Materials Science is always a topic for me, I love to do this. I don´t think I want to do something else with the same interest. I don´t only learn about materials at my work or at home, but I even do it when I travel. Actually when I go in holiday is a great opportunity for me to learn and observe how materials behave and how we are able to create objects and put them into assemblies using different methods and materials. Among to most fascinating object ever build by humans is that thing that make us fly: the plane.
To me it doesn´t matter how many times I experience turbulence when I travel by plane, I can never seem to stop the seeds of panic forming in my brain. Rationally, I know the wings aren´t going to snap – I have traveled across the world on long distances, the flying today is the most reliable, fast, save and convenient way of traveling. Accidents happen of course but are rare compared with other means of transport. But knowing how these planes are build and what we are able to do with them, this is beyond extraordinary, it´s amazing, it is a fantastic technology achievment. The wings are glued together and they are being mechanically tested in this condition with remarkably good results. But, despite this, the rational part of my brain is being ignored by my panicking neurons everytime I travel by plane. I know I´m not alone in this. Over the years, I´ve learnt not to talk about aircraft being glued together to other passengers; they generally don´t find that reassuring. But here on my blog I want to tell you something about these materials. I want to tell you about Glue.
The most frequent material we use for everything we want to put fast and easy into an assembly is the glue, which is nothing else but initially a sticky liquid. But glue used to build strong assemblies is a very specific and special material, glue is an adhesive substance used for sticking objects or materials together.
In the course of time since their first use from the Middle Pleistocene era (circa 200.000 years ago) and during their development, adhesives have gained a stable position in an increasing number of production processes. There is hardly any product in our surroundings today that does not contain at least one adhesive—be it the label on a beverage bottle, protective coatings on automobiles, or profiles on window frames or as mentioned on aeroplanes.
Many liquids are sticky – that is, they will stick to you if you put your finger in them. Oil sticks to us, water sticks to us, soup sticks to us, honey sticks to us. Thankfully they stick to other things better than us, which is why towels work.
For instance when you have a shower, water trickles down your body, sticking to your curves of your chest, belly and bum in defiance of gravity. This stickness is due to the low surface tension between the water and your skin. When the water comes in contact with the fibres of a towel, they act as little tiny wicks – just as candle wicks suck up liquid wax – so the micro-wicks of the towel suck the water off your body. Hence your skin gets dry, and the towel gets wet. The stickness of liquids, then, is not a property intrinsic to any particular liquid, but is determined by how they interact with different materials. However just because something is sticky doesn´t mean it can be used to glue an aircraft together. Wet your finger and dab it on a speck of dust and it will stick to you, and remain sticking to you until the water evaporates. That water loses its stickness when it evaporates is why, although it is sticky, it is not a glue.
Glues starts off as liquids and then, generally speaking, transform into a solid, creating a permanent bond. This is a material process humans have been playing with for a very long time. Our prehistoric ancestors made pigments like powdered charcoal or naturally occurring coloured rocks such as red ochre, and used them to draw pictures on the walls of caves. To get them stick to the walls, they mixed the pigments with sticky things like fats, wax and egg, and so invented paints. Paints are essentially coloured glues, and these earliest ones were permanent enough to last for thousands of years. Some of the oldest cave paintings still in existence are in the Lascaux caves in France, estimated to be about 20.000 years old. Tribal cultures have long used these coloured sticky substances as face paints, a central part of both sacred rituals and warfare. The tradition continues today with the modern cosmetics industry. Lipstick, for instance, is made up of pigments mixed with oils and fats that allow the colour to stick to your lips – hence the name. Getting the glue to stick to your lips for hours, but still be easy enough to remove at the end of the day, has always been an issue; ditto eyeliner and any other kind of make-up. The problem illustrates one of the main themes in glue design – namely, that unsticking is often as important as sticking. But more on that later – mastering sticking is hard enough for now. If you want to stick something together that needs mechanical strength, like the components of an axe, a boat or, indeed, an aeroplane, then you need something stronger than paint or lipstick.
