The aging of adhesives or adhesive bonding is defined as the change in the quality of adhesive bonding as a function of time and external effects that provoke the change.
There are three types of external factors that cause aging phenomenon both within the adhesive itself (cohesion) and the junction of the adhesive and the substrate (adhesion):
• Mechanical effects: including static loads, dynamic loads, cyclic loads and impact loads.
• Chemical effects: including effect of humidity and water condensation, effect of atmospheric oxygen, and concentrations of gases, solvents, salts and others in the environment.
• Physical effects: including temperature and ultraviolet radiation.
Each of these effects directly affect the adhesive joint, both in the adhesion and cohesion area. The result of the action of these factors on the adhesive joint depends on the type of substrate used and on the adhesive system (surface preparation of the substrate, activator, primer, adhesive, etc.)
For example, the presence of water on an adhesive joint can cause irreversible effect when using cyanoacrylate adhesives. This is produced because there is a hydrolytic cleavage of covalent bonds (hydrolysis) owing to water penetration through the phenomenon of diffusion, reducing the mechanical properties of the adhesive. However, this effect does not occur in adhesive joints using silicone adhesive or epoxy adhesives.
In order to prevent these effects, it is necessary to perform a series of accelerated ageing tests taking into account all factors that can act on the joint for life, in order to predict the behavior of the adhesive joint and select the appropriate adhesive system.
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