University of Michigan team, in partnership with Procter & Gamble, has developed a three-step process untangle superabsorbent polymers rom diapers and recycle them into materials similar to the gooey adhesives used in sticky notes and bandages. Their results are published in Nature Communications.
 
Anne McNeil, professor of chemistry and macromolecular science and engineering, and Takunda Chazovachii, who recently graduated from U-M with his doctoral degree in polymer chemistry, worked with Procter & Gamble to develop this process. The method needed to be energy-efficient and able to be deployed on an industrial scale.

Anne McNeil, professor of chemistry and macromolecular science and engineering, and Takunda Chazovachii, who recently graduated from U-M with his doctoral degree in polymer chemistry, worked with Procter & Gamble to develop a three-step process that turns superabsorbent polymers into a reusable material—in this case, adhesives. The method needed to be energy-efficient and able to be deployed on an industrial scale.

“Mechanical recycling is what most people think about: You separate different plastics based on their identities, chop them up into small pieces, melt them and reuse them, which lowers the quality of the product,” said U-M chemist Anne McNeil, corresponding author of the paper.
Mechanical recycling leads to lower quality materials because different companies’ plastics are constructed differently: The polymers can be different chain lengths or altered with different additives and dyes.

“There’s just so many problems, everything usually gets downcycled and ends up as carpet fibers or park benches,” said McNeil, whose lab focuses on the chemical recycling of plastics. “Chemical recycling is this idea of using chemistry and chemical transformations to make a value-added material, or at least a material as valuable as the original.”

The qualities that generally make plastics desirable, such as toughness and durability, are also responsible for their difficulty in recycling. In particular, polymers are difficult to break down because they are held together by stable bonds.

 “Superabsorbent polymers are particularly difficult to recycle because they are designed to resist degradation and retain water permanently,” Chazovachii said. “The superabsorbent polymers and adhesives are both derived from acrylic acid. This common origin inspired our recycling idea.”

The polymers in superabsorbent materials look like a loosely woven fishing net, McNeil says, except instead of a honeycomb mesh, these polymers have a crosslink every 2,000 units, which is more than enough to create an insoluble network structure. To recycle these materials, the researchers needed to find a way to delink the network polymer into water soluble chains. Chazovachii found that when these polymers are heated either in the presence of acid or base, their crosslinks are broken.

The results of this study are published in Nature Communications.