In the summer of 1991, 2 German tourists discovered the skeleton of a dead man while walking in the Italian Alps. The mummified man turned out to be 5.000 years old, and was later nicknamed Ötzi. His remains were extremely well-preserved because they´d been encased in ice since his death, as were his clothes and tools: he wore a cloack made of woven grass, a coat, a belt, leggings, a loincloth and shoes, all made of leather. All of his tools were ingeniously designed, but with respect to glue, it´s Ötzi´s axe that´s most interesting. The axe is made of yew wood, with a copper blade, bound together with leather straps stuck on with a birch resin. This gummy substance is produced by heating up birch bark in a pot, yielding a brownish-black goo that was widely used as an adhesive in the late Paleolithic and Mesolithic eras. It works for heavy tools like an axe because when it solidifies, it forms a tough solid. Our ancestors used it to glue arrowheads and flights, to make flint knives, to repair their pottery, and to make boats. The liquid is mostly made from a family of molecules called phenols. Their chemical name may be unfamiliar, but I´m sure you would recognize the smell: the major phenol in birch bark glue is 2-methoxy-4-methylphenol, which smells of smoky creosote. Phenol aldehyde smells of vanilla. Ethyl phenol smells of smoky bacon; indeed, whenever you smoke fish or meat, it´s the phenols that give them that distinctive flavor.
When you heat up the birch bark, you extract the phenols. The thick resin that´s produced is basically a mixture of a solvent called turpentine and phenols. The turpentine is the base of the liquid, but over the course of a few weeks the turpentine evaporates. This leaves just the phenol mixture, which turns from a liquid into a hard tar, a tar sticky enough to bond wood to leather and other materials. As it turns out, trees are excellent purveyors of sticky things. Pine trees exude nodules of resin that also make good glues. An adhesive popular for a thousands years, gum arabic, comes from the acacia gum tree. The resin of Boswellia tree is a particularly nice-smelling glue called frankincense. Myrrh, another aromatic resin, comes from a thorny tree called Commiphora. Resins were often used in medicines as well as in perfumes, perhaps because their active chemical components, like phenols, had potent antibacterials properties. Frankincense and myrrh were so highly valued in antiquity that they were given as presents to queens, kings and emperors, which is why their presence in the Christian nativity story is so significant. The stickiness of the tree resins is no accident. They evolved to be sticky so they could trap insects, and therefore provide a valuable form of defence for the trees. The gemstone amber is actually fossilized tree resin, and often there are insects and bits of debris trapped inside it, perfectly preserved. Without tree resins, it would have been very hard for our earliest ancestors to make tools and equipment, and so for our civilization to get off the ground.
Nevertheless, you wouldn´t want to glue an aeroplane together with them – they would certainly crack during flight. Phenol molecules don´t bond very strongly to other substances – the molecule itself is too self-contained, too happy sticking to itself. But once you´re in the tree, you don´t need to look very far for stronger glues. Consider birds: their wings are not bolted or screwed together. Their muscles and ligaments and skin are bonded via families of molecules called proteins. Our bodies are bonded together with them too. One of the most important of these proteins is called collagen. It is common to all animals and relatively easy to extract. Early humans used skins of fish and hides of wild game – they separated the fat and then boiled the skin in water. This extracts the collagen from the animals and creates a thick, clear liquid that turns into a solid, stiff material when it cools: gelatin. The collagen proteins in gelatin are long molecules made from a carbon and nitrogen backbone. In animals, collagen molecules, stick together to create strong fibrils that make up your tendons, skin, muscles and cartilage. But once they´ve reacted with hot water in the glue-making processes, the collagen molecules separate. They now have chemical bonds to spare that they want to satisfy. In other words, they want to stick to something else – they´ve become the animal glue gelatin.
