Superglue destroys foam by cooking it from the inside

How glue actually hardens, and why a material made mostly of air cannot survive it.

Superglue hardens through a chemical reaction that gives off heat as it happens, and a generous bead of it applied to expanded foam releases that heat faster than the foam, which is mostly trapped air held in a fragile skin of plastic, can carry it away, so the heat builds up right at the joint and melts the very cell walls the glue was meant to hold together.

A wing joined along its full length with a thick bead of superglue looked perfectly bonded the evening it was built, but by morning it had come apart on its own, and running a finger along the seam found not a clean glued joint but a shallow, collapsed trench where the foam itself had visibly shrunk and puckered away from the glue line, as though something had been dragged along it while it was still soft.

The chemistry behind a superglue cure

Superglue, chemically a cyanoacrylate, does not harden by drying in the way a pot of paint dries, since nothing evaporates out of it. It hardens because the liquid monomer links itself, molecule to molecule, into long solid chains the instant it meets a trace of moisture, and the ordinary film of water vapour sitting on almost any surface in a normal room is enough to trigger that reaction within seconds. Cyanoacrylate curing this way is exothermic, meaning the chemical reaction gives off heat as a byproduct rather than absorbing it, and a thin film of glue spread over a heat-conducting material barely warms at all, because there is so little of it reacting at once and whatever heat it does produce is carried away almost as fast as it forms. A thick bead applied all at once behaves very differently, since a larger mass of glue means a larger amount of the reaction happening together, releasing its heat into a much smaller volume in a much shorter time.

Where you have already felt this

A hot water bottle left too long pressed against skin under a thick duvet can feel scalding even though the room around the bed stays perfectly cool, because the duvet is doing exactly what it is built to do, trapping the heat rather than letting it escape outward, so the heat has nowhere to go except into whatever is touching it. Expanded foam is built the same way for the same reason, since the countless tiny sealed pockets of trapped air inside it, separated by walls of plastic only a fraction of a millimetre thick, are what make it such a good insulator in the first place, and an insulator that is good at keeping heat out is equally good at keeping heat in. The heat from a curing bead of superglue, released right at the surface of the foam, has almost nowhere to escape to, and instead of spreading out and cooling harmlessly the way it would in a solid block of wood or metal, it stays concentrated exactly where the thin plastic cell walls are least able to withstand it.

What follows from this

Because the damage comes from heat concentrated in a small volume rather than from the glue attacking the foam chemically, the fix is simply to reduce how much glue reacts in one place at one time. A thin film, applied sparingly and spread rather than pooled, releases its heat gradually enough for the surrounding foam to carry it away before any real damage is done, and gap-filling cyanoacrylates formulated to cure more slowly avoid the same problem by spreading the same total reaction out over a longer time. It also explains why the trench left behind runs precisely along the glue line and nowhere else, since the foam only a few millimetres away, never in contact with the reacting glue at all, is left completely untouched, which is the clearest sign that the destruction is thermal and local rather than some general weakness of foam against glue.