Research Highlight 3 - Adhesives from Waste Baby Diapers

Researchers from the University of Michigan, Prof. Anne J. McNeil and colleagues have recently upcycled baby diapers into a pressure sensitive adhesive, commonly used in office tapes, bandages, sticky notes, removable labels, and so on – which has an expected $13 billion global market by 2023. The superabsorbent crosslinked polymer in diapers – namely a sodium poly(acrylate) crosslinked with ~0.05 mol% of poly(ethylene glycol), was converted into a thermoplastic adhesive – poly(2-ethylhexylacrylate) via open-loop chemical recycling process – a term used for upcycling of polymer waste into a high value material in order to expand their lifetime. The work is published in the journal ‘Nature Communications’[1].

Proctor and Gamble (P&G), the author’s supplier, already has a diaper recycling facility – FaterSMART, for diaper acquisition, steam sterilization, shredding, and separation of the purified raw materials such as cellulosics, superabsorbent polymer, and polyolefins. The similarity in the polymer structure – both are based on polyacrylates, motivated the authors to recycle superabsorbent cross-linked polymer in diapers into a pressure-sensitive adhesive.

Recycling of crosslinked polymers is not that easy. Unlike thermoplastics, they cannot be recycled by applying heat or solvent due to their permanent covalently crosslinked three-dimensional structure. It is even extra challenging to recycle sodium polyacrylate superabsorbent cross-linked polymer due to the side chain degradation of the intermediate depolymerized product, polyacrylic acid – which is known to undergo dehydration and decarboxylation. Thus, main chain scission (C-C bond) to yield the starting monomer ‘acrylic acid’ is less favored. Hence, the author’s chose to upcycle it into another polymer which already serves a purpose in our daily life. "The author’s work presents an elegant example to deal with real world waste – approximately 1.7 million tons of diapers are disposed per annum in landfill – now there is a solution to create value from these waste".

The process involved two major steps – 1) de-crosslinking of superabsorbent polymer via mild acid-catalyzed hydrolysis into a linear polymer, poly(acrylic acid), and 2) its esterification using 2-ethylhexanol to create adhesive. A wide-range of pressure-sensitive adhesives can be accessed by employing an additional sonication step (after decrosslinking - 1st step) for different time intervals, which shortens the polymer chain length by solvodynamic shear forces without affecting its identity. The traditional petrochemical route to synthesize this pressure sensitive adhesive consists of acrylic acid monomer synthesis, and esterification of acrylic acid to 2-ethylhexylacrylate and then its free-radical polymerization. A life-cycle assessment revealed diaper recycling strategy has more environmental benefit than conventional petro-chemical route with 21-25% decrease in global warming potential and cumulative energy demand.

Reference:

[1]. Chazovachii, P. T.; Somers, M. J.; Robo, M. T.; Collias, D. I.; James, M. I.; Marsh, E. N. G.; Zimmerman, P. M.; Alfaro, J. F.; McNeil, A. J. Giving superabsorbent polymers a second life as pressure-sensitive adhesives. Nat. Commun. 2021,12, 4524.

Created by: Sini Nalakathu Kolanadiyil