following in an interview: “We’re at a critical point where we need to think about the infrastructure needed to modernise recycling facilities for future waste sorting and processing. If these facilities were designed to recycle or upcycle PDK and related plastics, then we would be able to more effectively divert plastic from landfills and the oceans”.

WATCH MORE: Energy Snack: plastic-eaters by Eniday Staff

about the author
Paola Arpino
Traveling with myself, through Rome, London and Milan, dreaming that one day I could be writing....