The most widely used hardeners for very fast curing adhesives are specialty mercaptans. These hardeners have a relatively safe toxicity profile and can be mixed with bare hands. For example, they are used for hand-moldable epoxy putties. However, mercaptans do have a very distinctive “rotten eggs” odor, which may not be acceptable in some plant situations. The most common products formulated with mercaptans are the so-called “5-minute” epoxy adhesives, which are used in a 1:1 mix ratio and are often supplied in convenient twin-syringe or cartridge dispensers. These products gel in 5 min and cure fully in about
24 hours at ambient temperatures.

However, much faster versions can be formulated, if required, to match the speed of cyanoacrylate instant adhesives. I have used versions that gel in 20 sec, but be aware that a lot of heat is generated on curing, which may affect some substrates. These products give good performance on a range of materials, but keep in mind that epoxies cured at ambient temperatures have fairly low glass-transition temperatures (T_g) and soften and lose strengths in the 50-60˚C range. For maximum bond strengths and heat resistance, single-component heat-cured epoxies are preferred, although gel and cure times are measured in hours.