Ask Dr. Dave
Answer: It is difficult to give you an exact answer without the full details of the adhesive or what type of hardener you use for the epoxy. However, I can make some comments and generalizations about this type of adhesive.
Compared to some other types of structural adhesives, such as cyanoacrylates or reactive acrylics, epoxies are highly crosslinked structures and tend to become highly viscous very early in their cure cycle. The result is that the molecules’ mobility becomes restricted, which limits their ability to collide and react.
Is not unusual to see a lot of unreacted epoxy when curing is carried out at moderate temperatures. Heat has two effects: it lowers the viscosity and increases the reaction rate. A good way to observe this is to perform differential scanning calorimetry (DSC) on your cured adhesive. DSC measures the energy absorbed or released as the adhesive is ramped up from low to high temperatures.
In the case of the adhesive cured at ambient temperature, you will normally see an exothermic peak (heat released) during the scan as the remaining epoxy cures; the area under this peak gives you an estimate of the amount of unreacted epoxy in the sample. When an adhesive is fully cured at higher temperatures, this peak will not be observed. Heat-cured adhesives give the maximum adhesive strength and physical properties, plus enhanced heat and fluid resistance. It is common to have long heat cure cycles for very high strength epoxy adhesives.