Designing Adhesive Aging Studies
Aging of an adhesive bond
When designing aging studies of a bonded joint consider both the adhesive aging (cohesion) and the bond between both substrates (adhesion). Several adhesive aging studies list the key factors are chemical, mechanical and physical effects. One piece that isn’t always listed is time and speed
Chemical effects on a bonded joint are increased with concentration and temperature.
Physical effects on a bonded joint like light and heat are increased if there are mechanical and chemical factors at play. It is also significant to identify the speed at which the temperature changes and at what mechanical load the bond is under during that transition
Mechanical effects like the stress and load can be affected by both chemical and physical effects.
A key contributor to varying results between the adhesive aging and the bond aging can be time. As time (or speed of change) can affect the bond more than it affects the adhesive.
For example, an adhesive bond that is exposed to an elevated temperature which is well within its service range may fail if it is cooled to the low end of the thermal range too quickly. This is especially true when bonding dissimilar materials with different coefficients of thermal expansion. The adhesive itself is designed to withstand the change, but the differential coefficients of thermal expansion are not always identified as a mechanical load.
Note that combinations of these effects can multiply the effects on the adhesive.
There are some instances where a product with a lower adhesive strength outperforms due to increased elongation and flexibility of the adhesive which acts as a shock absorber during the thermal cycle.
ASTM has several standard methods for adhesive aging, which detail the test conditions and test procedures.