4.5.3 Humidity and Water

The high polarity of water can lead to the weakening of bonded assemblies. All polymers absorb water to a certain extent. Although its effect on the interface is fundamental, water can also change the adhesive properties by changing the glass transition temperature and inducing cracks or chemical reactions.

The humidity of the environment can affect the adhesion force in two significant ways. Some adhesives will lose hardness, temperature resistance and, in the worst case, liquefy during exposure to hot and humid air. Water can also permeate the adhesive and displace the adhesive at the joint interface. This mechanism is the most common cause of reduced adhesive strength in humid environments.

The adhesion force deteriorates more quickly in an aqueous vapour environment than in liquid water due to faster water vapour penetration. Primers and surface treatments tend to prevent degradation of the adhesion force in humid environments.

In the case of substrates that are vulnerable to water and humidity, degradation of the substrate at the interface, regardless of any direct effect on the adhesive, will compromise the bond.