Here are two metal cylinders and a single drop of superglue. Once it sets, you can literally hang from that one drop of adhesive. Oh my God. That is crazy. So how is superglue so strong? Well, that's what I want to find out. And along the way, we're going to learn how it sets so quickly, why it's so good at sticking to skin, how it saves lives, and how it might even help solve our plastic pollution problem. Plus, if it's so strong, one single drop can easily lift over three tons. Why is it weak in certain circumstances? Mom is going to kill us. *scream* In 1942, the US was at war. And to accelerate the production of gun sights, the Eastman Kodak Company was looking for a clear plastic that could be cast instead of grinding glass lenses. Chemist Harry Coover was working on a compound called cyanoacrylate. It looked promising, but it had the unfortunate tendency to stick to everything it touched. Coover called it a severe pain. And as the war wound down, Kodak gave up on finding a plastic replacement and kept on making gun sights out of glass. By 1951, Coover was again trying to develop a clear plastic. This time for jet plane canopies. If he could just solve the stickiness problem, cyanoacrylate could work. Coover showed the material to his coworker Fred Joyner, but gave him strict instructions. I told him, look, don't take a refractive index of this material. If you do, you're going to wreck the refractometer. Cos it’s going to stick it together. But after testing 909 other compounds, Joyner had forgotten. So he prepped the 910 test by smearing cyanoacrylate between two prisms. After taking the measurement, Joyner discovered he couldn't pull the prisms apart. He panicked. The refractometer, worth nearly $10,000 adjusted for inflation, was ruined. But instead of getting angry, Coover had a flash of insight. He got a sample of cyanoacrylate and began gluing together anything within reach. Glass plates, rubber stoppers, metal spatulas, wood, paper, in all different combinations. He said “Everything stuck to everything almost instantly and with bonds I could not break apart.” With this discovery, he dubbed the compound Eastman 910 Adhesive, as it was the 910th substance tested in the company's search. But nowadays, everyone calls this super glue. So how does it work so quickly with such strength on so many different things? Bonds almost anything. A plastic knob, a plastic plug, a rubber boot, a metal brooch, a fishing rod, a cycle grip, model planes and model trains, a doorknob screw, a flashlight case, the broken trim on any car. Super glue in the tube is a liquid of identical monomer molecules. The molecule is ethyl cyanoacrylate. When you put it between two surfaces, the liquid flows into all the pores and crevices. Then the monomers start reacting with each other, joining to form long polymer chains. This turns the glue from a liquid into a solid, and at this point it can no longer be pulled out of the cracks and crevices, so it's stuck in place and the two surfaces are connected. If you're ever trying to glue surfaces that are too smooth, this is probably the reason super glue sticks poorly. There are few crevices or pores for it to cling to. To fix this, you can sand the surfaces to introduce some surface texture. That way when the liquid glue solidifies, it's stuck in the cracks. But what triggers the monomers to solidify instead of staying liquid like they are in the tube? Well, ethyl cyanoacrylate is primed to react because it has two double bonds and one triple bond close together. What makes it so reactive is you have a double bond attached to a nitrile group and an ester group. And the unique part about that is the chemistry of those groups that make that double bond so electron deficient. The oxygen and the nitrogen atoms attract electrons more than the carbons. So that leaves this carbon slightly positively charged. It is hungry. It is looking. And anything even slightly electronegative, it will attack and it'll start a reaction. In the presence of a negative ion, the carbon double bond breaks and four single bonds form. But the nitrile and ester groups are so electronegative they pull the extra electron on across the molecule, making this carbon negative. That invites another slightly positive carbon on a separate monomer to attack. And now they're chemically linked together. The start of a polymer chain. And once that goes now it has the ability to go react with all those other super glue hungry monomers and rapidly polymerize. It's a really, really chemically reactive species. That single initiator started a chain reaction. The next monomer again pulls across the electron and more and more monomers bond together, forming longer and longer chains until the super glue has solidified. It normally takes between 10 and 30 seconds to set. This was much faster than other adhesives available in the 1950s. A lot of glues like white glue and glue sticks work by drying out, so you have to wait for the water to evaporate. But with super glue it's almost the opposite. While I was prototyping snatoms, I 3D printed these plastic shells and super glued them together. One day I was trying to open a bottle of super glue where the cap had gotten glued on to the nozzle, so I tried twisting it off with my teeth, but the bottle exploded, filling my mouth with super glue. And I was thinking, well, I might have a few seconds to spit it out with the saliva in my mouth before it set. But the super glue actually hardened immediately. It stuck to my teeth and my tongue. I had to get my now wife to pick it out of my teeth with tweezers. Luckily, it didn't glue together anything essential. I think it's because it solidified so