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Pressure-sensitive adhesives are viscoelastic polymers used in tapes, labels, graphics and a variety of packaging. When the adhesives remain attached to paper and card, they make the re-pulping process more difficult, devaluing the recycled product and can even damage machinery. Similarly, there are difficulties in removing labels from glass bottles.
Caption: Example of peel test using office paper as a backing to correlate with label application
The challenge of recycling pressure-sensitive adhesives
The permanently tacky adhesives used in labels, stickers and tape (including domestic sticky tape) are called pressure-sensitive adhesives, or PSAs, because they adhere under light pressure, such as from a fingertip. Their required viscoelastic properties arise from the entanglement of their constituent polymer chains with some covalent crosslinks irreversibly binding the chains together. Unfortunately, the crosslinked network remains intact even in organic solvents and under harsh chemical conditions, which typically leaves behind adhesive residue on an adhering surface.
A high-performing PSA must combine viscous and elastic properties. It should be able to flow upon meeting a rough surface to make conformal contact. When debonding from a surface, such as when being peeled, it should be extended into strands, called fibrils, as a means to dissipate energy thereby making it more difficult to remove. A PSA must support stress under shear, but it must not be too rigid to make contact with a surface and to allow deformation when debonding. The presence of chemical bonds between chains, known as crosslinks, raises the elasticity of an adhesive. In the absence of crosslinks, the adhesive can fail easily when being debonded. That is why crosslinking is essential in most PSAs, including those based on acrylics.
Sustainable and new approach using thionolactone
Greater sustainability is a hot topic in adhesive development, particularly in the context of the circular economy. Sustainable approaches include, for example, the use of renewable and bio-based resources, the reduction of waste during production and end-use, and the promotion of material reuse or recycling. Researchers have developed polymer materials that allow adhesive residues to be degraded, which aligns with the growing demand for reusability and recyclability of bonded components and damaged products.
University of Surrey researchers have now prepared an adhesive containing degradable thioester connections in the network. The material performs similarly to conventional adhesives but when exposed to selective mild liquids, the networks are broken into smaller segments, and the adhesive becomes less elastic. Labels using the adhesive can be easily removed, leaving no residue behind. In a direct comparison, model labels attached with the new degradable adhesive detached up to 10 times faster than when a non-degradable control adhesive was used.
While other research groups have reported on degradable adhesives, they were chemically different from commercial adhesives already in use. The new material presented by Surrey researchers consists of one of the same polymers used commercially, poly(butyl acrylate), and contains only 0.25% of an additive, which is responsible for providing the degradable thioester connections in the network. In this particular research, a thionolactone is used as the additive.
As the polymer chains in adhesive networks usually are made from a vinyl monomer (such as butyl acrylate) and contain only carbon-carbon bonds along the length of the chain, the materials are stable for long periods. The bonds do not degrade. In the new materials, the thionolactone reacts with the growing chains in the polymerisation reaction. It inserts sulphur atoms into the chain in the form of a thioester. With a simple chemical treatment, such as exposure to an amine, the thioester connections in the chains are broken and the network is broken into shorter segments.
Thioesters are attracting attention worldwide for degradable networks, because their bonds can be broken quickly and selectively. Long times and harsh chemicals can be avoided in their recycling process. Even a mild treatment with a thiol can be used to decomposed the polymer networks. For instance, the researchers found that a non-toxic dietary supplement, called N-acetylcysteine, could dissolve the adhesive residue.
Conclusions and future work
This new approach to making degradable PSAs has been proven to be effective in tackling the problem of ‘stickies’. The research is expected to inspire the development of innovative approaches for creating more sustainable adhesives. It has the potential to increase the recyclability of soft adhesives, which could help manufacturers raise the sustainability of their packaging materials.
While the thionolactone used in the research at the University of Surrey, called “DOT”, can make degradable bonds in PSAs, it is not compatible with all types of vinyl monomers. The team is now developing second-generation thionolactones that have improved properties and can be produced on a large scale. They are also exploring other applications of thioester-functional polymers, which could include uses in degradable paints and coatings.
The full study was published in the journal Angewandte Chemie.