Researchers have successfully broken plastic into its molecular building blocks and recovered over 90% of them.
It’s a first step towards genuine plastic recycling.
The chemical industry has a long tradition of producing polymers. This involves turning small molecular building blocks into long chains of molecules that bond together. Polymers are the basis of all kinds of everyday plastics, such as PET and polyurethane.
However, while the formation of polymers is well established and well researched, scientists have given little attention to how polymer chains are broken down (a process called depolymerization) to recover their individual building blocks, called monomers.
Breaking down polymers is a complex process. Whether a polymer can be broken back down at all into its constituent parts depends on which of the different polymer manufacturing processes created it. In addition, the depolymerization processes used so far require a lot of energy, which has made them too expensive. And, recycled polymers are usually only used to make low-value products.
Athina Anastasaki, professor of polymeric materials at ETH Zurich, wants to change this. Her goal is producing polymers that can be easily broken down into their building blocks so that they can be fully recycled.
Anastasaki has taken a first step in this direction: A study by her group appears in the Journal of the American Chemical Society. In it, Anastasaki and her colleagues show that they can break down certain polymers into their basic building blocks—monomers—and recycle them for use in materials for further applications.
The polymers broken down are polymethacrylates (e.g. Plexiglass) that were produced using a specific polymerization technique called reversible addition-fragmentation chain-transfer polymerization, otherwise known as RAFT. This relatively new method, which is now also attracting the interest of industry, produces polymer chains of uniform length.
The researchers have succeeded in recovering up to 92% of the building blocks of polymethacrylates without adding a catalyst that would enable or accelerate the reaction.
“Our method could conceivably be developed even further to involve the use of a catalyst. This could increase the amount recovered even more,” says Anastasaki.
The chemical group present at the end of a polymer chain is crucial for the polymer’s breakdown. By heating the polymer solvent mixture to 120°C, the researchers created what are referred to as “radicals” at the end of a polymethacrylate chain, which triggered the depolymerization. Researchers at the Australian National University in Canberra confirmed the results mathematically.
According to Anastasaki, the building blocks recovered in this way can produce the same polymer or a completely different product—an insoluble hydrogel that can also be broken down into its monomers. The newly created products are of similar quality to the original ones. This is in contrast to previous products made from recycled polymers.
But there is a catch: “Products made with RAFT polymerization are more expensive than conventional polymers,” says Anastasaki. To address this drawback, she and her group are working to expand the method for large-scale use, which will make it more competitive and the resulting products cheaper. The researchers also aim to increase the amount retrieved and to recover all the building blocks of a polymer.
Anastasaki is also researching whether other polymers can be depolymerized. She is particularly interested in polystyrene, a widespread, low-cost plastic that has many everyday uses, such as Styrofoam.
Even if this new method raises hopes of solving humankind’s plastic waste problem, Anastasaki dismisses the idea for the time being. There is no quick fix to the problem. “It will take a lot of time and research before the process is established in the chemical industry,” she says. Nor will the method get rid of plastic waste: today’s polymers cannot be broken down in this way. Some new, suitable polymers have to come into circulation before their building blocks can be recovered. But the method has one advantage: its introduction and use require no new chemical plants.
“We are only at the beginning of our research into depolymerization. There are over 30,000 studies on developing new polymerization strategies, with only a handful of them addressing the subject of monomer recovery,” says Anastasaki.
Source: ETH Zurich