rapidly. And the reason it did that is because the polymerization of super glue is actually triggered by water. Specifically, it's the slightly negative oxygen atoms in the polar water molecule and the negative hydroxide ions in water that often break open the carbon double bond, initiating the formation of chains. And water is everywhere. I mean, there's moisture in the air, little bits of water on most surfaces, or absorbed into materials like fabrics. That's why super glue sets rapidly on almost every surface. Each water molecule can initiate the formation of a polymer chain. There's a forensic technique that uses super glue fumes to pull fingerprints from non-porous surfaces. When you grab something, your hand leaves behind moisture and oils that are perfect for super glue to bind with. And this also makes skin ideal to stick to. There are a lot of wrinkles and pores for super glue to seep into. Plus, the protein collagen has a number of negative regions that can initiate the polymerization reaction and bond directly with a monomer. So your skin is the perfect sort of surface for super glue to stick to. And that's why it's so hard to get off. Some of your molecules literally become part of the polymer chain. There's a medical case study where someone got cyanoacrylate all over his hands. So he quickly went to wash it off with soap and water. But that accelerated the polymerization reaction, and his hands were stuck together. So he went to multiple medical professionals, and he got terrible advice. They tried alcohol, water and soap. They also attempted physical separation. And he went to consult cosmetic surgery. But finally someone suggested acetone. Acetone, or nail polish remover, dissolved the glue and released his hands. Now, unfortunately, the bottles for some types of super glue look incredibly similar to those for eyedrops. So there are hundreds of cases of people putting super glue directly into their eyes. In that case, acetone is not the solution. Don't try to separate your eyelids, just seek medical attention. Super glue is strong. To demonstrate this and promote his new product, Coover went on a game show. He showed that with only a single drop of glue, he could lift himself and the host into the air. Here we go. One drop of glue. And after 24 hours, it would support something like 15,000 pounds. One drop of glue. Now, doctor, you must be very proud of this. I imagine many years of research went into its discovery. No, as a matter of fact, it wasn't Gary. This discovery was purely by accident. Bully for you. Super glue polymers are almost all single chains running linearly between the surfaces. They have a directionality to them, kind of like wood grain. The chains are densely packed with some cross-linking between them, and this makes the polymer fairly rigid. It's about as hard as a hard hat. This makes it strong in compression. Under tension, super glue is strongest when you pull in the same direction as the chains. The tensile strength of super glue can be upwards of 25 mega pascals, which is similar to other polymers, meaning a square patch just five centimeters on a side could suspend a fully grown African elephant. But super glue also has its weaknesses. It is brittle, so if there's a sudden impact, it just kind of falls apart. As a super glue is very quickly reacting. It usually produces really short polymer chains that have a matrix with built in stress. Any time you have stress in a material, it's a potential failure point. The thing that makes it a really good adhesive is kind of its Achilles heel. in some cases. Polyethylene, polypropylene, nylons, like those squishy plastics. Their polymer chains and their chemistry are such that they can absorb a lot of impact. If a force comes in, they'll kind of deform. Whereas in the chains of a super glue polymer, it doesn't have the ability to do that. So any impact will come and it'll physically break those rigid bonds. And it'll also kind of be absorbed in those stress points, and it'll cause the whole thing to fracture. Super glue is also weak if a force is applied perpendicular to the polymer chains. That is, it's weak in shear. During this shear test, the stress releases breaking the super glue bond. So shear is kind of a combination of forces. So you kind of have like a compression and like a friction going on. Tension is a pretty uni-directional force. And the polymer chains are able to deform enough. However, in shear you have a more dynamic set of forces going on. And and they're just kind of breaking up all at once. Shear force doesn't get spread evenly. It's highest at the edges and lower in the middle. And since super glue is so brittle, it can't redistribute this stress, causing the bond to fail. It's even worse if you grab one end of a surface and try to peel it back. In that case, all that force is on a few polymers, so the chains break one by one like a zipper unzipping. Two long bars super glued together could be strong since there's a lot of surface area. But if one end breaks, the rest can easily peel off. Now a bond breaking is bad, but there are some materials that super glue won't even stick to at all. So here we have a milk bottle, and this is made out of a plastic called polyethylene. I'm going to put one drop right here. But it's so weak you can see the glue has made a perfect little film and it doesn't stick whatsoever. The materials that super glue does not bond to. They fall in to category of materials that are known as chemically inert. And by inert it just means that they don't have reactive sites. So super glue is so reactive because it is so electron deficient. It's looking for any source of electrons. But you have a