Effect of Water on Joint Strength

Orman and coworkers were among the first to demonstrate the adverse effects of water on adhesive joints.They exposed aluminum bonded with a DGEBA/4,4 -diaminodiphenylmethane epoxy adhesive to various environments, including water and ethanol. The adhesive adsorbed ethanol readily, which caused up to a 30% reduction in the tensile strength while 

Brewis et al have studied the effect of exposure to warm moist air on aluminum bonded with a DGEBA/di(l-aminopropyl-3-ethoxy)ether adhes-ive. They observed a linear relationship between the loss in joint strength and the fractional water content of the joints, calculated under the assumption that the water entered the joint by diffusion through the adhesive. The excellent correlation observed strongly supports the view that the loss of strength is due to plasticization of the adhesive caused by water uptake. 

Although the exact conditions were not specified in their paper, it is reasonable to assume that the results referred to specimens prepared with solvent-degreased adherends, whereas those of Brewis et al were chromic acid etched. The differences in behavior observed in the two studies therefore probably arise because of the different surface treatments. The more durable chromic acid-etch interface was not appreciably attacked during the time of exposure, while the solvent-degreased interface was. 

There are many examples to show that adhesive joints exposed to typical laboratory conditions, or temperate climates, are often unaffected, even after long periods of time. The relative humidity of such environments can be as much as 50%, although the temperature does not normally deviate very much from 20°C. Thus adhesive joints under some conditions are able to tolerate a certain level of water uptake, without apparently any detrimental effects on the strength (see Section H.C.3.). 

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In addition, it has been shown that the decrease in the cohesive strength of epoxy resins in water is due to plasticization and is completely reversible67 681. Kerr and coworkers 66,69-70) and others71,72), however, have shown (Fig. 7)661 that although the effect of water on joint strength is largely reversible, it is at least partially irreversible. 

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