McNeil Lab: Recycling disposable diapers into sticky notes

In a world completely dependent on synthetic polymer materials that do not naturally degrade, it is becoming increasingly important to find innovative ways to recycle and/or repurpose these materials.

Now, researchers from the University of Michigan, in collaboration with Procter & Gamble (P&G), have identified an energy-efficient way to repurpose a common polymer into another useful material, opening the door to a new area of research in chemical recycling methods. This development in polymer processing could reduce single-use product waste, and a reduction of reliance on petroleum for polymer materials. The results are published in an article in Nature Communications.

“Superabsorbent polymer (SAP), made with crosslinked poly(acrylic acid), or PAA, is useful because it can absorb 10,000% of its weight in water, but once you use that material and dispose it in a landfill, it’s going to sit there forever,” said P. Takunda Chazovachii, lead author on the project, who did this research while a graduate student in  U-M Chemistry's McNeil group.

Polymers like PAA are long molecules made up of smaller building blocks called monomers. PAA is synthesized from the monomer (acrylic acid) to form the polymer that becomes the SAP used in diapers and feminine hygiene products. The global annual production of SAP is over 2 million tons, and about 97% of that is used in baby care diapers, adult incontinence pads, and feminine hygiene products.

P&G expressed a desire to develop environmentally beneficial end of life options for SAP,  Professor Anne McNeil explained. “They wanted us to take the absorbent material in diapers and either break it all the way down to a monomer and bring it back up to a polymer again so they could chemically recycle the SAP, or repurpose it to other materials.”

However, breaking down a polymer into its monomeric form can be extremely difficult because of the stable bonds that hold the polymer together. McNeil explained that it ended up being thermodynamically challenging to break the backbone of the polymer all the way down to the monomeric form. Instead, the researchers designed a series of steps to create a different type of polymer from the original. “We decided to do a route where we convert one value-added material into a different value-added material, so it’s still kind of chemical recycling. We break some bonds to break down the polymer a little bit, make it a bit more soluble, and then we functionalize it,” McNeil said.

For inspiration, the researchers began looking at other polymers that are made with similar building blocks. “We noted that the monomer that is used to make this polymer, acrylic acid, is also a building block for other polymers,” said Chazovachii. “The question we asked ourselves was: ‘what if we took the polymerized form of acrylic acid (polyacrylic acid) from the SAP and functionalized it into something that is made from acrylic acid?’ In this case, we looked at paints and pressure-sensitive adhesives.”

Pressure-sensitive adhesives are made using acrylic acid monomers, which are esterified into alkyl acrylate prior to polymerization. Acrylic acid monomers are traditionally extracted from petroleum sources, a nonrenewable resource.  To produce pressure-sensitive adhesives, the polymerization of alkyl acrylate is the last step. In this work, instead of polymerizing alkyl acrylate after it has been esterified, the esterification step is performed last, using the repurposed poly(acrylic acid) polymer from the diaper materials.  Because of this switch, the super absorbent polymer is recycled into a different useful polymer, giving it a second life.

Importantly, a life cycle analysis was performed in order to prove that the repurposing process is more efficient than a traditional petroleum-based synthesis with regards to both carbon dioxide emissions and the amount of energy required for the synthesis. The life cycle analysis showed that there was a 22% reduction in global warming potential and a 25% reduction in cumulative energy demand, as compared to the traditional pressure-sensitive adhesive synthesis method that creates polymer from petroleum-based sources. Chazovachii explained that “it’s not enough to just create a recycling pathway; a comparative assessment of the economic and environmental implications of the new methodology versus the conventional will be necessary.”

The broader implications of this work extend beyond this specific route of repurposing of superabsorbent polymers to pressure-sensitive adhesives. In working on this project, the researchers have identified general roadblocks in the field of polymer recycling and investigated solutions to these problems. For example, initial attempts to simply prepare aqueous solutions with the SAP were not successful due to the superabsorbent properties of the polymer – it simply absorbed all the water. Additionally, to further complicate the repurposing goal, this polymer is cross-linked, meaning that multiple strands of polymer are linked together. A hydrolysis step resolved both problems. Solutions to these issues could be useful as other researchers look to repurpose other types of polymer materials.

The effort to collect the superabsorbent polymers in disposable products is already underway. Some companies, such as P&G affiliate FaterSMART, are already isolating the requisite superabsorbent materials that have been used and would otherwise end in a landfill. Once PAA is isolated, the methods published in this article will turn it into the pressure-sensitive adhesives used in tapes, bandages, and sticky notes. For consumers who may be hesitant about used SAPs ending up in other consumable items, Chazovachii explained that the repurposing method contains harsh conditions that would not allow any bacteria to survive the processing.

There are nearly 2 million metric tons of superabsorbent polymer waste added to landfills each year. This repurposing method could be an industrially scalable and sustainable way to reduce that number by giving SAPs a second life. Chemical recycling is a relatively small field of study due to the inherent difficulties of breaking down stable polymer molecules, explains McNeil. However, this advancement, with its method development and efficiency, provides some remedies to the issues that are inherent to polymer repurposing and increases visibility to the field.

P. Takunda Chazovachii earned his PhD (July 2021) in the Department of Chemistry at the University of Michigan. Anne McNeil is the Carol A. Fierke Collegiate Professor of Chemistry and Arthur F. Thurnau Professor of Chemistry. This project was performed in collaboration with Dr. Dimitris Collias and Dr. Martin James, scientists from P&G, and with the collaboration of the groups of Paul M. Zimmerman, assistant professor of chemistry,  and Jose Alfaro, assistant clinical professor in  the School for Environment and Sustainability.