polypropylene or a polyethylene or a Teflon. And carbon loves carbon, and carbon is not sharing its electrons. So there's nothing on the surface that is willing to donate any reactivity. Even if you add initiators, say, by spraying water, it still doesn't work. That's because these materials are hydrophobic and non-porous. If you spray like, a polypropylene sheet, with water it'll just fall off, right? You know, if you can think of, like, the beating up of water on a hydrophobic surface. So if you pour super glue on top of water, it'll just like, you'll just get like a clump of super glue on it. And then you can just peel it off. Most super glue specifically warns against using it on polyethylene and polypropylene because it won't stick. But this is actually a good thing. You have to have something that super glue won't stick to, so it can be contained and stored without setting. Super glue quickly found industrial applications. Its first sale was to Mason and Hanger in 1956, who used it to assemble atomic bombs. Soon, different additives were devised to change the properties of pure ethyl cyanoacrylate. The monomers by themselves are very runny, almost like water, so companies added thickening agents like fumed silica to turn it into a gel. Fumed silica forms branched structures that increase viscosity. Some acid is often added to inhibit polymerization in the tube. If you want to speed up the setting of super glue, one way is to add more negative ions, the initiators that start the polymerization reaction. You can buy accelerators specifically for this purpose off the shelf, but a number of DIYers and handymen use baking soda - sodium bicarbonate. It reacts with moisture in the air to produce hydroxide ions. So when you add super glue, it sets even faster due to all those ion initiators. Plus, it forms a really hard composite substance. You can layer super glue and baking soda to strengthen a joint and fill gaps. It can even be drilled or sanded after setting. It's interesting to see how baking soda in solution speeds up the polymerization of super glue. If you take pure water and pour in super glue, you get a bunch of little plastic droplets. The glue sets quickly, but not fast enough to keep the whole stream connected. But with baking soda dissolved in the water, the super glue sets even faster. This creates a continuous length of polymer, which is really cool to see, but the resulting plastic is fragile and can easily be crushed. If you wanna glue things underwater with superglue, the key is actually to slow the polymerization down. With a gel cyanoacrylate, the thickeners slow the reaction, giving you enough time to apply and detach objects. - Wow, there it is: glued underwater. It's not even a perfect fit. I didn't put it together quite right, but it’s still stuck. - One day, Coover's eldest son was making a model, when he accidentally cut his finger. As a quick-thinking and experimentally-minded father, Coover got some super glue he brought home from the lab and applied it to the cut. It instantly sealed shut. Coover immediately saw its potential in medicine, and he went to work. He envisioned a glue that could completely replace sutures. But his team soon ran into three key problems: First, as super glue sets, all those bonds forming release heat. - This is a standard cotton ball. Just gonna add some liquid super glue. 86. Oh my gosh, it's starting to smoke. 93, 99, 108. Whoo, made my eyes water. 120. If you do this at home, wear goggles, and probably a respirator, and probably do it outside. I'm feeling it now. It is hot, hot to the touch. - The cotton has a lot of surface area, and it's absorbed a lot of water, so the superglue sets even faster than usual and releases all its heat at once. If I get it on my skin, the temperature increase wouldn't be as dramatic, but it's enough to irritate a wound. The second problem is that over time, super glue in the body breaks down, and some of the things it breaks down into are toxic chemicals like formaldehyde. And finally, super glue is hard and brittle, unlike us. We are mostly just squishy bags of meat and water. So an adhesive for living tissue needs to be soft and flexible for the entire duration of the healing process. Remarkably, Coover and his team found that all of these problems could be solved with a single change to the molecule: Simply increase the number of carbons in the alkyl chain. With longer carbon chains, it takes more time for the monomers to bind together, which slows down the rate of reaction. This slows the rate of heat released, so there's not a significant temperature increase all at once. The longer polymers also break down much more slowly, so the wound has enough time to heal before the glue starts releasing toxins into the body. It's removed before that happens. And finally, since the reaction is slower, there's more time for the monomers to float around and form longer polymers. These longer polymers can absorb stress better than shorter chains, meaning the glue can flex more without breaking. With the main problems addressed, Coover submitted an application to the FDA in 1964 for medical superglue. The US military was very interested in Coover's adhesive, and they developed a medical super glue spray for use in the Vietnam War. The spray saved lives. In one case, a bullet hit a 24-year-old soldier, passing through his kidney and liver. After part of his liver was removed, they gave him 12 liters of blood. That's enough to replace all the blood in his body twice over. But the bleeding just wouldn't stop. By conventional methods, he was gone. But then the surgeons sprayed superglue directly on the liver. The bleeding stopped, vital signs