Science

Use chemical recycling to prevent millions of tonnes of waste

Credit: Agilent / CC0 public domain

Every year, 3.5 million tons of moist diapers are dumped in landfills.


The super-absorbent material inside these diapers consists of a matrix of polymers that expand when moisture hits the diaper.The polymer is a long chain of repeating units, in this case the absorbent of the diaper polymer Polyacrylic acid.

A team at the University of Michigan has developed a technology to unravel these absorbent polymers and recycle them into materials similar to the sticky adhesives used in sticky notes and bandages.The results are published in Nature Communications..

Broadly speaking, recycling can be divided into mechanical recycling and chemical recycling.

“Mechanical recycling is what most people think. plastic Based on their identities, chopping them, melting them and reusing them will reduce the quality of the product, “said Anne McNeil, a UM chemist who is the corresponding author of this treatise.

Mechanical recycling reduces the quality of the material due to the different plastic structures of different companies. Polymers can have different chain lengths or can be modified with different additives and dyes.

“There are so many problems that usually everything is downcycled into carpet fibers and park benches,” said McNeill, a lab focused on chemical recycling of plastics. .. “Chemical recycling is the idea of ​​using chemistry and chemical transformations to create value-added materials, or at least as valuable as the original materials.”

The quality that makes plastics generally desirable, such as toughness and durability, also contributes to the difficulty of recycling. In particular, polymers are difficult to decompose because they are bonded by stable bonds.

McNeil, a professor of chemistry and polymer science and engineering, and Takunda Chazovachii, a recent graduate of UM with a PhD in polymer chemistry, are working with Procter & Gamble to turn highly water-absorbent resins into reusable materials. We have developed a three-step process to change. In this case, glue. This method needed to be energy efficient and deployable on an industrial scale.

“Highly absorbent resins are especially difficult to recycle because they are designed to resist deterioration and retain water forever,” says Chazovachii. “Both highly absorbent resins and adhesives are derived from acrylic acid. This common origin influenced our recycling ideas.”

According to McNeill, the polymer of superabsorbent material is like a loosely woven fishing net, except that instead of honeycomb mesh, there are crosslinks every 2,000 units, which is more than enough to create an insoluble network structure. Looks like. To recycle these materials, researchers needed to find a way to separate the network polymer into water-soluble chains. Chazovachii found that when these polymers were heated in the presence of acids or bases, their crosslinks were broken.

Researchers also needed to determine if these processes were feasible on an industrial scale. A life cycle assessment was conducted by collaborator Madeline Somers, a research assistant at the UM Graham Sustainability Institute, and Jose Alfaro, a researcher at the UM School for Environment and Sustainability. When they decrosslink a polymer using this acid method, they reduce the global warming potential by a factor of 10 based on carbon dioxide emissions, reducing the energy required by a factor of 10 of a base-mediated approach. I learned to be.

Second, researchers needed to shorten the long chains of polymers in the material in order to produce different types of adhesives. Chazovachii found that sonication, which uses small bursting bubbles to cut polymer chains, allows the chains to be broken into small pieces without altering the chemistry of the chains.

“What we really like about this method is that it’s a gentle and simple mechanical process,” he said. “It destroys the polymer, but leaves its components or acid groups intact, so you can actually use it to perform other reactions.”

Finally, Chazovachii converted the acid groups of the polymer chains into ester groups with the help of professor of chemistry Paul Zimmerman and his student Michael Robo. This changes the properties from water-soluble to organic-soluble, making it sticky like an adhesive. Additional Bonus: The reagents used in this reaction also act as solvents and can be recycled and reused. After testing the properties of the adhesive, Chazovachii realized that sonication was not required to target certain types of adhesive, further simplifying the approach.

Finally, researchers needed to show that developing adhesives from recycled polymers is easier on earth than making adhesives from petroleum, which is a typical route. Comparing the route to glue with the traditional one, the authors found that the route with recycled diapers reduced the global warming potential by 22% and energy by 25%.

Researchers say they are dealing with diapers that are already clean, but more and more companies are cleaning used diapers, such as Procter & Gamble’s affiliate Fatter SMART. In addition, Chazovachii says the conditions for chemical recycling will kill the surviving bacteria.

McNeill says he wants synthetic chemists who react with small molecules to pay attention to polymers.

“This is just one treatise, but I think it’s a really open opportunity for synthetic chemists to influence real-life problems, so I’ve moved most of my work in this direction,” McNeil said. Stated. “The global plastic problem is so big that chemists can play a very important role in rethinking what to do with this waste, so I want more people to think about it. think.”


Chemists create renewable plant-based polymers


For more information:
P. Takunda Chazovachii et al, Gives Highly Absorbent Resin Second Life as a Pressure Sensitive Adhesive, Nature Communications (2021). DOI: 10.1038 / s41467-021-24488-9

Quote: Turn diapers into sticky notes: Use chemical recycling to prevent millions of tonnes of waste (27 July 2021) 27 July 2021 https://phys.org/news/2021- Obtained from 07-diapers-sticky-chemical-recycling-millions.html

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