Instant glue

Why does a glue that sits liquid in its tube for years harden the moment it meets a fingertip?

A tube of superglue is stable for years. Squeeze a drop onto skin and it is solid in seconds. Both facts are about the same liquid, so whatever makes it set cannot be a property of the liquid alone — something the fingertip supplies must be missing from the tube.

The common guesses are that it dries, or that it reacts with air. Both are wrong, and it is worth seeing why they are wrong before reaching for the right answer, because the reasons rule out whole families of explanation.

Take drying first. Drying means a solvent leaves and the dissolved solid stays behind. Superglue is not a solution of anything — the liquid in the tube is the substance that becomes the solid, and the drop that hardened is essentially all still there. So the transition is chemical: small molecules joining into large ones.

Now air. If oxygen were the trigger, the open nozzle would seal itself the first time you used it, and a thin film spread in open air would set fastest. Instead the classic failure is the opposite — a joint in very dry conditions can take an unhelpfully long time — and what actually clogs the nozzle is a humid atmosphere. That points hard at water.

So: water, or something water brings. Ask what a molecule would have to look like for a trace of water to set off a chain of joinings. It needs a double bond that can be opened, and the opening must leave behind something that can attack the next molecule — otherwise you get one addition and nothing more. Cyanoacrylate is built for exactly that. Its carbon-carbon double bond carries two strong electron-withdrawing groups on the same carbon, a nitrile and an ester, which pull electron density away and leave the far carbon of the double bond unusually hungry for electrons.

Now a weak base — hydroxide from adsorbed surface moisture, or an amine — attacks that hungry carbon. The double bond opens and the negative charge lands on the carbon bearing the nitrile and the ester, where those same two groups spread it out and stabilise it. That is the crucial trick: the very groups that made the monomer easy to attack also make the resulting anion long-lived enough to attack again. So it does, on the next monomer, producing another identical anion at the new chain end.

Notice the shape of that. Each step consumes a monomer and regenerates the active site — the reaction carries its own trigger forward. One initiation can consume thousands of monomers, which is why a whisker of moisture on a fingertip solidifies a whole drop, and why setting is not gradual but arrives as a front sweeping through the liquid. This is anionic chain-growth polymerisation, and its defining feature is that initiations are few and additions per initiation are enormous.

Which finally explains the tube. If a trace of base starts it, the way to stop it is to keep the liquid slightly acidic and dry. Manufacturers dissolve a trace of an acidic stabiliser in the monomer, which protonates any stray anion the moment it forms and kills the chain before it goes anywhere. A radical inhibitor is usually present too, since these monomers can also polymerise by a radical route. The tube is not passive storage; it is a reaction held down by an inhibitor in slight excess. Extrude a drop onto a wet surface and that surface's basicity locally overwhelms the small acid reserve, and the chain runs.

Does the account survive its own predictions? It says a weak base should accelerate the cure, and amine sprays sold for the purpose are exactly that. It says cure should be slow on acidic surfaces — and acidic woods are a known problem case. It says the cure starts at the interface and works inward, so a thick layer should cure badly, and superglue is notoriously a thin-film adhesive that performs poorly in a gap. Three predictions, three matches. I do not know the specific stabiliser package in any given brand; those formulations are proprietary and vary, and I would not name one as though it were general.

The most useful correction is that superglue's grip is not a special stickiness. Two things are being conflated: the cure, which is the chemistry above, and the adhesion, which is the ordinary business of a liquid wetting a surface — getting into every irregularity while fluid, and being locked there when solid. The cyanoacrylate is remarkable for how fast and how completely it makes that transition, not for any unusual attraction to the substrate.

One smaller point follows from the mechanism: the reaction gives out heat, and a large amount soaked into cotton or wool, which offers enormous surface area and plenty of initiation sites, can release it fast enough to burn.

Finally, "instant" oversells it. What happens in seconds is enough polymerisation to fix the parts; full strength develops over hours as the reaction completes in the depth of the joint.

• The related case of anaerobic threadlockers, which cure when air is excluded — the mirror image of the guess we discarded.

Every term the collection defines is gathered in the glossary.