Development of Degradable Thioester-Functional Poly(n-Butyl Acrylate) Networks for Sustainable Pressure-Sensitive Adhesives

Pressure-sensitive adhesives (PSAs) are essential components in labels, adhesive tapes, and graphics and their ease of use, cost-effectiveness, and light weight compared to other alternatives have made them indispensable, especially in packaging applications.   PSAs consist of viscoelastic polymers that need to be liquid enough to wet a surface quickly and elastic enough to support stress, particularly in shear. These properties are typically achieved through the crosslinking of low-glass transition temperature polymers which involves covalent bonds that are critical in maintaining the cohesion and mechanical strength of the adhesive. However, this irreversible crosslinking is a significant challenge in recycling processes, specially for cardboard and glass because when removed from surfaces the adhesives retain their insoluble, crosslinked networks which results in the formation of “stickies.” That can complicate the recycling and repulping processes and increases reprocessing and waste disposal costs. Despite various chemical treatments, these insoluble “stickies” persist which lead higher costs and inefficiency in the recycling industry. Finding a solution requires the development of novel PSA materials that can degrade into soluble products, which facilitate easier and more effective in recycling. Moreover, to overcome the environmental challenge, researchers have tried different degradable PSAs such as polycarbonate-based PSAs made through cobalt-catalyzed copolymerization and polyester ABA triblock copolymers prepared via lactone ring-opening polymerization. However, these approaches often required extended timeframes and environmentally hazardous chemicals, which made them impractical for large-scale recycling operations. Moreover, the majority of industrial PSAs are synthesized using radical polymerization of acrylates due to their superior properties, including oxidation resistance, transparency, and tunable viscoelasticity. However, developing degradable PSAs entirely through radical polymerization is not possible because effective degradable PSAs need to ensure complete network degradation and selective, efficient cleavage of the crosslinks without compromising adhesive properties. To this end, new study published in the Journal Angewandte Chemie International Edition and conducted by Rohani Abu Bakar, Kyle S. Hepburn, Prof. Joseph Keddie, and Dr. Peter Roth from the University of Surrey in the UK  developed degradable PSAs that maintain the required performance during use but can be degraded after use into fully soluble products.

The researchers began by synthesizing the ABP photo-crosslinker and then copolymerized it with n-butyl acrylate (BA) and DOT through free-radical solution polymerization. The authors characterized the resulting copolymers, designated as BA-ABPx-DOTy, using ¹H NMR and FT-IR spectroscopy to confirm the successful incorporation of the desired functional groups. The molecular weights of the copolymers were measured using Size Exclusion Chromatography (SEC), and their glass transition temperatures were determined, showing values typical for PSAs around -45°C. They optimized the adhesive properties by varying the molar content of the ABP crosslinker and conducted probe tack and peel strength measurements to determine the optimal crosslinker content. The authors’ findings showed that increasing the ABP content led to higher plateau stress and shorter fibrillation plateaus which indicate an increase in the elastic modulus and crosslinking density. According to the authors, the copolymer with 0.05 mol% ABP exhibited the highest tack adhesion energy which suggest optimal adhesive properties and also if DOT was incorporated into the copolymers even higher tack adhesion energy and the formation of more extendable fibrils. The BA-ABP0.05-DOT0.25 copolymer showed the greatest tack adhesion energy and an ideal balance between elastic and viscous components which demonstrated superior adhesive performance compared to the control BA-ABPx copolymers without DOT. Moreover, the researchers wanted to confirm the degradability of the copolymers, so they subjected photo-crosslinked films of BA-ABPx-DOTy to aminolysis and thiolysis and found the BA-ABP0.05-DOT0.25 copolymer to have significant decrease in gel content from 20 wt% to less than 1 wt%, which indicates complete network degradation.  Furthermore, the team investigated the changes in adhesive properties upon degradation. Probe tack measurements before and after degradation showed a substantial reduction in tack adhesion energy and by this confirmed the loss of adhesive properties.

The researchers also tested the practical application of the degradable PSAs by preparing model labels and measuring their peel strength before and after degradation. Labels made with BA-ABP0.05-DOT0.25 showed spontaneous detachment from substrates within minutes when treated with ammonia or N-acetylcysteine, while the control BA-ABP0.05 labels took significantly longer to detach. After 24 hours of degradation, the peel strength of the DOT-containing labels was either immeasurable due to insufficient adhesion or significantly reduced, confirming the complete loss of adhesive properties. Additionally, the degradation was visually confirmed using dye-labeled copolymers where the prepared films of BA-ABP0.05-DOT0.25-NBDA0.25 and BA-ABP0.05-NBDA0.25 on glass slides and observed them under UV light before and after degradation. The DOT-containing film showed complete dissolution and disappearance of fluorescence after immersion in n-propylamine, while the control film retained residual fluorescence, further validating the effectiveness of the degradable PSA. In conclusion, Prof. Joseph Keddie and colleagues developed degradable PSAs which can be broken down on demand into soluble products under environmentally benign conditions, using non-toxic substances such as bio-ethanol and dietary supplements like N-acetylcysteine, which prevent the formation of “stickies.” This property is crucial for improving the recycling efficiency of glass and cardboard, leading to higher quality recycled materials and less waste. Moreover, their PSA can lead to fewer interruptions and lower maintenance requirements, thereby decrease overall recycling costs.