Pressure-sensitive adhesives: Why cohesion is just as important as adhesion

When talking about pressure-sensitive adhesives (PSAs), you could be forgiven for thinking that the product’s performance all just comes down to adhesion – the ability of one substance to stick to another.

However, it’s not quite as simple as that, because cohesion – the ability of a substance to stick to itself – also has an important role to play. We’ve probably all had the experience of attempting to peel off a glued-on paper label from a glass bottle or jar, only to find that delamination occurs within the label, leaving us holding shreds of paper and a sticky mess of glued-on paper on the bottle. This happens because the cohesion within the paper is the weakest point in the whole composite, meaning it’s the first part to break when force is applied.

As a result, we need to consider both adhesion and cohesion when considering what we want from a PSA.

It’s worth taking a moment to understand how cohesion arises. Whereas adhesion in PSAs relies upon van der Waals forces, cohesion in PSAs often involves other mechanisms too, such as entanglement, and here cooked spaghetti provides a useful analogy!

Long strands of spaghetti – rather like adhesives containing long molecular chains (or with cross-linking) – will entangle with each other very effectively, making it difficult to pull the whole structure apart. This corresponds to a high degree of cohesion. In contrast, if you were to cut up the spaghetti into short sections, then they would be easily separated, just like adhesives with short molecular chains (or no cross-linking). This corresponds to weak cohesion.

• Adhesion comes from the Latin words ad (to) and haerere (to stick), and refers to the attractive forces between two different substances.
• Cohesion comes from the Latin words com (with) and haerere (to stick), and refers to the attractive forces within a single substance.

When designing a PSA, the main factor is of course ensuring that the composite withstands the stresses it will encounter during the application. But beyond that, what are the main considerations regarding cohesion and adhesion that need to be made?

Things are fairly straightforward in cases where the bonding needs to be permanent. Here, in principle, it doesn’t matter where delamination happens, because if it does, the PSA has failed and that’s that. But in the industry, it’s common to design the PSA so that the cohesion within the substrates being bonded is the weakest point of the entire composite. This is because it’s usually the only component that cannot be changed, and so it acts as a limit on the strength of the whole system.

Things are a little different when the bonding needs to be reversible – as in the popular pads of sticky notes, removable films used to protect delicate surfaces during shipping or installation, or in fixings that need to be repositioned. Here, the boundary between the adhesive and the substrate needs to be the weakest point, so that delamination of the film brings the adhesive layer with it, leaving the substrate and film intact (and the substrate clean). Consequently, cohesion within all of the layers must be stronger than the adhesion between them, and the adhesive–substrate forces must be weakest of all.

Removable PSAs present an interesting challenge to the designer, because the forces between the adhesive and substrate must be weaker than between or within all the other layers.

This logic is taken to an extreme when considering release liners. Then, it’s essential that the adhesion between liner and adhesive is lower than the cohesion between the layers, so that the liner can be removed easily. For this reason, liners are often coated with silicones, fluorosilicones or fluoropolymers, which have very low surface energy and so form weak bonds to many adhesives.

Even so, for applications involving so-called ‘delicate functional materials’ this can be challenging. A good example is the silicone-coated mica sheets that provide thermal insulation in electric vehicle batteries, and which need to be securely fixed in place. In this case, the cohesion within the mica sheets is so low that even pulling off the backing paper could cause the mica sheets to split. Fortunately, even where careful selection of the adhesive isn’t sufficient on its own, there are other solutions, such as self-wound (linerless) or ‘fingerlift’ constructions.

The PSAs used to hold components of electric vehicle batteries in place must be carefully designed to account for the often delicate materials used.

Understanding cohesion and adhesion also has another benefit – troubleshooting. However well a PSA is designed, they occasionally fail, and if this happens, understanding whether adhesion or cohesion is at fault can help identify a solution. (As mentioned earlier, if there is a failure of cohesion within the substrates being bonded, then that is usually desirable, because it shows that the strength of the adhesive bond is not limiting the strength of the overall system).

• If there is a failure of adhesion, then the van der Waals interactions between the two surfaces haven’t been able to achieve full strength. This may be an issue related to (a) surface wetting, in which case, optimizing the surface energies might provide the answers, (b) physical or chemical contaminants on the surface, which need to be removed, or (c) the PSA not being given enough time to build the adhesive strength, suggesting the application method needs to be examined.
• If there is a failure of cohesion within the adhesive, then the adhesive itself needs to be optimized – perhaps with a different formulation featuring different chemistry, or with a greater degree of cross-linking.

In conclusion, understanding how a PSA will perform in a given scenario is a highly complex challenge, because it depends on predicting the forces at play within a complex multilayered composite. 

But by breaking down the interactions into cohesion and adhesion, we give ourselves a tool to understand this complexity, and so design PSAs that will provide optimum performance for a multitude of substrates under a wide range of application conditions.

If you need advice on identifying the best PSA for your application, then please try our Product Selector, or talk directly to an Avery Dennison specialist at andrew.christie@eu.averydennison.com.