Sticking Together. Steven Abbott

epoxy system might start with a 2-functional epoxy and a 3-functional amine (Figure 2.17). This will create a somewhat complex network. A 3-functional epoxy and a 4-functional amine would create an even more complex network.

      Figure 2.17 Multifunctional starting materials can form complex crosslinked networks.

      This sort of trick is especially important in acrylate systems. It happens to be easy to make 2-, 3-, 4-, 5- and 6-functional acrylates. We will see that mixtures of these systems are the key to many adhesive systems, from nail polish to dental fillings.

      For these crosslinking processes we often say that the system “cures” (or “sets”) rather than “polymerizes”. It makes no difference which word is used: the process of setting solid or becoming fully cured is a polymerization.

2.12 WATER AS THE INITIATOR

      The beauty of polymerization is that you just need to get it going and then it carries on, in principle, until all the monomer is used up. In practice, polymerizations terminate when they meet impurities or another reacting chain, so we need a moderate number of initiators to get a good conversion. For superglues, urethane glues and silanes the first step of the polymerization reaction can be initiated by water molecules. This is wonderfully convenient and is an element of the popularity of these types of adhesive. It is also frustratingly inconvenient.

       It is convenient because there is usually enough moisture in the air or sitting on the surface of the adherends to start things off. No need to add any other chemicals (“activators”). If the atmosphere or surface is too dry, a huff of moist breath or a spritz with water does the job.

       It is inconvenient because opening the adhesive package to squeeze out the adhesive allows at least a few water molecules to get into the bulk of the adhesive, which will eventually go solid.

      I always have problems with those tubes of silicone adhesives that are applied with a squeeze gun. The tubes are large enough for many jobs, yet I tend to need rather little for any single job, and the jobs are infrequent. For the next job I usually find that some portion of the silicone has gone hard; if I take off the cap to remove a solid blob from within the tube, I can do that job, but have added so much moisture that the tube is fully solid the next time I need it.

2.13 CATALYSTS

      Many polymerization reactions happen rapidly once the reactants mix or an initiating radical is created by heat or light. Some, however, are rather slow. This can be good for those jobs that require a lot of fiddling to put things properly into place. When we want such reactions to go faster, we add a catalyst. Catalysts are molecules that get involved in the polymerization reaction, greatly speeding it up. But after each step, the catalyst emerges unchanged, ready to help speed up the next step. A small amount of catalyst, therefore, can have a large effect on the rate of polymerization.

      Sometimes catalysts can be simple molecules like acids – which might be called “accelerators” rather than catalysts. Sometimes the systems require more sophisticated molecules. For example, the polymerization reactions for urethanes (general adhesives) and silicones (e.g. bathroom sealants) are catalyzed by tin-containing molecules. A problem with some catalysts is that they contain elements (such as tin) which carry small health or environmental risks so there is always pressure to reduce or eliminate them. Because catalysts are needed only at low levels, in general the risks from them are small.

      For professional workers, fast cures are desirable. For amateurs, speed can be a problem. Different versions of the same adhesive might use different levels of catalyst for different speeds. Or an “anti-catalyst” (retardant) might be added to slow things further.

2.14 THAT'S IT

      One more thing. Like most people, I had gone through much of my life tending to focus on the adhesive. During a very heated debate (scientists can get quite passionate) with some top scientists at a large chemical company I felt the frustration of positions hardening and drifting ever further apart, until one of their scientists, Dr Michele Seitz, said, “But adhesion is a property of the system”. That one sentence entirely changed the debate and it turned out that both sides had been focussing too much on specifics and not on the system. It also entirely changed my approach to adhesion, and I remain grateful to Dr Seitz for her momentous intervention.

      Now we have all the basics in place, it is time to start exploring adhesion. And a good place to start is without any adhesives at all. In other words, it is time to learn about geckos.

      CHAPTER 3

      Sticking Like a Gecko

      Congratulations! You have just been promoted to chief science officer of the geckos. And you have been presented with geckodom's biggest challenge yet. How are you going to climb up all those smooth glass structures that humans have created? You know that the smooth surface gives you nothing with which to grip. What are you going to do?

      Some of your fellow geckos suggest that you create a glue gun for your lizard feet, but you immediately reject this idea. “The problem with glue isn't how we are going to stick, but how we can unstick ourselves. A glue will be too good – we'll hang there, unable to move”. Translating this into scientific language you say: “What we want is the world's worst adhesive – just strong enough to hold a gecko, yet weak enough to be easily broken when we need to take another step”.

      Alone in your well-equipped lab, you look with annoyance at some dust on your equipment. It is everywhere, even on the sides. This gives you an idea. You get out your atomic force microscope (AFM) and attach a bit of dust to the end of the probe ready to measure the force between the dust particle and the surface. What you want to do is push the particle onto the surface and measure the force needed to pull it off, yet to your surprise, as the particle gets close to the surface it jumps into contact (Figure 3.1). It really wants to be on that surface!

      Figure 3.1 A bit of dust on the end of an AFM tip is pulled into contact with the surface. This is a surface energy effect.

      At first you think this is because the particle is statically charged, but even when you carefully neutralize the static with a deionizer, the particle still jumps into contact. You try many different types of dust and many different types of surfaces and you find that the attractive force does not change much. It seems to be a general property of all materials that they attract each other with a small force. [This general attraction is called the van der Waals force after the scientist who first characterized it properly.]

      As a good scientist you want to give this force a value. The force on its own doesn't tell you much because it depends on the area of contact. To make the value universal you decide to look at it in terms of the amount of energy in Joules, J, needed to separate one square metre of surface, and call it surface energy in units of J m⁻². For all surfaces you can find this varies from a low of 0.02 to a high of 0.06, with most being around 0.04 J m⁻² or, for convenience, 40 mJ m⁻².

      When you then do the surprisingly complicated calculations about how much of a gecko's weight could be supported if you only had those surface energies, you find a clear answer. If the whole area of your four feet was in contact with the smooth glass, you would never move again – it could support 100 kg. You therefore need only a modest fraction of your feet to be in perfect contact. Examining your feet you realize that they have a complex, multi-level design that allows lots of contact (Figure 3.2). For the simpler feet found on most other animals, the total contact area with smooth glass would be so small that there is no hope of holding on. The problem is