Tackling the “Stickies” Problem When Recycling Pressure-Sensitive Adhesives with Degradable, Hydrophilic Thioester Copolymer Networks

Pressure-sensitive adhesives are used widely in tapes and labels but create recycling challenges because their cross-linked polymer network is insoluble in solvents. When plastics and paper are recycled, they are soaked in hot aqueous alkaline solutions. Processing breaks adhesives into lumps of insoluble gel (“stickies”) that clog machinery, contaminate repulped paper, and divert recyclable material to landfill. Previously, dibenzo[c,e]oxepin-5(7H)-thione (DOT) was copolymerized with an acrylate monomer to introduce a cleavable linkage that was degradable only in amine solutions, which limited its utility in present-day recycling. Here, we address the stickies problem by copolymerizing hydrophilic poly(ethylene glycol) methyl ether acrylate (PEGA) with DOT and methyl acrylate (MA) to create a water-soluble copolymer that swells in aqueous NaOH solutions to facilitate chemical degradation by hydrolysis. Sparse cross-linking was achieved by the photo-cross-linker 4-acryloyloxy benzophenone (ABP). The PEGA:MA monomer ratio, cross-linking density, and chain length were varied to optimize the balance of viscous and elastic properties needed for good adhesion (achieving peel force: 4.5 ± 0.7 N/25 mm, adhesion energy: 140 ± 10 Jm2–, and shear holding time: 5 ± 1 min). A copolymer with a low gel fraction and a loose cross-linked network resulted in higher tack adhesion energies but lower shear holding times. With relative humidity <50%, water sorption is low, regardless of PEGA:MA ratio. Under higher relative humidities, there is greater sorption but water diffusivity slows. A cross-linked poly(PEGA-MA-ABP-DOT) network degrades into soluble molecular fragments in 0.5 M NaOH solutions at room temperature─leading to zero gel and avoidance of stickies. In an adhesive, substrate detachment was observed after only 4 min in an alkaline bath at room temperature, reducing energy demand. In contrast, a nonhydrophilic butyl acrylate-DOT adhesive, which did not swell in water, did not detach. Our poly(PEGA-MA-ABP-DOT) copolymer combines adequate adhesion with rapid, alkaline-triggered removal, providing a promising route toward greater recyclability.