THE GLUE – The oldest material used by humans to make assemblies of at least 2 different objects. – Part 2 – How glue creates advanced technology?

The sticky tape pioneered by Richard Drew as I mentioned in Part 1, although a useful invention, is not the technological innovation that led to the modern aircraft. That came from another American chemist, called Leo Baekeland, who succeeded in making one of the first plastics. He made his plastic by combining 2 liquids. The first was based on phenols, the main constituent of birch resin, and the other ingredient was formaldehyde, an embalming fluid. These 2 liquids react together to produce a new molecule that has a spare bond for more phenols to attach themselves to, which in turn produces bonds for still more reactions with more phenols, and eventually the whole liquid (if you get the proportions right) is chemically locked together into a solid. In other words, the reaction makes one giant molecule, and all the bonds holding it together are permanent, so whatever object you’ve created will be hard and strong. Baekeland used this new plastic to create a number of objects such as the new telephones that had just been invented. This was , of course, immensely useful and made Baekeland a fortune. But it had another impact. Chemists realized that the phenol and formaldehyde could be mixed and applied to the interface between 2 things – gluing them together as it hardened. This was the beginning of a whole new family of glues called 2-part adhesives, which were stronger than anything that had come before.

how 2 liquids, phenol and formaldehyde, create a strong adhesive

The more people used these 2-part adhesives, the more they understood just how useful they were. First of all, the different components – phenol and formaldehyde – could be stored in separate containers, and thus remain liquid until they needed to be used. And then, beyond that, you could alter their chemical composition through additives, and make them better or worse at wetting and then sticking to different materials, such as metals or wood. This new type of glue had a big effect on the world of engineers. They returned to thinking about plywood, first developed  in ancient Egypt. If you made plywood using a 2-part adhesive perfectly designed to bond with wood, you’d have plywood that was neither held together with the weak bonds of animal glue, nor sensitive to water. But, for this new plywood to take off, it still needed to be driven by a strong market demand. The simultaneous development of the aircraft industry provided just that. In the early 20th century, most planes were made of wood, but because of wood grain they were liable to crack. Plywood was the perfect solution – it could be moulded into aerodynamic shapes and was both reliable and resilient thanks to the new 2-part adhesives.The most famous plywood aeroplane ever built was the de Havilland Mosquito bomber. When it was introduced in the Second World War, it was the fastest aircraft in the sky. Because it could outrun every other plane, it wasn’t even outfitted with defensive machine guns. It remains, to this day, perhaps the most beautiful plywood object ever made. Its elegance and sensuousness come from the ability to mould the plywood into complex shapes while the glues set, a property that’s  maintained its popularity with designers for decades.

A de Havilland Mosquito bomber, which was made of plywood

After the war, plywood continued to revolutionize our world – this time with furniture. Two of the most innovative designers at the time were Charles and Ray Eames, who used plywood to reimagine wooden furniture. Their designs became classics, particularly what is now known as the Eames chair. These chairs are still made and imitated today – go into café or classroom and you are likely to see them. Other fashions in furniture have come and gone, but plywood has retained its appeal.

A plywood chair designed by Charles and Ray Eames

But while plywood furniture stood the test of time, aeronautical engineering had to move on. After the war, aluminium alloys became the pre-eminent material for making aircraft, not because they were stronger by weight than plywood, or even stiffer by weight than plywood. No, aluminium won out because it could be more reliably manufactured, pressurized and certified, especially as planes became bigger and started to fly higher. It´s very hard to stop plywood from absorbing water or from drying out. Plywood aircraft that spent a lot of time in arid countries would eventually dry out, causing the material to shrink and putting stress on the glued joints. Similarly , when aircraft were deployed in very  wet places, the plywood would expand (or even rot), again compromising the safety of the aircraft.

Aluminium doesn´t suffer from these defects; in fact, it´s incredibly resistant to corrosion, and as such was the basis of aircraft structures for the next 50 years. But it is by no means perfect – it isn´t stiff enough or strong enough by weight to create truly lightweight, fuel-efficient aircraft. So even when aluminium aircraft production was at its height, a generation of engineers were scratching their heads about what the ideal material for the skin of an aeroplane would be. They wondered whether it was another metal, or something else entirely. Carbon fibre looked promising since it was 10 times stiffer by weight than steel, aluminium or plywood. But carbon fibre is a textile, and, at the time, no one could make a plane wing out of it. The answer, it turned out, was epoxy glue. Epoxies are another 2-part adhesive formulation, but at their core is always a single molecule called epoxide.

