Superglue for Repairs

Our illustrious editrix Erin asked me about the potential dangers of using superglue to repair an item that would come into contact with drinking water, which is something that may come up if you're using your gear for a while in rough conditions. Things break, and knowing how to safely repair them is important.

CAUTION: chemistry ahead!

Cyanoacrylic glues (there are a few different formulas) may sound like they would be poisonous due to the part of their name which sounds like “cyanide”. This is incorrect; the actual root of the cyan- prefix is merely the Greek word for blue (kyanos), because many blue dyes contain some form of cyanide salt. Cyanide (CN-) is a naturally-occurring anion that reacts with other materials and forms numerous different molecules that form the cyano group, not all of which are dangerous.

Most people know that CN- was used in various gas-chambers over the years, but it was actually Hydrogen Cyanide (HCN) that did the killing. The spy movie cyanide capsules were Potassium or Sodium Cyanide (KCN or NaCN), which are very toxic and would also create HCN when it reacted with the acid in a person's stomach. The reaction with acid is how most capital punishment gas chambers were set up: a container of acid under the prisoner's chair would have a bag of KCN or NaCN lowered into it by remote control, forming a strong cloud of HCN gas inside the sealed room. HCN kills by blocking the use of oxygen inside the body and it was used as a battlefield chemical weapon in WW1. The Zyklon B used in the gas chambers of Nazi concentration camps was a liquid form of HCN absorbed into diatomaceous earth, HCN melts at 8° F and boils at 78° F, so it will vaporize at room temperature.

Once dry, cyanoacrylic glue doesn't have any of those dangers. Before I even looked up the specifics, I was able to assure Erin that superglue was food-safe because I've known several people that used it to repair broken dentures over the years with no ill effect. The “liquid sutures” that you see used to bind the edges of minor cuts are nothing more than cyanoacrylic glue. I have used it on myself and various animals, and it does a good job of sealing skin to skin - ask any kid who has superglued his fingers together how strong the bond is. Idiots will concentrate liquid cyanoacrylic glue in a container and “huff” it for the nearly-lethal “high” they will get as their bodies starve for oxygen, but once dried it is safe.

Looking into the subject a bit deeper, I found that surgeons in the 1960's used it to close wounds on internal organs like the liver, but it wasn't FDA certified for that use until recently because the patent had expired and nobody wanted to spend the millions of dollars that the testing would have required. Cyanoacrylic glue is considered non-toxic, with a rather high LD50* of 5 grams per kilogram of body weight. Compared to NaCN and KCN with a LD50 of 5-10 mg/kg, cyanoacrylic glue is literally a thousand times less toxic.

* LD50 is a common measure of toxicity. It stands for Lethal Dose for 50% of the test population. If you were to give 100 rats that weigh 1 kg each a dose of 5 g of cyanoacrylic glue, you could expect half of them to die from the dose. Various small mammals are used in such testing, and the results extrapolated based on body weight. It's not an exact measure, but it gives a baseline to measure toxicity. For example, a 100 kg human would require 100 times the dose that a 1 kg rat would. That means that the average American adult would have to ingest about a pound (~500 g) of superglue before hitting the 50/50 lethal dose. Since the glue is sold in packages measured in grams, you'd need a couple of hundred tubes just to get to that 50/50 dose.

There is however a type of repair that you don't want to use superglue for, and that is anything which will get heated. This will cause the glue to break down, releasing a small amount of HCN gas. The failure of the repair will probably be more of a hazard than the minor amount of toxic gas released, but it is still a hazard.