Blistering as a form of degradation in adhesive joints

Water-filled blisters were observed to form during accelerated aging experiments with aluminum adherends and two structural epoxy adhesives. Both closed adhesive joints and open-face specimens were affected. The blisters grew with time, originated both in the adhesive and at the epoxy-aluminum interface, and were found only at 100% relative humidity at both 65 degrees C and 85 degrees C; blisters were never observed at 85% relative humidity or lower. The same water-soluble ionic species were found in the blister liquid, the two adhesives and water that had been in contact with adhesive samples for an extended period. It is proposed that the blisters grew under the influence of osmosis, originating in water clusters at microscopic voids. Contamination of the aluminum adherends by residual etching solution, although not a necessary precondition for blistering, could facilitate this process by lowering the partial pressure at which water condenses and by creating higher osmotic pressures.