Question: How can I tell if my epoxy is fully cured?

Answer: This is a good question, as many users assume that mixing a 2-part epoxy and curing at room temperature will eventually completely cure the resin. However, this is not the case; epoxies are quite viscous to start with and, as curing proceeds, the viscosity increases dramatically to a gelled state and then to a solid. As this reaction continues, the molecular mobility decreases and reactive groups on the resin and the hardener are unable to react in order to increase the molecular weight and crosslinking. Maximum properties for epoxies are only achieved by curing at high temperatures for long periods of time to complete the reactions.

A useful way to look at a cured epoxy is to carry out differential scanning calorimetry (DSC). DSC measures the energy input or output of the solid resin as it is scanned from low to high temperatures. A typical curve for a 2-part epoxy will show two transitions: first, an endothermic (energy being absorbed) change in slope. This is the glass-transition temperature (T_g), which is important because this is the temperature at which the material changes from a rigid solid to a softer, more rubbery state, with a corresponding decrease in properties such as tensile strength and heat resistance.

As you continue to scan to higher temperatures, you will see an exothermic (energy being emitted) peak, which is the remaining epoxy being cured. If you then scan the sample a second time, you will find that the T_g is at a higher temperature (more resin has cured and crosslinked) and the exothermic peak has disappeared; that is, all the resin has cured. If you don’t have access to a DSC, university labs can often run samples for you. That is not to say that resins cured at room temperature are not useful; they are widely used and offer tremendous convenience over heat-cured systems—as long as the properties desired are not too demanding.