Sticking Together. Steven Abbott

for which it is wonderful, welding. A good way to make anything fail is to put a hole in it or to heat parts of it while other parts remain cool. When things are held together by nails, screws or rivets, stresses are focussed on the hole and crack cans start there. Those fixing devices become instant sources of potential failure. The bio-inspired hook-and-loop type of fasteners developed and commercialized under the name Velcro are an ingenious variant on the old hook-and-eye system. Although they are a welcome addition to the world of fasteners thanks to their ability to distribute a load over a wide area, because they are purely mechanical, they feature no further in this book. And, of course, these fasteners themselves are often attached via a strong adhesive.

      Adhesives, in contrast to conventional fixings, spread the load and do not require you to add defects to your object. This means that they are used just about everywhere.

      The high-tech adhesives industry is very much alive and well. For phones, cars, aircraft, medical devices, construction, packaging – just about everywhere – customers demand not so much better adhesives as better adhesive systems that deliver a package of benefits.

      As we will discuss later, there is a constant set of trade-offs to be battled. Prioritizing strength and hardness can lead to systems that are too brittle and cannot cope with shocks. Making things flexible and resilient can lead to compromises in resistance to those steady, long-term forces that can produce creep (slow deformation) within the bond.

      Providing controlled low adhesion is especially tricky for consumers.

       People like easy-open packages and get frustrated if they are too hard to pull or give a raspy “stick-slip” release. But no one likes it when they are so easy to open that this has happened during storage.

       Similarly, we all want hooks that are easy to stick to a surface and that securely support a large weight. But we also want them easily removed and repositioned with zero damage to the surface.

      In addition to the functional challenges, everyone says that they want “green” adhesives. Going back to the bad old days of collagens and milk protein, which will be discussed in Chapter 1, is not a sensible option, both technologically and in terms of those who would object to animal products in, say, their smartphone. Trying to replace petroleum-based adhesive ingredients with those made from renewable resources is neither straightforward nor provably green. A life cycle analysis that takes into account, for example, the water and fertilizer used in growing a crop, the energy needed to harvest it and transport it, plus all the processing and waste disposal does not automatically show that such materials are preferable. And that is before taking into account the alternative use of that land: growing crops for food. This is not just my personal view; some major EU studies on, for example, bio-based plastics confirm that there are often major downsides to switching from petrochemical alternatives. I have had conversations with urethane manufacturers who are frustrated by the fact that the variability of bio-based raw materials has a serious impact on performance, with whole batches sent to waste (definitely not green) because the adhesive performance was sub-standard.

      In the foreseeable future the greenness of the adhesives will consist of doing more with less; minimizing the amount of adhesive in each bond, which in turn requires greater precision in the parts to be adhered. You can't use a 1 µm layer of adhesive if the parts have a 5 µm roughness. And as more and more parts of products such as cars and aircraft are made from (fibre-reinforced) plastics, the environmental upsides from using a small amount of a high-performance “non-green” adhesive far outweigh any environmental downsides of the adhesive itself.

      Nature has many examples of ingenious modes of adhesion that excite a lot of well-intentioned publicity when scientists find ways to use similar principles. As we shall see in the final chapter, the principles are usually more to be admired than to be copied. If I am flying in an aircraft mostly held together by adhesive, I far prefer a wholly synthetic, high-tech, apply-and-forget adhesive to a “smart” bio-inspired one that needs a constant supply of chemicals in water to keep it in good shape.

1.1 WHICH ADHESIVE SHOULD I USE?

      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