the ring of an epoxide molecule being opened up by a hardner, allowing it to form a polymer glue

There is the ring at the centre of the epoxide molecule with 2 carbon atoms connected to one oxygen atom. Breaking these bonds opens up the ring, allowing the epoxide to react with other molecules to create a strong solid. The hardening reaction won´t start until the ring is opened up by breaking the carbon-oxigen bonds, and this is typically done by adding a “hardener”. One of the major advantages of epoxies is that the reaction is temperature-dependent; you can mix it up and it won´t start bonding until you want it to. This is critical to the production of the complex-shaped, fibre-reinforced parts that make up an aeroplane wing, all enormous and requiring weeks of manufacture. When you are finally ready to transform the glue into a strong solid, you put it into a pressure oven, heat the wing up to the right temperature, and hey presto. These ovens are called autoclaves and they can be the size of aircraft. All air is removed from aircraft moulds before they are heated up, solving another problem of glues – they often trap air inside a bond, forming a bubble that, once it has hardened, becomes a weak spot. Another major advantage of epoxies is that they are chemically very versatile. Chemists can attach different components to the epoxide ring, which allows it to bond to different materials, such as metals, ceramics and yes, carbon fibre.

You may be wondering why the epoxy resins sold in hardware stores don´t need to be heated in autoclaves before you can use them to repair broken crockery or glue the knob back on the metal lid of your juicer. These have different chemical hardeners from the ones used to make aircraft, and they´ve been designed to react with the epoxide molecule at room temperature. The glue is sold in 2 containers, which you have to mix together. One tube holds the epoxide resin and the other tube contains a hardener and various accelerators that speed up the reaction, allowing the glue to become solid faster. These domestic epoxies are not as strong as the aerospace versions, but they are still very powerful. Maybe this all sounds easy, but it took decades to develop the fundamental understanding and technology of composite structures to the point where everyone would trust these carbon fibre planes in flight. First, carbon fibre composites were tested on the ground in racing cars, and proved to be highly successful. Racing cars even have carbon parts in their engines now, and yes, you guessed it, we´ve designed epoxies that can be used in that high-temperature environment. After racing cars, carbon fibre composites were applied to prosthetics, a great innovation because they are stiffer and stronger than many metals, and a great deal lighter too. The “blades”  you see being used by disabled runners are made of carbon fibre composites. The material has also been used to make bicycles, and to this day the highest-performing bikes in the world are made of carbon fibre composites glued together using epoxies. And of course, the latest commercial passenger aircraft from Boeing and Airbus are made from carbon fibre composites.

Just as bolts and rivets have given way to glues and adhesives in prosthetics and aerospace, it feels highly likely that stitches and screws will give way to glues in hospitals. For instance, if you by accident you hurt yourself and you are wounded by a cut on your arm you go to the hospital and the doctor will first clean your wound and then he will produce a tube of cyanoacrylate adhesive. He will squirt this on both sides of your wound before holding them together for 10 seconds and will send you home. And this is not just some crank doctor trying to save time, this has become standard practice in hospitals.

a water molecule opening up a cyanoacrylate molecule to create a polymer glue

Cyanoacrylate glue is best known as Superglue, and it is a very odd liquid. On its own, the liquid is an oil, and behaves like one. But if exposed to water, the H2O molecules react with the cyanoacrylate. This opens up the double bond that holds it together, making it available to react with another cyanoacrylate molecule. This creates a double molecule with an extra chemical bond ready to react with something else. And so it does, reacting with still another cyanoacrylate molecule, creating a triple molecule with another extra bond, and then that reacts too, and so on and so on. As this chain reactions continues, a longer and more interlinked molecule is produced. This is already clever, but it gets even more so when you realize that a thin layer of cyanoacrylate liquid only needs the water vapour that´s already in air to transform into a solid. While many glues don´t adhere in a wet environment because all the water makes it impossible for them to stick to a surface, superglue works anywhere. Conversely, as anyone who´s had a run-in with it knows, it makes it stupidly easy to glue your fingers together, which is why chemists are on the hunt for a way to unstick superglue quickly and comfortable.

Fingers aside, glues are holding together a lot of the world these days, and it´s quite likely there´s more ahead because, as the aircrafts currently in use today are amply demonstrating by withstanding turbulence at 800km/h, glues are up to it. We probably haven´t even scratched the surface of what glues can do, especially when you consider just how many strong, sticky substances other living organisms are using. Hardly a day  goes by without some  scientist discovering a new glue used by plants, shellfish or spiders.  

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