Question: Which hardeners are recommended for very fast curing epoxy adhesives?

Answer: There are several alternative hardeners for this type of application; my answer depends on what you are looking for in terms of performance and handling. Aliphatic amines can be used, as can materials like N-aminoethylpiperazines. Unfortunately, most of these hardeners have health and safety implications such as high toxicity or severe skin and eye irritation or damage.

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.