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What everyone, manufacturer or user, wants to know is whether this specific adhesive will do a great job on this specific adhesive problem. We have the whole of Chapter 3 to see how adhesion is tested in the lab. What about testing it at home?

Mostly we have one-off jobs such as sticking a drawer handle back on, or fixing the leg of a chair. Our “test” therefore is whether the job worked out OK.

If we regularly do an adhesion task then we can try out a few different adhesives and a few different application methods, working out which is the best balance of cost, speed and effectiveness. Most of the time we don't have that luxury – 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.