Bonding of Aluminium Alloy with some Phenolic Adhesives and a Modified Epoxide Adhesive, and Strength Changes on Exposure to Moist Air at 50°C

Abstract

Single lap joints of aluminium alloy bonded with three phenol-based and one modified epoxide adhesive have been aged at 100% relative humidity (r.h) or at 50% r.h. for up to 10,000 hours at 50°C. Whilst joints are not significantly weakened on exposure at 50% r.h., at 100% r.h. strength falls over about 2000 h and then tends to remain steady. This fall in strength is controlled by the rate at which water enters the adhesive layer. Some joints were exposed firstly at 100% r.h. for 5000 h., and then at 50% r.h. for a further 5000 h whereupon some recovery of strength took place. The effect of moisture on joint strengths can be interpreted in terms of water, by virtue of its high permittivity, weakening ion-pairs at the interface. X-ray photoelectron spectroscopy has been used to examine fracture surfaces at all stages, showing that visual inspection can lead to false conclusions about the mode of failure.