'Infinitely' recyclable polymer shows practical properties of plastics

April 26, 2018, Colorado State University
Eugene Chen's lab has developed a completely recyclable polymer, shown here. Credit: Bill Cotton/Colorado State University

The world fell in love with plastics because they're cheap, convenient, lightweight and long- lasting. For these same reasons, plastics are now trashing the Earth.

Colorado State University chemists have announced in the journal Science another major step toward waste-free, sustainable materials that could one day compete with conventional plastics. Led by Eugene Chen, professor in the Department of Chemistry, they have discovered a with many of the same characteristics we enjoy in plastics, such as light weight, heat resistance, strength and durability. But the new polymer, unlike typical petroleum plastics, can be converted back to its original small-molecule state for complete chemical recyclability. This can be accomplished without the use of toxic chemicals or intensive lab procedures.

Polymers are a broad class of materials characterized by long chains of chemically bonded, repeating molecular units called monomers. Synthetic polymers today include plastics, as well as fibers, ceramics, rubbers, coatings, and many other commercial products.

The work builds on a previous generation of a chemically recyclable polymer Chen's lab first demonstrated in 2015. Making the old version required extremely cold conditions that would have limited its industrial potential. The previous polymer also had low and molecular weight, and, while plastic-like, was relatively soft.

But the fundamental knowledge gained from that study was invaluable, Chen said. It led to a design principle for developing future-generation polymers that not only are chemically recyclable, but also exhibit robust practical properties.

The new, much-improved polymer structure resolves the issues of the first-generation material. The monomer can be conveniently polymerized under environmentally friendly, industrially realistic conditions: solvent-free, at room temperature, with just a few minutes of reaction time and only a trace amount of catalyst. The resulting material has a high molecular weight, thermal stability and crystallinity, and mechanical properties that perform very much like a . Most importantly, the polymer can be recycled back to its original, monomeric state under mild lab conditions, using a catalyst. Without need for further purification, the monomer can be re-polymerized, thus establishing what Chen calls a circular materials life cycle.

This piece of innovative chemistry has Chen and his colleagues excited for a future in which new, green plastics, rather than surviving in landfills and oceans for millions of years, can be simply placed in a reactor and, in chemical parlance, de-polymerized to recover their value - not possible for today's petroleum plastics. Back at its chemical starting point, the material could be used over and over again - completely redefining what it means to "recycle."

"The polymers can be chemically recycled and reused, in principle, infinitely," Chen said.

Chen stresses that the new polymer technology has only been demonstrated at the academic lab scale. There is still much work to be done to perfect the patent-pending monomer and polymer production processes he and colleagues have invented.

With the help of a seed grant from CSU Ventures, the chemists are optimizing their monomer synthesis process and developing, new, even more cost-effective routes to such polymers. They're also working on scalability issues on their monomer-polymer-monomer recycling setup, while further researching new chemical structures for even better recyclable .

"It would be our dream to see this chemically recyclable polymer technology materialize in the marketplace," Chen said.

Explore further: Recyclable bioplastics cooled down, cooked up in chem lab

More information: J.-B. Zhu el al., "A synthetic polymer system with repeatable recyclability," Science (2018). science.sciencemag.org/cgi/doi … 1126/science.aar5498

H. Sardon el al., "Plastics recycling with a difference," Science (2018). science.sciencemag.org/cgi/doi … 1126/science.aat4997

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6 comments

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Eikka
4.7 / 5 (3) Apr 27, 2018
"completely redefining what it means to "recycle.""


It doesn't re-define what it means to recycle - everybody else is just abusing the term when they really mean "downcycle" - like when people talk about recycling concrete or glass when they just grind it down to rubble and use it for land construction.

Recycling has always meant using scrap to make more of the same or equal product. Downcycling means producing items of lesser value, and upcycling means producing items of greater value. There's just a great amount of greenwashing going on where people call downcycling recycling to get good press, public funding, grants, or free raw materias through subsidized collection programs.

Eikka
5 / 5 (2) Apr 27, 2018
Zhu et al. developed a polymer based on a five-membered ring cyclic monomer derived from γ-butyrolactone


If this plastic has similiar properties to polycaprolactones (PCL), then it's also biodegradeable.

This can be a blessing and a curse. I remember reading about a Swedish experiment with PCL plastic shopping bags, which had to be refridgerated in storage because any moisture from the environment left the piles of bags rotting away before they even reached the stores.
TrollBane
not rated yet Apr 27, 2018
So how does this affect the advice for new graduates? ;)
Eikka
not rated yet Apr 28, 2018
Even common polyethylene terephthalate in soft drink bottles can be depolymerized back to the starting monomers


Not easily or cheaply, which is why PET bottles are hardly recycled at all. They melt it and blow it into packaging peanuts or extrude plastic filaments from it.

The biggest issue with plastics recycling is that it's very hard to separate them and keep them separate in the trash cycle. Contaminants and other materials like paper labels have to be washed off before they can be used, because the processes used for hydrolyzing the plastic would also melt these other substances and create a mixture of monomers that is tainted and unsuitable for producing the same plastic. The quality degrades on each pass, so a maximum of about 10% recycled material can be put back into the cycle.

Eikka
not rated yet Apr 28, 2018
Same thing as with glass recycling. Everyone uses a slightly different formulation of glass for different purposes, but you can't see the chemical composition of the glass by looking at it, so it's nearly impossible for consumers to sort them out - and even if they did, someone would still chuck the wrong type of glass in the collection box and the whole effort would go in vain.

So recycling glass bottles back into glass bottles would quickly result in bottles that are off-color, leach dangerous chemicals, or break in use.
Eikka
not rated yet May 04, 2018

5.9 million tons of flake.
500,000 tons to produce bottles


So about 8% recycling rate.

Again, a small amount of recycled PET can be put back through the process to produce clear PET for bottles and product packaging, but more reduces clarity because of the contaminants and degraded polymers, so most of the "recycled" PET bottles are used for lower value stuff like polyester fibre to make shoddy clothes, brooms and brushes, or pillow stuffings. Then after that it's dumped or burned.

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