Structural adhesives are categorised according to their chemistries: cyanoacrylates, acrylics, urethanes and epoxies. Of them all, structural epoxy provides the best overall bond strength on metal substrates and comes in widest range of properties. The standard rigid epoxy is best suitable for applications with no impact and relatively low stress. But there is also the flexible epoxy for assembly of parts that need some flex or bend; it can better withstand impacts and has higher peel strengths than rigid epoxy. For highest peel, impact, shear, fatigue and vibration resistance, toughened epoxy is the best option. It incorporates elastomeric regions that effectively absorb impacts, making it perfect for the most demanding applications.
However, the use of structural epoxy requires that any oil must not be present on metal joint surfaces to be assembled. In case oil cannot be removed, a one-part heat cure epoxy can still be used. When it comes to processing and handling considerations, one-part and two-part epoxies differ significantly. Unlike two-part epoxies, a one-part epoxy doesn’t need to be measured and mixed as it comes pre-mixed. But it is more prone to shelf life problems and requires curing at around 250° to 350°F. Therefore, it needs more careful handling than two-part epoxies. However, one-part epoxies are easy to dispense and offer the highest shear strengths and best resistance to high temperatures.