Will two-part epoxy eventually harden with insufficient hardener mixed in?

I mixed up a batch last night to glue something important, it seemed to be setting up just fine, but this morning it was still somewhat soft. It’s 5-minute epoxy.

Will it eventually harden anyway?

No, it won’t harden properly.

Depending on which compound is lacking, it may eventually cure, but using an incorrect mixture results in the polymer chains being short, having side branches, forming rings and other odd shapes that destroy the rigidity of the plastic. It will always remain putty-like and won;t bond to the surface properly.

Eventually is a long time…
If it hasn’t cured by now, it probably will never be strong enough, but you could try heating it with a hair dryer for a while and see if it helps harden it.

Damn… okay, thanks.

Heating it will not help because two-part epoxy doesn’t work by simple dehydration. The hardener needs to strip the side branches of the epoxy to produce a free radical. If the ratio is wrong then heating won’t do a damn thing.

If the mix is exposed you could try exposing it to strong UV light, which can be enough to initiate free radical production in some epoxies and might help if the mix is deficient in hardener rather than epoxy. Of course most cases where people use epoxy the mix is between two substances and can’t be exposed to light.

Heat increases the rate of the reaction.