Adhesion Unraveled: The Energy Dilemma

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This article is intentionally theoretical, but don’t worry, I’ll keep it simple, with a few examples. I promise not to use any math, just common sense… and without losing even a shred of rigor! Warning: if physical chemistry isn’t your thing, if deep thinking bores you, or if you find me too academic, this is your exit point. Don’t waste your precious time.

The key to adhesion is the dissipation of fracture energy. If the term “dissipation” confuses you, feel free to call it “delocalization”. It works just the same, trust me!?

What does this mean??

For adhesion to fail, a crack must start, which could be natural or deliberate, and then it needs to propagate along the fault line, much like a precise strike on a crystal’s cleavage plane. This entire process involves energy, and since it doesn’t transfer to “any” other place, it’s used exclusively for breaking… it just breaks!

This happens because energy is conserved (or turns into mass, according to Einstein, but let’s save that for another day).?

Where does that energy come from??

From your hands or a machine. By the way, laminated or bonded materials don’t weaken naturally—meaning, you provide the energy, and anything that absorbs, dissipates, or relocates it will reduce the likelihood of the crack spreading, hence increasing adhesion.

In real life, if you score a piece of glass and then tap it lightly, the crack spreads because nothing absorbs the energy from the tap, so it’s used to cut the glass. But if instead of glass, you use rubber and make the same mark, no amount of tapping will propagate it.?

Where did the energy go??

It dissipates molecularly within the rubber (it heats up). These examples are extreme, but believe me, inks and adhesives lie “somewhere in between” glass and elastomeric polymers.

But… what happens if we formulate an ink with “dissipation” in mind (this is done with plasticizers)??

This is where one of the paradoxes of ink formulation begins… Do you lean towards adhesion or cohesion? And you thought the life of an ink formulator was easy!

Before your head explodes, let me tell you, this isn’t the end. Adhesion is a property of the system, not just its surface energy (in fact, inks for untreated materials don’t peel off… but they don’t adhere either). Also, if you test adhesion by pulling on the tape, you won’t get the same result as if you do it by “peeling” the film… did you notice that?

Most people believe adhesion comes from corona treatment… partly true, but those bonds break with very little energy, about 1 joule/m2. Don’t believe me? Think about cracked glass. Do you think the bonds that hold the glass solid are weak? (By the way, glass, physically speaking, is a very high-viscosity liquid, not a “solid”).?

Adhesion is the result of molecular crosslinking, it’s the “entanglement” that holds things together… the rest is, in many cases, negligible.

One could write volumes on this, but, in summary, if you want to improve adhesion, work on energy dissipation or enhance molecular crosslinking… don’t waste too much time or money on corona treatment… it helps, but not too much!

Bon appétit!