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 who has overcooked any starchy food has created an adhesive. You don't even have to cook it – a paste of wheat or corn starch creates an equivalent glue. It just isn't very good, especially if the joint ever gets wet. Bacteria and moulds love to feast off the nutrients, so a starch-based glue will go off in storage and a joint might fail via mould growth.
Birch bark tar (pitch) has been a wonderful adhesive for thousands of years and birch forests are common. To get the tar you have to heat the bark; the problem is that if you heat it in the presence of oxygen it gets burned to something useless. If you were in a birch forest and had to do anaerobic (“without oxygen”) heating of tons of birch bark to make large quantities of adhesive for your tribe, how would you go about doing it if you did not have access to modern tin cans?
Birch bark tar is known to have been used by the Neanderthals; archaeologists have studied tar-hafted arrow heads and discovered nearby lumps of tar ready to be used for gluing. One team of scientists, therefore, had to have a go at working out how the Neanderthals might have done it. The team tried out various methods, using embers, holes in the ground or a more complex raised structure, seeing how much usable tar they could create for a given amount of time, bark and firewood. They looked at wrapping some birch bark in fresh fibres and surrounding the bundles with embers; they dug a hole, placed a birch basket at the bottom, laid on some bark then threw in some hot embers. And the team tried something similar but covering it all with earth and lighting a large fire. In all three cases it turns out that you (can) get respectable quantities of tar, with the covered structure giving the most. I find it wonderful that 21st century scientists will spend considerable time, resource and ingenuity to find out how people did things ∼100 000 years ago. More recently, by looking at the birch bark gum used for Scandinavian hunting weapons 10 000 years ago, it is clear that the gum was first chewed. Teeth marks show that both children and adults were involved in the process, and female DNA extracted from the gum shows that this was not a male-only activity.
Then there are the glues from dead animals: fish, rabbits, horses, cattle. Boil up the skins, cartilage, hooves and bones and… you can make gelatine, which is not useful, except in food. The trick is to control the boiling so that the collagen, the tough protein in all those parts of the animal, is broken down sufficiently to become a meltable glue yet not so much as to be reduced to gelatine (Figure 2.2).
Figure 2.2 The tough, insoluble collagen from bones, skin etc., (left) needs to be boiled sufficiently to break it down into soluble glue molecules (middle) without being broken further into smaller lumps that constitute gelatine (right).
It is a genuinely difficult challenge to get these processes to work at all – imagine having to provide the quality control to ensure that they work day after day!
One useful additive to cartilage-based glues sounds vaguely amusing to us now: urine. It is at first surprising how often urine appears in ancient recipes and processes. We now know that the main chemical in urine, urea, is one of the few molecules that interacts strongly with proteins such as collagen to make them more soluble. Camel urine was for centuries a hair-care essential because it allowed the keratin protein in hair to be made flexible before being shaped into whatever was the current fashion. Urea is frequently used in skin cream formulations and is an essential part of “natural moisturizing factor” created by our skins. As with all complex products, adding too little or too much urea would cause problems; it would be interesting to know what quality control procedures were used to ensure consistent urea additions from such a variable raw material.
How do we know what sorts of glues were being used a long time ago? Archaeology shows that locals used the resources to hand, and some of them are easy to identify even after millennia: Aztecs used rubber; Mesopotamians in 4000 BC used bitumen to attach ivory eyeballs to statues; and from the Neanderthals onwards, the plentiful