Sticking Together. Steven Abbott
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have left a residue.
My own experiences wrestling with this issue have often concluded, reluctantly, that surface energy on its own is not sufficiently trustworthy. Despite the simplicity and low cost of these systems, the practical choice is a more expensive film (often clear PET which is always too crystalline to give reliable surface energy adhesion) coated with a low level, but reliable, pressure sensitive adhesive (PSA). Smartphone screen protectors which need to be secure (yet removable if desired) are stuck on with a very expensive, high quality PSA.
Writing a book is an adventure in challenging one's own assumptions and statements. In the same week that I wrote the original draft of this chapter I had two demonstrations that my earlier assertion that “In general it is impossible to get good surface energy contact…” was wrong.
First, a friend observed that items on his painted windowsill became stuck over time. He showed me an example where a picture frame was indeed slightly stuck to the paint. With a bit of peel motion it was easily removed, and there was no visible mark on the paint. Clearly the paint surface had flowed on the micron scale over weeks or months to create significant surface contact. He had never had this problem with old-fashioned solvent-based paints. As we shall see in Chapter 8, water-based paints need additives to help the paint particles to coalesce into a film. My guess is that these additives are present at a level sufficient to keep the particles slightly mobile, hence their slow flow into contact with the picture frame.
Second, I found myself having to do some work on a “cold seal” adhesive system. I had never thought about cold seal even though there is a well-known example, envelope adhesives which do not stick (much) to anything else but readily stick to themselves when pressure is applied. It turns out that these adhesives are similar to normal adhesive tapes but they don't create quick tack with another surface. The trick is to get them to produce adequate adhesion when pressed hard together, allowing the polymers to flow sufficiently to create the same type of adhesion as is generated spontaneously with a standard adhesive tape. The adhesion itself is not due to surface energy – these seals work via the same principles as pressure sensitive adhesives. By being on the borderline between pure surface energy and pressure-induced flow, they are an interesting example of surface energy at the margins.
3.3 LIQUID GECKO ADHESIVE
To get good gecko-style adhesion, all we need is perfect surface-to-surface contact. This is impossible to achieve (even for geckos) outside highly controlled laboratory conditions. When we start exploring common household adhesive tapes in Chapter 6, we will find that they must meet the “Dahlquist criterion” which states that the adhesive must be relatively soft and squidgy so that it can spontaneously flow into perfect contact. If it is not soft enough it will not give perfect contact with the surface. A PSA needs the good contact to provide full surface energy adhesion which in turn gets amplified by the adhesive itself. The cold seal surfaces of the previous section do not have the required softness to create good, spontaneous contact with other surfaces, yet together can provide adequate contact and adhesion when enough force is applied.
It is even more impossible to get two real-world solid surfaces into perfect contact without extreme pressures that force them to flow (the cold welding mentioned above). This is why liquid glues were invented.
If you can squeeze a little bit of liquid between two surfaces, it will make perfect contact with both sides. Some readers might know enough about “wetting” to immediately disagree with this statement. After all, a drop of water on a piece of plastic does not spontaneously flow to cover the surface. Although this is true, a drop of water placed between two pieces of the same plastic will spontaneously fill the gap. We will return to this apparent puzzle later.
Let us be specific and say that we have two pieces of a broken cup where we are well able to match the two jagged edges at the large scale but where they are simply too jagged to match at the nm scale. The liquid spontaneously fills the gap and we squeeze reasonably hard so that excess liquid is pushed out. Now, via some magic, let the liquid solidify (Figure 3.4). We have four surfaces in perfect contact: upper cup surface to upper glue surface and lower glue surface to lower cup surface. As long as we don't get a crack going along the interface, we now have surface energy holding the cup together.
Figure 3.4 No practical pressure can force together the two solid surfaces, but a liquid can easily fill the gap. If the liquid turns to a solid then we have, at the very least, perfect surface energy contact between all four surfaces (two interfaces).
The specific modern example of this is superglue. The liquid is a cyanoacrylate which is stable in its plastic tube. As it is squeezed onto the piece of broken cup it picks up moisture from the air. As shown in Chapter 2, a water molecule turns a cyanoacrylate molecule into a cyanoacrylate ion which can readily react with another cyanoacrylate, creating an ionic dimer which reacts with another cyanoacrylate … until (we hope) all the cyanoacrylates have been joined together.
We all agree that these superglues are amazing. Now ask yourself the question: “Do I trust this superglue fix on my favourite coffee (or tea) cup?” Experience tells us that superglues are wonderful for jobs where there aren't any sudden or oddly-angled stresses. A small jug for watering orchids that has been fixed with glue will stay together no problem. We can pick it up, fill it with water, put it down with a bit of a bump and have no fear of it falling apart. But would you trust a glued handle on your hot cup? I wouldn't. Just pouring in the hot drink can create thermal shocks that might break the joint. And it just needs one careless motion to impose an unexpected stress on the handle with a heavy load of hot drink for the joint to break and for your drink to spill all over you.
I am so old that I remember the superglue revolution. Before that we always had a problem. The only way to make a liquid adhesive was via a solvent (water, toluene, acetone …). If you put the joint together too early for something non-porous such as a cup handle, the solvent had not evaporated so there was no viable adhesion. If you waited too long for the solvent to evaporate then the adhesive was too solid to give the intimate contact needed for surface energy adhesion. By getting it just right you had adequate adhesion which improved over, say, 24 hours as the remaining solvent molecules escaped along the edges of the joint.
Those solvent-based glues worked wonderfully on porous surfaces such as cloth, paper, leather or wood, because any excess solvent had an escape route (Figure 3.5). Superglues were a revolution because as long as they picked up enough moisture before you squeezed the surfaces together, they solidified and you had good adhesion rather quickly.
Figure 3.5 The problem with solvent-based glues is the solvent has to go somewhere. For impermeable surfaces the solvent escapes (too) slowly through the edges. For permeable surfaces such as leather the solvent can easily escape. Superglue goes solid without solvent evaporation so works for both systems.
The other types of household glue available to us were the epoxies, where we had to mix two components which solidified over time. Because epoxies have the potential for an extra and highly significant degree of adhesion, we will discuss them in detail later in the book. For the moment we will take a simple case that highlights the problem with this sort of surface energy-only adhesion.
During the writing of this book I happened to need to use a nail brush to get rid of some glue on my fingers from a failed experiment. My hands and the brush were wet and suddenly the brush's handle fell off. I'd forgotten that I had broken it some years ago and had glued it back on with some superglue. It had been fine for years. Now the combination of a bit of pressure in the wrong direction and, I assume, the water meant that the handle came off. This then reminded me of a beautiful experiment described by one of the Big Names of adhesion science, Prof. Kevin Kendall in his famous Molecular Adhesion book.
He coated a thin layer of epoxy onto glass and, in his words, “… the adhesion was good. The film required