Nappies stacked in shop shelves

Superabsorbent nappy polymers could be recycled into stationery

Image credit: Dreamstime

Researchers from the University of Michigan have developed a technique to untangle the chains that make up the superabsorbent polymers (SAPs) in personal hygiene products such as sanitary towels and nappies. This could allow them to be recycled into materials similar to the gooey adhesives used in Post-it Notes and bandages.

The study focused on disposable nappies, of which 3.5 million tonnes end up in landfill every year. The superabsorbent properties of nappies come from SAPs: a matrix of polymers which expand when damp. In nappies, the material is based on polyacrylic acid.

There are two broad types of recycling: mechanical recycling and chemical recycling. Mechanical recycling – by far the most common – involves separating plastics by type, shredding them, melting them, and forming fresh feedstock. Although this feedstock is still useful, it is lower quality than raw plastic feedstock because it contains plastic manufactured by various companies containing all sorts of polymers.

“There’s just so many problems, everything usually gets down-cycled and ends up as carpet fibres or park benches,” said Professor Anne McNeil, a polymers expert and co-author of the Nature Communications paper. “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 make plastics so useful, such as their durability, also cause difficulties in recycling. For instance, polymers are hard to break down because they are held together by stable bonds.

Dr Takunda Chazovachii, who worked with McNeil and Proctor & Gamble on the study, explained: “[SAPs] are particularly difficult to recycle because they are designed to resist degradation and retain water permanently. The SAPs and adhesives both derive from acrylic acid. This common origin inspired our recycling idea.”

The polymers in SAPs resemble a loosely woven fishing net with a crosslink every 2,000 units, more than enough to create an insoluble network structure. To recycle these materials, the Michigan chemists had to find a method of delinking the structure into water-soluble chains.

Chazovachii found that when these SAPs are heated in the presence of acid or base, their crosslinks are broken; this formed the basis of a three-step process to turn SAPs into a reusable material. However, they had to determine whether this initial process would be energy efficient and scalable. They found that using acid to separate the polymers would exhibit a 10-times lower global-warming potential (based on CO2 release) and would require 10 times less energy than the base-mediated approach.

Next, they shortened the long polymer chains through sonication; this meant using tiny bursting air bubbles to break the chains. Sonication allows then to cut the chains without changing their chemical properties. “What we really liked about this method is that it is a mild and simple mechanical process,” said Chazovachii. “It breaks the polymer but leaves its building blocks, or acid groups, intact, so you can actually do other reactions with it.”

The final step of the recycling process involves converting acid groups on the polymer chains to ester groups (containing a carbon atom bound to three other atoms); this changes the properties from water soluble to organic soluble, and they become tacky, like an adhesive. After testing the adhesive, the researchers realised that sonication wasn’t necessary to target one type of adhesive, streamlining the approach.

If the idea of using recycled nappies sounds unpleasant, there is no real cause for concern; SAPs undergoing this multi-stage process are 'cleaned', as the conditions of the chemical recycling kill surviving bacteria.

Finally, they demonstrated that developing these gooey adhesives from recycled polymers was more sustainable than making them from raw petroleum. They found a 22 per cent reduction in global-warming potential and 25 per cent reduction in energy for their recycling process compared with the conventional process. McNeil said she hopes that synthetic chemists working on reactions for small molecules may turn their attention to polymers.

“This is just one paper, but I’ve moved most of my research in this direction because I think it’s a really open opportunity for synthetic chemists to make an impact on a real-world problem,” she said. “I want more people to be thinking about this because the global plastics problem is so huge and chemists can play a really important role in reimagining what we do with this waste.”

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