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Polymers

Reversible polymer chemistry could make a truly recyclable plastic

Polymer forms through click chemistry reactions and then reverts back to monomer building blocks in strong acid

by Sam Lemonick
May 1, 2019 | APPEARED IN VOLUME 97, ISSUE 18

 

09718-scicon7-vitrimer.jpg
Credit: Nat. Chem.
A new recyclable polymer dissolves in sulfuric acid after 12 hours. The vials contain increasing concentrations of sulfuric acid from left (none) to right (5 M).

When recycling plastics, contamination and chemistry conspire to degrade the quality of each recycled generation. In a perfect world, a polymer could be converted to its monomer building blocks, then made into new plastic over and over. Researchers have been working to develop such chemistry, and Brett Helms and colleagues from Lawrence Berkeley National Laboratory now demonstrate a new approach (Nat. Chem. 2019, DOI: 10.1038/s41557-019-0249-2).

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Their polymer is based on a network of reversible diketoenamine bonds formed in a ball mill using click chemistry reactions between triketones and polyamines like tris(2-aminoethyl)amine. Helms says that the resulting plastics, which the group refers to as poly(diketoenamine)s (PDKs), have mechanical properties on par with thermoplastic polyurethanes, which are used in products such as mobile phone cases and shoe soles. PDKs are in a class of plastics called vitrimers that are characterized by a crosslinked polymer network. The materials, which behave like glasses, have been explored as recyclable plastics by other chemists.

To convert the PDKs back to their monomer building blocks, the group uses 5 M sulfuric acid. The researchers could recover more than 90% of the triketone and polyamine monomers after 12 h in the acid. They could then separate the monomers from additives and other polymers left in the acid solution. For example, they could dissolve and extract the triketones and amines from dyed plastics, mixtures of different plastic types, and plastics mixed with flame retardants.

This research shows more of what vitrimers can do, says Eric Drockenmuller, a polymer chemist at Claude Bernard University Lyon 1. “Besides showing properties typical of the best vitrimers developed so far,” he says, Helms’ group also demonstrated a new approach to so-called closed-loop recycling.

But Jinwen Zhang, who studies vitrimers at Washington State University, points out using a strong acid comes with its own environmental concerns in terms of waste disposal. He also says for PDKs to be commercially practical, they will need a cheap feedstock. Helms says that his team has made renewable feedstocks a priority from the start.

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Comments
Jorge Soto (May 8, 2019 2:14 PM)
Having worked in the chemical industry for a long time, the first question I have is (never mind kinetics):
1. How are the monomers removed from the sulfuric acid solution? You will need to use large amounts of base, generating a large amount of salt. How about the disposal of the salts? Recycle as ...?
2. What will be the cost associated with the steps needed to achieve the neutralization in 1) above?
3. What market is such poly(diketoenamine) material getting into? In other words, what is the size of said market compared to the markets of PE, PS, PP, SBR, nylons and similar large markets?
4. I may be wrong but the material seems to be a thermoset. Or is it a thermoplastic? Therefore, again, what is the market it will be getting into? Food, automobile, transportation, ... applications? Hoses, coatings, paints, ....
Thanks!
Hew Turner (May 23, 2019 8:04 PM)
Would this application even be possible in regards to super absorbent polymer products?
Arvind Kr Gupta (May 9, 2019 1:32 AM)
Dear Team,
Great invention. However, kindly let us know:
1. How to dispose of the Sulfuric Acid with Dissolved plastic? What is the implication on soil and Water where this material will be disposed off - treated or untreated?
2. Can this plastic or objects made from this plastic be painted?
3. What is the strength of this plastic?
4. typically which Industries can use this plastic?

Thanks and Regards,
Patrick Breen (May 9, 2019 8:10 AM)
The header at the top of this article says it appeared in Volume 97, number 18. That's a lie, it is not there, nor in the next issue, number 19, or the subsequent one, number 17. Profoundly irritating, it appears that this article in fact was never published in the periodical. Why is that?

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