It was animal glues that replaced wood resins as the mainstay of early human technologies. The Egyptians, for instance, used animal glue to make furniture and decorative inlays. In fact, it appears that the Egyptians were the first people to use glue to get around one of the main mechanical problems of wood – that is has a grain. The density and arrangement of the cellulose fibres in wood give it its grain, which is determined not just by the biology of trees but also their growth environment. Thus the grain varies from species to species and from tree to tree. The upshot is that wood is strong across the grain, but has a tendency to crack along it. This is useful if you are splitting logs for a fire, but if you are building a house, a chair, a violin, an aeroplane or pretty much anything out of wood, it presents a design problem. The thinner the piece of wood, the more cracking becomes an issue. Counterintuitively, the solution to this problem is to cut the wood into even thinner pieces called veneer. The Egyptians were the first to make veneer. They stuck pieces of it on top of each other, so that the grain of each layer was perpendicular to the one above. This allowed them to construct an artificial piece of wood that didn´t have a weak direction: we now call this plywood. They used animal glues to stick the plywood together and that worked reasonably well. But as you´ll have seen if you´ve ever cooked with gelatin, animal glue dissolves in hot water. Unless kept absolutely dry, furniture made from animal glues falls apart. This seems like a huge defect, but Egypt is and was a very dry place, and so they managed.
And as mentioned earlier, there are distinct advantages to having a glue that can be unstuck. Historically, the designers of classic musical instruments, like Antonio Stradivari, known as the greatest violin maker of all time, used animal glue to construct their instruments. This would have allowed Stradivari to unstick any joints that were faulty during production and so produce almost perfect instruments. Today, in order to repair a wooden instrument, craftspeople unstick the joints with stream. This causes the bond between the glue and the wood to weaken and then dissolve. Thus the wood comes away undamaged and clean, extending the lifetime of the instrument, and increasing its value. Indeed most people who work in furniture restoration use animal glues precisely because they can be easily unstuck using heat.
But when it comes to making wings, heat can be real problem, or at least that´s what legend tells us. Just look at what happened to King Minos, who ruled the Mediterranean island of Crete and was a very beautiful snow-white bull from the sea god Poseidon. King Minos was instructed to sacrifice the bull to honour Poseidon but he sacrificed another bull instead, because he did not want to kill the more beautiful one. To punish him, Poseidon made King Minos´wife fall in love with the bull, and the offspring of that union was a creature that was half man and half bull – a Minotaur. This Minotaur grew up to be a terrifying beast that ate humans, and so King Minos got his master craftsman Daedalus to construct a prison for the Minotaur in the form of an elaborate maze called the Labyrinth. To prevent Daedalus from telling others about the secrets within, King Minos imprisoned him in a tower along with his young son, Icarus. Daedalus, though, was a hard man to contain. He constructed wings by glueing feathers together with wax: one pair for himself and one for Icarus. On the day of their escape Daedalus warned his son not to fly too close to the sun. but during their flight Icarus was so exhilarated that he began to soar higher and higher. The wax melted, the feathers came unstuck, and Icarus fell to his death.
If you are wondering whether a modern aircraft could come unglued as it flies higher and higher, I should point out that the myth of Icarus defies logic. By flying higher, Icarus would have experienced colder temperatures, not hotter ones. Temperature decreases by 1°C for every 300 meters of altitude you gain, because the atmosphere is cooled by the radiation of heat into space- At 12.000 meters, the altitude a passenger plane flies, the temperature is at approximately -50°C, a temperature at which all waxes are solid.
