Why would Super Glue take an hour to harden

adhesive

I'm used super glue beginning to cure within a minute. I have a batch of tiny single-purpose Super Glue Gel tubes from 3M of unknown age. I want to use them to fix tiny parts to a circuit board. So the blob is exposed to a lot of air as opposed to disappearing between two mated parts. But after an hour it was still unset, and I could wipe most of it off.

Why? Too old? Bad batch? Too big a sample? Wait longer? Humidity too low?

Relevant portion of instructions: "Hold until set."

(Manually migrated from lifehacks.)

Best Answer

Superglue works best when spread in a thin layer, a "blob" will take longer to harden because a thin skin forms on the surface, slowing the curing:

https://en.wikipedia.org/wiki/Cyanoacrylate

In general, cyanoacrylate is an acrylic resin that rapidly polymerises in the presence of water (specifically hydroxide ions), forming long, strong chains, joining the bonded surfaces together. Because the presence of moisture causes the glue to set, exposure to normal levels of humidity in the air causes a thin skin to start to form within seconds, which very greatly slows the reaction. Because of this cyanoacrylate is applied thinly, to ensure that the reaction proceeds rapidly and a strong bond is formed within a reasonable time.

This question over on electronics.stackexchange.com talks about glues used to attach components to PCB's. That's probably a better site to ask any followup questions about the best way to glue components to PCB's.

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