Tech In Plain Sight: Superglue [Hackaday]

View Article on Hackaday

Many inventions happen not by design but through failure. They don’t happen through the failure directly, but because someone was paying attention and remembered the how and why of the failure, and learns from this. One of these inventions is Super Glue, the adhesive that every tinkerer and engineer has to hand to stick pretty much anything to anything, quickly. Although it was a complete failure for the original uses it was developed for, a chemist with good memory and an eye for a helpful product created it in a process he described as “one day of synchronicity and ten years of hard work.”

Super Glue was initially invented in 1942, when the chemist Harry Coover was working on a team trying to develop a clear plastic gun sight that would be cheaper than the metal ones already in use. The team cast a wide net, trying a range of new materials. Coover was testing a class of chemicals called cyanoacrylates. They had some promise, but they had one problem: they stuck to pretty much everything. Every time that Coover tried to use the material to cast a gun sight, it stuck to the container and was really hard to remove. 

When the samples he tried came into contact with water, even water vapor in the air, they immediately formed an incredibly resilient bond with most materials. That made them lousy manufacturing materials, so he put the cyanoacrylates aside when the contract was canceled. His employer B. F. Goodrich, patented the process of making cyanoacrylates in 1947, but didn’t note any particular uses for the materials: they were simply a curiosity. 

It wasn’t until 1951 when Coover, now at Eastman Kodak, remembered the sticky properties of cyanoacrylates. He and his colleague Fred Joyner were working on making heat-resistant canopies for the new generation of jet fighters, and they considered using these sticky chemicals as adhesives in the manufacturing process. According to Coover, he told Joyner about the materials and asked him to measure the refractive index to see if they might be suitable for use. He warned him to be careful, as the material would probably stick in the refractometer and damage it. Joyner tested the material and found it wasn’t suitable for a canopy but then went around the lab using it to stick things together. The two realized it could make an excellent adhesive for home and engineering use.

So, they patented the use of cyanoacrylate as an adhesive, a patent granted in 1954. Eastman Kodak started selling it as Eastman 910 in 1958, and that exact formula is still sold today. On one memorable demonstration, Dr. Coover lifted the 150-lb host of the TV show I’ve Got a Secret with a single drop of the glue., and lifted a Corvette using the glue.  

Dr Harry Coover was awarded the national medal for Technology and Innovation in 2010, but he never made that much money out of Super Glue because the product didn’t catch on until the patent expired and others started making their own versions. That wasn’t the only thing that Dr Coover worked on, though: he is listed on over 350 patents. He always claimed that Super Glue was his favorite and that he was most proud of how the glue was used to save lives in the Vietnam War.

That happened when the US Army realized that you could use it to seal bullet wounds on the battlefield. Because it reacted with the water in the blood, it quickly staunched bleeding and closed a wound until it could be adequately operated on. It forms a thin, strong layer, but it can be fairly easily removed because, as anyone who has Super Glue on their hands knows, once it sets, it has low shear strength. If you get it stuck between your fingertips and it sets, you won’t be able to pull them apart. You can, however, remove it by either waiting until a layer of sweat forms under the glue, or by moving your fingertips back and forth against each other, shearing the bond between glue and finger. Combine this with the fact that cyanoacrylate is not poisonous, and you have a great way to seal a wound quickly.

Modern Super Glues

Modern cyanoacrylate glues come in a wide variety of types, including ones that mix cyanoacryaltes with epoxy to make a stronger bond, and ones that add materials to improve the bonding on porous surfaces. There are also chemicals you can add, making it set quicker if you need to fix something quickly.

How Super Glue Glues

The chemistry of Super Glue is fascinating. Although the specific chemicals used in brands and types differ, all of them are cyanoacrylates, composed of very short chains (called monomers) of carbon atoms surrounded by oxygen and nitrogen and tipped with hydrogen atoms. The chain is relatively stable, but things start to happen as soon as you introduce water. 

Water is composed of two hydrogen atoms linked to a single oxygen atom. Still, the bonds that hold these together sometimes fray, leaving hydroxide ions, consisting of an oxygen atom with only one hydrogen atom attached. These float around, looking for something to bond to. When one of these bumps into a cyanoacrylate molecule, it latches onto one of the carbon atoms, but this leaves the carbon atom with a spare electron, where it wants to bond with something. Carbon loves to bond to other carbon atoms, so two of these unbonded cyanoacrylate molecules bond together. This process releases more ions that start bumping into more water molecules, creating more hydroxide ions, and so on. So, once started, the process repeats, creating a long chain of these molecules, a polymer. That is called chain growth polymerization, and it requires very little water to happen. That’s why if you leave the cap off a container of Super Glue, it sets because the moisture in the air sets off the polymerization process. 

Polymerization of Cyanoacrylate [Wiki Commons]

This process will also happen because of traces of water in the glue, so even when stored sealed, the polymerization will occur eventually so that the glue will set in the tube.

Water is not the only thing that Super Glue will react with: it also bonds with the oils and proteins in fingerprints. That’s why it can be used to find fingerprints: if you heat it to make a vapor, some will stick to the oil and protein in a fingerprint, which will polymerize more, creating a visible fingerprint impression. This also explains perhaps the most common application of cyanoacrylates: gluing your fingers either together or to the project at hand.

So think about this the next time you reach for a tube of Super Glue to fix a project: this is available because an observant chemist remembered his failures and realized they were not failures; they had just not found the correct use yet.

Featured image: “Super Glue” by Andrew Gustar