returned to normal, and the soldier recovered. Despite its success on the battlefield, medical superglue was held up in bureaucratic red tape for years, so long that Coover had to abandon the project. It wasn't until 1998 that he saw his dream of a medical glue approved: a 2-octyl cyanoacrylate called Dermabond. Medical super glue has now grown to be a $900-million-a-year industry. In the 74 years since Coover's accidental discovery, cyanoacrylate has grown to become a $3 billion industry. But its impact doesn't stop there, because now scientists are turning to Coover's original research, exploring its use as a plastic, and it could well solve one of the biggest problems the planet faces: how to recycle the mountain of plastic we produce each year. Manufacturers can shred, melt, and reform other plastics, but the polymers degrade, so the quality is worse and the process generates microplastics. - So you can only recycle mechanically and thermally like that a certain number of times before you get a material that's kind of useless. It still will sit in the environment, and it won't break down, but it's not useful. - [Derek] But superglue is unique. If you heat it up to 210 degrees Celsius, it'll break back down into pure monomers. These can be distilled and then reactivated back into fresh polymer. - We have a starting material, we make it into a plastic, and then, under a certain stimulus, we can turn it back to that starting material. So let's see if we can make a plastic from this, you know, 'cause the goal of a depolymerizable plastic is that it's sustainable, right? - There are just two problems: First, how do you cast something that sticks to basically everything? And second, super glue on its own is brittle, so how do you stop it from breaking? - The big problem is the handleability. That was kind of the original problem. They found that, okay, this isn't a great plastic because it sticks to everything. That's how they discovered that it was an excellent adhesive. So we were faced with that same problem. - But Coover didn't have the modern plastics we have today. Polypropylene, polyethylene, and Teflon are so inert, they don't activate superglue. So they are the perfect materials to handle the monomers. - So we were able to actually handle it, so that was, you know, a huge check. We can actually work with this material. - Next, to reduce brittleness, you need super glue to form long chains that tangle together. And this problem can actually be broken down into two parts. First, a lot of initiators means lots of short polymer chains. Second, as the superglue sets, the existing chains freeze and they can't connect together. There also might be some unused monomers that can't get into the right spot to attach. If you can solve both these issues, you get longer chains. The first fix is to use a very weak base. By mixing in a little bit of this weak base, there are enough initiators to start the polymerization reaction, but not too many, so the polymer chains end up being longer. - We use DMSO, dimethyl sulfoxide, which, in no other context, would be used as an initiator, but it was just electronegative enough that we were able to initiate a reaction that could go slowly and proceed in a way that produced a nice solid plastic. - [Derek] Next, you need a solvent, in this case, acetone. - And what a solvent is, is it's just a medium that dilutes the conditions. It does not participate in the reaction. Usually, it provides additional mobility. - [Derek] After mixing the glue and initiator into the acetone, the mobile polymers are able to form even longer, more stable chains than usual. And after a while, all the superglue sets, the acetone evaporates, and the new plastic is removed. To reuse it, just heat it up and distill it to get back superglue monomers. - So I would, on average, get around 93%, which is excellent. - [Derek] Compared to that, even the most widely recycled plastics can't return to the same quality, and they can only be downcycled once or twice before ending up in landfill. - I'm super glue's biggest fan, obviously, but I hope that it can actually have a real impact in terms of sustainability and kind of revolutionizing how we look at plastics and how we look at materials that, you know, we've existed with for decades. You know, okay, what if we use this for something else? - Coover said it took the right mindset to see cyanoacrylate as a great adhesive. The first time he worked with it, he was thinking about gun sights and nothing but gun sights. Its sticky qualities were a pain. The second time he encountered it, his colleagues fixated on the broken refractometer, but he could finally see the quality that frustrated him as a benefit. And then, after decades of researchers only viewing it as a useful adhesive, it took another flash of inspiration to see its potential as a plastic once more. As Coover said, this should serve as a reminder to all of us to be open-minded and curious enough to pursue unexplained events and unexpected results, which may unlock new secrets and lead to new and exciting discoveries of the future. Coover's ability to look beyond roadblocks and see the breakthroughs on the other side is a skill that every great innovator shares. It's not some kind of rare genius, it just takes strong critical thinking and problem-solving skills, which anyone can learn. And if you wanna start building these skills yourself, right now for free, look no further than today's sponsor, Brilliant. Brilliant will help you build real skills through thousands of interactive lessons on everything from math and science to programming and technology. 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