Effect of Water on Adhesive-Bonded Structures

Water must be considered a dominant factor in determining the permanence of adhesive-bonded joints in most weathering service environments. The structure may be affected or involved in a variety of ways, depending on whether water can penetrate into the adhesive structure itself to produce significant property changes, or move via some pathway to reach the interfacial areas and debond the adhesive. 

The unique effect of water on joint durability has been studied by many investigators. Falconer et published some of the earliest results followed by Scott,(70) and Kerr, 

In other experiments Orman and Kerr(72) showed that epoxy-bonded aluminum joints could show a significant loss in joint strength when exposed in 100% RH conditions during the 

FIGURE 4. Bond strength of aluminum/epoxy joints and tensile strength of epoxy resin after exposure to water and ethanol at 194 F (90°C). (From Reference 25.) 

Comyn et have followed the pattern of joint strength decline of aluminum/epoxide joints in 100% RH at 122°F (50°C) for up to 10,000 hours. They obtained a decline during the first 2000 hours followed by a plateau condition. Some joint strength restoration was possible if the humidity was then lowered to 50%. They proposed an explanation in terms of the high permittivity of water and a weakening of ion-pairs at the interface. 

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