Super Absorbent Polymer Recycling to Pressure Sensitive Adhesives

University of Michigan Background
Crosslinked sodium polyacrylate-based superabsorbent polymers (SAP) are used in a variety of common consumer products, including disposable personal hygiene products such as baby diapers, adult incontinence products, and feminine hygiene products. The global annual production of this superabsorbent material is estimated to be over two million metric tons, with disposable diapers claiming 74% of the market. Because SAPs are used predominately in disposable articles, a high percentage of SAPs are disposed of in landfills. Because mechanical recycling cannot be used with polymers that do not reversibly melt (e.g., crosslinked SAPs), most diaper recycling efforts have focused on the cellulosic components and ignored the SAPs.
Technology Overview
Researchers at the University of Michigan, in collaboration with a Fortune 500 consumer products company, have developed a practical method to upcycle sodium polyacrylate-based SAPs to pressure sensitive adhesives (PSAs). The open-loop recycling approach involves de-crosslinking the SAP, an optional chain-shortening step, and functionalization via esterification to produce the PSA. The resulting materials exhibit low-to-medium storage and loss moduli and are structurally nearly identical to commercially available pressure-sensitive adhesives. They are suitable for use as general-purpose adhesives for products including tapes, bandages, and sticky notes.
Furthermore, a life cycle analysis demonstrated that the adhesives synthesized via this approach outcompete the same materials derived from petroleum feedstocks on nearly every metric, including carbon dioxide emissions and cumulative energy demand. This potentially scalable route to recycling diapers and feminine hygiene products could keep two million metric tons of polymer waste from landfills each year and simultaneously reduce carbon emissions associated with conventional PSAs.
Globally, there have been significant efforts toward recycling the other components of diapers. For example, FaterSMART, has developed and implemented a diaper recycling facility that includes used diaper acquisition, steam sterilization, shredding, and separation into the purified raw materials (cellulosics, superabsorbent polymer, and polyolefins). The Michigan technology solves an important technical gap faced by such efforts regarding recycling of the separated SAP component.
Benefits

Chemical recycling provides scalable, practical, and flexible process
Performance and structure of the adhesives produced are very similar to commercially available PSAs
Environmental benefit through diversion of polymer waste from landfills
Sustainability benefit over petroleum derived adhesives via reduced CO2 emissions and reduced energy demand

Applications

Recycling of superabsorbent polymers from disposable hygiene products
Upcycling of baby diapers, feminine hygiene products, and adult incontinence products
Production of pressure sensitive adhesives for tapes, bandages, sticky notes, etc.

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