I should also say at this point that modern aircraft are not glued together using wax – nowadays we have much better glues. The intellectual journey of their discovery starts with rubber. Rubber is of course, another sticky tree product. It´s extracted by tapping the bark of the Pará rubber tree, which is indigenous to South and Central America. The Mesoamerican cultures made many things with it, including the bouncing balls they used in their ritualistic games. When European explorers reached the continent in the 16th century, they were amazed by the rubber. They had never seen anything like it before: it has the softness and pliability of leather, but is far more elastic and completely resistant to water. But, despite its obvious value, no one in Europe could find an immediate economic use for it, until the British scientist Joseph Priestley found that it was good for rubbing pencil marks off paper – which is how rubber got its name. Natural rubber consists of thousands of small isoprene molecules bonded together in a long chain. This molecular trick of linking together units of the same chemical to make a completely different one is common in nature. These types of molecules are called polymers – “poly” meaning “many“ and “mer” meaning “unit”. Isoprene is the “mer” in natural rubber. The long polyisoprene chains in rubber are all jumbled up like spaghetti. The bonds between each chain are weak, which is why there is not much resistance if you pull rubber: the chains just unravel. This is what makes rubber so stretchy. It´s rubber`s stretchiness that makes it so sticky. It can mould itself easily and so wedge itself into any space, including the crevices in your hand, which is what makes it grippy.
That grip is the reason why rubber is perfect for putting on the handlebars of a bicycle, or for making car tyres – it sticks the car to the road strongly enough to create the friction needed to move your wheels forward, but not so strongly that the car gets stuck to the road permanently; similarly it holds your hands to your bicycle´s handlebars firmly enough for them not to slip off accidentally, but you don´t need to worry about getting struck to the bike for ever.
One of the unnoticed, but most ingenious, uses of rubber is on Post-It notes. Post-Its have an adhesive layer of rubber that remains stuck to the notes when you pull them from their pad, so they can be attached to walls, tables, computer monitors, books and more, without damaging them or leaving a mark. The microscopic spheres of rubber that make up the glue on the Post-It bond strongly to the note itself, but when pressed on to a surface they create only a small adhesive force. Which is why, when you pull the Post-It off whatever it’s stuck to, the rubber stays put on the paper. Thus the Post-It is repositionable and reusable. Genius? Well, actually, this not-very-sticky glue was, in fact, an accidental invention, stumbled upon in 1969 by Dr. Spencer Silver, a chemist from the 3M company, while he was trying to make a super-strong adhesive.
Many other culture-changing adhesive products emerged in the 20th century. One of the most important of these was sticky tape, invented in 1925 by another 3M company inventor, called Richard Drew. Drew’s tape is composed of 3 key layers. The middle layer is made of cellophane, a plastic made from wood pulp that gives the tape its mechanical strength and transparency. The bottom layer is an adhesive and the top layer – the crucial layer- is a non-stick material, like Teflon, which has a high surface tension with most other materials and so cannot be easily wetted by them (which is why we use it in non-stick pans). Its use in tape really is genius; it means the tape can be placed on top of itself without permanently sticking to itself, allowing it to be manufactured as a roll. And a roll of tape – well, what home is complete without one? Or ten, in my case 🙂
You can tell a lot about someone from how they handle a roll of sticky tape. I have to admit straight away that I’m a tearer, not a cutter. Ask me for a bit of tape and I’ll grab the roll and enthusiastically try to rip a piece off for you. I probably won’t get it right first time. Most likely I’ll mangle a few pieces first, either tearing them off at a crazy angle or snapping them off clean, and inevitably I will somehow allow the sticky parts to get stuck to one another. I’m not proud of this; it actually sends me into a rage. I get increasingly furious with the tape, which in turn seems to goad me by sticking itself back on to the roll so seamlessly that I can’t find the end. At this point I have to resort to running my thumb around the roll, trying to locate the end by feel alone. This sometimes takes so long that I started shouting at the tape. Then I throw it across the room – and wonder why it is that I still don’t own a tape dispenser. Gaffer tape suits my personality better. It’s designed to tear without scissors. It’s reinforced with fabric that runs across the roll, and makes the easy tear possible. The strength of the tape comes from the fabric’s fibers, while the stickiness and flexibility come from the plastic and adhesive layers. I love gaffer tape so much, I confess I envy people with jobs that require them to carry it around with them on their belt.