Moisture is the substance that causes the greatest difficulties in terms of environmental stability for many bonded or sealed joints. Water can be an exceptional problem because it is very polar and permeates most polymers. Other common fluids, such as lubricants and fuels, are of low or zero polarity and are not as likely to permeate and weaken adhesive or sealant joints.
Moisture can degrade a completed joint in three distinctive ways.
* Moisture can degrade the properties of the bulk adhesive or sealant itself.
* Moisture can degrade the adhesion properties at the interface.
* Moisture can also degrade the properties and cause dimension changes of certain adherends.
Ambient moisture can also affect certain types of uncured adhesive, either as it is being mixed and applied to a substrate or as it is stored in a container waiting to be applied. This degradation mechanism, however, will not be examined in this article. Rather, this article will focus on the affects and attack mechanisms of moisture on cured adhesive and sealant joints after they are placed into service.