Thinking about epoxies

We’ve had a lot of questions recently about curing epoxies from everyone from a knifemaker to a researcher in academia, so we thought it might be useful to discuss them a bit.

Expoxies are used just about everywhere from knifemaking to aerospace to electronics to construction. Because of this, a lot of folks realize that tracking the degree of cure is important. Many folks have developed approaches to estimating this and the most effective is looking at residual cure in the DSC. However, this approach does miss that the properties of a cured system are often path dependent.  Curing too fast or too slow might give different results, as would excessive ageing.  In some systems, different mechanisms occur and overlong storage at low temperature might give a different result than exposure to high storage temperature.

An example of this would be the adhesive commonly used to attached wood or composite handles to kitchen knives. If the final cured material is rubbery, rather than hard, at room temperature, is it because the cure was incomplete, because the material was stored wrong, or because the cure profile took us to the wrong structure? (For example, curing too fast so we get localized crosslinks only.)

A more thorough method of characterizing the cure that should be more commonly used, but for some reason isn’t, is the Gilman-Enns or TTT (Time-Temperature-Transition) diagram. While more time-consuming, this approach generates a diagram like a phase diagram for the curing process.  An example is below.  A similar diagram can be developed for non-isothermal cures as well as for photocures where light intensity replaces temperature.

TTT diagram from Dr Bilyeu’s thesis. Used with permission

For more information, you can look at the three articles on epoxies Drs. Brostow and Bilyeu did with Dr Menard in 2000.  Most of the information is still good.  Full-text copies are available on ResearchGate from Dr Brostow’s LAPOM page.