Superglue
From Chempedia
Superglue: The Functional Use of Polymers
Ever wondered what super glue actually is? Maybe even how it works or where it is used? Take for instance, a space capsule. The heat generated by re-entering the atmosphere is extremely hot. A heat protective shield is attached to the bottom of the capsule using, you guessed it, superglue. This use was started back in the 1960’s, by NASA, but was discontinued because the heat that was generated caused the bonds to break between the capsule and the shield.[1] Superglue is a clear, acrylic resin consisting of methyl alpha-cyanoacrylate (see picture 1) and is also known as cyanoacrylate or CA glue – a more generic name.
Other uses for CA glue became known as time passed. Dentists began using CA glue to repair crowns and bridges and the glue was also experimented with in the Vietnam War. However, the enzymes in saliva were discovered to cause bond breakage, and since medical uses have been altered. The most common current use of superglue still remains to be everyday household projects.
An interesting use was developed in the war to help soldiers that were bleeding to death. The glue was tested unofficially on soldiers with wounds and it was observed that it stopped bleeding quite effectively. A spray was then invented that, when a warrior had a gaping wound the glue was sprayed on the injury to stop the bleeding until they could get the victim to a hospital.
Picture 1 – Cyanoacrylate Monomer: 
[2]
Now you may ask yourself, if such a fast acting strong glue exists, how is it packaged? While CA glue is still in its initial package, it will not stick to the inner surfaces. This is due to the presence of an acidic stabilizer which acts to inhibit the polymerization. Polymerization is the formation of a polymer from several monomers. The glue will also not react because there is no water in the container. In order to trigger the solidification process, hydroxide ions (OH-), found almost any where in our atmosphere, must be present. Very few of these ions are needed to trigger the reaction. Exposure to air, even if short in duration, can cause solidification – which is the physical state of the bond.
As hydroxide ions are introduced into the CA glue, polymerization begins to form chains of methyl alpha-cyanoacrylate. The acidic stabilizer previously used to prevent the liquid CA glue from changing states is rendered ineffective when exposed to the bonding structures. This is because it is neutralized by the alkalinity of the substance being bonded or by the partially ionized water molecules present microscopically on most surfaces. As the reaction proceeds, the monomers continue to bond, forming a durable, strong mesh-like structural polymer. This polymer allows for a bond between the desired structures.
Polymerization occurs because the CN and COOCH3 of the methyl alpha-cyanoacrylate are powerful electron-withdrawing groups, which means that they attract electrons. To form the first link of the chains, an anion precursor is needed and is provided by the CN and COOCH3. The structures of these two molecules also create a very stable "foundation-bond" and as such act as the stepping stone for the chain of bonds to come.
Once the chains can no longer freely move, the reaction is complete. This can take up to twenty-four hours, but some variations of CA glues take as little as thirty seconds. When exposed to liquids with hydroxide ions in them, they can complete the reaction instantly. This is shown clearly in this statement that, "It’s virtually impossible to swallow the glue because it will cure as soon as it gets into the mouth."[3]
The reaction can be quickened even more by the further introduction of an alkali, such as baking soda, because its presence will increase the number of hydroxide ions present. This will increase the rate at which the stabilizing acid is neutralized. Likewise, the reaction can also be slowed by the introduction of even slightly acidic solutions.
Temperature is another factor to consider in effectiveness of CA glue. If heated to temperatures above around 180 ºF, CA glues will generally lose their effectiveness as demonstrated in the space capsule example. CA glue also becomes brittle when it’s exposed to cold temperatures as high as 30 ºF. Acetone (CH3COCH3), commonly found in nail polish remover, also causes the degradation of the structural polymer.
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Footnotes
- ^ http://glassattic.com/polymer/glues-Diluent.htm
- ^ http://en.wikipedia.org/wiki/Super_glue
- ^ http://www.greatplanes.com/faq/adhesives.html
- ^ http://ludicrosity.net/galleries/airtoons/imagepages/superglue.html
Researched and written by: Molly Stammer and Spencer Service
