– we have to make a one-off decision to use this adhesive, applied in this manner to stick these things together. If it works, no one will ever comment. If it fails, then you risk anything from embarrassment to significant loss.
One aim of this book is to help you make the right choice of adhesive for any given job, and in Chapter 6 we review many common systems for sticking A to B. To make the right choice you need a key fact that is missing from most accounts of adhesion and adhesives. Here it is:
“Adhesion is a Property of the System”.
You will have no problem remembering this phrase because, I make no apology, it appears many times throughout the book.
The biggest mistake any of us can make when thinking about an adhesion problem is to focus on the adhesive, rather than the system. If your system is going to involve lots of peel, then, as we shall see, don't bother with a superglue. If your system involves lots of shear, then (all this will be explained clearly later) the strength of the bond depends as much on the thickness and modulus of the adherends (the things you are adhering) as on the adhesive. If you can increase the thickness and/or modulus of the adherends you are already improving things, even without thinking about the adhesive.
Then you need to worry about speed, and its equivalent, temperature (yes, the two are strongly inter-related as we shall discover). If your problem is long, slow loads and/or higher temperatures then a “strong” adhesive will be right. If the problem involves short, sharp shocks (and/or cold temperatures) then a strong adhesive might be catastrophically brittle and you will need something far more forgiving. The common PVA wood glues are used extensively not because they are amazingly strong (which they're not) but because they are amazingly forgiving when the woods in the joints (they might be different types of wood or in different grain directions) expand or shrink with the rise and fall of humidity.
Adhesives have, as we will see, moduli, viscosities, glass transition temperatures, curing speeds, degrees of cure, crosslink densities. Each of these can be measured and a supplier could, if necessary, give you all those values. What no supplier can give you is a meaningful statement about how strong it is, because no adhesive has “a” strength, because Adhesion is a Property of the System.
A supplier can say that this adhesive can survive X N m−2 when tested against Test Standard XY92, and you can compare that to a different adhesive tested against the same standard and it is possible that the test is relevant to the type of loads you are trying to resist. But we are not likely to have such a situation in our day-to-day fixing jobs.
I now want to flip all these negatives into a positive. You are the one who knows what you are sticking to what, for what reasons, and you know the sorts of assaults the joint will receive over its required lifetime. You also know the restrictions of contamination, access space, time, temperature, weather, sunlight. You are the world expert on your system. Now that you know that Adhesion is a Property of the System, and that you are the expert on that system, you don't have to be taken in by adverts for glues that work only under the precise conditions created for the advert. You don't have to be fooled by statements like “sticks anything to anything”, with a little asterisk pointing you to a set of disclaimers in small print.
With help from the chapters that follow, you will be able to:
Understand what will or won't help with surface preparation
Look at advertising claims with a healthy scepticism
Choose between a “strong” or a “tough” adhesive
Choose between a thick or thin layer of adhesive
Choose between a good general-purpose adhesive and one (allegedly) specifically designed for your sort of system
Know whether the adhesion promoters present in some adhesives will help (and how) or hinder (and why)
Understand why too much of a good thing is a bad thing
Find out how to reduce adhesion when you need to
You cannot do these things well if you assume that everything is down to the adhesive. By understanding the system, you go a long way towards understanding how to get the best out of what you have to hand.
1.2 HOW THE BOOK UNFOLDS
We start by admiring those pioneers of adhesion who managed to take crude raw materials such as birch bark tar or boiled bones to create really rather impressive adhesives. We then switch to some necessary basics to become familiar with the few core ideas needed to understand the rest of the book. By looking at how geckos manage to stick to walls, we see the sort of adhesion we mostly don't want, getting ready to find out how to get the (usually) strong adhesion we do want. But before getting to strong adhesion we need to know how to measure if our adhesion is strong. Because adhesion is a property of the system, this is by no means obvious. Then we can get to understand how strong adhesives work (and when they will fail). Because much of strong adhesion depends on strong polymers, we need then to switch to pressure sensitive adhesives (common tapes) that give strong adhesion thanks to very weak polymers. What unites the strengths of both types of adhesives is that they each manage to dissipate the energy of a potential crack; adhesion is much more about dissipation than it is about “strength”. That completes the next five chapters and provides all the principles we need. The final five chapters are about specific systems and how they work with the principles we've worked hard to understand.
CHAPTER 2
Background Ideas
We take adhesion for granted because most of the time it just works. We only notice it when it goes wrong – when the thing we fixed at home breaks again or when the removable adhesive isn't so easy to remove. One reason for us taking adhesion for granted is that modern adhesives are so good. It wasn't always like that.
Take yourself back hundreds of thousands of years. Adhesion is now a matter of life or death. If you can reliably stick a flint arrowhead into a slot at the front of an arrow (and stick some feathers onto the back) then you will be able to eat tonight (Figure 2.1) – if not you starve. How hard can it be? Just get some sticky stuff and, well, stick it. Ah, that pine tree has some sticky stuff, let's try it. The real sticky stuff is too soft, so you try the hardened version, using a fire to melt it. Pour it around the arrowhead and stick and, when it has cooled take a test shot. As soon as the head touches the target, the brittle pine glue shatters. You won't eat tonight. After a few decades or centuries, chance or a course in advanced nanotechnology leads someone to mix the pine resin with the right sort of finely-ground charcoal. Now the adhesive is shatter-proof.
Figure 2.1 Gluing a flint arrowhead into a wooden haft is a big technical challenge. It is impressive that Neanderthals managed to do it with birch bark tar that was neither too soft nor too brittle.
This story of the development of a working adhesive captures the frustrations of adhesive developers today. It is easy to make an adhesive that is too soft, it is easy to make one that is too hard and brittle. Finding the right balance remains a deep challenge, especially because, as emphasized in this book, adhesion is a property of the system, not just of the adhesive.
There has never been a shortage of things that might produce a useful glue. Anyone
