CHAPTER 1
Introduction
Modern society would fall apart without adhesives.
Your smartphone has at least 25 adhesive joints in it, with the all-important glass screen and the display held with adhesives, as are the chips, microphones, speakers and aerials inside. The screen protector is also stuck on.
Now step into your car. Manufacturers are under intense pressure to reduce weight. Every screw or rivet is a potentially avoidable weight (yes, automobile manufacturers agonize over each gram in each component) and the interior of a modern car is held together by clips (for removable parts) and adhesives. External parts such as bumpers (fenders) used to be metal. Now they are made of polypropylene, strongly adhered to the metal shell. Even the metal shell itself is rapidly heading for history as plastic/carbon fibre takes over, held together with adhesives.
What about something really important such as an aircraft? The interiors are stuck together – the weight, inconvenience and aesthetics of screws and rivets make them unacceptable. The rivets we see on the outside give the impression that they are the key structural element, yet they are now mostly for backup or are redundant. Even the (by now) old all-metal aircraft are largely held together with adhesives, while the latest generation of civil aircraft such as the lightweight, efficient Boeing Dreamliner and Airbus 350 are predominantly glued together.
And yet, the hidden nature of adhesives (if you can see them, they probably haven't been applied properly), allied with their avoidability (people often prefer to try other methods first), has led to a rather dismissive attitude to the whole technology. When I mention my interest in adhesives to friends and neighbours, the general response is that adhesives are trivial, unsatisfactory or excessive, i.e. they are only an occasional feature of their lives, they often don't work well and can be (like my wife's orchid jug mentioned in the Preface) unsightly when used; as such they are not of any great importance. There is a medieval notion that to “really” put things together you need nails, screws, rivets and welding.
Talk to anyone who really needs to put things together and you find a horror of nails, screws, rivets and, with the exception of steel 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.
